Aspects of the biology of Labeo cylindricus (Pisces: cyprinidae) in Lake Baringo, Kenya

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UAEM #481236, VOL 13, ISS 2

Spatial dynamics of chromophoric dissolved organicmatter in nearshore waters of Lake Victoria

Steven A. Loiselle, Nicholas Azza, John Gichuki, Luca Bracchini, AntonioTognazzi, Arduino M. Dattilo, and Claudio Rossi

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TABLE OF CONTENTS LISTINGThe table of contents for the journal will list your paper exactly as it appears below:Spatial dynamics of chromophoric dissolved organic matter in nearshore waters of Lake VictoriaSteven A. Loiselle, Nicholas Azza, John Gichuki, Luca Bracchini, Antonio Tognazzi, Arduino M. Dattilo,and Claudio Rossi

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Spatial dynamics of chromophoric dissolved organicmatter in nearshore waters of Lake Victoria

Steven A. Loiselle,1∗ Nicholas Azza,2 John Gichuki,3 Luca Bracchini,1

Antonio Tognazzi,1 Arduino M. Dattilo,1 and Claudio Rossi11University of Siena, Dipartimento Farmaco Chimico Tecnologico, CSGI, 53100 Siena, Italy5

2Directorate of Water Resources Management, Entebbe, Uganda3Kenyan Marine Fisheries Research Institute, Kisumu, Kenya

∗Corresponding author and present address: Steven A. Loiselle [email protected] University of Siena,via Aldo Moro 1, 53100 Siena, Italy

The underwater light conditions in the African Great Lakes depend on the complex dynamics ofecological and hydrological forces, and are strongly influenced by local environmental conditions andglobal biogeochemical cycles. Changes in the optical conditions in these lakes have direct impacts onecosystem productivity, carbon dynamics and nutrient availability. A central role in the underwater lightclimate is played by dissolved organic matter which is present in all aquatic ecosystems. The chromophoricfraction of these compounds can mediate ecosystem change through its influence on the attenuation ofultraviolet and PAR radiation, microbial carbon cycling and radiative transfer. In the African Great Lakes,little information is available regarding the dynamics of dissolved organic matter and those sources andsinks which control its presence in the water column. We present an extensive spatial analysis of three majorbays on the Kenyan and Ugandan shores of Lake Victoria. We use these data to examine the dynamicsof chromophoric dissolved organic matter in different bays and we develop a model to estimate its flowfrom these bays to the Lake, considering both conservative mixing and photodegradation processes. Whilesome bays release chromophoric dissolved organic matter practically unmodified into the Lake, increasedresidence time and exposure to solar ultraviolet radiation create conditions where chromophores are lostbefore entering the open lake.

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Keywords: organic carbon dynamics, photodegradation, conservative mixing, Africa25

Introduction

Variations in the penetration of solar radiationcan strongly modify the chemical and biologi-cal environment of aquatic ecosystems. A key el-ement which controls the spectral conditions in30tropical ecosystems is dissolved organic matter.Whether produced by coastal vegetation, phyto-plankton biomass or catchment activities, the spec-tral properties and concentration of dissolved or-

ganic matter can influence primary productivity, 35secondary production and radiative transfer. Photo-transformations of dissolved organic matter have re-cently received much study (Helbling and Zagarese,2003). Photodegradation and photobleaching (de-fined here as the loss of absorption in the ultravi- 40olet and visible wavelengths) of chromophoric dis-solved organic matter (CDOM) will depend on thespectral intensity of the incident irradiance in thewater column, photolability of the CDOM present

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Aquatic Ecosystem Health & Management, 13(2):1–11, 2010. Copyright C© 2010 AEHMS. ISSN: 1463-4988 print / 1539-4077 onlineDOI: 10.1080/14634988.2010.481236

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2 Loiselle et al. / Aquatic Ecosystem Health and Management 13 (2010) 1–11

Figure 1. Map of Lake Victoria, showing (clockwise) the main sampling areas in Uganda (UG) and Kenya (KE): Katonga Bay,Nsonga Bay, Murchison Bay, the coastal area near Yala swamp, and Nyanza Gulf.

and exposure time (Bertilsson and Tranvik, 2000).45Photochemical transformation of CDOM can resultin the production of low molecular weight organiccompounds with lower absorbivity in the wave-lengths absorbed (Carrillo et al., 2002). The resul-tant consequences on the aquatic ecosystem can be50multiple, including: the formation of toxic photo-products (Osburn and Morris, 2003; Zepp, 2003),the increase in the pool of bioavailable carbon sub-strates (Bertilsson and Tranvik, 1998) and the in-crease of secondary productivity, as well as inducing55changes in the spectral attenuation of the water col-umn with implications on productivity and radiativetransfer (Neale, Helbling and Day, 2007).

In the present study, we examine the spectralconditions of the inshore waters of Lake Victoria,60concentrating on three large bay areas (Katonga Bay,Nsonga Bay and Nyanza Gulf), each with differentoptical and hydrological conditions, and each withdifferent influences on the open lake (Fig. 1). Inthese areas, catchment and littoral zone conditions65play an important role in controlling the variabilityof the chemical and physical conditions of the watercolumn (Lung’Ayia et al., 2000; Cozar et al., 2004,Bracchini et al., 2007). Recent increases in nutri-ent loads, changes in the stability and extension of70the vertical stratification and major changes in thetrophic web have brought these waters to eutrophicconditions (Mugidde, 1993; Talling and Lemoalle,1998).

The dominant wetland vegetation in Lake Victo-75ria is a highly productive emergent C4 sedge (Cype-rus papyrus L.) (Saunders et al., 2007) which cangenerate large quantities of organic carbon in favor-

able conditions of high solar insolation and abun-dant water. The anaerobic accumulation of organic 80matter and its eventual release into the water col-umn is strongly linked to wetland extension andmanagement. Likewise, intensive land use withinthe catchment has been shown to lead to major in-creases in the transport of organic and nutrient rich 85matter (Njiru et al, 2008).

To better understand the importance of CDOMon the optical conditions of these important areas,we characterise and compare spatial conditions ofconservative and non-conservative components of 90the water column. We develop a model of photo-bleaching to help explain differences in the spatialvariability of optical conditions.

Methods

We examined the optical and chemical conditions 95of coastal areas throughout the Ugandan and Kenyanareas of Lake Victoria, in May 2003 and June 2004(Loiselle et al., 2008). Measurement stations (277)represent all major coastal types.

Vertical profiles of temperature (accuracy 100±0.1◦C) and specific conductance (accuracy±2.0 µS cm−1) (Hydrolab Datasonde 4, Hach Com-pany, Loveland, CO) and solar irradiance (PARwaveband, 313 nm, 340 nm with a PUV 514, Bio-spherical Instruments, San Diego, CA) were mea- 105sured at each sampling site, beginning just below thewater surface and measuring to near the lake bot-tom. The profiling radiometer was manually loweredfrom the sunny side of the boat, obtaining diffuse ir-radiance measurements in the PAR waveband every 110

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Loiselle et al. / Aquatic Ecosystem Health and Management 13 (2010) 1–11 3

0.3 s together with depth. On average, 400 measure-ments (irradiances and water depth) were obtainedfor each profile. Profiles in which changes in surfacesolar irradiance occurred (clouds) were discardedand the measurements repeated. The resulting ra-115diation profile was corrected by removing the darkcurrent signal (<0.00001 W m−2 nm−1) and plottedagainst the water depth. The diffuse attenuation co-efficient Kλ was calculated by fitting the best expo-nential curve to the profile data (Iz = Ioe(−Kλ(z−z0)))120where the fitting of the exponential decay was supe-rior to R2 > 0.98. Specific conductance was deter-mined from conductivity normalized to a temper-ature of 25◦C. The conductivimeter was calibratedevery day using a standard KCl solution (Crison,125Spain). Incident solar photosynthetically availableradiation (PAR) was measured 3 m above the wa-ter surface (Skye Instruments LTD, UK) during allmeasurements.

Mixing depth was defined graphically by the in-130tersection of trend lines fitted to the thermal pro-files, one through the upper part of the epilimnionand the second through the metalimnion(Wetzel,2001). The determination of chlorophyll a and dis-solved organic matter was obtained from samples135obtained at a depth of 0.5 m. Chlorophyll a concen-trations were estimated using a calibrated portablefluorometer (Turner designs: wavelength excitation= 485 nm, measured wavelength = 680 nm) whichwas calibrated daily using a solid standard (Turner140Designs). Fluorometer calibration was made usingmeasurements of chlorophyll-a concentrations ac-cording to Strickland and Parsons (1972) from dif-ferent lakes including Lake Victoria. Despite of thedifferent sources of variability in the chlorophyll-145a fluorescence (e.g. quenching, temperature), themeasured concentrations and fluorescence signalsshowed a good linear correlation. CDOM sampleswere filtered using single use 0.22 µm polyethersul-fone filters (Millex GP Filter Unit, Millipore S.A.,150Malsheim, France) following Grzybowski (2000).Samples were stored in acid washed glass bottles,after rinsing each bottle with filtered lake water.Filled bottles were covered and stored in the dark at4◦C for 1 to 5 days before absorbance measure-155ments. Absorbance scans were performed using aLambda 25 spectrophotometer (PES, Perkin Elmer,speed scan of 4500 nm min−1). A single quartz cu-vette (pathlength 0.01 m) and a Milli-Q blank wereused. After acclimation in a temperature controlled160room (18◦C), three scans were performed for eachsample and average values computed for all sam-

ples where the differences between scans were lessthan 10−3. Absorbance (aλ.) was measured over awavelength range of 200 to 700 nm with a spectral 165resolution of 1 nm. The absorbance measurement at700 nm was subtracted from each measurement toremove variations due to baseline changes. Absorp-tion coefficients (Aλ) were calculated using Aλ =2.303 l−1 ∗ aλ. where l is the pathlength in meters. 170

A small number of filtered samples (4) weretested for photo-lability using quartz bottles with ahigh UV transparency (average transmittance 94%in UVB and UVA). These samples had been pre-viously filtered in-situ (0.22 µm) after sampling in 175Nyanza Gulf (Figure 3c) and stored in the dark at4◦C for 10 days prior to the photobleaching experi-ment. The quartz bottles were placed in a controlledtemperature bath (20 ± 1.8◦C), together with MilliQblanks and exposed for three days to natural solar 180radiation in the Siena laboratories. Prior to initiatingthe experiment, samples were stored for 8 hours inthe dark at room temperature to examine changes inoptical properties not related to photodegradation.Surface irradiance measurements were obtained ev- 185ery minute throughout the experiment using singlechannel sensors (SKR 420, 430, Skye InstrumentsLTD, UK) for ultraviolet A (UVA, 315-380nm) andultraviolet B (UVB, 280-315nm). The sensors arecosine corrected with cosine error of 3% to 80 de- 190grees, linearity error < 0.2%, absolute calibrationerror of about 3%. Total dose (kJ m−2) was calcu-lated by considering the average solar irradiance inthe UV wavelengths, the total time of exposure andspectral transmission of the quartz bottle, weighted 195for solar spectral irradiance in the UV wavelengths.Samples within the bottles were mixed twice dailyduring sampling. Samples (15 ml) were obtainedtwice daily (pre and post exposure) from each bottleafter rinsing the external parts with sterilized wa- 200ter. Samples were housed in dark glass containers,covered and stored at 4◦C until spectrophotometricanalysis at the conclusion of the experiment.

Results

The optical variables showed significant spatial 205variability in all study areas. Diffuse vertical atten-uation coefficients were constantly higher near tothe littoral zone (Fig. 2a, b, c). Nearshore KPAR val-ues averaged well over 2 m−1 while KPAR values inthe open lake averaged below 1 m−1. In all cases, a 210horizontal gradient was present, which is more pro-nounced for the measurements of diffuse vertical

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attenuation of solar UV radiation, determined at313 nm (K313) (Fig. 3a, b, c) and 340 nm. Absorp-tion of solar UV radiation was related to CDOM215absorption, with a significant and linear correlationbetween K313 and A365 (df = 95, R2 = 0.75).

Measurements of absorption of the dissolvedfraction (A365) were highest nearest to the littoralzone, especially when wetland lined rivers flowed220into the bay (Katonga River into Katonga Bay,

Sondu and Nyando rivers into Nyanza Gulf). Ab-sorption decreased as the CDOM rich nearshorewaters moved into the open lake. This reductionin CDOM absorption can be associated to two prin- 225ciple processes, degradation of chromophores andthe conservative mixing of CDOM rich bay waterswith the open lake water. The two processes arenot exclusive and contribute in different degrees tochanges in CDOM absorption over time and space. 230

Figure 2. Measurements of the diffuse vertical attenuation of photosynthetic available radiation (KPAR, m−1) in a) Katonga Bay,Uganda, b) Nsonga Bay, Uganda, c) Nyanza Gulf, Kenya. Sampling sites referenced using UTM coordinates and interpolationbetween sites made by kriging. (continued on next page)

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Figure 2. Continued.

In ocean estuaries, several authors (eg. Stedmonand Markager, 2003) have compared changes insalinity and changes in CDOM adsorption to dif-ferentiate between conservative mixing and pho-todegradation processes. As riverine waters enter235into the sea, there is a gradual increase in salinity. Ifsalinity can be considered as a conservative property(ignoring evaporation), relative changes in salinity

can be compared to changes in CDOM absorptionover space. Such a comparison can be used to deter- 240mine whether losses in CDOM absorption are due toconservative mixing with low concentration CDOMwaters or due to the non-conservative sinks (loss ofchromophores) such as photodegration.

In Lake Victoria, an inverse situation occurs. 245About 84% of water input into Lake Victoria comes

Figure 3. Measurements of the diffuse vertical attenuation of solar ultraviolet radiation at 313 nm (K313, m−1) in a) Katonga Bay,Uganda, b) Nsonga Bay, Uganda, c) Nyanza Gulf, Kenya. Sampling sites referenced using UTM coordinates and interpolationbetween sites made by kriging. (continued on next page)

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as rain (Talling and Lemoalle, 1998), resulting in alake with a very limited ionic content with respectto the ion rich waters from the catchment whereanthropogenic activities are present. In such con-250ditions, differences in specific conductance havebeen used to trace water inflows from the catch-

ment to the lake (Cozar et al. 2004), treating spe-cific conductance as a near conservative tracer ofwater movement. Differently from estuaries, a de- 255crease in specific conductance is expected to occurfrom riverine waters to open lake waters. Wheredecreases in CDOM absorption and the specific

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conductance follow similar patterns, conservativemixing is likely to dominate the loss of CDOM ab-260sorption. Where changes in CDOM concentrationsand specific conductance are unlinked, sources or

sinks of CDOM should be present. In Katonga Bayand Nyanza Gulf, major changes in both conduc-tance and CDOM absorption occurred (Fig. 4a, b). 265In both cases, a reduction in the specific conductance

Figure 4. Measurements of the CDOM absorption at 365 nm (A365, m−1, right axis) and specific conductance (µS cm−1, left axis)with increasing distance from inner bay sampling site in a) Katonga Bay, Uganda, b) Nyanza Gulf, Kenya. Distance from coastmeasured in linear meters, c) loss of absorption with increasing UV irradiance for samples from Nyanza Gulf (samples obtainedfrom inner bay (K1, UTM36 696375 9978346), mid bay (K8, UTM36 664700 9966800), Rusinga Channel (K15, UTM36 6393059958044) and open lake waters (K20, UTM36 625551 9976948). (continued on next page)

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for inner bays (>100 µS cm−1in Katonga Bay and>140 µS cm−1 in Nyanza Gulf) to open water (<80 µS cm−1) occurs. Nsonga Bay, without a majorriver source presents less significant differences in270specific conductance.

In Katonga Bay, a comparison between specificconductance and CDOM absorption (Fig. 4a) showsthat both measurements behave similarly, while inNyanza Gulf CDOM absorption and specific con-275ductance show different trends (Fig. 4b). This isalso confirmed by examining the correlation be-tween specific conductance and CDOM absorption.In the case of Katonga Bay, a significant correla-tion was observed (df = 19, R2 = 0.77) while the280correlations for Nyanza Gulf (df = 18, R2 = 0.27)and Nsonga Bay (df = 22, R2 = 0.34) were notsignificant (p > 0.001).

Measurements of photodegradation, performedusing quartz bottles on 4 samples from Nyanza285Gulf show the gradual reduction in UV absorptionover increasing dose (Figure 4c). The resultant pho-todegradation rate (change in absorption over in-creasing UV dose) ranged from 0.001 – 0.0001 mkJ−1 m−2. Higher rates characterised the inner Gulf290(K1) and lower rates characterised the outer Gulf(K15, K20). Blank measurements of MilliQ watershowed no absorption and samples not exposed to

solar radiation did not exhibit a reduction in absorp-tion over time. 295

Discussion

Exponential decay of CDOM absorption and thereduction in K313 in relation to distance from thecoast occurred in both Katonga Bay and NyanzaGulf. This reduction in CDOM absorption is related 300to both dilution and photodegradation of CDOMfrom most probably terrestrial sources (Kowalczuket al., 2003). While the degradation of phytoplank-ton biomass may also contribute to CDOM absorp-tion (DeGrandpre et al.,1996; Nelson et al. 2004), no 305significant correlation between chlorophyll a con-centrations and CDOM absorption was observedin the study bays. Undoubtedly, the importance ofphytoplankton biomass in the production of CDOMincreases in the open lake waters (Loiselle et al. 3102008).

Conservative mixing between the open lake wa-ters and the CDOM rich inner bay waters will de-pend on river flowrate, overall rainfall and the hori-zontal mixing regime between the lake and the bay. 315While the rainfall and river flow, normalised for bayarea are a similar magnitude for both Nyanza andKatonga, the exchange rate between the bay and the

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lake are significantly different. Katonga Bay is anopen bay (with the Bay aperture making up 28% of320the total Bay perimeter) which allows for a signifi-cant exchange with open lake waters. Nyanza Gulfis closed by Rusinga Island and the Mbita cause-way and presents a reduced channel (Rusinga Chan-nel) with respect to its overall area (<5% of total325perimeter). The high exchange (and mixing) ratebetween Katonga Bay and the lake is clearly seenin the quick reduction of specific conductance overdistance. This differs significantly from the spatialdistribution of specific conductance in Nyanza Gulf.330

As mixing between the river borne CDOM richwaters and the open lake waters is favoured in Ka-tonga Bay, the short residence time does not favourphotodegradation within the bay. This is further in-hibited by the high attenuation of solar UV radiation335(K313) in the inner Bay (due to high concentrationsof dissolved and particulate organic matter) furtherreducing overall UV dose of the CDOM rich wa-ter. The result is that conservative mixing, ratherthan photodegradation appears to be the dominat-340ing mechanism in modifying the optical properties(absorption) of CDOM in Katonga Bay. In suchconditions, the organic matter released into the bayfrom the Katonga River and surrounding wetlandsdoes not undergo major modification before it en-345ters into the open lake. Using the average flowrateof the Katonga River (5.1 m3 sec−1, COWI 2002)and roughly estimating dissolved organic carbonconcentrations from CDOM absorption (Mazzuoliet al., 2003), it is possible to estimate that approxi-350mately 7 × 103 kg of dissolved organic carbon enterthe bay each day, with a wide variation in relationto season floods. This CDOM is then released, rel-atively unmodified, into Lake Victoria. It should benoted that such a rough calculation will underesti-355mate the overall flowrate of organic carbon as we donot consider the release of organic carbon from pa-pyrus wetlands (through diffusion and wave action)located in other parts of the Bay.

Nyanza Gulf presents different conditions, both360in the behaviour of CDOM and specific conduc-tance. Due to its morphology and extension, there isa reduced exchange between the Gulf and the Lake,in comparison to Katonga Bay. This reduced ex-change (increased residence time) and higher pene-365tration depth of solar UV radiation create conditionsin which photodegradation may play a relatively im-portant role in modifying the optical properties ofCDOM. To better understand such modifications,we modelled the combined effects of conservative370

mixing and photobleaching of CDOM in relation todistance from the riverine sources, the Sondu andNyando rivers, using the measured photobleachingrates. For the photo-induced loss of absorption, wecalculated a hypothetical reduction in CDOM ab- 375sorption (A365) in relation to distance from the river-ine sources (residence time),

�Ai = AK 1 + Ti · IU V · βU V (1)

where AK 1 is the absorption at 365 nm at thenearest coastal site (K1), T is the estimated resi-dence time at each point in the Gulf, IU V is the solar 380UV irradiance dose and βU V is the photobleach-ing rate considering the relative distance betweenthe coast and the open lake waters. We assumed adaily dose of 780 kJ m-2 (UVB + UVA) just belowthe lake surface after correcting for surface reflec- 385tion at higher angles from nadir. We used spatiallydistinct measurements of vertical attenuation coef-ficients (K313 for UVB, K340 for UVA) to determinethe average solar UV dose in the mixed water col-umn. Residence time (T ) approximations consid- 390ered a plug flow with published average flowratesfrom the major rivers (42 m3 sec−1 for Sondu R.,18 m3 sec−1 for Nyando R., COWI 2002) and thecross sectional area of the Gulf. We used an averagephotodegradation rate (βU V = 0.0005 m kJ−1 m-2) 395as we did not have sufficient spatial information forβU V to determine the distribution of this parame-ter within the Gulf. In this manner, it was possi-ble to estimate the change in absorption that river-ine CDOM would undergo in its movement from 400the inner Gulf to the Lake, due to photobleaching.This was then combined with a conservative mixingmodel based on changes in specific conductance,where losses of specific conductance (C) are re-lated to increasing dilutions (D%) of riverine waters 405(CCoast ) by open lake water (CLake) according toD% = (CCoast − Ci )/(CCoast − CLake). The resultis an estimate of the expected CDOM absorptionconsidering both photobleaching and conservativemixing. 410

Ai = (1 − D%) · (Ti · IU V · βU V ) − D% · AL AK E

(2)

As was expected, the estimated distribution ofCDOM absorption over distance considering pho-tobleaching is closer to that measured in varioussites in the Gulf (Fig. 5). Differences between themodel and the measurements can be associated with 415

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Figure 5. Modelled and measured variation of CDOM absorption (A365) in relation to linear distance from the inner bay samplingsite in Nyanza Gulf, Kenya. Open circles represent measured A365, crossed triangles indicate estimated A365 if conservativemixing dominated the variations of CDOM over distance, half filled circles indicate modelled A365 when considering both variablephotodegradation rates and conservative mixing (assuming no other source or sinks of CDOM). Distance from coast measured inlinear meters.

the shortcomings of our modelling approach, inparticular that CDOM sources are limited toriver/wetlands in the inner Gulf and CDOM sinks arelimited to photodegradation. Further informationon CDOM sources from photoplankton/macrophyte420production and secondary sinks (microbial degrada-tion, flocculation) within the Gulf need to be con-sidered to refine the estimate of the fate of organiccarbon.

Using the average water flowrate of the Sondu425and Nyando rivers and estimating dissolved or-ganic carbon concentrations from CDOM absorp-tion (Mazzuoli et al. 2003), it is possible to estimatethat approximately 17 × 103 kg of dissolved organiccarbon are released into the Gulf each day. How-430ever, due to the relatively limited mixing betweenthe Gulf and the lake, photodegradation processesare allowed to modify CDOM absorption (loss ofchromophores) before Gulf waters enter the lake.Of the 17 × 103 kg of chromophoric dissolved or-435ganic carbon released, approximately one third (7 ×103 kg) reach the open Lake. The remaining twothirds are lost due to sinks.

The comparison between Nyanza Gulf andKatonga Bay points to major differences in the440roles that bays play on Lake Victoria. Semiclosedbays such as Nyanza Gulf (and to a lesser degreeMurchison Bay) provide conditions where riverine

and wetland released dissolved organic matter havesufficient time to undergo degradation mechanisms 445before they enter the open lake. Open bays suchas Katonga, Nsonga or Entebbe Bay, Karungu Bay,Muhuru Bay release largely unmodified CDOM intothe Lake. Mixing, residence time and UV penetra-tion depths needs to be investigated in major bays 450such as Speke Gulf and Murchison Bay to betterunderstand the dynamics of sources and sinks ofdissolved carbon in Lake Victoria.

Acknowledgements

This research was supported by the 455European Commission RTD INCO programme(ICA4CT2001-10036) and the Italian Interuniver-sity Consortium CSGI. We thank our colleaguesat the Kenyan Marine Fisheries Research Instituteand the Ugandan Department of Water Resources 460Management for their collaboration in data gath-ering and analysis. We thank R.T. Heath for hisconstructive comments to the manuscript.

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Aspects of the biology of Labeo cylindricus (Pisces: cyprinidae) in Lake Baringo, Kenya

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