Climate change does not explain historical changes in the pelagic ecosystem of Lake Kariba (Zambia–Zimbabwe)

Recent studies on the pelagic ecosystem of Lake Kariba identified a number of changes in its thermal regime, planktonic communities and fishery production, concluding they were the result of climate change, particularly warming. This study re‐examines these conclusions and suggests alternative explanations for these changes. Historical data suggest there was no warming of the lake until at least the 1990s. Furthermore, lack of recent data makes it difficult to conclude that the lake’s temperature has increased by 2 °C. It is also not clear that the pattern of thermal stratification has changed, or that the thermocline has risen and become more stable. Although one of the suggested effects of climate change was a decreased number of larger zooplankton, this change occurred in the 1970s, when there had been no change in the lake temperature. Rather, there is strong evidence that the zooplankton composition changed as a result of selective predation by the introduced clupeid, Limnothrissa miodon. Furthermore, the loss of large grazing zooplankton species could have affected the composition of the phytoplankton, although this phenomenon also has been attributed to climate change. Although the phytoplankton communities in the lake are not as well documented as the zooplankton communities, it is clear that many changes in the lake actually began in the 1970s. Finally, the decline in the pelagic fishery for Limnothrissa was also linked to temperature changes. However, because the fisheries on the Zambian and Zimbabwean sides exhibited very different behaviours, there is little evidence to support this conclusion. It is concluded that the impacts of climate change on Lake Kariba are likely to be complex and that possible over‐simplification in identifying these impacts will not facilitate our understanding of these complexities.     

Lake—Wide Seasonal Changes in Limnological Conditions in Lake Michigan in 1976

Data collected on lake-wide cruises in 1976 were used to study seasonal and vertical variations in water temperature, transparency, chlorophyll a, and nutrients in Lake Michigan. Data were analyzed according to subsets corresponding to the northern and southern open lake. Comparisons (t-tests) of data from the open lake indicated that the average water temperature was cooler and average water transparency was greater in the northern lake than in the southern, but with the exception of total phosphorus, average nutrient concentrations did not differ between the northern and southern parts. It was found that physical-chemical characteristics of nearshore and Straits of Mackinac stations differed significantly from open lake stations. Seasonal phytoplankton dynamics in the open lake were related to seasonal and vertical changes in silica and nitrate nitrogen. The spring phytoplankton bloom occurred before the lake was strongly stratified thermally. After thermal stratification was well developed, epilimnetic concentrations of chlorophyll a decreased, probably due to some combination of nutrient limitation and zooplankton grazing, and maximum chlorophyll a concentrations were found below the thermocline. Epilimnetic silica concentrations decreased after thermal stratification and diatoms were replaced in the phytoplankton assemblage by green and blue-green algae in late summer. Total phosphorus averaged only 8 μg/L on a lake-wide basis and thus only small but significant reductions in absolute concentration can be expected from phosphorus control programs. However, over a period of several years, these small reductions in concentration may be difficult to verify from total phosphorus measurements which have relatively large sampling errors and variances.     

Disrupted seasonal cycle of the warm-adapted and main zooplankter of Lake Biwa,Japan

The seasonal cycle is an important feature of life, leading to the succession of different species and allowing them to share a common environment. Disruption in the seasonal cycle of many species has been reported, but the source of disruption varies from species to species. Lacustrine zooplankton species are widely acknowledged to respond to climate-induced, thermal and trophic variations. Here, we discuss the case of a warm-adapted copepod in Lake Biwa (i.e., Eodiaptomus japonicus), which experienced changes in trophic status and thermal regime over four decades (1966–2010). We investigated the phenological response of E. japonicus to these changes over this period and aimed to identify the sources of the observed variations. The combined results of wavelet analysis and cluster analysis indicated that E. japonicus exhibited different seasonal cycles during the study period. The common unimodal seasonal cycle of the copepod was disrupted on several occasions during which it presented sometimes two or three modes. Wavelet coherence analysis revealed a strong stationary correlation with lake temperature for the total abundance, the clutch size, and the birth rate, but a transient correlation with the body size of females at the annual scale. No coherence was found with food proxies. Discriminant analysis between unimodal and plurimodal seasonal cycles highlighted the effect of low temperature and high predation in leading to plurimodal cycles. Our study emphasizes the need for considering the seasonality of both lower and higher trophic levels for understanding zooplankton phenology.     

Temporal patterns in phytoplankton,zooplankton and fish composition,abundance and biomass in Shirati Bay,Lake Victoria,Tanzania

Shirati Bay is among the important breeding and nursery sites for major fish species in Lake Victoria. Weekly samplings were conducted to assess the temporal patterns in phytoplankton, zooplankton and fish composition, abundance and biomass in relation to prevailing water quality parameters. The study also determined the influences of plankton dynamics and water quality on the fish catch composition and biomass. It was hypothesized that temporal patterns in the composition, abundance and biomass in the plankton in the bay are controlled by water quality parameters that, in turn, affect the composition and biomass of fish catches. The phytoplankton comprised mainly cyanophytes and bacillariophytes, while the zooplankton were dominated by copepods. The heavy rain season exhibited a significantly higher plankton abundance and biomass than the dry season. The plankton abundances in both seasons exhibited significant positive correlations with water temperature and transparency. The phytoplankton community was controlled by calanoid and cyclopoid species. At higher trophic levels, Lates niloticus juveniles, Oreochromis niloticus juveniles and haplochromines controlled Cladocera and Cyclopoid copepods, while Tilapia rendalli juveniles controlled the Rotifera. This study revealed that Cyanophyta and Bacillariophyta are the dominant phytoplankton, whereas cyclopoids dominate the zooplankton species in the bay. These dominant plankton groups are partly controlled by rainfall, water temperature and transparency. Fish biomass, zooplankton and phytoplankton exhibit a typical predator–prey inverse relationship. Thus, evaluation of the plankton composition, abundance and biomass should be mandatory during fisheries stock assessments to effectively manage the fishery resources in the bay.     

Environmental state of Lake Kariba and Zambezi River Valley: Lessons learned and not learned

Lake Kariba, still the largest reservoir in the world by volume, is 60 years old. It has undergone changes in its thermal properties, associated with global warming, which reflect in turn on its limnology. These changes include a shallower eipilimnion, higher heat content and increased tropicality to near equatorial status. The role of Lake Kariba with regard to its energy characteristics is discussed in light of global warming findings. The lake’s water residence time has increased from 3.7 years to ≈5.7 years, attributable to a reduced inflow from the Zambezi River. The phytoplankton communities have changed towards a cyanophyceae‐dominated community, leading to a decline in entomostracan zooplankton, and a near collapse of the planktivorous Limnothrissa miodon fishery. Prolonged use of pesticides to control Glossina has led to measurable ecosystem level impacts on both terrestrial and aquatic biota. The impacts of the forced relocation of the Tonga people were still evident during this study. Siltation from resettlement areas has led to the loss of habitat and biodiversity in the inflowing streams to the lake. Unplanned shoreline development in the early history of the lake poses health problems. It is projected that global warming will cause the lake temperature to rise by ≈4 °C by the end of the century. Higher temperatures will be accompanied by windier conditions, thereby enhancing the risks from storms on the lake. The appropriateness of administrative structures intended to manage the Zambezi River Basin environment also is discussed herein. It is concluded that the management protocol is institutionally a non‐inclusive process lacking the capacity to involve other stakeholders in managing the lake’s resources, and even less so in the integrated management of the basin.     

Management criteria for lake ecosystems applied to case studies of changes in nutrient loading and climate change

The aim of this paper is twofold: to present and discuss practically useful management criteria from different perspectives of lake management (fishery, recreation, conservation, monitoring of water quality and use of water for irrigation and drinking), and to put these criteria into the context of a holistic lake ecosystem model, LakeWeb, which accounts for production, biomasses, predation and abiotic/biotic feedbacks related to nine key functional groups of organisms constituting the lake ecosystem. These are phytoplankton, benthic algae, macrophytes, bacterioplankton, herbivorous zooplankton, predatory zooplankton, zoobenthos, prey fish and predatory fish. The LakeWeb model also includes a mass‐balance model for phosphorus and calculates bio‐uptake and retention of phosphorus in these groups of organisms. It also includes submodels for the depth of the photic zone and lake temperature. The LakeWeb model is driven by few and readily accessible driving variables and it has been extensively tested and shown to capture fundamental lake foodweb interactions very well, which should lend credibility to the scenarios discussed in this paper regarding the conditions in Lake Batorino, Belarus. The LakeWeb model offers a tool to address important, often very complex, scientific problems in a realistic manner. The first scenario describes the changes after 1990 when there was a drastic reduction in the use of fertilizers in agriculture because of political changes and the corresponding changes in lake characteristics and foodweb structures utilizing the given management criteria. The second scenario describes, for comparative purposes, the probable alterations in the lake foodweb related to global climatic changes; in this case, warming and increased temperature variations. This study indicates that there are several similarities between eutrophication and increases in temperatures, which are discussed in this paper along with the mechanistic reasons related to such changes by using a set of general management criteria.     

Using 3D modeling and remote sensing capabilities for a better understanding of spatio-temporal heterogeneities of phytoplankton abundance in large lakes

Lake biological parameters show important spatio-temporal heterogeneities. This is why explaining the spatial patchiness of phytoplankton abundance has been a recurrent ecological issue and is an essential prerequisite for objectively assessing, protecting and restoring freshwater ecosystems. The drivers of these heterogeneities can be identified by modeling their dynamics. This approach is useful for theoretical and applied limnology. In this study, a 3D hydrodynamic model of Lake Geneva (France/Switzerland) was created. It is based on the Delft3D suite software and includes the main tributary (Rhône River) and two-dimensional high-resolution meteorological forcing. It provides 3D maps of water temperature and current velocities with a 1?h time step on a 1?km horizontal grid size and with a vertical resolution of 1?m near the surface to 7?m at the bottom of the lake. The dynamics and the drivers of phytoplankton heterogeneities were assessed by combining the outputs of the model and chlorophyll-a concentration (Chl-a) data from MERIS satellite images between 2008 and 2012. Results highlight physical mechanisms responsible for the occurrence of seasonal hot-spots in phytoplankton abundance in the lake. At the beginning of spring, Chl-a heterogeneities are usually caused by an earlier onset of phytoplankton growth in the shallowest and more sheltered areas; spatial differences in the timing of phytoplankton growth can be explained by spatial variability in thermal stratification dynamics. In summer, transient and locally higher phytoplankton abundances are observed in relation to the impact of basin-scale upwelling.     

Seasonal variation in trophic structure and restoration effects in a deep perialpine lake (Lake Lugano,Switzerland and Italy)

Using monitoring data from the South Basin of Lake Lugano (Switzerland and Italy), we examined seasonal responses of phytoplankton and herbivorous zooplankton biomass to nearly three decades (1989–2017) of phosphorus (P) management to ask: [1] what is the trophic structure of the lake, [2] whether trophic structure and the effects of nutrient management varied seasonally, and [3] what are the implications of the existent trophic structure for the restoration of the lake. Trophic structure varied seasonally, including a structure consistent with strong consumer control (exploitation food chain) in spring and fall, and an unexpected structure in summer, characterized by a negative correlation between phytoplankton biomass and phosphorus. This structure was explained by accumulation of inedible phytoplankton (mainly cyanobacteria) at low P concentrations. Owing to a lack of apparent resource (P) control, P-management had no detectable effects on phytoplankton biomass. The trophic structures identified in the lake provides explanations for this lack of response to P-management and for the differences between the responses of the South Basin of Lake Lugano and other perialpine lakes. Based on the results, lake restoration practice would benefit from a greater understanding and an increased ability to predict trophic structure within and across lakes.     

Documenting Complex Surface Temperature Patterns from Advanced Very High Resolution Radiometer (AVHRR) Imagery of Saginaw Bay,Lake Huron

Interannual differences in nutrient concentrations, phytoplankton densities, and the dispersal of zooplankton and fish life history stages (e.g., planktonic larvae, resting eggs) in embayments often are contingent upon bay and coastal water exchange and movements. However, predicting water mass exchange is difficult in that many factors influence the circulation of waters within bays. The central theme of this contribution is that circulation phenomena derived from simple models can be identified and classified using satellite-obtained lake surface temperature maps, and the incidence of events related to gross temperature regime (season) and winds (shorter-term perturbations). To illustrate this potential, the high sampling frequency of AVHRR imagery was used to examine seasonal surface temperature patterns between Saginaw Bay and Lake Huron over a 3-year period. Linear regressions of CoastWatch IMGMAP and OCNMAP daytime sea surface temperature (SST) algorithms against shipboard bulk temperatures were highly significant, with r² values of 0.98 and 0.94. Synoptic reconnaissance of lake surface temperatures from AVHRR verified many previously known general seasonal events, yet provided much better spatial coverage. The thermal bar persisted in the bay for approximately 6 weeks from late April until mid-June. The combination of shallow depths and impounded river discharges caused inner bay waters to warm more rapidly than outer bay and open lake waters. A thermal gradient of 6 to 10°C persisted between the inner bay and Lake Huron waters throughout the summer. The persistence of major spatial thermal gradients restricted mixing of inner bay waters with the outer bay and open lake, whereas inner and outer bay temperatures converged during fall months, increasing the likelihood of mixing. However, frequent wind-induced effects can cause circulation reversals and move surface waters in quite complex patterns across the bay. AVHRR image classification confirms the sensitivity of inner bay waters to wind stress-related circulation, aiding interpretations of historical data sets.     

Ecological implications of artificial mixing and bottom-sediment removal for a shallow urban lake, Lake Sheldon, Colorado

This paper describes the results of the investigation of a shallow urban lake before, and a second year after, restoration to determine the effects of aeration and bottom‐sediment removal on the trophic state and environmental variables controlling the lake's phytoplankton, macrovertebrates and zooplankton compositions. The anthropogenic eutrophic process of the lake is characterized by increased nutrient concentrations from urban run‐off inputs. This addition triggers a chain of events that start with a massive increase in the growth of primary producers, as these are generally growth‐limited by nutrients in freshwater ecosystems. Although the lake was artificially mixed, nuisance cyanobacteria were dominant as primary producers the second year after restoration, during the summers of 2004 and 2005, until this population collapsed and were replaced by diatoms after periods of high winds, rainfall and a decreased temperature. The absence of macrophytes in the main basin because of the removal of bottom sediment had a major effect on macroinvertebrates and zooplankton. Daphnia sp. was the dominant zooplankton species before restoration, being replaced by Bosmina sp., possibly because of the lack of refuge by macrophytes. As an alternative, Bosmina sp. used the surface blooms of cyanobacteria in summer as refuge from grazing by planktivorous fish. The greatest disturbance on the macroinvertebrate community richness and evenness was the decline of scrapers and shredders, because of the lack of macrophytes after removal of the lake bottom sediment.     

Phytoplankton composition and biomass in tropical soda Lake Shala: seasonal changes in response to environmental drivers

Although soda lakes are valuable, sensitive aquatic resources where phytoplankton play a decisive role for the entire ecological functions, they are among the least‐studied ecosystems. Seasonal variations in phytoplankton composition, abundance and biomass in relation to some environmental parameters of the little known, deep, large, volcanic and saline–alkaline Lake Shala were investigated over an annual cycle. The lake phytoplankton community consisted of relatively diverse taxa (23) belonging to Bacillariophyceae, Cryptophyta, Cyanoprokaryota and Dinophyta. Bacillariophyceae and Cryptophyta were the dominant groups throughout the annual cycle, accounting for about 57% and 22% of the total number of species, and 28% and 69% of the total abundance of the phytoplankton community, respectively. Cryptomonas spp. were most abundant throughout nearly all months, contributing about 59%–95% of total phytoplankton abundance, followed by Thalassiosira sp. (1%–35%). The chlorophyll‐a concentration, as a proxy for algal biomass, was generally low (mean 17 μg L^?1), exhibiting only small seasonal variation. The strong, inverse relation of chlorophyll‐a with water transparency (r = ?0.69; n = 11) and the persistent dominance of species adapted to low‐light conditions and mixing suggest the overriding importance of these factors in controlling the lake's phytoplankton. The results of the present study generally suggest the phytoplankton composition and biomass in Lake Shala exhibited muted seasonal changes, despite the environmental perturbations, probably because of the lake's high buffering capacity against allochthonous impacts because of its voluminous nature.     

Spatio-temporal variation of microcystins and its relationship to biotic and abiotic factors in Hongze Lake,China

An investigation of three particle-associated microcystin (MC) congeners (MC-LR, MC-RR, and MC-YR) was performed in 2013 from August to November in Hongze Lake, China. All MCs congeners exhibited significant spatial and temporal variability. Particle-associated MCs were commonly found in this lake, but MC-LR was present at low levels (0.075–1.252 μg/L). All three MC congeners were found to be significantly correlated with non-toxic phytoplankton (e.g., chlorophyte, cryptophyte and diatoms), while only MC-YR and total MC were correlated with cyanobacteria biomass. Copepods, rotifers and three genera of Cladocera were positively correlated with MC concentrations, but feeding habits of these zooplankton species might have different effects on different MC congeners and total MC distributions. Linear mixed effects (LME) model results showed that the interactions between cyanobacteria and other non-toxic phytoplankton or zooplankton species may drive MCs dynamics. Physiochemical parameters also may affect MC congener variability individually or through their effects on toxic cyanobacteria. The results of this study suggested that MCs distribution in Hongze Lake was site-specific and could be influenced by biotic and abiotic factors both individually and through their interactions.     

Mass balance model of Lake Volta fisheries: The use of Ecopath model

A mass balance model of trophic interactions among ten key functional producer and consumer groups in Lake Volta was constructed using the Ecopath model to study the energy flows and species interactions in the lake. The present study was based on secondary and primary data on fish catch, diet composition, phytoplankton and zooplankton biomasses, collected in 2015 and 2016. Additional information on growth parameters of major species required for balancing the Ecopath model was obtained from sampling and FishBase. The functional groups were detritus, phytoplankton, zooplankton, benthos, prey fish, Tilapia, Bagrus, Chrysichthys, Alestes and Synodontis species. Four trophic levels were identified in the Lake Volta ecosystem, with the energy flow occurring mainly within the first three trophic levels. The calculated ecotrophic efficiency value of the primary producers (phytoplankton: 0.17; detritus: 0.22) indicated they were least exploited, compared to the secondary producers, zooplankton (0.80) and benthos (0.50). All secondary consumers had ecotrophic efficiency values higher than the primary producers, indicating they are exploited in the ecosystem. The main energy flows in the lake were from phytoplankton and detritus at trophic level I, and Bagrus species, the top predator, at a level of 3.30. The network analysis, illustrating a connectance index of 0.43 and an omnivory index of 0.06, in the lake system indicated the ecosystem is unstable, somewhat immature and still in a developing stage.     

Decline of zooplankton food resources of Limnothrissa miodon fishery in Lake Kariba: Global warming‐induced ecosystem disruption by Cylindrospermopsis raciborskii

The present study was conducted on the Lake Kariba portion of the Middle Zambezi Biosphere Reserve. Magadza (2011) previously reported the pelagic Lake Tanganyika sardine (Limnothrissa miodon) fishery introduced into Lake Kariba was declining. This phenomenon was attributed to observed climate‐change‐related warming of the lake, facilitating establishment of the invasive Cylindrospermopsis raciborskii, currently the dominant alga in the lake. Chlorophyceae contribute to less than 10% of the phytoplankton biomass. Accordingly, it was postulated that its dominance in the lake resulted in a food chain disruption because this cyanobacterium was not preferred by zooplankton grazers. Accordingly, the present study examined the feeding behaviour of L miodon with reference to available food items in the Sanyati Basin of Lake Kariba. The results highlighted a virtual absence of entomostraca zooplankton in the Sanyati Basin of Lake Kariba, in contrast to previous findings of previous studies, the avoidance of C. raciborskii in the diet of L. miodon, as measured by the Ivlev electivity index, a low stomach fullness index, the movement of L. miodon to macroinvertebrates associated with the littoral zone as a food source, and unfavourable dietary indicators of L. miodon. These results collectively suggest a food deficit for L. miodon in Lake Kariba, leading to the previously observed decline in the lake fishery.     

Biomanipulating the drinking water reservoir of Estonia's capital city: Prospects for success

The possibility of utilizing biomanipulation to improve the water quality of Tallinn's drinking water reservoir (Lake Ülemiste) was analysed on the basis of water quality data, test fishing by different methods, and earlier studies on aquatic plants, light climate and sediments. Eutrophic, polymictic Lake Ülemiste is characterized by a prevalence of high filamentous cyanobacteria biomass, rotifers in the zooplankton community, mature bream in the fish biomass, and a high density of planktivores (YOY perch). Several prerequisites for being a potential target for biomanipulation were identified, including (i) decreased external total phosphorus (TP) loading; and (ii) a sufficient stock of piscivorous fish in the lake. Prohibition of fishing should favour biomanipulation efforts. The potential for recolonization of macrophytes and large Daphnia species in Lake Ülemiste could be judged from historical data. The in-lake TP concentrations are within the limits that facilitate successful restoration. As phytoplankton biomass was significantly limited by phosphorus in Lake Ülemiste, its water quality improvement will be driven primarily by reduced nutrient concentrations, and then by zooplankton grazing. Internal TP loading and a new phytoplankton species community were assessed as major challenges for successful implementation of biomanipulation as a means of improving the water quality of Lake Ülemiste.     

Phytoplankton primary production, chlorophyll-a and nutrient concentrations in the water column of mountainous Lake Phewa, Nepal

The seasonal patterns of phytoplankton primary production, chlorophyll‐a concentration, cell number and several other limnological variables in Lake Phewa, located in the active monsoon zone in Central Himalaya, Nepal, were studied for a year beginning in April 2001. During the study period, the gross primary production and chlorophyll‐a concentrations were relatively low during the monsoon season. The phytoplankton cell number, represented by 24 genera, also fluctuated seasonally, but tended to increase in the pre‐ and post‐monsoon period. These results suggest that the monsoon plays a crucial role in the primary production and phytoplankton dynamics for Lake Phewa. Among the phytoplankton species, Microcystis aeruginosa, a representative species for eutrophic lakes, was the dominant phytoplankton. At the same time, however, it is clear that the lake is not yet heavily eutrophic. The present study suggests that the exchange of lake water during the monsoon season contributes to maintaining the health of the lake against further degradation. Nevertheless, the silt carried in the monsoon rain run‐off from the lake's catchment area suggests increasingly serious degradation problems for this small mountainous lake.     

Variation in Lake Michigan Plankton: Temporal,Spatial, and Historical Trends

Lake Michigan has been impacted by excessive material loading and invasion by exotic species; however, few studies have evaluated the recent basin-wide response of the lake to these changes, particularly given the reduction of phosphorus loads since the 1970s. From 1994–95, quarterly measurements were made of physical-chemical conditions, plankton biomass, and plankton species composition at 18 stations throughout the lake (n = 111). Sampling sites were clustered according to their physical-chemical similarity; these zones corresponded with depositional regions in the lake (Deep water, Shallow water, and Impacted regions). While plankton biomass did not vary among lake-zones, species composition was zone-specific suggesting that several factors (food web structure, nutrient cycling, and physical mixing) may determine the distribution of species throughout the lake. Plankton biomass and gross composition (phyla) were variable in time (seasons), and exhibited predictable succession patterns. Phytoplankton peaked in June corresponding with the upward mixing of nutrients, while zooplankton peaked during mid-stratification (August) when water temperatures were most warm. Finally, the basin-wide estimates for both total phosphorus and phytoplankton biomass were lower compared with historical estimates (measured in 1970s) and significant differences were not observed between near- and offshore regions. Despite this, the data also show that phytoplankton species composition varies widely throughout the lake, and that some nearshore sites do support impacted assemblages.     

Phytoplankton biomass and primary production dynamics in Lake Kariba

The present study examined spatial, seasonal and depth variations in phytoplankton biomass and primary production (PP), compared with those reported for other tropical African lakes, to determine whether or not measured phytoplankton changes might be linked to climate warming. The biomass of three major phytoplankton classes (Cyanophyceae; Chlorophyceae; Bacillariophyceae) and net PP were measured during the midwinter and midsummer at six different depths at 35 sampling sites distributed across the lake’s five basins. A more rigorous sampling regime was used in the fifth basin, with phytoplankton biomass and PP rates measured every second month over a 24 month period at six different depths at ten sampling sites located in riverine and lacustrine waters. Cyanophyceae, which displayed a gradient of decreasing biomass from Basins 2 to 5, contributed 69% of the total phytoplankton biomass in the lake’s five basins during summer. This percentage was approximately four times greater than that contributed by the Bacillariophyceae and about ten times greater than that contributed by the Chlorophyceae. During winter, Bacillariophyceae biomass was equivalent to that of the Cyanophyceae, but displayed an opposing gradient of increasing biomass from Basins 1 to 3, with a subsequent biomass decline from Basins 3 to 5. Chlorophyceae exhibited no distinct biomass gradient across the five lake basins, being undetectable during winter. The biomass of all three phytoplankton classes and the net PP varied in magnitude and direction monthly between the lacustrine and riverine waters, with increasing water depth and with no distinct seasonal patterns being evident. The monthly variations in biomass were related to the thermal stratification cycle, hydrological gradients and the extent of water mixing, being similar to those reported for other tropical African lakes. It is noteworthy that total phytoplankton biomass and PP in Lake Kariba have declined by about 95% and 50%, respectively, since the 1980s. These declines correspond to an upward shift in the depth of the thermocline, associated with an average temperature increase of 1.9 °C and a 50% reduction in the depth of the euphotic zone, since the 1960s.     

Seasonal dynamics of plankton populations and phytoplankton photosynthetic activity in a highland fish pond in tropical West Africa

Compared to the East/Central regions of Africa, little research efforts have focused on West African tropical limnology. This study, conducted on a freshwater ecosystem, Panyam fish pond, in the highland region (Plateau State) of tropical West Africa, provides knowledge on the hydrochemical characteristics and seasonal or annual biomass oscillations corresponding to the distinguishable climatic phases characteristic of the region. Physiographic phenomenon ‘Harmattan’ (North‐East Trade Winds) induced low temperature conditions (<11°C) during December to January. Moderately dilute waters are reflected by relatively low ionic content (average conductivity of 77 μS cm^?1), conforming to the category II of African waters, based on the classification of Beadle (1981) . Seasonal algal succession was marked by its abundance relative to the beginning of the rainy season (May to September). The sequence of annual phytoplankton dominance was Chlorophyceae>Cyanophyceae>Bacillariophyceae. The dominance for zooplankton was Rotifera>Crustaceae. The occurrence of fewer Cladocera and Copepoda in the Panyam pond is an observation consistent with the findings for other tropical water systems. These results further support the contention that seasonal or annual biomass oscillations in the tropics are not systematically lower than in the temperate zone. The observed phytoplankton productivity rates (average value of 4.86 g C m^?2 day^?1) are higher than those reported by Talling (1965) for African waters. A considerable proportion of the net production (64.3%) was comprised of gross production (1760 g C m^?2 day^?1). The observed relatively enhanced respiratory values (25.4–41.5%) are characteristic of tropical waters. Further, more robust studies will work to remedy the paucity of knowledge regarding tropical West African limnology.     

Seasonal Features of Zooplankton Assemblages in the Nearshore Area of Southeastern Lake Michigan

Zooplankton in a nearshore area in southeastern Lake Michigan displayed strong seasonal and depth-related trends in abundance (#/m³) during the period 1971 to 1977. Depth-related assemblages defined by principal component analysis varied with season. From mid-spring to mid-autumn, zooplankton assemblages were strongly related to water depth. In general, zooplankton were least abundant in the inner region (5–10 m) where small organisms such as nauplii, Asplanchna spp., and Bosmina longirostris and epibenthic organisms such as Eurytemora affinis dominated. Zooplankton were most abundant between 20 and 30 m, with a peak July 1972–1977 mean density of 87,000/m³. Larger zooplankton, including Cyclops spp., Diaptomus spp., Daphnia spp., and Eubosmina coregoni dominated the outer region (20–50 m). During autumn, the inner region was dominated by cladocerans while the outer region was dominated by copepods. After thermal stratification disappeared in late autumn and before the thermal bar moved beyond the survey area in early spring, zooplankton tended to occur in higher densities in the shallow depths of the survey area. Also, in contrast to the mid-spring to mid-autumn period, zooplankton occurred in similar compositions over the survey grid. Factors implicated in producing seasonal differences in abundance patterns include temperature regimes, phytoplankton standing stocks, and predation by fish and invertebrates.