Zooplankton Communities in the Shallow Lake Guiers (Senegal,West Africa)

Seasonal variations of zooplankton communities and their interactions with phytoplankton and environmental parameters were studied over a twelve‐month period in Lake Guiers (Senegal, West Africa). Zooplankton was most abundant during the warm and rainy season corresponding to the flood event (July to October) with peaks for rotifers and cyclopoid copepods in July–August and September–October, respectively. Filamentous cyanobacteria were the dominant phytoplankton during the warm season (August to November), while diatoms were dominant during the cool season (December to April). Temperature increase and river flooding both played an important role in these successions, favoring intense development of cyanobacteria and cyclopoid copepods. Trophic interactions also seem to be an important factor in structuring of the plankton community. Overall, the plankton composition displayed a trend toward a eutrophication state. Our study confirms the existence of seasonality in zooplankton communities and the importance of using biological indicators such as phyto‐ and zooplankton to monitor water quality, in particular in tropical freshwater ecosystems. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spatial and seasonal changes of net plankton and zoobenthos in Lake Tonle Sap, Cambodia

To clarify spatial and seasonal differences in net plankton and zoobenthos in Lake Tonle Sap, Cambodia, quantitative surveys were carried out at 14 stations in the north and south basins in high- and low-water seasons during 2003–2005. In the phytoplankton communities, a diatom Aulacoseira granulata dominated throughout the lake in the high-water seasons, while blue-green algae, mostly composed of Microcystis, surpassed other algae in the low-water season when the lake water was very turbid and the Secchi disk readings were only a few centimeters. In the low-water seasons, a bloom of floating blue-green algae occurred everywhere, especially prominent in the coastal areas. Protozoans and rotifers dominated the zooplankton communities. In the open-water stations, diversity was higher in high-water seasons in phytoplankton, while it was not significantly different between seasons in zooplankton. Composition of plankton communities in Lake Tonle Sap appears to have changed little since the 1950s, at least in phytoplankton, while the phytoplankton density appears to be higher in the present study. Among the macrozoobenthos, mollusks, oligochaetes and chironomids dominated in density, and mollusks exceeded others in biomass in both basins and seasons. The total densities of macrozobenthos were not high, being fewer than 1,300 m⁻² throughout the stations and seasons. Possible reasons for the low zoobenthos abundance in the lake may include high predation pressures by benthivorous fish or unfavorable unstable and flocculant substrates.     

Factors influencing the spatial distribution of zooplankton and fish in Loch Ness, UK

• 1 The vertical and horizontal distribution of phytoplankton, zooplankton and fish in Loch Ness, Scotland, were monitored during one day‐time and one night‐time survey in July 1992. The vertical samples were collected at a site located at the northern end of the loch and the horizontal samples along a longitudinal transect.
• 2 The vertical distribution surveys demonstrated that the phytoplankton, the zooplankton and the fish were concentrated in the top 30 m of water above the seasonal thermocline. Within this layer, Cyclops stayed much closer to the surface than Eudiaptomus but both species moved towards the surface at night.
• 3 The most important factor influencing the horizontal distribution of the phytoplankton was the north‐ south gradient in productivity. The sub‐catchments surrounding the north basin contain a greater proportion of arable land than those to the south and the concentrations of nitrate‐nitrogen and phytoplankton chlorophyll increased systematically from south to north.
• 4 Zooplankton distribution patterns were influenced by wind‐induced water movements and the dispersion of allochthonous material from the main inflows. The highest concentrations of Cyclops were recorded in the north, where there was more phytoplankton, and the highest concentrations of Eudiaptomus in the south, where there were higher concentrations of non‐algal particulates.
• 5 There was no spatial correlation between total zooplankton and total fish abundance but the highest concentrations of small (1–5 cm) fish were recorded in the south where there was a large patch of Eudiaptomus. The number of Eudiaptomus at specific locations within this patch were, however, negatively correlated with the numbers of small fish. These results suggest that the fish were actively foraging within the patch and were depleting their zooplankton prey in the areas where they were most abundant.

     

The plankton community of an acid blackwater South Carolina power plant impoundment

Robinson Impoundment, an acid blackwater system in the South Carolina sandhills, serves as a cooling pond for a 854 MW power plant. It is divided by a causeway into an upper and lower impoundment. The upper impoundment is narrow, shallow, exhibits higher flow, and much more periphytic and rooted aquatic macrophyte vegetation. The lower impoundment is wide and deep with many sandy shoreline areas and little macrophyte vegetation.The zooplankton and phytoplankton communities of both the lower and upper impoundment were investigated over a three-year period (1980–1982) in an effort to determine power plant discharge effects on the plankton. Primary productivity rates were low, characteristic of an oligotrophic system. The phytoplankton community was dominated by the Bacillariophyceae and Chlorophyceae in cell density and the Chlorophyceae in species richness. The zooplankton community was dominated numerically by copepods, rotifers, and cladocerans, respectively. Principal zooplankton species includedDiaptomus mississippiensis, Bosmina longirostris, Diaphanosoma brachyurum, Keratella americana, K. cochlearis, Pompholyx sulcata, andConochiloides coenobasis. The rotifers were dominant in species richness, while crustacean zooplankton species richness was usually quite low. Statistical analyses performed using phytoplankton and zooplankton population variables indicated that in most cases upper impoundment densities were significantly lower (p < 0.05) than lower impoundment densities which were likely a function of the lotic upper impoundment habitat. No significant differences were found between the discharge area and the rest of the lower impoundment stations suggesting that localized power plant discharge effects did not occur during the study. Peak midsummer thermal discharge (36.0 ° C) may have caused temporary reductions inD. mississippiensis and several rotifers. Thermal stimulation during summer may have caused population increases for the filter-feeding cladocerans,B. longirostris andD. brachyurum.     

Rotifer community structure in the south basin of Lake Biwa

Seasonal changes in abundance and species composition of rotifers were surveyed at five locations under different physical and chemical conditions in the south basin of Lake Biwa during July 1987–June 1988. Total density of rotifers showed similar seasonal fluctuation with three peaks, although the maximum density showed north (low)-south (high) gradient. Polyarthra spp. (P. vulgaris and P. dolichoptera with low density) dominated except during July–October 1987 and April–May 1988. In the former period the species of Brachionus, Trichocerca, Filinia and Hexarthra, and in the latter, those of Synchaeta, Keratella and Kellicottia had somewhat different proportions in the communities of north and south stations, respectively. The difference in composition during July–October suggests a difference of trophic state between the northern and southern areas in the south basin of Lake Biwa. However, the dominance of Polyarthra and the difference in the composition during April–May 1988 could not be explained by such a difference in trophic state. No critical difference was observed in the community structure of the eastern area of the south basin, where the seasonal fluctuations in nutrient levels and phytoplankton community structure were different from other areas in the south basin. The present results, therefore, suggest that physical and chemical conditions were not effective in controlling the rotifer community structure in the south basin of Lake Biwa.     

An experimental study of the effects of nutrient supply and Chaoborus predation on zooplankton communities of a shallow tropical reservoir (Lake Brobo, Côte d'Ivoire)

1. Based on two mesocosm experiments and 10 in vitro predation experiments, this work aimed to evaluate the impact of nutrient supply and Chaoborus predation on the structure of the zooplankton community in a small reservoir in Côte d'Ivoire. 2. During the first mesocosm experiment (M1), P enrichment had no effect on phytoplankton biomass (chlorophyll a) but significantly increased the biomass of some herbivorous zooplankton species (Filinia sp, Ceriodaphnia affinis). During the second experiment (M2), N and P enrichment greatly increased phytoplankton biomass, rotifers and cladocerans (C. affinis, C. cornuta, Moina micrura and Diaphanosoma excisum). In both experiments, nutrient addition had a negative impact on cyclopoid copepods. 3. Larger zooplankton, such as cladocerans or copepodites and adults of Thermocyclops sp., were significantly reduced in enclosures with Chaoborus in both mesocosm experiments, whereas there was no significant reduction of rotifers and copepod nauplii. This selective predation by Chaoborus shaped the zooplankton community and modified its size structure. In addition, a significant Chaoborus effect on chlorophyll a was shown in both experiments. 4. The preference of Chaoborus for larger prey was confirmed in the predation experiments. Cladocerans D. excisum and M. micrura were the most selected prey. Rotifer abundance was not significantly reduced in any of the 10 experiments performed. 5. In conclusion, both bottom‐up and top‐down factors may exert a structuring control on the zooplankton community. Nutrients favoured more strictly herbivorous taxa and disadvantaged the cyclopoid copepods. Chaoborus predation had a strong direct negative impact on larger crustaceans, favoured small herbivores (rotifer, nauplii) and seemed to cascade down to phytoplankton.     

Can tropical freshwater zooplankton graze efficiently on cyanobacteria?

Zooplankton may at times graze cyanobacteria. However, their top-down effects are considered to be low, particularly in tropical regions dominated by small-size grazers that may be unable to consume efficiently filamentous or colonial species. Recently, cyanobacteria blooms were reported in the Senegal River hydrosystem. We conducted feeding experiments to assess the ability of copepods (Pseudodiaptomus hessei and Mesocyclops ogunnus), cladocerans (Moina micrura and Ceriodaphnia cornuta), and rotifers (Brachionus angularis, B. falcatus, and Keratella sp.) to control different cyanobacteria (Cylindrospermopsis raciborskii, Anabaena solitaria, A. flos-aquae, and Microcystis aeruginosa). None of the zooplankton species ingested M. aeruginosa. Mesocyclops ogunnus did not consume any of the cyanobacteria. Both cladocerans consumed the smallest filaments of cyanobacteria, whereas all the rotifers and P. hessei consumed a broader food-size spectrum. The functional feeding responses suggest that the concentration and size of the filaments are not the sole criteria for food consumption. The high zooplankton community grazing rates, estimated by applying the clearance rates measured in the laboratory to the in situ zooplankton abundance, indicate that grazing by zooplankton potentially constitutes an important controlling factor for the filamentous cyanobacteria in the tropics.     

Zooplankton Dynamics in the Hyper‐Eutrophic Nakdong River System (Korea) Regulated by an Estuary Dam and Side Channels

Zooplankton in the main channel of the Nakdong River and in three tributaries was sampled from June 1994 to September 1995. Planktonic rotifers (Brachionus spp., Keratella spp., and Polyarthra spp.), cyclopoid nauplii and small cladocerans (Bosmina longirostris) were numerically dominant. There was considerable longitudinal variation of zooplankton biomass in the main channel as well as spatial heterogeneity among the major tributaries. In the middle region of the main channel, between river kilometer (RK) 170 and 150 above the estuary dam, total zooplankton abundance sharply increased from less than 100 ind. L^—1 to more than 1,000 ind. L^—1. In a downstream direction toward the estuary dam, phytoplankton biomass increased while total zooplankton biomass decreased. However, as shown by the increasing transport of zooplankton biomass, zooplankton was diluted in the reach of the estuary dam. Advective effects from major tributaries appear to be the contributory factor for the higher zooplankton biomass in the middle region. Overall, rather the external factors (flushing, retention) than internal factors (e.g., phytoplankton) appear to be responsible for changes in zooplankton abundance toward the river mouth.     

Seasonality and abundance of some dominant crustacean zooplankton in Lake Awasa,a tropical rift valley lake in Ethiopia

The zooplankton of a Rift Valley lake in Ethiopia, Awasa, was sampled at 3 stations for 2 years (1986 and 1987) concurrently with various meteorological and limnological measurements. The spatial and temporal variation in abundance of some numerically dominant crustaceans, Mesocyclops aequatorialis similis (Copepoda), Thermocyclops consimilis (Copepoda) and Diaphanosoma excisum (Cladocera) is discussed. Temporal (months, sampling dates) rather than spatial (station) variability accounts for more than 50% of the total variance in zooplankton abundance but horizontal patchiness exists during periods of high zooplankton density. Sampling errors were generally low, except for counts of cyclopoid nauplii (subsampling) and Diaphanosoma (inter-replicate variance). Zooplankton showed distinct seasonality associated with the mixing cycle of the lake. Total numbers increased to more than 200 000 m⁻³ during the unstratified period (July to September). Low numbers were evident during stratification (February to May) when zooplankton numbers did not exceed 15 000 m⁻³. Individual zooplankton species and age classes showed variable seasonal amplitudes, ranging from 6.4 (nauplius 3) to 44.8 (copepodite 3 of Mesocyclops). We discuss some possible causes for zooplankton seasonality in Lake Awasa, and also review zooplankton seasonal cycles in other tropical lakes, especially African ones.     

Secondary production of crustacean zooplankton and biomass of major rotifer species in Lake Bosumtwi/Bosomtwe,Ghana, West Africa

Studies have shown a strong linkage between zooplankton and fisheries' potential in tropical lakes. High zooplankton production provides the basis for fish production, but knowledge of zooplankton production dynamics in African lakes is extremely limited. Crustacean zooplankton production and the biomass of dominant rotifers in Lake Bosumtwi were assessed over a 2‐year period. The crustaceans comprised an endemic and extremely abundant cyclopoid copepod, Mesocyclops bosumtwii and the cladoceran Moina micrura. Mean standing stock of the crustaceans was 429 mg dw m^?3, whilst annual production averaged 2.1 g dw m^?3 y^?1. Production doubled from 1.4 g dw m^?3 y^?1 in 2005 to 2.8 g dw m^?3 y^?1 in 2006. Copepods accounted for 98.5% of crustacean production. The biomass of the dominant rotifers Brachionus calyciflorus and Hexarthra intermedia was less than 1% of total zooplankton biomass. Daily turnover rate and turnover time of the crustaceans was 0.19 day^?1 and 6.2 days respectively. Crustacean production yielded no statistical relationship with phytoplankton biomass. Production was well within the range of tropical lakes. Peak crustacean production synchronized maximum rainfall, lake mixing and phytoplankton production. Most importantly, no one year's set of dynamics can be used to characterize zooplankton production in the lake.     

Planktivory in the changing Lake Huron zooplankton community: Bythotrephes consumption exceeds that of Mysis and fish

1. Oligotrophic lakes are generally dominated by calanoid copepods because of their competitive advantage over cladocerans at low prey densities. Planktivory also can alter zooplankton community structure. We sought to understand the role of planktivory in driving recent changes to the zooplankton community of Lake Huron, a large oligotrophic lake on the border of Canada and the United States. We tested the hypothesis that excessive predation by fish (rainbow smelt Osmerus mordax, bloater Coregonus hoyi) and invertebrates (Mysis relicta, Bythotrephes longimanus) had driven observed declines in cladoceran and cyclopoid copepod biomass between 2002 and 2007. 2. We used a field sampling and bioenergetics modelling approach to generate estimates of daily consumption by planktivores at two 91‐m depth sites in northern Lake Huron, U.S.A., for each month, May–October 2007. Daily consumption was compared to daily zooplankton production. 3. Bythotrephes was the dominant planktivore and estimated to have eaten 78% of all zooplankton consumed. Bythotrephes consumption exceeded total zooplankton production between July and October. Mysis consumed 19% of all the zooplankton consumed and exceeded zooplankton production in October. Consumption by fish was relatively unimportant – eating only 3% of all zooplankton consumed. 4. Because Bythotrephes was so important, we explored other consumption estimation methods that predict lower Bythotrephes consumption. Under this scenario, Mysis was the most important planktivore, and Bythotrephes consumption exceeded zooplankton production only in August. 5. Our results provide no support for the hypothesis that excessive fish consumption directly contributed to the decline of cladocerans and cyclopoid copepods in Lake Huron. Rather, they highlight the importance of invertebrate planktivores in structuring zooplankton communities, especially for those foods webs that have both Bythotrephes and Mysis. Together, these species occupy the epi‐, meta‐ and hypolimnion, leaving limited refuge for zooplankton prey.     

The plankton community of Lake Matano: factors regulating plankton composition and relative abundance in an ancient,tropical lake of Indonesia

Recent evidence reveals that food webs within the Malili Lakes, Sulawesi, Indonesia, support community assemblages that are made up primarily of endemic species. It has been suggested that many of the species radiations, as well as the paucity of cosmopolitan species in the lakes, are related to resource limitation. In order to substantiate the possibility that resource limitation is playing such an important role, a study of the phytoplankton and zooplankton communities of Lake Matano was implemented between 2000 and 2004. We determined species diversity, relative abundances, size ranges, and total biomass for the phytoplankton and zooplankton, including the distribution of ovigerous individuals throughout the epilimnion of Lake Matano in three field seasons. The phytoplankton community exhibited very low biomass (<15 μg l^?1) and species richness was depressed. The zooplankton assemblage was also limited in biomass (2.5 mg l^?1) and consisted only of three taxa including the endemic calanoid Eodiaptomus wolterecki var. matanensis, the endemic cyclopoid, Tropocyclops matanensis and the rotifer Horaella brehmi. Zooplankton were very small (<600 μm body length), and spatial habitat partitioning was observed, with Tropocylops being confined to below 80 m, while rotifer and calanoid species were consistently observed above 80 m. Less than 0.1% of the calanoid copepods in each year were egg-bearing, suggesting very low population turnover rates. It was concluded that chemical factors as opposed to physical or biological processes were regulating the observed very low standing crops of phytoplankton which in turn supports a very minimal zooplankton community restricted in both species composition and abundance. As chemical factors are a function of the catchment basin of Lake Matano, it is predicted that resource limitation has long played an important role in shaping the unique endemic assemblages currently observed in the food web of the lake.     

Cyanobacterial chemical warfare affects zooplankton community composition

1. Toxic algal blooms widely affect our use of water resources both with respect to drinking water and recreation. However, it is not only humans, but also organisms living in freshwater and marine ecosystems that may be affected by algal toxins. 2. In order to assess if cyanobacterial toxins affect the composition of natural zooplankton communities, we quantified the temporal fluctuations in microcystin concentration and zooplankton community composition in six lakes. 3. Microcystin concentrations generally showed a bimodal pattern with peaks in early summer and in autumn, and total zooplankton biomass was negatively correlated with microcystin concentrations. Separating the zooplankton assemblages into finer taxonomic groups revealed that high microcystin concentrations were negatively correlated with Daphnia and calanoid copepods, but positively correlated with small, relatively inefficient phytoplankton feeders, such as cyclopoid copepods, Bosmina and rotifers. 4. In a complementary, mechanistic laboratory experiment using the natural phytoplankton communities from the six lakes, we showed that changes in in situ levels of microcystin were coupled with reduced adult size and diminished juvenile biomass in Daphnia. 5. We argue that in eutrophic lakes, large unselective herbivores, such as Daphnia, are ‘sandwiched’ between high fish predation and toxic food (cyanobacteria). In combination, these two mechanisms may explain why the zooplankton community in eutrophic lakes generally comprise small forms (e.g. rotifers and Bosmina) and selective raptorial feeders, such as cyclopoid copepods, whereas large, unselective herbivores, such as Daphnia, are rare. Hence, this cyanobacterial chemical warfare against herbivores may add to our knowledge on population and community dynamics among zooplankton in eutrophic systems.     

Nutritional quality of biofilms with respect to light regime in Lake Saint-Pierre (Québec, Canada)

1. In situ experiments were conducted using specialised incubation devices to grow biofilms under varying light regimes and grazing intensities (by excluding fish and large‐sized zooplankton, >2 mm) both within and between two sites in Lake Saint‐Pierre. 2. Biofilms growing under greater in situ UVR and light exposures found in the south water mass were characterised by a greater biomass and nutrient content, but their total fatty acid (FA) contents and ratios of elemental nutrients were not significantly different from the north. There was a relatively greater abundance of chlorophytes and cyanobacteria in the south water mass, along with a greater proportion of low nutritional quality saturated fatty acids (SAFA). Conversely, biofilms growing in the north had a greater relative abundance of diatoms, as well as greater eicosapentaneoic acid (20:5ω3) and docosahexaneoic acid (22:6ω3) concentrations (two FAs implicated in the physiological competency of grazers). 3. The prevailing community structures created differences in terms of nutritional status of the biofilms for benthic grazers and their predators at the two sites. The biofilms from the southern site were characterised by greater food quantity at the expense of quality, while biofilms from the northern site contained less food of a better quality. Despite this, the nutritional regime in the south supported a greater productivity at higher trophic levels. The secondary treatments (light and grazing by fish and macro‐invertebrates) had lesser effects on food quality.     

Etude préliminaire des effets de la mise en eau du Réservoir de LG-2 (Territoire de la Baie James,Québec) sur le seston grossier et le zooplancton des rivières et des lacs inondés

Preliminary Study of the Effects of Impoundment of LG-2 Reservoir (James Bay Territory, Quebec) on the Net Seston and the Zooplankton of Impounded Rivers and Lakes The effects of impoundment on the biomass of net seston and zooplankton in lakes and rivers of Northern Quebec were investigated from 1978 to 1980, before, during and after the completion of LG2 Reservoir on the River La Grande (53° 54′ N, 76° 78′ W). In lotic stations, a 41–77% decrease in net seston was observed due to sedimentation of mineral particles (36–80 mg/m³ in 1978 to 10–21 mg/m³ in 1979). The ratio of organic to total seston increased from an average of 0.18 before the impoundment to 0.65 after. The flooding phase brought about a trophic upsurge; the biomass of the zooplankton increased by one to two orders of magnitude at the various sites; the greatest increases occurred in the central stations near the dam, the smallest in the upper stations near the inflow. In lake stations, no significant difference between years could be detected; the mean zooplankton biomass for all three years ranged from 6.59 mg/m³ to 34.34 mg/m³. Spatial variations between lakes were however significant. Results are compared with those in other natural lakes in Canada and reservoirs in the United States. Comparative examination of phytoplankton biomass before and after impoundment of LG2 Reservoir suggests that bacterioplankton and allochthonous organic material are key elements in the pelagic food chain after impoundment.     

Organic Matter,Carbon, Nitrogen and Phosphorus Contents of the Mesozooplankton,Mainly Acartia clausi,in a Tropical Brackish Lagoon (Ebrié Lagoon,Ivory Coast)

Mesozooplankton samples were collected over a period of one year in five stations of the Ebrié lagoon and analysed for organic, carbon, nitrogen and phosphorus contents. When zooplankton was dominated by A. clausi, body composition was relatively stable compared with the broad variability of environmental factors (mainly salinity, food abundance and composition), although a negative hypothetic influence of salinity may have occurred. Body composition was comparable when zooplankton was dominated by Pseudodiaptomus hessei and freshwater species, but contents were significantly lower for assemblages dominated by more exclusively marine species (Paracalanus spp., Oithona spp., marine cladocerans, chaetognates, etc.).     

Top‐down regulation of filamentous cyanobacteria varies among a raptorial versus current feeding copepod across multiple prey generations

1. Although considered a key functional trait, little is known about how zooplankton feeding mode affects top‐down regulation of phytoplankton communities. Indeed, copepods are expected to promote the dominance of toxic phytoplankton by selective removal of their edible competitors; however, empirical evidence comparing the effect among calanoid and cyclopoid copepods is lacking.
2. We compared the top‐down effects of two copepods with contrasting feeding modes—the calanoid Notodiaptomus iheringi (current feeder) and the cyclopoid Thermocyclops decipiens (ambush feeder) — on the relative and absolute biomass of the filamentous cyanobacterium Raphidiopsis raciborskii co‐cultured with the nutritious eukaryotic phytoplankton Cryptomonas obovata in a week‐long laboratory assay.
3. The current feeder had a stronger top‐down effect on the biomass of both prey throughout the experiment, with mass‐specific clearance rates 3–5× higher than ambush feeder. By the end of the experiment, the current feeder significantly reduced cyanobacteria biomass compared to controls while the ambush feeder did not. During the week‐long experiment, the current feeder switched from grazing on edible prey to cyanobacteria as the former became less abundant.
4. Contrary to expectation, neither of the copepod species promoted cyanobacterial dominance by the end of the experiment. This is because both grazers, but especially the current feeder, initially increased but subsequently decreased the relative contribution of cyanobacteria to total phytoplankton biomass. Moreover, both copepods decreased the length of cyanobacteria filaments by c. 70%
5. Current feeders can switch from edible prey to cyanobacteria when the abundance of shortened filaments surpasses the abundance of edible prey. While top‐down regulation of phytoplankton can be stronger for current feeding copepods, ambush feeding copepods can have a significant role during blooms by shortening cyanobacterial filaments. Hence, the broader role of contrasting copepod feeding traits on phytoplankton communities merits further study.

     

Phytoplankton pigments and community composition in Lake Tanganyika

1. A 2‐year (2002–2003) survey of chlorophyll and carotenoid pigments is reported for two off‐shore stations of Lake Tanganyika, Kigoma (Tanzania) and Mpulungu (Zambia), and from three cruises between those sites. Chlorophyll a concentrations were low (0.3–3.4 mg m^?3) and average chlorophyll a integrated through the 100 m water column were similar for both stations and years (36.4–41.3 mg m^?2). Most pigments were located in the 0–60 m layer and decreased sharply downward. Chlorophyll a degradation products (phaeophytins and phaeophorbides) were detected at 100 m depth, whereas carotenoids became undetectable. Temporal and seasonal variation of the vertical distribution of pigments was high. 2. The biomass of phytoplankton groups was calculated from marker pigment concentrations over the 0–100 m water column using the CHEMTAX software. On average for the study period, chlorophytes dominated in the northern station, followed by cyanobacteria T1 (type 1, or Synechococcus pigment type), whereas cyanobacteria T1 dominated in the south. Cyanobacteria T2 (type 2, containing echinenone), presumably corresponding to filamentous taxa, were detected in the rainy season. Diatoms (and chrysophytes) developed better in the dry season conditions, with a deep mixed layer and increased nutrient availability. Very large variation in the vertical distribution of algal groups was observed. 3. Our observations on phytoplankton composition are broadly consistent with those from previous studies. Our pigment data provide evidence for the lake‐wide importance of picocyanobacteria and high interannual variation and spatial heterogeneity of phytoplankton in Lake Tanganyika, which may render difficult assessment of long‐term changes in phytoplankton driven by climate change.     

Evaluation of eutrophication control in Lake Chivero,Zimbabwe, by multivariate analysis of zooplankton

Zooplankton was a useful tool in evaluation of eutrophication control in Lake Chivero, a tropical man made impoundment. Principal component analysis revealed both spatial and temporal changes in zooplankton community structure which were related to nutrient inflows as well as changes in nutrient content of the lake. Studies on filtration rates of different algae by Bosmina longirostris and Daphnia lumholtzi demonstrated that the response of zooplankton to eutrophication was related to their inability to utilize colonial algal species that develop in a nutrient rich-environment.     

The role of macroinvertebrates and fish in regulating the provision by macrophytes of refugia for zooplankton in a warm temperate shallow lake

1. The zooplankton often undergoes diel horizontal migration (DHM) from the open water to the littoral of shallow lakes, thus avoiding predators in the former. This behaviour has functional impacts within the lake, as it enhances zooplankton survival, increases their control of phytoplankton and tends to stabilise the clear water state. However, most of the evidence supporting this migration pattern comes from cold north temperate lakes, and more evidence from tropical and subtropical areas, as well as from southern temperate areas, is needed. 2. We conducted a field study of the diel horizontal and vertical migration of zooplankton, and the horizontal distribution of potential predatory macroinvertebrates and fish, over two consecutive days in the summer in a temperate lake in the southern hemisphere. We took zooplankton samples at two depths, at three sampling stations (inside beds of aquatic macrophytes, at their edge and in open water) along three transects running from the centre of a bed of Ceratophyllum demersum to open water. At each sampling station, we also took samples of macroinvertebrates and fish and measured physical and chemical environmental variables. 3. Zooplankton (pelagic cladocerans, calanoid copepods and rotifers) avoided the shore, probably because of the greater risk from predators there. Larger and more vulnerable cladocerans, such as Diaphanosoma brachyurum and Moina micrura, were two to four times more abundant in open water than at the edge of or inside beds of macrophytes, respectively, by both day and night. Less vulnerable zooplankton [i.e. of medium body size (Ceriodaphnia dubia) or with the ability to swim fast (calanoid copepods)] were distributed evenly between open water and the edge of the plant beds. Small zooplankton, Bosmina huaronensis and pelagic rotifers, showed an even distribution among the three sampling stations. Accordingly, no DHM of zooplankton occurred, although larger organisms migrated vertically inside C. demersum stands. 4. Macrophytes contained high densities of predatory macroinvertebrates and fish. The predator assemblage, composed of large‐bodied macroinvertebrates (including odonates and shrimps) and small littoral fish, was permanently associated with submerged macrophytes. None of these groups moved outside the plant beds or changed their population structure (fish) over the diel cycle. 5. Submerged macrophyte beds do not represent a refuge for zooplankton in lakes where predators are numerous among the plants, implying a weaker top‐down control of phytoplankton biomass by zooplankton and, consequently, a more turbid lake. The effectiveness of macrophytes as a refuge for zooplankton depends on the associated assemblage of predatory macroinvertebrates and fish among the plants.