Spatial and seasonal variation in the phytoplankton community of Lake Victoria’s Mwanza Gulf,compared to northern parts of the lake

We investigated the phytoplankton species composition and abundance in two seasons in Mwanza Gulf, Lake Victoria (Tanzania). Phytoplankton was sampled and chlorophyll a content was measured in the dry and wet seasons of 2010–2011 at three stations, from the southern land-inward end of the Gulf towards the open lake. Cyanobacteria, mostly small colonial and filamentous species (e.g., Aphanocapsa spp., Planktolyngbya spp., Merismopedia spp.) dominated at each station (76–95 %), followed by Chlorophyta (5–21 %), whereas the contribution of Bacillariophyceae was small (0–6 %). Phytoplankton densities were generally higher in the rainy season and strongly increased going land-inward from the open lake. Low abundance of N-fixing phytoplankton species suggests that N-fixation was low. The chlorophyll a content in the mouth of the Gulf was low (mean values 4–6 µg/L) compared to values reported previously. Also, chlorophyll a values (means 11–14 µg/L) at land-inward stations of Mwanza Gulf were much lower than those in the northern gulfs (Napoleon Gulf, Murchison Bay and Nyanza Gulf). Between 2002 and 2009 the phytoplankton composition of Mwanza Gulf changed from a community mostly dominated by Bacillariophyceae into a community dominated by Cyanobacteria. In the open water of Lake Victoria, Bacillariophyceae and Cyanobacteria were both abundant. Cyanobacteria dominated both in the three northern gulfs and Mwanza Gulf, but all four showed substantial differences in species and genus compositions. Phytoplankton composition and abundance in Mwanza Gulf differs in many respects from the open water of Lake Victoria and its three northern gulfs.     

Exotic introductions to the fishery of Lake Victoria: What are the management options?

Lake Victoria is Africa's most important source of inland fishery production, exhibiting annual catches of ≈ 400 000 mt. The predatory Nile perch, Lates niloticus, and the herbivorous tilapiines, Oreochromis niloticus, Oreochromis leucostictus, Tilapia zillii and Tilapia rendalii, were introduced in Lake Victoria in the 1950s and 1960s. Nile perch were introduced to convert the abundant, but bony, haplochromines to fish flesh, while the tilapiines were introduced to boost the declining fishery. Since that time, the fisheries of Lake Victoria have undergone dramatic social and ecological changes. The catches increased tremendously, changing the fishery from artisanal to commercial, in turn increasing fisher income and employment opportunities. However, there was a decline and, in some cases, the disappearance of many indigenous fish species, especially the haplochromine cichlids. This reduction was attributed to overexploitation, predation, and competition and hybridization with the introduced species. The decline of the native fish species has had impacts on the trophic and ecological status of the lake. Nile perch now dominate the formerly complex food web. The loss of phytoplanktivorous haplochromines has contributed to an increase in algal blooms, reduction in water quality and occasional fish kills. Water hyacinth, Eichhornia crassipes, invaded Lake Victoria in 1988, with high rates of infestation in shallow waters and bays, which are breeding and nursery grounds for most fish species. Catches of Nile perch decreased following the infestation, while those of Nile tilapia, lung fish (Protopterus aethiopicus) and mud fish (Clarias gariepinus) increased. Haplochromines species also showed some signs of recovery. In view of all these changes, the future of the Lake Victoria fishery is uncertain.     

Growth and population parameters of Nile tilapia,Oreochromis niloticus (L.) in the open waters of Lake Victoria,Kenya

The Nile tilapia (Oreochromis niloticus) was introduced into Lake Victoria in the early 1950s and 1960s and has since become the dominant tilapiine in the lake. This study investigated the growth and population parameters of O. niloticus in Lake Victoria on the basis of length–frequency data collected during the period June 2014 and June 2015. The asymptotic length (L_∞) had a mean (±SE) value of 46.24 ± 0.04 cm TL, growth curvature (K) of 0.69 ± 0.25 year^?1, total mortality (Z) of 2.18 ± 0.80 year^?1, a natural mortality (M) of 1.14 ± 0.28 year^?1, a fishing mortality (F) of 1.05 ± 0.53 year^?1, an exploitation rate (E) of 0.46 ± 0.08, a growth performance index (?) of 3.14 ± 0.17 and a length at first capture (L_C50) of 20.31 ± 0.40 cm TL. Comparing the results of this study with previous studies indicates the parameters K, Z and M have increased, whereas ?, F, E and L_C50 have decreased. Changes in these parameters could be attributed to the existing high fishing capacity, and changing lake conditions. Thus, management measures should include continued restriction on illegal fishing methods and gears, such as the use of undersized gillnets (<5 in. mesh size) and beach seines. More attention also should be directed to the implementation of measures to control pollution of the lake from its various sources.     

Changes in pollution indicators in Lake Victoria,Kenya and their implications for lake and catchment management

Monitoring of aquatic pollution is important for ascertaining the relationship between fisheries and the general ecosystem health of a lake. This study evaluated the use of changes in pollution indicators in Lake Victoria, Kenya, as a decision support tool for fisheries management and productivity. Principal component analysis (PCA; R² ≥ 0.5, P < 0.05) of physical and chemical parameters delineated sampling sites into ecological cluster zones consisting of the inner gulf (C1), mid‐gulf (C2) and open lake (C3). Test results for lead (Pb) and mercury (Hg) levels in the Nile perch tissues were found to be compliant with EU standards. The inner and mid‐gulfs of the Winam Gulf had high levels of total (1818.8 ± 102–1937.78 ± 94 cfu 100 mL^?1) and faecal (390 ± 21 cfu 100 mL^?1) coliforms attributable to urban sewage and industrial effluents exceeded WHO standards. Similarly, Winam Gulf was more polluted than the open lake, with higher total phosphorus and nitrogen concentrations, turbidity levels and electrical conductivity. Low phytoplankton biovolume and a low number of macroinvertebrates genera, and high zooplankton densities and pollution‐tolerant catfishes (e.g., Schilbe victoriae; Clarias gariepinus) were observed in Winam Gulf. Faecal coliforms and dissolved oxygen influenced the abundance of tolerant fish species (e.g., S. victoriae) in the lake. This study indicated a declining trend of ecological integrity in the Winam Gulf, compared with the open waters of Lake Victoria. An integrated management approach directed to minimizing pollution levels, especially in the Winam Gulf, is recommended to enhance fishery production.     

Phosphorus and nitrogen loading to Lake Huron from septic systems at Grand Bend,ON

Groundwater nutrient loading to L. Huron was assessed along a 1.7 km section of beach at Grand Bend, ON, Canada, where septic systems are used for wastewater disposal. The artificial sweetener acesulfame (ACE) was detected in all groundwater samples (7–842 ng/L, n = 78), revealing that the entire surficial aquifer was impacted by septic system wastewater. Nitrate concentrations (3.5 ± 1.4 mg/L, n = 78) were correlated with ACE (r² = 0.54), indicating that septic systems contribute to nitrate loading in the aquifer. Chloride was also elevated (37 ± 11 mg/L, n = 78), but was not correlated with ACE (r² = 0.008), indicating a non wastewater source was dominant, likely road salt. Soluble reactive phosphorus (SRP) values were low (5.3 ± 9.3 μg/L, n = 77) and were not correlated with ACE (r² = 0.006). Sediment profiling below two of the septic system drain-fields, showed that the sand grains had distinct secondary coatings containing P, indicating that mineral precipitation reactions played a role in limiting P concentrations present in the aquifer. Groundwater nutrient loading to the lake was estimated at 13,000 kg N/year from NO₃^? and 1.9 kg P/year from SRP. These amounts are insignificant compared to nutrient loading from a stream that drains an agricultural catchment and discharges to the lake at the north end of the study site (Parkhill Creek). This calls into question, in some cases, the rationale of decommissioning properly functioning septic systems as a mitigation measure for reducing nutrient loading to nearby water courses.     

Characteristics of nearshore water quality of Lake Ontario coast under Credit Valley Conservation Jurisdiction,Ontario, Canada

The 28.5 km Lake Ontario shoreline along Credit Valley Conservation’s jurisdiction is habitat for native flora and fauna, the source of drinking water for over two million residents, and the location of outfalls for Region of Peel’s two wastewater treatment plants. Some of the significant sources of pollutants to this region are urban tributaries and the Credit River. Instream, shoreside, and nearshore water quality of this region were characterized under this Lake Ontario Integrated Shoreline Strategy study. Eight Parameters of Concern (PoC); water temperature (WT), total suspended solids (TSS), total phosphorous (TP), orthophosphate (OP), nitrate-nitrogen (NO₃-N), Escherichia coli (E. coli), specific conductivity (SC), and pH were studied from 2011 to 2013. Amongst all the PoCs TP was of concern since its instream (75th percentile range: 51.0 µg/L to 188 µg/L), shoreside (75th percentile range: 9.00 µg/L to 53.75 µg/L), and nearshore (75th percentile range: 9.75 µg/L to 19.00 µg/L) concentrations, exceeded their respective objectives of 30 µg/L and 10 µg/L. Other parameters in shoreside/nearshore samples met their respective provincial/federal objectives/guidelines; however, instream E. coli and specific conductivity levels were elevated, reflecting the effects of urbanization. The Lake water temperature moorings recorded lake stratification dynamics, including eight downwelling and five upwelling events during 2011 deployment period. These results provide baseline data for future climate change-related assessment studies.     

Broad screening of toxic and bioactive metabolites in cyanobacterial and harmful algal blooms in Lake of the Woods (Canada and USA), 2016–2019

Cyanobacteria produce toxic/bioactive metabolites that affect human and ecosystem health via inhibition of proteases, carboxypeptidases, or phosphatases. From surface water samples collected between 2016 and 2019 inclusive (n = 78), we report intracellular concentrations of more than 30 such compounds in Lake of the Woods (LOW), a large lake system of multinational importance. Although most surface biomass blooms were dominated by the cyanobacteria Dolichospermum (max. ～50 mg/L) and Aphanizomenon (max. ～60 mg/L), there was also significant biomass of other cyanobacteria (Gloeotrichia echinulate along shoreline, max. ～40,000 mg/L; Woronichinia naegeliana max. ～100 mg/L; Lyngbya max. ～10 mg/L; Planktothrix agardhii max. ～6 mg/L) and phytoplankton (Stephanodiscus niagarae max. ～6 mg/L). Microcystins (MCs) were present in ～80 % of samples, peaking at ～33 µg/L (MC variant-LR). Approximately 50 % of samples contained MC concentrations below 0.1 µg/L. Alkaloid metabolites detected included neurotoxin anatoxin-a (～60 % < 0.1 µg/L) and cytotoxic cylindrospermopsins (～30 % < 0.1 µg/L). Anabaenopeptins A/B/F were prevalent (>96 %) and exceeded MCs by >100 fold (max. >4,000 µg/L). Aeruginosamide B was present in ～75 % of samples (max. ～10 µg/L). Cyanopeptolin 1040 MB, microginin 690 methyl ester, and oscillaginin A were not detected however, traces of micropeptin 1106 were detected. Microcystin and anabaenopeptin genes were also detected in the majority of samples (78 % and 79 %, respectively), while saxitoxin, cylindrospermopsin, and anatoxin genes occurred at lower frequencies with 59 %, 36 %, and 38 % detection, respectively. These results will aid assessment of risk from cyanobacterial blooms in LOW and inform ongoing binational lake management and policy development.     

Variations in zooplankton community structure and water quality conditions in three habitat types in northern Lake Victoria

Lake Victoria is vulnerable to increasing eutrophication, which has become manifested in ecological changes not yet fully understood. From October 2009 to January 2010, the influence of water quality on zooplankton community structure in three habitats in northern Lake Victoria, including sewage lagoons at the lake shores, Napoleon Gulf (NG) and the interface between the lake and the Nile River (also known as Source of the Nile River), was examined. Selected physico‐chemical parameters (dissolved oxygen concentration; water temperature; electrical conductivity; water depth) were measured in‐situ, while water samples were collected for chlorophyll‐a determinations. Zooplankton was sampled with conical plankton net (mesh size 60 μm; 0.25‐m mouth diameter). The NG and Source of the River Nile (SN) sampling sites exhibited significantly higher species richness, relative to the Sewage Lagoons (SL) site (F_2,69 = 68.533; P < 0.05). Higher mean densities and dry biomass of zooplankton was generally a characteristic of the SL site (8715 ± 3241 ind L^?1; 1862 ± 451 μg L^?1), compared to the NG (119 ± 24 ind L^?1; 53 ± 8 μg L^?1) and SN sites (151 ± 26 ind L^?1; 58 ± 9 μg L^?1). Copepoda constituted a high numerical composition of the zooplankton at the NG and SN sites (>90% for both sites), while Rotifera dominated the zooplankton community at the SL site (97%). The mean values (±SE) of soluble reactive phosphorus (4060.7 ± 776.6 μg L^?1) and nitrate–nitrogen (2121.7 ± 355.5 μg L^?1) were much higher for the SL site, compared with the SN (8.2 ± 1.1 μg L^?1; 28.6 ± 5.3 μg L^?1, respectively), and NG site (7.8 ± 0.8 μg L^?1; 32.7 ± 5.4 μg L^?1, respectively). This study indicated the nutrient‐rich conditions observed at the SL site suppress the zooplankton species diversity, but favour species‐specific abundance and biomass. These study results indicate the zooplankton community structure can be used as a biological indicator of water quality in the Lake Victoria region.     

Some aspects of the biology and life‐history strategies of Oreochromis variabilis (Boulenger 1906) in the Lake Victoria Basin

Oreochromis variabilis (Boulenger), a fish species endemic to Lake Victoria, was abundant, forming an important component of the indigenous fisheries stocks before and up to the late‐1950s. Catches declined drastically thereafter, and only sporadic catches are currently found in Lake Victoria. Remnants population of the species, however, are found in several small waterbodies (SWBs) within the lake basin. The life‐history characteristics of O. variabilis in Lake Victoria, including, sex ratio, reproduction and length–weight relationship, were compared to those in selected three SWBs in the lake basin. Fish samples were collected by monofilament gillnets of 30–255 mm between 2001 and 2005. Males predominated over females from all the sampled sites (sex ratio 1.00:0.33). Length at first maturity (Lm₅₀) had mean (±SE) of 18.48 ± 1.50 cm TL for males, and 16.87 ± 0.95 cm TL for females, and did not exhibit any significant differences between habitats. Fecundity ranged between 73 and 14 800 eggs for fish of 13.5–18.6 cm TL, respectively. Absolute fecundity of O. variabilis was proportional to the body weight, but nearly proportional to the cube of the fish length. Egg diameter varied from 0.3 to 5.19 mm, with a mean (±SE) of 3.44 ± 0.08 mm. Growth was allometric in both male and female, being significantly different from the expected value of 3 (P < 0.05). The life‐history strategy of O. variabilis is discussed within the context of changes in the lake and the SWBs.     

A review on the status of some major fish species in Lake Victoria and possible conservation strategies

Lake Victoria, the second largest freshwater lake in the world, supports an enormous flora and fauna biomass, with a large human population around the Lake. The lake is a source of food (fish), water for domestic use and crop and livestock production, transportation and tourism, among other uses. As a result of these varied uses, human intervention within the lake and its catchment has resulted in several ecological changes in the lake in recent years, with profound effects on its fish resources. One of the most notable changes is the reduction and risk of extinction of some fish species, including the endemic tilapiine species (Oreochromis esculentus and Oreochromis variabilis), catfishes (Xenoclarias eupogon), haplochromines and cyprinids (Labeo victorianus and Barbus altinialis) in the lake. The reduced biodiversity, as well as extinctions within the lake, has been attributed to the introduction of alien fish species (e.g. Nile perch (Lates niloticus) and Nile tilapia (Oreochromis niloticus) into the lake, habitat loss and cultural eutrophication. If left unchecked, these changes will have devastating effects on the lake's resources, as well as to people living around the lake and beyond who depend on the lake fisheries for their livelihood. Thus, there is an urgent need for management measures based on sound scientific research to be implemented in order to curb the loss of ichthyodiversity within the lake, as well as to save the livelihoods of those stakeholders who are either directly or indirectly dependent on the lake.     

Review of current state of knowledge of microcystin and its impacts on fish in Lake Victoria

Microcystins are part of algal toxins produced intracellularly within algal cells, being in the family of hepatotoxic cyclic peptides from various species of blue‐green algae. Blue‐green algae are widely abundant in many equatorial eutrophic lakes, including Lake Victoria, with microcystin mainly from cyanobacterial blooms released into the water column, with different effects along the aquatic ecosystem trophic levels. Depending on the length of exposure and exposure route, microcystin effects on fish can include embryonic hatching perturbations, reduced survival and growth rates, changes in behavior, osmoregulation, increased liver activities and heart rates, as well as histopathological effects. While bioaccumulation is confirmed among fish, biomagnification along food webs is debatable. Lake Victoria the second largest freshwater lake in the world, and the source of livelihoods to millions reported near the gulf and shore MCs of 190 ± 51 to 543 ± 26 ng MC/g DW, respectively. Little is known, however, on the effects of microcystin on the Lake Victoria fishery and, ultimately, on the human population against the WHO recommended human microcystin intake levels of 0.04 µg/kg, thereby being the basis for this review.     

Alternative Ecological Pathways in the Eastern Lake Ontario Food Web—Round Goby in the Diet of Lake Trout

Round goby (Neogobius melanostomus) range expansion and their possible inclusion in the diet of lake trout (Salvelinus namaycush) were investigated. Fish community index bottom trawls in eastern Lake Ontario (Kingston basin) during summer 2003 and 2004 indicated the presence of the round goby at relatively low densities (3.72 × 10^–2 ± 5.24 × 10^–3 fish/m²) in depths up to 30 m. Lake trout (mean fork length = 585 ± 78 mm and mean weight = 2,770 ± 1,134 g) stomach contents showed round goby to be the second most abundant diet item at almost 20% by number (36% by mass). Round goby ingested by lake trout ranged in total length from 50 to 110 mm. The most important prey species in terms of abundance (68%) and mass (56%) was alewife (Alosa pseudoharengus). Alewives were the most important diet item for all sizes of lake trout sampled, except those in the 550 - 650 mm size class, which ingested more round goby by mass than alewife. Round goby range expansion to deep water and prominence in the diet of lake trout signal significant change in the eastern Lake Ontario food web.     

Lake Victoria fisheries: Outlook and management

The past subsistence of the Lake Victoria fishery was dominated by rich, diverse haplochromine cichlids. This multispecies fishery has undergone a decline over the past four decades, evolving into a commercial fishery consisting mainly of Nile perch (Lates niloticus), Nile tilapia (Oreochromis niloticus) and cyprinid (Rastrineobola argentea species). To better understand Lake Victoria fisheries, studies dating as far back as the 1920s have been carried out to assess the status of the fish stocks. These past studies indicated the lake fisheries were declining because of numerous major challenges, including intense fishing, invasive species, loss of biodiversity, ecological alterations, climate change, inadequate information to inform management and unharmonized policies. Numerous policies and regulations have been developed and implemented over the years to address these issues and manage the fisheries sustainably. Most of the interventions have been sectorial, disjointed and unharmonized and have not reduced the declining fish catch rates. With reestablishment of the East Africa Community (EAC) with several institutions in 1994, the Lake Victoria riparian states initiated an ecosystem approach to manage the Lake Victoria fishery resources in a sustainable manner. This study reviews the development of the Lake Victoria fisheries, outlines major past and present management challenges and provides a set of new strategies to manage the lake's fisheries resources, with emphasis on an ecosystem approach.     

Assessment of the nutritional quality of fish cultured in Samdihi,an open cast coalpit at the Raniganj Coal Field areas,West Bengal,India

The present study assessed the nutritional quality of four major teleostean fishes (Catla catla; Labeo rohita; Notopterus notopterus and Oreochromis niloticus) cultured in the Samdihi, an opencast coalpit (OCP). The physicochemical parameters were within the pisciculture standard limits, and the heavy metal contents of the fish were within safe concentrations and below the target hazard quotient (THQ) level (<1). The zooplankton abundance and density were sufficient as food item for the fish faunas. Ash and moisture contents confirmed the good quality of the fish as food. The results of the other analysed nutritional parameters indicated the total protein content was highest in the herbivorous fish Labeo rohita (85.32 ± 0.61 mg/g), compared to Catla catla, Notopterus notopterus and Oreochromis niloticus (50.34 ± 0.43, 71.32 ± 0.49 and 75.14 ± 0.53 mg/g, respectively). The order of decreasing lipid content was Notopterus notopterus > Catla catla > Labeo rohita > Oreochromis niloticus (7.04 ± 1.02 > 6.01 ± 0.01 > 1.83 ± 0.03 > 1.46 ± 0.04%). The percentage of carbohydrates was highest in the omnivorous Oreochromis niloticus (18.2 ± 0.8), followed by Notopterus notopterus, Catla catla and Labeo rohita (15.1 ± 0.7, 13.9 ± 0.5 and 9.4 ± 0.4, respectively). The sodium content was higher in the column feeders Labeo rohita (82.06 ± 0.61 mg/g) and Notopterus notopterus (81.87 ± 0.61 mg/g) than in the surface feeders Catla catla (69.16 ± 0.55 mg/g) and Oreochromis niloticus (79.33 ± 0.60 mg/g). The potassium content exhibited similar trends, being higher in the column feeders Notopterus notopterus (204.8 ± 1.52 mg/g) and Labeo rohita (199.5 ± 1.81 mg/g) than in the surface feeders Oreochromis niloticus (190.8 ± 1.13 mg/g) and Catla catla (183.2 ± 1.3) mg/g). The trend in iron content was in the order of Oreochromis niloticus > Labeo rohita > Notopterus notopterus > Catla catla (5.60 ± 0.01 > 4.806 ± 0.01 > 4.25 ± 0.01 > 1.303 ± 0.01 mg/g, respectively). The results of the present study highlight the potential and feasibility of conducting pisciculture in the abandoned OCP as a means increasing the economic livelihoods of the core group of people in and around the mining areas of RCF regions. The estimated fish nutritional index (EFNI) analysis also revealed the fish cultured in this OCP exhibited a good nutritional quality standard.     

Spatial-temporal dynamics of water hyacinth, Eichhornia crassipes (Mart.) and other macrophytes and their impact on fisheries in Lake Victoria,Kenya

Invasive aquatic macrophytes in Lake Victoria including water hyacinth (Eichhornia crassipes) exhibit periodic cyclical patterns of decline and proliferation with attendant ecological and economic impacts. This study aimed to monitor the extent of macrophytes and other invasive weeds on Lake Victoria to establish their impact on fisheries. The study employed a combination of remote sensing and geographical information system (GIS) techniques to estimate the coverage of water hyacinth and other invasive macrophytes. Data on fish landings and their respective market values were acquired from the Electronic Fish Market Information Service (EFMIS) database, which is hosted by the Kenya Marine and Fisheries Research Institute (KMFRI). Analysis of consistent temporal satellite data showed that the weeds frequently cover sheltered bays and river mouths. These areas of Winam Gulf had higher coverage (average 5000?ha) of macrophytes than the open waters (<200?ha). The proliferation of the invasive weeds showed fluctuations over the study period reaching the highest peak between September and November 2016. Other aquatic plants that have invaded the littoral areas of Lake Victoria include Egeria densa, Ceratophyllum demersum and Potamogeton spp. Increased coverage of macrophytes was found to be correlated inversely with the commercially important tilapia Oreochromis niloticus, but not with other species. The study concludes that there is a need for sustained monitoring of the invasive macrophytes alongside ecosystem modelling studies using the available time series data to clearly identify the ecological factors that drive water hyacinth dynamics and predict more precisely its impact on the fishery.     

Empirical modeling of chlorophyll a from MODIS satellite imagery for trophic status monitoring of Lake Victoria in East Africa

We detail our attempts at empirical modeling of MODIS derived Chlorophyll a (Chl a) distribution on Lake Victoria in East Africa and consequently its trophic status. This was motivated by the need for Lake Victoria specific algorithms, as the current satellite based standard algorithms overestimate derived Chl a. In situ Chl a data was hence collected in three field campaigns in November 2014, March 2015 and July 2015. In situ reflectances were collected during the July campaign only. We first developed models from in situ reflectances and in situ Chl a, which when applied to MODIS bands performed dismally (R² = 0.03). We then proceeded to derive empirical models by directly comparing MODIS bands with in situ Chl a based on data collected in November 2014 and July 2015. The March 2015 dataset couldn’t be used due to cloud cover hence no matchups could be obtained. The best model derived (R² = 0.88) was based on the ratio 488 nm/645 nm, and was then used to determine the trophic status of Lake Victoria using Carlson’s Chl a Trophic State Index (TSI). The results show that large areas of the lake are mesotrophic with eutrophic displays closer to the shores. The modeled TSI was then validated against in situ TSI derived from the March dataset and posted an 80% matchup. One of the main challenges, however is the prevalence of cloud cover, which hinders synoptic mapping of the lake. That notwithstanding, the study demonstrates the potential of earth observation in providing accurate TSI information for improved management of Lake Victoria.     

Phytoplankton Productivity and Standing Crop in the Vicinity of the Lake Huron-Saginaw Bay Front

Continuous underway sampling was used in four research cruises near the mouth of Saginaw Bay in order to locate the bay-lake front, where eutrophic bay water mixes with oligotrophic Lake Huron water. Although mixing between these water masses normally occurs in outer Saginaw Bay, the distribution was variable, depending on wind force and direction as well as water temperature. Large algal crops in inner Saginaw Bay had reduced soluble nutrients, particularly silicate and nitrate-nitrogen, in the outer bay to levels lower than the adjacent lake water. Nonetheless, outer bay water supported high algal biomass (up to 14 μg/L chlorophyll a) and ¹⁴C uptake (up to 73 μg /L/h). The phytoplankton community at the nutrient front between bay and lake waters was characterized by high chlorophyll a biomass ($\overline{x}$ = 4.47 μg/L), low ¹⁴C uptake ($\overline{x}$ = 7.12 μg/L/h), and high bacterial activity (V_max$\overline{x}$ = 1.33) indicating the presence of a senescent assemblage. Decomposition and settling of phytoplankton in the vicinity of the front apparently served to remove much of the bay plankton from the water column before mixing into the open lake. But, during mid-summer, occasional large inputs of nutrients and/or phytoplankton from the bay into the open lake were observed.     

Food‐web patterns and diversity in tropical fish communities

The food webs for three Sri Lankan reservoirs, Minneriya (ancient and shallow), Udawalawe (young and shallow) and Victoria (young and deep), were compared. The species richness of the fish communities was highest in Minneriya (30 species), intermediate in Udawalawe (21 species) and lowest in Victoria (18 species). The fish species belonged to 11 families, with Cyprinidae dominating the community in terms of both abundance and species richness. The daily quantity of food consumed per fish species was used to distinguish weak from strong trophic interactions in these food webs. The fish community consumption pattern was characterized by a few strong interactions and many weak ones. The number of major consumers (i.e. contributing >5% of the total fish community consumption) for each reservoir was small and similar for all three reservoirs. Dawkinsia singhala and Amblypharyngodon melettinus were the two major consumers in all three reservoir food webs. Puntius chola was a major consumer in Minneriya and Victoria, although not in Udawalawe, where it fed less on detritus than for the other two food webs. The fish community fed mainly at the bottom of the food web, primarily on algae, macrophytes and detritus in all three reservoirs, with very little piscivory occurring. The average food‐web length was measured as the mean trophic index weighted for the consumption rates of the various fish populations that together comprised the fish community. The average food‐web length in these three Sri Lankan reservoirs relatively short, compared with most other tropical lake/reservoir food webs in Asia and Africa for which relevant data were available. Furthermore, traditionally unexploited species (Oreochromis mossambicus; O. niloticus), in contrast to species of commercial fisheries interest, are major role players in the trophic dynamics of reservoir ecosystems. Thus, based on this study, the potential of their exploitation should be considered in the management of reservoir fisheries.     

Potamoplankton of the Maumee River during 2018 and 2019: The relationship between cyanobacterial toxins and environmental factors

While algal blooms are common in eutrophic lakes, blooms can also occur in tributaries that load nutrients into the lake. We sampled six sites along a 122-km stretch of the Maumee River May through October 2018 and 2019 at weekly to biweekly intervals to determine if algal blooms occur, in particular toxic cyanobacteria, and to provide insights on potential environmental drivers of blooms. Samples were analyzed for concentrations of potamoplankton (=riverine phytoplankton), chlorophyll a, nutrients, cyanobacterial toxins, microcystins and saxitoxins, and cyanotoxin genes (mcyE and sxtA). Extreme precipitation in 2019 resulted in more high discharge events during 2019 than in 2018. Chlorophyll a ranged from 50 µg/L to 300 µg/L during periods of low discharge (<50 m³/s), and green algae and diatoms accounted for the majority of the chlorophyll a. In both years, cyanobacteria comprised a low proportion of all chlorophyll a, usually<20 %, but microcystins and saxitoxins were detectable in 38.7 % and 16.7 % samples, respectively, and mcyE and sxtA were detected in 36.2 % and 59.7 % samples, respectively. Therefore, cyanotoxins were present even when cyanobacteria were not at bloom densities. Chlorophyll a, cyanotoxin genes, and microcystins negatively correlated with discharge rate measured on the date of sample collection. Together our results suggest that cyanotoxins can occur in any portion of the Maumee River during low discharge conditions. Climate change is expected to reduce precipitation during the warm summer months in the Maumee River watershed and thus possibly increase the frequency of low discharge conditions that favor cyanobacteria.