Composition and seasonal variations in abundance of Copepod (Crustacea) populations from the northern part of Lake Tanganyika

peer reviewedCopepod has an important ecological role as a main food of several fishes on which commercial fisheries are based in Lake Tanganyika. However, very little multi-annual monitoring programs have been conducted for this group of crustacean zooplankton. This study was conducted in the northern part of Lake Tanganyika by weekly sampling at both pelagic and littoral sites over three consecutive years (2012–2014). The analyzed samples showed that Lake Tanganyika copepod was essentially composed of three suborders including Calanoida (30% by number) with a single endemic species, Cyclopoida (69%) with four dominant species and Harpacticoida (1%). These taxa showed variations in their abundance respectively at pelagic and littoral sampling sites. The average densities of copepods did slightly vary during the sampling period in both areas although higher peaks were observed in 2013. For the post-nauplii stage, Tropocyclops tenellus predominated while the nauplii of T. simplex predominated at both sampling sites. Seasonal fluctuations of copepod densities showed higher peaks in September/October and April/May, respectively, for the three most common species. T. simplex and M. aequatorialis ovigerous females showed higher peaks in the rainy season in 2013. These results can benefit pelagic fisheries research and lake environmental management efforts as the copepod abundance seems to be closely correlated to that of sardines in Lake Tanganyika. Copyright © 2016 AEHMS

Feeding habits of four species belonging to the genus Haplochromis (Teleostei, Cichlidae) in Ihema lake (Rwanda)

peer reviewedThe food of four haplochromine species is investigated in this study. These species represent 27, 9 % of the total weightcaught by experimental fishing. This work shows that each species takes advantage of specific food items. Haplochromis (Haplochromis) sp. feeds mainly on Chaoboridae, during the night, near the surface. H. (Gaurochromis) sp. is also an insect feeder whose main prey are Chaoboridae, but during the day and on the bottom. H. (Harpagochromis) sp. is a predator feeding mainly on fishes. The food of H. (Paralabidochromis) sp. is composed of benthic organisms, especially Chironomidae and Oligochaetes in the coastal zone. This high specialisation seems an efficient way to avoid food competition. © Gauthier-Villars, 1992

Analysis of trophic relationships in two shallow equatorial lakes Lake Naivasha (Kenya) and Lake Ihema (Rwanda) using a multispecifies trophic model

peer reviewedA multispecifies trophic model called ECOPATH II, which can be used to describe the trophic relationships in aquatic ecosystems on a quantitative basis, is briefly presented. When properly used, it can help to explain the trophic relationships in ecosystems and possible evolution of fishstocks after modifications of the environment (eg eutrophication, introduction of a new population and/or a significant increase of the fishing effort), and to compare the trophic structure of several ecosystems. Examples are provided on two shallow lakes: Lake Ihema and Lake Naivasha

Observations of a desmodesmus (Chodat) s.s. ann, friedl & e. hegewald (sphaeropleales, chlorophyta) and two didymocystis korshikov (chlorellales, chlorophyta) in lake tanganyika

peer reviewedThe occurrence of three species belonging to the small coenobia forming green algae Desmodesmus and Didymocystis is reported from the northernmost part of Lake Tanganyika, East Africa. During a monitoring between September 2011 and October 2013 the highest number of cells of Desmodesmus bicellularis and Didymocystis comasii were observed in August and September, at the end of the dry season. On the other hand, the third taxon, Didymocystis cf. inermis, was only sporadically observed between September 2011 and March 2012. According to existing databases, it should be the first records from Africa of these three species. However, D. bicellularis (as Didymocystis bicellularis) was already reported in 2003 in phytoplankton samples of the pelagic zone from more southern located places in Lake Tanganyika. © The authors

Unusual massive phytoplankton bloom in the oligotrophic Lake Tanganyika

peer reviewedBackground and aims – Massive algae growth resulting in a phytoplankton bloom is a very rare event in the meromictic and oligotrophic Lake Tanganyika. Such a bloom was observed in the north of the lake in September 2018. Phytoplankton species composition during this bloom is compared to a documented bloom in 1955, and to the composition in September 2011–2013. Meteorological observations suggest hydrodynamics could explain the occurrence of the 2018 bloom. Material and methods – Phytoplankton net samples were taken in the pelagic and littoral zone near Uvira during five consecutive days of the bloom in 2018. For the period 2011–2013, quantitative phytoplankton samples were obtained during a weekly sampling at the same sites. Samples were analysed with an inverted microscope and relative abundances of the algal species were compared. Key results – Dolichospermum flosaquae (Cyanobacteria) initially dominated the bloom followed by high relative abundance of Limnococcus limneticus (Cyanobacteria) on the third sampling day in September 2018. In the pelagic zone an increase of Nitzschia asterionelloides (Bacillariophyta), and Dictyosphaerium and Lobocystis (Chlorophyta) was observed while in the littoral zone increasing abundances of dinophytes were noted. Dolichospermum flosaquae was also responsible for the bloom reported in 1955, but was only sporadically observed in the 2011–2013 samples. Although Limnococcus limneticus was present in 2011–2013, it never reached relative abundances as high as during the 2018 bloom. Meteorological data indicate that 2018 experienced different conditions compared to previous years: strong south-east winds from May to September with a more eastern direction of the wind, and a well-marked drop in atmospheric pressure between August and September. Conclusion – After a very windy season, the combination of strong hydrodynamics, calmer lake conditions, and high solar radiation and air temperature in September 2018 was favourable for a massive Cyanobacteria bloom in the north of Lake Tanganyika. © 2021 Christine Cocquyt, Pierre-Denis Plisnier, N’sibula Mulimbwa, Muderhwa Nshombo

Observations on the limnology and phytoplankton community of crater Lake Kyaninga (Uganda), with special attention to its diatom flora

peer reviewedBackground and aims - With a depth of at least 220 m, Lake Kyaninga is the deepest known maar crater lake in western Uganda. We studied its limnology and phytoplankton community to determine how the frequency and depth of water-column mixing influences nutrient cycling and seasonality in this aquatic ecosystem. Methods - Water-column temperature was measured continuously during a full annual cycle between August 2007 and August 2008. Other physical and chemical variables as well as diatom and other phytoplankton communities were investigated on three occasions, namely during the dry season in August of 2007 and 2008, and during the main wet season in April 2009. Key results and conclusions - The water column of Lake Kyaninga is permanently stratified (meromictic) below ~ 100 m depth. Above this depth, mixing frequency varies from daily (down to 8-12 m depth) over at least once per year (down to 39-47 m depth), to once in several years or decades (between 39-47 and ~ 100 m depth). Nutrient and chlorophyll concentrations as well as phytoplankton data classify the lake as low in aquatic productivity (oligotrophic). Its pelagic, open-water phytoplankton community is dominated by Cyanobacteria (blue-green algae) and Chlorophyta (green algae). Bacillariophyta (diatoms) contribute only a minor part of total phytoplankton biomass in both wet and dry seasons, and are characterized by an assemblage of small Nitzschia species. Epiphytic and epipelic diatoms are relatively few, because steep rocky crater slopes limit the littoral zone even though water-column transparency is high. The composition of recently deposited diatom assemblages preserved in offshore surface sediments gives a good, annually integrated representation of the present-day pelagic diatom community. The documented species richness of the diatom flora of Lake Kyaninga is moderate with about 150 taxa. Only ~ 17% of these are biogeographically restricted to tropical Africa; and most of these belong to the genus Nitzschia. © 2010 National Botanic Garden of Belgium and Royal Botanical Society of Belgium

Spatial and seasonal variation in reproductive indices of the clupeids Limnothrissa miodon and Stolothrissa tanganicae in the Congolese waters of northern Lake Tanganyika

peer reviewedKnowledge on the reproductive biology of the endemic clupeids Limnothrissa miodon and Stolothrissa tanganicae, two main target species of the pelagic fisheries of Lake Tanganyika, is constrained by fragmented monitoring activities. Here, we investigate the nursing areas of L. miodon, the timing of reproductive activities of littoral and pelagic L. miodon, and the timing of reproductive activities of pelagic S. tanganicae in the Congolese waters of the northern end of Lake Tanganyika (Bujumbura sub-basin). Nursing areas were determined year-round (2009–2010) based on the presence of clupeid larvae at two sandy and two stony beaches. The gonadosomatic index (GSI) and the proportion of fish having ripe gonads were used to study variation in reproductive indices in space (littoral vs. pelagic zones) during one year (2013–2014), as well as in time (dry vs. rainy season) during three years (2013–2016). Larvae of L. miodon were more frequently encountered on sandy than on stony beaches. Mature L. miodon females were more abundant in the littoral than in the pelagic zone, while the proportion of mature males in both habitats was similar. Irregular, low amplitude peaks could be distinguished in the GSI and proportion of mature males and females, but averages only differed between the dry and the rainy season in males. In contrast, GSI and proportions of mature males and females in S. tanganicae were higher in the dry season than in the rainy season. The reproductive effort of males and females of S. tanganicae and littoral L. miodon, but not pelagic L. miodon, was strongly synchronized. Interestingly, reproductive investment was also synchronised between pelagic male L. miodon, and pelagic S. tanganicae. Our time series strongly supports the view that L. miodon reproduces year-round in the littoral zone, while reproduction in S. tanganicae is seasonal. For fisheries management, we recommend year-round protection of sandy beaches, which are the main breeding grounds for L. miodon. © 2022, Royal Belgian Institute of Natural Sciences. All rights reserved

Spatial and seasonal variation in reproductive indices of the clupeids Limnothrissa miodon and Stolothrissa tanganicae in the Congolese waters of northern Lake Tanganyika

Knowledge on the reproductive biology of the endemic clupeids Limnothrissa miodon and Stolothrissa tanganicae, two main target species of the pelagic fisheries of Lake Tanganyika, is constrained by fragmented monitoring activities. Here, we investigate the nursing areas of L. miodon, the timing of reproductive activities of littoral and pelagic L. miodon, and the timing of reproductive activities of pelagic S. tanganicae in the Congolese waters of the northern end of Lake Tanganyika (Bujumbura sub-basin). Nursing areas were determined year-round (2009-2010) based on the presence of clupeid larvae at two sandy and two stony beaches. The gonadosomatic index (GSI) and the proportion of fish having ripe gonads were used to study variation in reproductive indices in space (littoral vs. pelagic zones) during one year (2013-2014), as well as in time (dry vs. rainy season) during three years (2013-2016). Larvae of L. miodon were more frequently encountered on sandy than on stony beaches. Mature L. miodon females were more abundant in the littoral than in the pelagic zone, while the proportion of mature males in both habitats was similar. Irregular, low amplitude peaks could be distinguished in the GSI and proportion of mature males and females, but averages only differed between the dry and the rainy season in males. In contrast, GSI and proportions of mature males and females in S. tanganicae were higher in the dry season than in the rainy season. The reproductive effort of males and females of S. tanganicae and littoral L. miodon, but not pelagic L. miodon, was strongly synchronized. Interestingly, reproductive investment was also synchronised between pelagic male L. miodon, and pelagic S. tanganicae. Our time series strongly supports the view that L. miodon reproduces year-round in the littoral zone, while reproduction in S. tanganicae is seasonal. For fisheries management, we recommend year-round protection of sandy beaches, which are the main breeding grounds for L. miodon

Trophic structure of Lake Tanganyika: Carbon flows in the pelagic food web

peer reviewedThe sources of carbon for the pelagic fish production in Lake Tanganyika, East Africa, were evaluated in a comprehensive multi-year study. Phytoplankton production was assessed from seasonal in situ 14C and simulated in situ results, using on-board incubator measurements and knowledge of the vertical distributions of chlorophyll and irradiance. Bacterioplankton production was measured on two cruises with the leucine incorporation method. Zooplankton production was calculated from seasonal population samples, the carbon contents of different developmental stages and growth rates derived from published sources. Fish production estimates were based on hydroacoustic assessment of pelagic fish biomass and data on growth rates obtained from length frequency analyses and checked against daily increment rings of fish otoliths. Estimates for primary production (426-662 g C m-2 a-1) were 47-128% higher than previously published values. Bacterioplankton production amounted to about 20% of the primary production. Zooplankton biomass (1 g C m-2) and production (23 g C m-2 a-1) were 50% lower than earlier reported, suggesting that the carbon transfer efficiency from phytoplankton to zooplankton was low, in contrast to earlier speculations. Planktivorous fish biomass (0.4 g C m-2) and production (1.4-1.7 g C m-2 a-1) likewise indicated a low carbon transfer efficiency from zooplankton into planktivorous fish production. Relatively low transfer efficiencies are not unexpected in a deep tropical lake, because of the generally high metabolic losses due to the high temperatures and presumably high costs of predator avoidance. The total fisheries yield in Lake Tanganyika in the mid- 1990s was 0.08-0.14% of pelagic primary production, i.e. within the range of typical values in lakes. Thus, no special mechanisms need be invoked to explain the productivity of fisheries in Lake Tanganyika