6580我首付了1400、分期的金额本金是352.53共18期、18期就是6336、被坑了吗我们的生活Q人?2-我首付了1400、分期的金额本金是352.53共18期、18期就是6336、被坑了吗-爱问知识网
6580我首付了1400、分期的金额本金是352.53共18期、18期就是6336、被坑了吗
?2-01 03:23
分期付款就这样啊，你想买又付不了全款，要么不买，要么就得多付点
?2-01 03:34
有手续费用的！还有利息
?2-01 03:25SOLENOID OPERATED, TWO WAY DIVERTER VALVE FOR FLUID LINE WASHING APPARATUS
United States Patent 3670744
A washing apparatus including a fluid circuit for conducting a cleansing solution through conveying conduits, a drying means for blowing heated air through the conduits after they are cleaned, and improved, solenoid operated two way valves for selectively diverting the washing and drying fluids. An improved, solenoid operated, two way diverting valve for fluids.
Inventors:
BENDER LLOYD F
Application Number:
Publication Date:
06/20/1972
Filing Date:
01/11/1971
Export Citation:
LLOYD F. BENDER
Primary Class:
Other Classes:
International Classes:
B08B3/10; B08B9/02; B08B9/032; (IPC1-7): B08B3/10; B08B9/02
Field of Search:
134/166,169,57R,58R 137
View Patent Images:
&&&&&&PDF help
US Patent References:
3500839Bender
Primary Examiner:
Roberts, Edward L.
1. A system for automatically washing and drying fluid conduits, said system comprising, a fluid tank having an openable drain, drain closing means, supply means for supplying water to said tank, fluid circulating means in fluid receiving communication with a conduit to be washed and dried, a dryer for hea a two way intake valve connected to said conduit and when in one position being in fluid receiving communication with said tank whereby fluid from said tank is conducted through said conduit to wash the latter, said intake valve when in another position being in air receiving communication with said dryer whereby air is forced through said condu a two way discharge valve having a fluid receiving connection with said fluid circulating means and also having a discharge to said tank, whereby when said discharge valve is in one position fluid from circulating means is discharged into said tank, and when said discharge valve is in another position fluid from said circula each of said two way valves comprising, a housing having an upper nipple, an intermediate nipple, and a lower nipple, baffle plate means in said housing and defining an upper chamber and a lower chamber therein, a shiftable control rod extending into said housing and having an inner end in said housing and an outer end outside of said housing, a stopper fixed on the inner end of said rod and engageable with said seat for sealing said lower chamber from said upper chamber, said upper nipple and said intermediate nipple being in fluid communication with said upper chamber and said lower nipple being in fluid communication with said lower chamber, and means for shifting said stopper between an open position in which said lower chamber communicates with said upper chamber and a closed position on said seat in which said lower chamber is not in communication wit and electrical control means including a timer, said control means operative to (1) actuate said water supply means to fill said tank, (2) actuate said fluid circulating means, (3) actuate said drain closing means, (4) shift the positions of said two way valves, and (5) actuate said dryer, all in timed sequence.
2. A system for automatically washing and drying fluid conduits, said system comprising, a fluid tank having an openable drain, drain closing means, supply means for supplying water to said tank, fluid circulating means in fluid receiving communication with a conduit to be washed and dried, a dryer for hea a two way int a housing having an upper nipple, an intermediate nipple, and a lower nipple, baffle plate means in said housing and defining an upper chamber and a lower chamber therein, a shiftable control rod extending into said housing and having an inner end in said housing and an outer end outside of said housing, a stopper fixed on the inner end of said rod and engageable with said seat for sealing said lower chamber from said upper chamber, said upper nipple and said intermediate nipple being in fluid communication with said upper chamber and said lower nipple being in fluid communication with said lower chamber, said stopper being shiftable between an open position in which said lower chamber communicates with said upper chamber and a closed position on said seat in which said lower chamber is not in communication wit said intermediate nipple having a fluid connection with said conduit to be washed and dried, said upper nipple having a fluid receiving communication with said tank whereby fluid from said tank is conducted through said conduit to wash the latter, said lower nipple having an air receiving communication with said dryer whereby air is forced through said conduit to dry the latter, a two way discharge valve having a fluid receiving connection with said fluid circulating means and when said discharge valve is in one position fluid from circulating means is discharged into said tank, and when said discharge valve is in another position fluid from said circula and electrical control means including a timer, said control means operative to (1) actuate said water supply means to fill said tank, (2) actuate said fluid circulating means, (3) actuate said drain closing means, (4) shift the positions of said two way valves, and (5) actuate said dryer, all in timed sequence.
3. The system set forth in claim 2 further characterized in that said two way disch a housing having an upper nipple, an intermediate nipple, and a lower nipple, baffle plate means in said housing and defining an upper chamber and a lower chamber therein, a shiftable control rod extending into said housing and having an inner end in said housing and an outer end outside of said housing, a stopper fixed on the inner end of said rod and engageable with said seat for sealing said lower chamber from said upper chamber, said upper nipple and said intermediate nipple being in fluid communication with said upper chamber and said lower nipple being in fluid communication with said lower chamber, said stopper being shiftable between an open position in which said lower chamber communicates with said upper chamber and a closed position on said seat in which said lower chamber is not in communication with said upper chamber, said upper nipple having said fluid receiving communication with said circulating means, said lower nipple dischargeable into said tank, and said intermediate nipple open to dump said fluid.
Description:
BACKGROUND OF THE INVENTION The present invention pertains generally to the cleaning of fluid conveying equipment and particularly to two way diverter valves which find particular utility when used in such systems. An example of prior art valves used in such systems is shown in FIGS. 1 and 4 of my U.S. Pat. No. 3,500,839, issued Mar. 17, 1970, and entitled "Automated Washing System for Cleaning, Sanitizing and Drying Flexible Tubing or the Like." SUMMARY OF THE INVENTION The present invention provides an improved solenoid operated, two way diverter valve for various fluids, and which valve finds particular utility in combination with an automatic washing system for cleaning and drying flexible tubing of milk handling equipment or the like. These and other objects and advantages of the present invention will appear hereinafter as this disclosure progresses, reference being had to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a washing system embodying the present invention, certain parts being shown as broken away or in section for cla FIG. 2 is a sectional view, taken along line 2--2 in FIG. 1, but on an enlarged scale, and showing the solenoid operated slide valve for FIG. 3 is a plan view of the control box shown in FIG. 1, but on an enlarged scale, with certain parts shown as being bro FIG. 4 is a cross sectional view of one of the two way valves shown in FIG. 1, but
FIG. 5 is an electrical wiring diagram used with t FIG. 6 is a cross sectional view similar to FIG. 4 and showing the other two- and FIG. 7 is a chart showing the timer sequence. DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 shows the general organization of an arrangement using the invention and includes a fluid circulating means FCM which takes the form of an electrically timed washer- solution tank T for holding the clear water, washing solution or rinse solution, the electrical automatic control means AC, the air heating means such as an electric dryer D; the improved solenoid operated two way valves VI and VD, one valve being an intake valve VI and the other a discharge valve VD; and the flexible tubing FT which is to be completely cleaned and dried. It will be appreciated that other forms of fluid circulating means such as a pressure pump (not shown) may also be used with the present invention. FLUID CIRCULATING MEANS Referring in greater detail to FIG. 1, the washer-releaser type of fluid circulating means FCM may be of the general type shown and described in my U.S. Pat. No. 3,273,514, issued Sept. 20, 1966. The washer-releaser would include the hollow vessel 1 of spherical shape and transparent material which has an integral fluid discharge conduit 3 extending downwardly from its lowermost side, and a one-way dump valve 3a, such as shown in my U.S. Pat. No. 3,352,248 of Nov. 14, 1967, for releasing fluid when the vessel is opened to atmosphere. The vessel has a fluid inlet 4 at its upper end and a vacuum conduit 5 adjacent its upper end and to which is attached by a conduit 6, a slide valve SV. The slide valve is electrically operated by a timer 7 driven by an electric motor 7a so as to alternately subject the vessel to a vacuum from conduit 8, or to the atmosphere via an opening 9 in the slide 10 of the valve. If a more complete description of the valve SV is deemed to be either necessary or desirable, reference may be had to the said U.S. Pat. No. 3,273,514. It is believed sufficient to say that a vacuum pump motor 11 provides the vacuum to which conduit 8 is attached. As the timer 7 causes either one of the solenoids 13 or 14 (FIGS. 2 and 5) to shift the slide in one direction or the other, either vacuum is drawn from the vessel 1 or alternatively, the vessel is opened to atmosphere. When the vessel is subjected to vacuum, it draws fluid from conduit 15 attached to its inlet 4, and when the vessel is opened to atmosphere, fluid is released via valve 3a and to the conduit 17 attached to the vessel discharge conduit 3. Thus the washer-releaser FCM acts as a pump to draw fluid into the vessel and then release the fluid. DRYER The dryer D is an air heater and blower and may be of the type shown in U.S. Pat. No. 3,067,756, issued Dec. 11, 1962, and includes an electric motor which draws air in through its enlarged end 18 and forces the air, after it is heated, out of its restricted outlet 19. TANK The tank T includes a closeable drain 20 at its bottom and when the drain is closed, as will appear, the tank can hold either clear water, a washing solution, or a rinse solution. The tank is filled or emptied by the action of the electrically operated, automatic control means AC, to be described. CONTROL MEANS The box 22 for the control means AC is preferably mounted directly on the tank and houses some of the components shown in the electrical diagram of FIG. 5. Drain closing means in the form of a drain stopper 23 is fixed on the lower end of a rod 24 which extends downwardly from the box 22. The rod 24 is vertically shiftable in a bushing 25 fixed on the bottom of box 22 and a spring 26 biases the rod downwardly to normally close the drain. An electric solenoid 27 is connected to the top end of rod 24 and is energized by a timer 29, via cam 30, to cause the rod and stopper to raise, thus opening the tank drain 20. A pair of water hoses 33 and 34 are connectable to conventional spigots (not shown) of cold and hot water lines, respectively, for furnishing the necessary water. The water flows through lines 33 and 34 (FIG. 3), through a mixer valve 35 which is operated by a solenoid 36 and down a feeder tube 37 into the tank T. The timer 29 also has a cam 42 (FIG. 5) causing actuation of the solenoid coils of the solenoids 36 or 39 (FIGS. 3 and 5). As will appear later, the timer is arranged so that the tank will first fill, for example, with water at about 95° temperature for the first rinse. The tank later fills with hot water including detergent and after that washing cycle, the tank is again filled with clear water for the final rinse during which a measured amount of acid, from a container (not shown) may be dispensed into the rinse solution. More specifically, the addition of the acid is regulated by cam 44 (FIG. 5) which actuates the solenoid coil 43b which in turn operates a vacuum valve 43c that admits vacuum via line 43d to pump 43a. The level of the fluid in tank T is controlled by a normally closed pressure switch 45 which causes the closing of the solenoid operated water valve. The pressure switch 45 is activated by a predetermined level of the fluid in a pressure tube 46 which extends from the switch 45 down into the tank T, and the switch 45 thus regulates the water valves and consequently the flow of water into the tank. TWO WAY VALVES The housing H of the valve VI (FIG. 4) is preferably molded from plastic and is of two parts, a lower part HL and an upper part HU, each having outwardly extending flanges secured together by bolt means 49. A baffle plate 48 is located and held between the flanges of the housing parts. An upper chamber 47 and a lower chamber 47a are thus formed in the housing. A bevelled seat 51 extends through the plate 48. The intake valve VI is operated by a solenoid 50 (FIGS. 3 and 5), and returned by gravity. The solenoid shifts a rod 50a carrying a rubber, frustoconical, stopper 50b at its end, closing the valve by pulling the stopper against a seat 51. The housing H has an upper nipple 52 to which is attached the conduit 53. This valve is of the two way type and its stopper 50b is shiftable so that (1) when stopper 50b is closed, it permits fluid from the tank T to be picked up in conduit 53 and conducted via conduit 54 to the flexible tubing FT or (2) when stopper 50b is in an open position, then hot air can be blown from the dryer D which is driven by electric motor 57, through conduit 55 and into conduit 54 for drying the tubing FT. The housing H has an intermediate height nipple 54a to which conduit 54 is connected, and also has a lowermost nipple 55a to which the dryer conduit 55 is connected. Valve VI has a check ball CB in its side nipple 52 which can seated against the valve seat 60. Stop pin 60a is located on the side of seat 60 to hold the ball in proximity to the seat. The ball CB acts to close on seat 60 when the dryer is being used to dry the tubing, at which time the stopper 50b is in an open position, and dry air is forced out conduit 54 to dry the tubing FT. The discharge valve VD is similar to valve VI except it does not have a check ball CB. Valve VD serves to return solution, which is received via line 17 and intermediate nipple 54b from the releaser vessel 1 to the tank T via its upper nipple 58 and conduit 58a (FIGS. 1 and 6), or it diverts or dumps fluid to a waste drain or the like via its lower nipple 59 and conduit 59a. The discharge valve is operated by a solenoid 61 which is energized by a cam 62 of the timer 29. The timer 29 also has another cam 65 which, through coil 66 of a 40 ampere contactor 67, actuates the vacuum pump motor 11 and the timer 7 of the fluid circulating means FCM. The 25 ampere contactor 51 controls the dryer D. OPERATION When it is desired to clean the flexible tubing FT without unwinding it from the portable tank cart 75 on which it is mounted, conduits 15 and 54 are connected to the flexible tubing at the quick detachable points 15a and 54a, respectively, to place it in communication with the fluid circulating means FCM and the intake valve VI. Alternatively, the flexible tubing can be connected directly to the washer-releaser and valve VI without the use of separate conduits 15 and 54. The timer knob 77 is set to the "on" position and the tank fills with 95° water. After the tank is filled, the motor 11 will start to operate the fluid circulating means for a rinse cycle, say 5 minutes, the normally open intake valve VI permitting fluid from the tank T to flow to the flexible tubing. The discharge valve VD at this time diverts the rinse fluid to the drain. The drain stopper in the tank then is opened when the solenoid 27 is energized for say 3 minutes, to allow the tank to drain. Solenoid 50 is then de-energized and valve VI shifts by gravity, and dryer motor 57 operates. After the tank is emptied, solenoid 27 is de-energized and drain stopper 23 is then closed by spring 26 and the dryer D starts blowing hot air for a period, say 30 minutes, preset on the dryer timer DT. The tank T then fills with hot water mixed with detergent and the vacuum pump motor 11 then operates, for say, 10 minutes, to wash the tubing FT. The drain stopper 23 then again opens the tank drain for 3 minutes to drain the tank T. The drain stopper 23 is closed again by spring 26. The tank T again fills to the set level with 95° water. After the tank T is filled, the vacuum pump starts, and fluid circulating means FCM functions for a 5 minute rinse period. After about 1 minute has elapsed in the rinse period, a measured amount of acid will be dispensed into the rinse solution. The drain stopper then opens for 4 minutes. The drain stopper closes and the dryer starts blowing warm air for the period preset on the dryer timer, and then it also stops.
& 2004-. All rights reserved.On-line version ISSN X
Acta Limnol. Bras. vol.24 no.2 Rio Claro Apr./June 2012
Epub Sep 25, 2012
http://dx.doi.org/10.-975X6
Biogeochemical
cycling of urea in the aquatic systems of Pindar& and Turia&u
River basins, a pre-Amazonian floodplain, Baixada Maranhense, Brazil
Ciclo biogeoqu&mico
da ureia nos sistemas aqu&ticos das bacias hidrogr&ficas do Pindar&
e Turia&u, v&rzea pr&-Amaz&nica, Baixada Maranhense,
Osamu MitamuraI;
Nobutada NakamotoII; Maria do Socorro Rodrigues Iba&ezIII;
Paulo Roberto Saraiva CavalcanteIV; Jos& Policarpo Costa NetoIV;
Ricardo BarbieriIV
of Environmental Sciences, University of Shiga Prefecture, Hikone, Shiga 522-0057,
Japan e-mail:
IIDepartment of Applied Sciences, Shinshu University, Ueda, Nagano
386-8567, Japan e-mail:
IIIDepartamento de Ecologia, Universidade de Bras&lia – Unb,
Campus Darcy Ribeiro, Asa Norte, CEP , Bras&lia, DF, Brazil
IVDepartamento de Oceanografia e Limnologia, Universidade Federal
do Maranh&o – UFMA, Campus do Bacanga, CEP , S&o Lu&s,
MA, Brazil e-mail: ;
work is aimed at extending the understanding of urea cycle in freshwater ecosystems.
Its degradation rate concerning microorganisms activities was measured in the
turbid waters of Pindar& and Turia&u rivers located on the pre-Amazonian
floodplain, B
METHODS: The Pindar& and Turia&u aquatic systems have distinct
dry and rainy periods. The investigation was developed in the middle of the
rainy period, and field activities were carried out in the extensive water bodies
from the middle to lower reaches. The rate of degradation in the surface waters
of the two aquatic systems was determined with in situ simulation technique
using 14C-labelled urea. Photosynthetic rates were determined by the radiocarbon
technique simultaneously with experimental measurements of urea
RESULTS: Urea degradation rates (sum of carbon incorporation into particulate
matter and CO2 liberation into water) were 2.0&mg urea&C&m–3
day–1 in the Pindar& and 17.1&mg urea C m–3 day–1 in the Turia&u
waters. Daylight values were obviously higher than those in the dark, and the
urea degradation rates in Turia&u showed much higher values than those
in Pindar&. Most of the urea degradation occurred during the CO2 liberation
phase. The average of chlorophyll a specific urea degradation rate was 0.13
and 0.83&mg urea C&mg chl.a–1 day–1 in both river waters. The ratio
of urea carbon degradation to photosynthetic carbon assimilation in both waters
averaged 1.2 and 4.2%, respectively. The residence time of urea in the surface
water was calculated as 2.3 to 4.5 days in the Pindar& and 0.21 to 0.50
days in the Turia&u. Much shorter residence times were obtained in the
Turia&u due to the high degr
CONCLUSION: The correlation coefficient between the urea degradation
rate and chlorophyll a or photosynthetic rate showed a statistically significant
value in the Turia&u. This indicates that in the Turia&u system
the urea degradation rate was proportional to the standing crop of phytoplankton
and their photosynthetic rate. A strong relationship between the urea carbon
incorporation rate and photosynthetic rate in the light bottles was observed,
indicating that the carbon incorporation into the phytoplankton cells was also
related to the photosynthetic rate. The present brief residence time indicates
that the urea was being rapidly recycled in the euphotic zone of the investigated
systems in the rainy period.
urea degradation, phytoplankton, pre-amazonian floodplain
O objetivo deste trabalho & ampliar o conhecimento do ciclo da ureia
em ecossistemas de &gua doce. A taxa de sua degrada&&o
em rela&&o &s atividades dos microorganismos foi medida
nas &guas turvas dos rios Pindar& e Turia&u localizados
na v&rzea pr&-Amaz&nica, Baixada Maranhense, B
M&ETODOS: Os sistemas aqu&ticos dos rios Pindar&
e Turia&u t&m per&odos diferentes de chuva e seca. A investiga&&o
foi realizada no meio do per&odo chuvoso, e as atividades de campo foram
feitas ao longo dos extensos corpos d'&gua compreendendo &guas
de m&dio a baixo cursos. A taxa de degrada&&o nas &guas
superficiais nas respectivas esta&&es de ambos os sistemas foi
determinada com a t&cnica de simula&&o in situ usando 14C-
ureia. As taxas fotossint&ticas foram determinadas pela t&cnica
de radio carbono simultaneamente com medidas experimentais de degrada&&o;
RESULTADOS: As taxas de degrada&&o (somat&rio da
incorpora&&o de carbono na mat&ria particulada e libera&&o
de CO2 na &gua) foram 2,0&mg de ureia C m–3 dia–1 nas &guas
do Pindar& e 17,1&mg ureia C m–3 dia–1 nas do Turia&u. Os
valores no claro foram naturalmente maiores que os obtidos no escuro, e as taxas
de degrada&&o no Turia&u foram muito mais elevadas que
as do Pindar&. A maior parte da degrada&&o ocorreu durante
a fase de libera&&o de CO2. As taxas de degrada&&o
espec&fica de ureia da clorofila a foram calculadas com valor m&dio
de 0,13 e 0,83&mg ureia C&mg chl.a–1 dia–1 nas &guas de ambos
os rios. As taxas de degrada&&o de ureia carbono para a assimila&&o
fotossint&tica de carbono em ambas as &guas foram em m&dia
1,2 e 4,2%, respectivamente. O tempo de resid&ncia desse composto org&nico
na superf&cie da &gua foi entre 2,3 a 4,5 dias no Pindar&
e 0,21 a 0,50 dias no Turia&u. Tempos de resid&ncia muito mais
curtos foram achados nas &guas do Turia&u devido & elevada
taxa de degrada&&o;
CONCLUS&AO: O coeficiente de correla&&o entre a taxa
de degrada&&o da ureia e a clorofila a ou a taxa fotossint&tica
foi calculada como um valor estatisticamente significativo nas &guas
do Turia&u. Isto indica que no sistema Turia&u a taxa de degrada&&o
desse nutriente foi proporcional & quantidade de fitopl&ncton e
sua taxa fotossint&tica. Foi observada uma s&lida rela&&o
entre a taxa de incorpora&&o de ureia carbono e a taxa de fotoss&ntese
nas garrafas no claro, indicando que a incorpora&&o de carbono
no fitopl&ncton est& relacionada com a taxa fotossint&tica.
O breve tempo de resid&ncia indica que a ureia estava sendo rapidamente
reciclada na zona euf&tica dos sistemas investigados no per&odo
Palavras-chave:
degrada&&o da ureia, fitopl&ncton, v&rzea pr&-amaz&nica
Introduction
Amazon River ecosystem
has been thoroughly researched (Fisher and Parsley, 1979; Sioli, 1984; Forsberg&et&al.,
1988; Fisher&et&al., 1988; Marlier, 1967; Melack and Fisher, 1990;
Richey&et&al., 1990; Devol&et&al., 1995; Junk and Weber,
1996; Iba&ez, 1997; Bozelli and Garrido, 2000; Carneiro&et&al.,
2002; Farjalla&et&al., 2002; Roland&et&al., 2002; Guenther
and Bozull, 2004). Freshwater bodies in this floodplain provide high levels
of productivity, which are sustained by various biogeochemical cycles. There
are also data from lentic waters in the Baixada Maranhense published by Reid
and Turner (1988), Barbieri&et&al. (1989) and Iba&ez&et&al.
Urea is one of
the important nitrogen sources for phytoplankton (Eppley&et&al., 1971;
McCarthy, 1972; Harvey and Caperon, 1976; Kristiansen, 1983; Harrison&et&al.,
1985; Mitamura and Saijo, 1986a; Price and Harrison, 1988; Chang&et&al.,
1995; Presing&et&al., 2001; Twomey&et&al., 2005) and plays
a significant role in the nitrogen cycle (Mitamura and Saijo, 1986b; Mitamura&et&al.,
2006) of freshwater lakes and oceans. The degradation of urea by phytoplankton
has been demonstrated using 14C-labelled urea to elucidate the metabolism of
nitrogenous compounds in freshwater lakes (Carpenter&et&al., 1972a;
Remsen&et&al., 1972; Webb and Haas, 1976; Mitamura, 1986; Mitamura
and Saijo, 1986a; Mitamura&et&al., ). Urea is presumably
degraded more effectively by phytoplankton than by bacteria, although urea-degrading
bacteria are extensively distributed in natural waters (ZoBell and Feltham,
1935; Taga, 1972; Satoh and Hanya, 1976; Cho and Azam, 1995; Cho&et&al.,
Some data are available
on urea degradation for lacustrine ecosystems. In the euphotic zone of freshwaters,
urea degradation is associated with the photosynthesis of phytoplankton (Mitamura
and Saijo, 1986a; Mitamura&et&al., 2000). Several investigators have
reported using 15N-labelled urea that it plays a significant role as a nitrogen
source for phytoplankton growth (Mitamura and Saijo, 1986a, 1986b; Gu&et&al.,
1997; Presing&et&al., 2001; Mitamura&et&al., 2006). Mitamura
and Saijo (1986a) suggested that urea is mainly degraded during the urea nitrogen
uptake by phytoplankton, and that the urea carbon is assimilated simultaneously
by phytoplankton as one of their carbon sources.
Knowledge of urea
degradation and its cycling in diversified tropical freshwater bodies is quite
limited. The current investigations were carried out in a typical pre-Amazonian
floodplain ecosystem, the Baixada Maranhense, Maranh&o, Brazil, located
in the southern part of the Amazon River estuary. Our aim is to understand the
urea cycle in a freshwater ecosystem by measuring its rate degradation in relation
to the activities of microorganisms in the Turia&u and Pindar&
aquatic systems, which differ in salinity and turbidity.
Material and
Investigation
Baixada Maranhense
is an Environmental Protection Area (APA in Brazil), with an area of almost
18,000&km2 (2° 00'-4° 00' S and 44° 20'-45° 30' W) and distinct dry and
rainy seasons. This wetland area is subject to a seasonal flooding cycle with
medium-scale river water level fluctuations (4-6 meters). During our investigation
the Pindar& and Turia&u aquatic systems were in the middle of
the rainy period. Water levels, therefore, were consequently considered low.
Field investigations were carried out of extensive water bodies located in the
middle to lower reaches of the Pindar& and Turia&u Rivers in March
Urea degradation
rate in surface waters was measured at five stations (Sta.1; 2° 17.72' S and
45° 22.98' W, water depth of 2.6 m, Sta.2; 2°&18.66'&S and 45° 22.74'
W, 3.5 m, Sta.3; 2°&18.96' S and 45°&20.59' W, 2.5 m, Sta.4; 2° 17.13'
S and 45°&20.08' W, 3.4 m, Sta.5; 2° 15.83' S and 45° 19.40' W, 4.7 m)
of the Turia&u River, and at seven (Sta.1; 3° 19.52' S and 45° 00.36'
W, 9.0&m, Sta.2; 3° 19.31' S and 45° 01.00' W, 5.5 m, Sta.3; 3° 13.69'
S and 45° 00.95' W, 1.8 m, Sta.4; 3° 13.51'&S and 44° 59.99' W, 1.5 m,
Sta.5; 3° 15.83'&S and 44° 59.74'&W, 1.5 m, Sta.6; 3° 14.61'&S
and 45° 01.70'&W, 1.4 m, Sta.7; 3° 14.15' S and 45° 03.49'&W, 1.5
m) of the Pindar& River aquatic systems, respectively. The present investigation
stations were lentic in character, exhibiting the geomorphometric property of
a shallow lake located at the middle-lower reaches of both aquatic systems.
Procedure of
chemical analyses
Water temperature,
pH and electric conductivity were measured in the field using a boat equipped
with a water quality monitoring system (YSI, model 33). Measurements of transparency
were taken with a Secchi disk. Electric conductivity was equated to the value
at 25 °C. Water samples (approximately 0.2&m depth) were collected at the
respective stations with a plastic pail. These samples were used for turbidity
measurements in the laboratory using a Turbidmeter (Hach, 2100N). After filtration
with a 0.2 um membrane filter, the concentrations of six major elements (Na,
K, Mg, Ca, Cl, SO4) were determined with an Ion Chromatograph (Dionex, DX-120).
For the determination
of biogeochemical constituents and chlorophyll a concentrations, the waters
were immediately filtered through glass fiber filters (Whatman GF/F) purged
of organic matter by ignition at 420 °C. Both the filters and filtrates were
then frozen solid at –20 °C until chemical analyses in the laboratory. Urea
concentration was determined by Newell&et&al. method (1967). Ammonia
was determined by Sagi method (1966); nitrite after Bendschneider and Robinson
(1952), nitrate after Mitamura (1997), and phosphate (DIP) after Murphy and
Riley (1962). Dissolved organic carbon (DOC) and nitrogen (DON) were determined
with an infra-red Total Organic Carbon Analyzer (Shimadzu, TOC-5000A) and an
Automatic Total Nitrogen Analyzer (Yanaco TN-301P), respectively. Chlorophyll
a was determined with a fluorometer (Turner Designs, 10-AU) according to Holm-Hansen&et&al.
(1965). Particulate organic carbon and nitrogen were determined with a CHN Corder
(Yanaco, MT-5) after the samples were free of carbonate carbon using 1 M HCl
Measurement
of urea degradation and photosynthesis
For measuring urea
degradation rates, 20&mL of water samples were taken from the surface layer
in the two respective aquatic systems and, after, this volume was placed into
three series of clear plastic bottles. Crystalline 14C labeled urea (A
sp act, 1.85 GBq&mmol–1) was dissolved in sterile, deionized distilled
water, and the 14C labeled urea stock solution was then stored at –20 °C. After
adding 0.5&mL of the diluted 14C labeled urea solution (containing 1.0&nmole
12C urea and 1.85 kBq 14C urea) into each bottle, 0.2&mL of concentrated
formaldehyde solution was immediately added to the series of control bottles.
The second series of bottles was wrapped in a black sheet for determination
of the urea degradation rate in the dark regime. The series of transparent and
dark bottles were incubated in a water tank which was placed in the field. Incubation
temperature was similar to that of the sampling stations. After incubating the
bottles during the period of sunlight, the biological activity was stopped by
adding formaldehyde solution. The weather condition on the respective incubation
days was pallid sky. The sample water in each bottle was then filtered through
a 0.2 um Nuclepore filter. The filter was put in a scintillation vial, and 10&mL
of Bray scintillation fluid (Bray, 1960) was added. The radioactivity was then
measured with a liquid scintillation spectrometer (Aloka LSC-651) to determine
the rate of urea carbon incorporation into the particulate matter. The filtrate
of each sample was poured into a separate 50&mL glass bottle with a screw
cap, and a CO2 absorption tube containing 0.5&mL of n-ethanolamine was
inserted into each bottle to absorb the 14CO2 liberated from the water sample
solution by acidification. After adding 0.5&mL of 1&M sulfuric acid
solution to each filtrate, the bottles were then sealed tightly and left for
four days at room temperature. After adding the scintillation fluid to n-ethanolamine
containing 14CO2 liberated from the water sample in the bottle, the radioactivity
was determined as described above.
Photosynthetic
rates were measured by Steemann Nielsen's radiocarbon technique (1952) simultaneously
with the experimental measurements of the urea degradation rate. The water samples
were poured into both light and dark bottles and inoculated with 14C bicarbonate
solution to a final concentration of 185 kBq L–1. Thereafter, measurements of
the photosynthetic rates were conducted using the same procedure as that for
the urea carbon incorporation rate as described above. The concentration of
total carbon dioxide in the water sample was determined with an infrared carbon
dioxide analyzer (Horiba VIA-510), as described by Satake&et&al. (1972).
Results and
Discussion
General features
There were no appreciable
differences in the vertical distribution of the physico-chemical variables determined
at the respective investigation stations, indicating that water column was well
mixed vertically.
Water temperature
ranged from 27.4 to 31.6 °C and pH exhibited a weak acidic property. Electric
conductivity revealed a considerable difference between the two aquatic systems,
ranging from 45&±&0.1 to 51&±&0.1 uS&cm–1 (as an average
value with standard deviation) in the Turia&u and 22&±&3 to
28&±&1 uS&cm–1 in the Pindar& ().
In the Pindar& high salinity of 144&±&6&mg L–1 was calculated
as estimated from the sum of the six major ionic elements (Na+, K+, Mg2+, Ca2+,
Cl–, SO42–), while in the Turia&u salinity presented a fairly low value
of 26&±&1&mg L–1. The major ionic constituents (Na+, K+, Mg2+,
Ca2+, Cl–, SO42–) in waters of the present aquatic systems showed a variation
depending on the type of chemicals. Transparency measured with a Secchi disk
was 0.1 to 0.2&m in the Pindar& and 0.4 to 0.5&m in the Turia&u.
Turbidity ranged from 82&±&5 to 107&±&6 NTU and 19&±&3
to 23&±&2 NTU in the Pindar& and the Turia&u systems,
respectively.
Distribution
of urea and biogeochemical components
Urea concentrations
and other biogeochemical variables in the surface waters at respective stations
in the Pindar& and Turia&u aquatic systems were listed in Table&1.
Urea concentrations ranged from 0.36&±&0.17 to 0.66&±&0.09
uM, as an average with standard deviation (average 5.9 ug C L–1 as urea carbon
and 13.7 ug N L–1 as urea nitrogen) in the Pindar&, and 0.30&±&0.08
to 0.64&±&0.11 uM (5.5&ug C L–1 and 12.8 ug N L–1) in the Turia&u.
The urea concentration displayed no appreciable change among stations in either
of the aquatic systems. Total nitrogenous nutrient (TNN; sum of urea, ammonia,
nitrite and nitrate nitrogen) amounted to 68&±&11 ug N L–1, as an
average value with standard deviation, in the Pindar&, and 42&±&8&ug&N
L–1 in the Turia&u. This indicates that in both aquatic systems these
nitrogenous compounds were not mostly loaded from their watersheds and/or were
rapidly consumed by microorganisms. The principal nitrogenous nutrients were
ammonia and nitrate. An appreciable amount of urea nitrogen in the TNN was observed,
ranging from 15 to 27% and 24 to 38% of TNN in both the Pindar& and Turia&u.
In freshwater lakes, urea was found to make an appreciable contribution to the
nitrogenous nutrients (Satoh&et&al., 1980; Mitamura and Saijo, 1981).
The present concentrations of urea in the waters of the Pindar& and Turia&u
systems were comparable to those reported by the above investigators in oligotrophic
to eutrophic waters (Mitamura&et&al., 1995, Saijo&et&al.,
1997). The urea levels in the present aquatic systems seem to constitute the
source of one of the essential nitrogenous nutrients for phytoplankton growth
in the euphotic zone of both aquatic systems.
Contributions of
urea carbon to DOC averaged 0.15% in the Pindar& and 0.08% in the Turia&u
waters, while the percentages of DON, on the other hand, were 3.4% and 1.5%
in both waters. Mitamura and Saijo (1981) have reported that urea comprised,
as an average value, 0.29% of DOC and 13.8% of DON in Lake Biwa. Mitamura and
Hino (personal communication) have noted that the contributions of urea carbon
or nitrogen were counted as 0.05 to 0.24% of DOC or 0.9 to 5.0% of DON, as an
average percentages in each of four tropical lakes. The contributions in the
present study were comparable in their values to those in tropical lakes but
lower than those in Lake Biwa. Only a small amount of DOC was represented by
urea. Thus, in the present aquatic systems, urea seems to furnish a rather insignificant
contribution to the DOC and DON variations.
The DIP concentration
in the waters was 1.9 to 5.7 ug P L–1. Both nitrogen and phosphorus were the
limiting nutrients for phytoplankton growth in the euphotic zone from the two
systems as the data seem to indicate, regarding the nitrogen and phosphorus
levels and their ratio compared with that of the Redfield stoichiometric ratio
(Redfield, 1958).
Photosynthetic
activity and Urea degradation rate
Concentrations
of PC and PN averaged 12&g Cm–3 and 890&mgNm–3 in the Pindar&
and 5.3&g C m–3 and 640&mg Nm–3 in the Turia&u waters, respectively.
Chlorophyll a amounts were, respectively, 16&±&2&mg chl.a m–3
and 21&±&3&mg&chl.a m–3 in the Pindar& and Turia&u,
showing a distribution similar to those of PC and PN. To compare the particulate
matter characteristics between both aquatic systems, the ratio of PC concentration
to turbidity or the chlorophyll a amount was calculated. The organic carbon
content and phytoplankton biomass in the particulate matter of the Pindar&
system was quite low, with a low ratio of PC to turbidity and a high ratio of
PC to chlorophyll a. The Turia&u system, however, with their high ratio
of PC to turbidity and low ratio of PC to chlorophyll a was characterized as
turbid water rich in phytoplanktonic particulate matter that might contain the
fragments of macrophytes supplied by a dense macrophyte zone.
Photosynthetic
rates, determined simultaneously during our urea degradation experiments, ranged
from 163 to 233&mg C m–3 day–1 in the Pindar& and 335 to 505&mg
C m–3 day–1 in the Turia&u ().
Photosynthetic activity (photosynthetic rate per unit amounts of chlorophyll
a during one day) was low, ranging from 8.1 to 12.4&mg C&mg&chl.a–1
day–1 in the Pindar& and 16.7 to 21.9&mg C&mg&chl.a–1&day–1
in the Turia&u (),
with activity levels generally lower than those observed in natural clear lakes.
The distribution
of urea degradation rates at respective stations in the Pindar& and Turia&u
aquatic systems are on .
The hourly rate of urea degradation (sum of hourly rates of carbon incorporation
into particulate matter and CO2 liberation into water, originating in the urea
carbon) during the incubations ranged from 0.08 to 0.13&mg urea C m–3 hr–1
in the light bottle and 0.06 to 0.08&mg urea C m–3 hr–1 in the dark one
at the Pindar& stations, and from 0.68 to 1.16&mg&C&m–3&hr–1
and 0.42 to 0.68&mg C m–3 hr–1 in both bottles at the Turia&u stations.
The range of the daily rates of urea degradation in the surface water, estimated
from the light and dark values, was 1.6 to 2.4&mg urea C m–3 day–1 (average
2.0&mg urea C m–3 day–1) in the Pindar& and 13.2 to 20.7&mg
C m–3 day–1 (16.3&mg C m–3 day–1) in the Turia&u. The rates of carbon
incorporation and CO2 liberation phases in the urea degradation ranged from
0.17 to 0.32&mg&C m–3 day–1 and 1.40 to 2.12&mg C m–3 day–1 in
the Pindar&, and 3.1 to 5.6&mg C m–3 day–1 and 10.1 to 17.5&mg
C m–3 day–1 in the Turia&u stations, respectively. The urea degradation
rates in the Turia&u showed much higher values than the ones in the Pindar&.
shows the contribution of the carbon incorporation phase to the urea degradation.
9 to 27% of daily urea degradation was incorporated into the particulate matter.
The contributions in the Turia&u stations were high, whereas the percentages
at the Pindar& stations were low. Most of the urea degradation occurred
during the phase of CO2 liberation, especially in the Pindar& waters.
In the light incubations, an appreciable contribution occurred in the carbon
incorporation phase, whereas the values in the dark incubations were
there tended to be no differences in either of the aquatic systems.
These percentages in light and dark bottles were in the same range as those
in Brazilian tropical lakes (Mitamura&et&al., 1995) and in temperate
lakes, rivers and oceans (e.g., Webb and Haas, 1976; Mitamura and Saijo, 1980,
1986a), but showed high values compared with those of Irmisch (1991) in coastal
The urea degradation
rates were obviously higher in light than in dark regimes. The ratios from dark
to light values for CO2 liberation rates revealed considerably high values,
ranging from 0.62 to 0.97 in the Pindar& and 0.79 to 0.96 (as carbon
ratio) in the Turia&u, while reaching only negligible levels during the
phase of carbon incorporation ().
A similar tendency was reported in coastal waters by Tamminen and Irmisch (1996).
Relationship
between urea degradation and phytoplankton photosynthesis
Specific urea degradation
rates, using chlorophyll a as a cell parameter, were calculated to range from
0.09 to 0.15&mg urea C&mg chl.a–1 day–1 at the respective stations
in the Pindar&, and from 0.68 to 0.96&mg urea C&mg chl.a–1
day–1 in the Turia&u ().
The specific rates in the Turia&u waters were approximately ten times
higher than those in the Pindar&. The chlorophyll a specific degradation
rates in the Turia&u proved to cover the same range as those reported
by previous investigations in temperate and tropical lakes (Mitamura and Saijo,
1986a; Mitamura&et&al., 1995).
The distribution
of the chlorophyll a specific urea degradation rate was somewhat similar to
the pattern observed in photosynthetic activity ().
The ratio of the urea carbon degradation rate to the photosynthetic carbon assimilation
rate in the Pindar& and Turia&u systems was calculated to range,
respectively, from 0.008 to 0.018 and 0.039 to 0.044 during the day. The contribution
of carbon incorporation into cells (particulate matter) from urea was between
0.11 and 0.16% of the photosynthetic carbon assimilation in the Pindar&,
i.e., between 0.90 and 1.18% in the Turia&u. The current contributions
of urea carbon to the carbon source of phytoplankton (particulate matter) were
almost negligible.
To clarify the
relationship between the urea degradation rate and the phytoplankton biomass
or its photosynthetic rate, the regression equation of the urea degradation
rate against the chlorophyll a amount or photosynthetic rate was calculated
by linear regression analysis. The linear regression equations of the urea degradation
rate (U;&mg urea C m–3 day–1) against the chlorophyll a amount (C; mg chl.a
m–3) were: U&=&–0.001C&+&2.0, R2 = 0.01, p & 0.5 in the
Pindar&, and U&=&1.44 C&–&12.6, R2&= 0.86, p
& 0.05 in the Turia&u. The correlation coefficient between the urea
degradation rate and photosynthetic rate showed a high value at a statistically
significant level (R2 = 0.98, p & 0.001) in the Turia&u, in contrast
to a low value in the Pindar& (R2 = 0.03, p & 0.5).
Cycling of urea
The residence time
of urea in water, supposing a steady state, can be expressed as the time necessary
to degrade an amount of urea equivalent to the environment concentration. The
residence time of urea was calculated from the daily urea degradation rate and
urea concentration (). The residence time in
surface water was calculated to be between 2.3 and 4.5 days in the Pindar&,
and 0.21 and 0.50 days in the Turia&u. There were no appreciable differences
among the respective stations in the two aquatic systems. Much shorter residence
times were obtained in the Turia&u due to the rapid degradation rate
of urea. Several studies have estimated the residence time for urea using 14C-labelled
urea in freshwaters (Mitamura and Saijo, 1986a; Mitamura&et&al., 2000).
The residence times in the current investigations were shorter than the values
in freshwater lakes and reservoirs in temperate and tropical zones.
In the turbid water
of the Pindar& and Turia&u aquatic systems, urea was presumably
degraded by phytoplankton. Although a small amount of urea carbon was incorporated
into the phytoplankton cells, a good part of the carbon was liberated into the
water as CO2 phase during the urea degradation. In the dark regime, in particular,
the incorporation phase was a negligible contribution in the urea degradation.
Several studies (Carpenter&et&al., 1972a; Irmisch, 1991; Mitamura&et&al.,
; Tamminen and Irmisch, 1996) reported a negligible amount of carbon
incorporation from degraded urea, findings that agreed with the tendency noted
in the current results. The urea was better degraded in the light condition
than in the dark one. The current findings suggest that during the day urea
degrades into the two phases of carbon incorporation and CO2 liberation, while
during the night it degrades only into CO2 liberation. In the Pindar&
and Turia&u waters, the present ratios might increase with depth upon
an attenuation of light intensity. The variation in the ratio of dark to light
values might be partly caused by the differences in phytoplankton species, as
suggested by Webb and Haas (1976), by the physiological condition of the microorganisms,
as well as by their lower levels of irradiance due to turbid waters. Although,
light intensity was not determined, the turbidity and the high suspended solids
concentrations were in the same range as those in Solim&es river during
the low water season according to previous studies by Mitamura&et&al.
(2000) about grain size distribution of particulate matter and sediment. The
irradiance, in the present aquatic systems, might be a key parameter affecting
the dark to light ratio in urea degradation.
Urea degradation
activity bears a close relationship to the utilization of urea nitrogen by phytoplankton,
as suggested by Carpenter&et&al. (1972b) and Mitamura and Saijo (1986a).
They also reported that the urea was mainly degraded during the urea nitrogen
uptake by phytoplankton, and that, simultaneously, the urea carbon was assimilated
by phytoplankton as a carbon source. The principal nitrogenous compounds which
sustain the standing crop of phytoplankton populations in natural waters are
thought to be urea, ammonia and nitrate, as reported in several studies (McCarthy&et&al.,
1977; Mitamura and Saijo, 1986a; Presing&et&al., 2001; Twomey&et&al.,
2005; Mitamura&et&al., 2006). In the current investigation, variations
in the relationship between the urea degradation rate and the photosynthetic
rate or the chlorophyll a amount might be due to a difference in the contribution
of urea nitrogen as a nitrogen source for phytoplankton growth. The strong relationship
(R2 = 0.72, p & 0.1) between the urea carbon incorporation rate and photosynthetic
carbon assimilation rate was noted in light bottles from the Turia&u,
whereas a low value (R2 = 0.25, p & 0.5) was observed in the Pindar&.
This suggests that the urea degradation rate was proportional to the standing
crop of phytoplankton and its photosynthetic rate, and that the degradation
may have been due to the autotrophs. In the Pindar& waters, a weak relationship
might be due to a difference in the contribution of urea nitrogen as a nitrogen
source for phytoplankton growth, as well as to the variations in bacterial degradation
among respective stations in this aquatic system. This indicates that in the
Turia&u system an appreciable amount of urea degradation, particularly
in the case of carbon incorporation into particulate matter, is associated with
the photosynthesis of phytoplankton. Therefore, in the present results, the
carbon incorporation into particulate matter in the urea degradation can be
considered as due to the carbon uptake process into phytoplankton cells. The
variation in the urea degradation rate might result in a difference in the utilizable
activity of phytoplankton species and their physiological conditions, as noted
by Fan&et&al. (2003), although such a variation must take into account
the levels of the bacterial utilization for the urea degradation as described
Several investigators
have reported that natural bacterial assemblages could utilize urea (Satoh and
Hanya, 1976; Mitamura&et&al., 1994; Park&et&al., 1997; Joergensen,
2006). However, very little urea degrading activity occurs under natural conditions.
This would suggest that the urea concentration in natural waters is below the
critical level for bacterial utilization (Mitamura&et&al., 1994),
or that the urease activity of bacteria is inhibited by the presence of ammonia
(Magana-Plaza and Ruiz-Herrera, 1967). The degradation of urea in natural waters
occurs in connection with the photosynthesis of phytoplankton, as suggested
by several studies using 14C-labelled urea (Mitamura and Saijo, a;
Webb and Haas, 1976; Mitamura&et&al., 2000). Mitamura&et&al.
(1995) indicated that the urea in tropical lakes was degraded by phytoplankton
rather than by bacteria during both dry and rainy seasons. Webb and Haas (1976)
considered that urea degradation seemed to be an exception to the general rule
of competition between phytoplankton and bacteria for the utilization of dissolved
organic compounds. On the other hand, Mitamura&et&al. (1994) pointed
out that the bacterial contribution to urea degradation was recognized in polluted
river waters. In the pre-Amazonian floodplain, in terms of urea degradation,
the phytoplankton in both the Turia&u and Pindar& aquatic systems
seemed to compete with bacteria. However, in both turbid waters, heterotrophic
bacteria attached to the surface of suspended solids may also have contributed
to urea degradation at the same time as autotrophic phytoplankton. Both the
current and previous results imply that the utilization of urea by phytoplankton
seems to be one of the important functions in the urea degradation of natural
tropical waters, although its detailed physiological pathway remains obscure.
In the waters of
both aquatic systems, a short residence time for urea cycling was observed.
Mitamura and Saijo (1986b) suggested that the urea regeneration rate from excretion
and mineralization is equivalent to their degradation rate, and that these processes
are in a state of dynamic balance. The current brief residence time indicates
that the urea was rapidly recycled in the euphotic zone of the tropical aquatic
ecosystems in the pre-Amazonian floodplain.
In summary, the
present findings indicate that in the aquatic systems of the pre-Amazonian floodplain
of Baixada Maranhense, Brazil, the degradation of urea occurred in connection
with the photosynthesis of phytoplankton, and that the urea might be more readily
degraded by phytoplankton rather than by the bacteria with which phytoplankton
compete, though the bacterial role in urea degradation remains obscure. To fully
understand urea cycling in the pre-Amazonian floodplain, the total dynamics
of urea in the water system, including the shading effect by high turbidity
on urea degradation activity, ought to be carefully examined. Furthermore, the
phytoplankton taxa and their structure, their physiological state and specific
growth rate, and the allochthonous input of urea from their watersheds, as well
as the bacterial contribution to urea degradation must be incorporated into
further investigations as key parameters influencing urea cycling.
Acknowledgements
The authors wish
to thank the following ongoing members of the cooperative study: the students
J. P. Pontes, S. C. C. Santana and C. L. M. Serra. They also wish to express
their gratitude for the financial support provided by the Japan International
Cooperation Agency (JICA) to japanese scientists. Thanks are also due to Dr.
M. Nakayama, Division of Natural Sciences, Osaka Kyoiku University, for his
valuable advice in the use of radioactive carbon, and to the members of the
Limnological Laboratory, University of Shiga Prefecture, for their generous
assistance in the chemical analyses.
We thank Roler
Iba&ez for English final revision.
References
BARBIERI, R., IBA&NEZ,
MSR., ARANHA, FJ., CORREIA, MMF., REID, JW. and TURNER, P.&1989. Plankton,
primary production and some physico-chemical factors of two lakes from Baixada
Maranhense. Revista Brasileira de Biologia, vol.&49, p.&399-408.
&&&&&&&&[  ]
BENDSCHNEIDER,
K. and ROBINSON, RJ.&1952. A new spectrophotometric method for the determination
of nitrite in sea water. Journal of Marine Research, vol.&11, p.&87-96.
&&&&&&&&[  ]
BOZELLI, RL. and
GARRIDO, AV.&2000. Gradient of inorganic turbidity and responses of planktonic
communities in an Amazonian lake, Brazil. Verhandlungen Internationale Vereinigung
f&r Theoretische und Angewandte Limnologie, vol.&27, p.&147-151.
&&&&&&&&[  ]
BRAY, GA.&1960.
A simple efficient liquid scintillator for counting aqueous solutions in a liquid
scintillation counter. Analytical Biochemistry, vol.&1, no.&4-5, p.&279-285.
&&&&&&&&[  ]CARNEIRO, LS.,
BOZELLI, RL. and ESTEVES, FA.&2002. Long-term changes in the density of
the copepod community in an Amazonian lake impacted by bauxite tailings. Amazoniana,
vol.&17, p.&553-566.
&&&&&&&&[  ]
CARPENTER, EJ.,
REMSEN, CC. and SCHROEDER, B.&1972a. Comparison of laboratory and in situ
measurement of urea decomposition by a marine diatom. Journal of Experimental
Marine Biology and Ecology, vol.&8, no.&3, p.&259-264. &&&&&&&&[  ]CARPENTER, EJ.,
REMSEN, CC. and WATSON, SW.&1972b. Utilization of urea by marine phytoplankters.
Limnology and Oceanography, vol.&17, no.&2, p.&265-269.
&&&&&&&&[  ]
CHANG, FH., BRADFORD-GRIECE,
JM., VONCENT, WF. and WOODS, PH.&1995. Nitrogen uptake by the summer size-fractionated
phytoplankton assemblages in the Westland, New Zealand, upwelling system. New
Zealand Journal of Marine and Freshwater Research, vol.&29, p.&147-161.
&&&&&&&&[  ]
CHO, BC. and AZAM,
F.&1995. Urea decomposition by bacteria in the Southern California Bight
and its implications for the mesopelagic nitrogen cycle. Marine Ecology Progress
Series, vol.&122, p.&21-26. &&&&&&&&[  ]CHO, BC., PARK,
MG., SHIM, JH. and AZAM, F.&1996. Significance of bacteria in urea dynamics
in coastal surface waters. Marine Ecology Progress Series, vol.&142, p.19-26.
&&&&&&&&[  ]DEVOL, AH., FORSBERG,
BR., RICHEY, JE. and PIMENTEL, TP.&1995. Seasonal variation in chemical
distributions in the Amazon (Solim&es) River: A multiyear time series.
Global Biogeochemical Cycles, vol.&9, no.&3, p.&307-328. &&&&&&&&[  ]EPPLEY, RW., CARLUCCI,
FA., HOLM HANSEN, O., KIEFER, D., McCARTHY, JJ., VENRICK, E. and WILLIAMS, PM.&1971.
Phytoplankton growth and composition in shipboard cultures supplied with nitrate,
ammonium, or urea as the nitrogen source. Limnology and Oceanography, vol.&16,
no.&5, p.&741-751.
&&&&&&&&[  ]
FAN, C., GLIBERT,
PM., ALEXANDER, J. and LOMAS, MW.&2003. Characterization of urease activity
I three marine phytoplankton species, Aureococcus anophagefferens, Prorocentrum
minimum, and Talassiosira weissflogi. Marine Biology, vol.&142, no.&5,
p.&949-958.
&&&&&&&&[  ]
FARJALLA, VF.,
ESTEVES, FA., BOZELLI, RL. and ROLAND, F.&2002. Nutrient limitation of
bacterial production in clear water Amazonian ecosystem. Hydrobiologia, vol.&489,
no.&1-2, p.&197-205. &&&&&&&&[  ]FISHER, TR., MORRISSEY,
KM., CARLSON, PR., ALVES, L F. and MELACK, JM.&1988. Nitrate and ammonium
uptake by plankton in an Amazon River floodplain lake. Journal of Plankton Research,
vol.&10, no.&1, p.&7-29. &&&&&&&&[  ]FISHER, TR. and
PARSLEY, PE.&1979. Amazon lakes: water storage and nutrient stripping by
algae. Limnology and Oceanography, vol.&24, no.&3, p.&74-92.
&&&&&&&&[  ]FORSBERG, BR.,
DEVOL, AH., RICHEY, JE., MARTINELLI, LA. and DOS SANTOS, A.&1988. Factors
controlling nutrient concentrations in Amazon floodplain lakes. Limnology and
Oceanography, vol.&33, no.&1, p.&41-56.
&&&&&&&&[  ]
GU, BH., HAVENS,
KE., SCHELSKE, CL. and ROSEN, BH.&1997. Uptake of dissolved nitrogen by
phytoplankton in a eutrophic subtropical lake. Journal of Plankton Research,
vol.&19, no.&6, p.&759&#. &&&&&&&&[  ]GUENTHER, M. and
BOZULL, R.&2004. Factors influencing algae-clay aggregation. Hydrobiologia,
vol.&523, no.&1-3, p.&217-223.&10.1023/B:HYDR..05034.32&&&&&&&&[  ]HARRISON, WG.,
HEAD, EJH., CONOVER, RJ., LONGHURST, AR. and SAMEOTO, DD.&1985. The distribution
and metabolism of urea in the eastern Canadian Arctic. Deep-Sea Research, vol.&32,
no.&1, p.&23-42. &&&&&&&&[  ]HARVEY, WA. and
CAPERON, J.&1976. The rate of utilization of urea, ammonium, and nitrate
by natural populations of marine phytoplankton in an eutrophic environment.
Pacific Science, vol.&30, no.&4, p.&329-340.
&&&&&&&&[  ]
HOLM-HANSEN, O.,
LORENZEN, CJ., HOLMES, RW. and STRICKLAND, JDH.&1965. Fluorometric determination
of chlorophyll. Journal du conseil/Conseil international pour l'exploration
de la mer, vol.&30, no.&1, p.&3-15. &&&&&&&&[  ]IBA&NEZ,
MSR.&1997. Phytoplankton composition and abundance of a central Amazonian
floodplain lake. Hydrobiologia, vol.&362, no.&1-3, p.&79-83.
&&&&&&&&[  ]IBA&NEZ,
MSR., CAVALCANTE, PRS., COSTA NETO, JP., BARBIERI, R., PONTES, JP., SANTANA,
SCC., SERRA, CLM., NAKAMOTO, N. and MITAMURA, O.&2000. Limnological characteristics
of three aquatic system of the pre-amazonian floodplain, Baixada Maranhense
(Maranhao, Brazil). Aquatic Ecosystem Health and Management, vol.&3, no.&4,
p.&521-531. &&&&&&&&[  ]IRMISCH, A.&1991.
Investigations on the urea uptake by phytoplankton in the Baltic Sea. Acta Hydrochimica
et Hydrobiologica, vol.&19, no.&1, p.&39-44. &&&&&&&&[  ]JOERGENSEN, NOG.&2006.
Uptake of urea by estuarine bacteria. Aquatic Microbial Ecology, vol.&42,
no.&3, p.&227-242.
&&&&&&&&[  ]
JUNK, WJ. and WEBER,
GE.&1996. Amazonian floodplains: a limnological perspective. Verhandlungen
des Internationalen Verein Limnogie, vol.&26, p.&149&#.
&&&&&&&&[  ]
KRISTIANSEN, S.&1983.
Urea as a nitrogen source for the phytoplankton in the Oslofjord. Marine Biology,
vol.&74, no.&1, p.&17-24. &&&&&&&&[  ]MAGANA-PLAZA, I.
and RUIZ-HERRERA, J.&1967. Mechanisms of regulation of urease biosynthesis
in Proteus rettgeri. Journal of Bacteriology, vol.&93, no.&4, p.&.
PMid:6032508.
&&&&&&&&[  ]
MARLIER, G.&1967.
Ecological studies of some lakes of the Amazon Valley. Amazoniana, vol.&1,
p.&91-115.
&&&&&&&&[  ]
McCARTHY, JJ.&1972.
The uptake of urea by natural populations of marine phytoplankton. Limnology
and Oceanography, vol.&17, no.&5, p.&738-748.
&&&&&&&&[  ]
McCARTHY, JJ.,
TAYLOR, WR. and TAFT, JL.&1977. Nitrogenous nutrition of the plankton in
Chesapeake Bay. I. Nutrient availability and plankton preferences. Limnology
and Oceanography, vol.&22, no.&6, p.&996&#.
&&&&&&&&[  ]
MELACK, JM. and
FISHER, TR.&1990. Comparative limnology of tropical floodplain lakes with
an emphasis on the central Amazon. Acta Limnologica Brasiliensia, vol.&3,
&&&&&&&&[  ]
MITAMURA, O.&1986.
Urea metabolism and its significance in the nitrogen cycle in the euphotic layer
of Lake Biwa. II. Half-saturation constant for nitrogen assimilation by fractionated
phytoplankton in different trophic areas. Archive f&r Hydrobiologie, vol.&107,
p.&167-182.
&&&&&&&&[  ]
MITAMURA, O.&1997.
An improved method for the determination of nitrate in freshwaters based on
hydrazinium reduction. Memoirs of Osaka Kyoiku University Series III, vol.&45,
p.&297-303.
&&&&&&&&[  ]
MITAMURA, O., CHO,
KS. and HONG, SU.&1994. Urea decomposition associated with the activity
of microorganisms in surface waters of the North Han River, Korea. Archive f&r
Hydrobiologie, vol.&131, no.&1, p.&231-242.
&&&&&&&&[  ]
MITAMURA, O., KAWASHIMA,
M. and MAEDA, H.&2000. Urea decomposition by picophytoplankton in euphotic
zone of Lake Biwa. Limnology, vol.&1, p.&19-26.
&&&&&&&&[  ]
MITAMURA, O., MAEDA,
H., SEIKE, Y., KONDO, K., GOTO, N. and KODAMA, T.&2006. Seasonal changes
of carbon and nitrogen productivity in the north basin of Lake Biwa, Japan.
Verhandlungen des Internationalen Verein Limnogie, vol.&29, p.&;1920.
&&&&&&&&[  ]
MITAMURA, O., NAKAMOTO,
N., IBA&NEZ, MSR.,COSTA NETO, JP. and BARBIERI, R.&2000. Grain Size
distribution of particulate matter and sediment in extensive freshwater bodies
in pre-Amazonian floodplain, Baixada Maranhense, Brazil. Verhandlungen des Internationalen
Verein Limnogie, vol.&30, no.&&6, p.&964-970.
&&&&&&&&[  ]
MITAMURA, O. and
SAIJO, Y.&1975. Decomposition of urea associated with photosynthesis of
phytoplankton in coastal waters. Marine Biology, vol.&30, no.&1, p.&67-72.
&&&&&&&&[  ]MITAMURA, O. and
SAIJO, Y.&1980. In situ measurement of the urea decomposition rates and
its turnover rate in the Pacific Ocean. Marine Biology, vol.&58, no.&2,
p.&147-152. &&&&&&&&[  ]MITAMURA, O. and
SAIJO, Y.&1981. Studies on the seasonal changes of dissolved organic carbon,
nitrogen, phosphorus and urea concentration in Lake Biwa. Archive f&r Hydrobiologie,
vol.&91, p.&1-14.
&&&&&&&&[  ]
MITAMURA, O. and
SAIJO, Y.&1986a. Urea metabolism and its significance in the nitrogen cycle
in the euphotic layer of Lake Biwa. I. In situ measurement of nitrogen assimilation
and urea decomposition. Archive f&r Hydrobiologie, vol.&107, p.&23-52.
&&&&&&&&[  ]
MITAMURA, O. and
SAIJO, Y.&1986b. Urea metabolism and its significance in the nitrogen cycle
in the euphotic layer of Lake Biwa. IV. Regeneration of urea and ammonia. Archive
f&r Hydrobiologie, vol.&107, p.&425-440.
&&&&&&&&[  ]
MITAMURA, O., SAIJO,
Y. and HINO, K.&1995. Cycling of urea associated with photosynthetic activity
of phytoplankton in the euphotic zone of tropical lakes, Brazil. Japanese Journal
of Limnology, vol.&56, no.&2, p.&95-105.
&&&&&&&&[  ]
MITAMURA, O., SAIJO,
Y., HINO, K. and BARBOSA, FAR.&1995. The significance of regenerated nitrogen
for phytoplankton productivity in the Rio Doce Valley Lakes, Brazil. Archive
f&r Hydrobiologie, vol.134, no.&2, p.&179-194.
&&&&&&&&[  ]
MURPHY, J. and
RILEY, GA.&1962. A modified single solution method for the determination
of phosphate in natural waters. Anaytica Chimica Acta, vol.&27, p.&31-36.&&&&&&&&[  ]NEWELL, BS., MORGAN,
B. and CUNDY, J.&1967. The determination of urea in seawater. Journal of
Marine Research, vol.&25, p.&201-202.
&&&&&&&&[  ]
PARK, MG., SHIM,
JH. and CHO, BC.&1997. Urea decomposition activities in an enriched freshwater
pond. Aquatic Microbial Ecology, vol.&13, p.&303-311. &&&&&&&&[  ]PRESING, M., VOROS,
L., HERODEC, S. and ABRUSAN, G.&2001. Nitrogen uptake and the importance
of internal nitrogen loading in Lake Balaton. Freshwater Biology, vol.&46,
p.&125-139.
&&&&&&&&[  ]
PRICE, NM. and
HARRISON, PJ.&1988. Uptake of urea C and urea N by the coastal marine diatom
Thalassiosira pseudonana. Limnology and Oceanography, vol.&33, no.&4,
p.&528-537.
&&&&&&&&[  ]
REDFIELD, AC.&1958.
The biological control of chemical factors in the environment. American Scientist,
vol.&46, no.&3, p.&205-221.
&&&&&&&&[  ]
REID, JW. and TURNER,
PN.&1988. Planktonic Rotifera, Copepoda and Cladocera from Lake A&u
and Viana, State of Maranh&o, Brazil. Revista Brasileira de Biologia,
vol.&48, no.&3, p.&485-495.
&&&&&&&&[  ]
REMSEN, CC., CARPENTER,
EJ. and SCHROEDER, BW.&1972. Competition for urea among estuarine microorganisms.
Ecology, vol.&53, p.&921-926.
&&&&&&&&[  ]
RICHEY, JE., HEDGES,
JI., DEVOL, AH., QUAY, PD., VICTORIA, R., MARTINELLI, L. and FORSBERG, BR.&1990.
Biogeochemistry of carbon in the Amazon River. Limnology and Oceanography, vol.&35,
no.&2, p.&352-371.
&&&&&&&&[  ]
ROLAND, F., ESTEVES,
FA. and BARBOSA, FAR.&2002. Relationship between antropogenically caused
turbidity and phytoplankton production in a clear Amazonian floodplain lake.
Amazoniana, vol.&17, p.&65-77.
&&&&&&&&[  ]
SAGI, T.&1966.
Determination of ammonia in seawater by the indophenol method and its application
to the coastal and off-shore waters. The Oceanographical Magazine, vol.&18,
SAIJO, Y., MITAMURA,
O., HINO, K., IKUSIMA, I., TUNDISI, JG., MATSUMURA-TUNDISI, T., SUNAGA, T.,
NAKAMOTO, N. FUKUHARA, H., BARBOSA, FAR., HENRY, R. and SILVA, VP.&1997.
Physicochemical features of rivers and lakes in Pantanal wetland. Japanese Journal
of Limnology, vol.&58, no.&1, p.&69-82.
&&&&&&&&[  ]
SATAKE, K., SAIJO,
Y. and TOMINAGA, H.&1972. Determination of small quantities of carbon dioxide
in natural waters. Japanese Journal of Limnology, vol.&33, p.&16-20.
&&&&&&&&[  ]
SATOH, Y. and HANYA,
T.&1976. Decomposition of urea by the larger particulate fraction and the
free bacteria fraction in a pond water. Internationale Review gesamten Hydrobiologi