Littoral microcrustacea in Lake 302S in the Experimental Lakes Area of Canada: acidification and recovery

• 1 The littoral microcrustacean community (Cladocera and Copepoda) was examined from 1989 to 1991 in a lake experimentally acidified to pH 4.5, and from 1992 to 1997 during the early stages of pH recovery.
• 2 Cladoceran abundance declined significantly from 1989 to 1991 (pH 4.5), but species richness did not change. Acantholeberis curvirostris, Simocephalus serrulatus, Latona spp. (Latona setifera, L. parviremis), and all species of chydorid Cladocera declined markedly in abundance while at pH 4.5. The abundance of cyclopoid copepods was low and Ceriodaphnia and calanoid copepods were absent.
• 3 Recovery of the community was subsequently monitored as pH was incrementally changed to a target pH 5.1 in 1992 and 1993, and to 5.8 in 1994–97. Species richness remained unchanged. Chydorid cladocerans remained at low abundance in 1992, and only Chydorus cf. brevilabris increased substantially from 1993 to 1996. Non‐chydorid Cladocera increased in abundance in 1992, declined again in 1993, then gradually increased (mainly due to Ophryoxus gracilis) in 1994–96. All species declined in 1997 as minnows recolonized the lake. The calanoid copepod Leptodiaptomus minutus was present in low numbers in 1997.
• 4 The microcrustacean community in the littoral zone of Lake 302S has not yet shown consistent signs of recovery from acidification.

     

Long-term changes in benthic Cladocera populations in Lake Myvatn, Iceland

Benthic Cladocera were monitored at five sites in Lake Myvatn, Iceland, over a decade (1990–1999), as part of a programme documenting the population fluctuations of animals at different trophic levels in the lake. The species composition remained relatively stable over the first seven years, but in 1997 the population of Eurycercus lamellatus was greatly reduced at all sites. The following year saw a mass occurrence of Alona rectangula and Alonella nana that were previously abundant only locally and rare at most sites. Alona affinis, A. quadrangularis, Acroperus harpae and Chydorus sphaericus were not affected. In 1999 the Cladocera assemblages returned to the pre-1997 situation. The shift was from large-bodied epibenthic and planktonic species (Eurycercus, Daphnia) to small infaunal (Alona rectangula) and ubiquitous (Alonella nana) species. Medium sized (Alona, Acroperus) and some small cladocerans (Chydorus) were not affected. The course of events was reminiscent of a trophic cascade caused by a change in size-selective predation pressure. If so, the impact of a predatory fish population (three-spined stickleback, Gasterosteus aculeatus) depended on whether cyclic chironomid populations were in a high or a low phase. The change in the Cladocera coincided with profound changes in the sediment characteristics associated with low chironomid abundance. We suggest that the relative competitive ability of the Cladocera species is reversed when the sediment has become homogeneous and nutrient-poor after overexploitation by the dominant, tube building and detritivorous chironomid Tanytarsus gracilentus.     

Zooplankton dynamic of Lake Prespa (Macedonia)

The composition and temporal abundance patterns of zooplankton community in Lake Prespa pelagic zone were studied throughout an annual cycle (October 2008–September 2009). Eighteen species (10 Rotifera, 5 Cladocera, 2 Copepoda, 1 Mollusca) were evidenced. It includes Arctodiaptomus steindachneri, an endemic calanoid of the Western Balkans. The zooplankton density varied during the investigated period and certain seasonal successions in temporal transpositions of their maximal development were noticed. The total zooplankton density was much higher in spring and summer period. The representatives of subclass Copepoda were dominant during the whole year. The dominant species was A. steindachneri. At the second place (with exception of the winter period) were the Cladocera representatives. The dominant species was Daphnia cucullata. At the third and fourth place were rotifers and larval form of Dreissena polymorpha, respectively. The quantitative presence of the Copepoda representatives was significantly lower in terms of the previous investigations. The Cladocera quantity reached high values during this period, particularly in summer. Such altering in the quantitative ratio between Copepoda and Cladocera in favor of the Cladocera, especially high quantitative presence of D. cucullata, typical for eutrophic lakes, imply of significant changes in zooplankton community and changes of the water quality in Lake Prespa pelagial.     

Spatial and temporal variation of benthic Cladocera (Crustacea) studied with activity traps in Lake Myvatn, Iceland

Benthic Cladocera were studied with a modified type of an activity trap in Lake Myvatn in 1990–1992. After feasibility experiments, the operation time and the distance of the traps from the benthic substrate were adjusted in order to minimize the effects of diurnal variation and the trapping of planktonic organisms. The trap catches of Eurycercus lamellatus were positively correlated with their abundance at the bottom as estimated by grab sampling (r = 0.910, P < 0.001). The usefulness of the activity trap was demonstrated by: (1) a lake-wide survey of the benthic Cladocera; (2) a study of the seasonal variation in the size distribution, abundance and sex ratio of E. lamellatus; and (3) a study of the seasonal succession of Chydorus sphaericus and Alona affinis. The variation of benthic Cladocera among 21 trap sites distributed on a 1–2-km scale across the lake exceeded the within-site variation. The sampling sites could be divided into five main groups based on cluster analysis. Eurycercus lamellatus was the most common species in the mat of filamentous green algae (Cladophorales). Alonella nana dominated the area of spring water inflow in the north basin and Macrothrix hirsuticornis the area of spring water inflow in the southeastern part of the lake. In other parts of the lake either Chydorus sphaericus or Alona quadrangularis tended to dominate. The size distribution and sex ratio of E. lamellatus was followed at two sites through one summer. In early summer most individuals were females less than 1.45 mm long. Around mid-summer they had grown to 0.69–3.1 mm. By the end of August the size distribution had become bimodal, with a large number of small males and a smaller number of much larger females. The seasonal succession in the abundance of E. lamellatus, A. affinis and C. sphaericus was followed at four sites over two seasons. With some exceptions the abundance of a species followed a similar seasonal trajectory on the different stations in any one year. There was, however, a marked difference between the two years (1991 and 1992), probably related to different temperatures.     

Projected shifts in fish species dominance in Wisconsin lakes under climate change

Temperate lakes may contain both coolwater fish species such as walleye (Sander vitreus) and warmwater fish species such as largemouth bass (Micropterus salmoides). Recent declining walleye and increasing largemouth bass populations have raised questions regarding the future trajectories and management actions for these species. We developed a thermodynamic model of water temperatures driven by downscaled climate data and lake‐specific characteristics to estimate daily water temperature profiles for 2148 lakes in Wisconsin, US, under contemporary (1989–2014) and future (2040–2064 and 2065–2089) conditions. We correlated contemporary walleye recruitment and largemouth bass relative abundance to modeled water temperature, lake morphometry, and lake productivity, and projected lake‐specific changes in each species under future climate conditions. Walleye recruitment success was negatively related and largemouth bass abundance was positively related to water temperature degree days. Both species exhibited a threshold response at the same degree day value, albeit in opposite directions. Degree days were predicted to increase in the future, although the magnitude of increase varied among lakes, time periods, and global circulation models (GCMs). Under future conditions, we predicted a loss of walleye recruitment in 33–75% of lakes where recruitment is currently supported and a 27–60% increase in the number of lakes suitable for high largemouth bass abundance. The percentage of lakes capable of supporting abundant largemouth bass but failed walleye recruitment was predicted to increase from 58% in contemporary conditions to 86% by mid‐century and to 91% of lakes by late century, based on median projections across GCMs. Conversely, the percentage of lakes with successful walleye recruitment and low largemouth bass abundance was predicted to decline from 9% of lakes in contemporary conditions to only 1% of lakes in both future periods. Importantly, we identify up to 85 resilient lakes predicted to continue to support natural walleye recruitment. Management resources could target preserving these resilient walleye populations.     

Bird abundance and species richness on Florida lakes: influence of trophic status,lake morphology,and aquatic macrophytes

Data from 46 Florida lakes were used to examine relationships between bird abundance (numbers and biomass) and species richness, and lake trophic status, lake morphology and aquatic macrophyte abundance. Average annual bird numbers ranged from 7 to 800 birds km^–2 and bird biomass ranged from 1 to 465 kg km^–2. Total species richness ranged from 1 to 30 species per lake. Annual average bird numbers and biomass were positively correlated to lake trophic status as assessed by total phosphorus (r = 0.61), total nitrogen (r = 0.60) and chlorophyll a (r = 0.56) concentrations. Species richness was positively correlated to lake area (r = 0.86) and trophic status (r = 0.64 for total phosphorus concentrations). The percentage of the total annual phosphorus load contributed to 14 Florida lakes by bird populations was low averaging 2.4%. Bird populations using Florida lakes, therefore, do not significantly impact the trophic status of the lakes under natural situations, but lake trophic status is a major factor influencing bird abundance and species richness on lakes. Bird abundance and species richness were not significantly correlated to other lake morphology or aquatic macrophyte parameters after the effects of lake area and trophic status were accounted for using stepwise multiple regression. The lack of significant relations between annual average bird abundance and species richness and macrophyte abundance seems to be related to changes in bird species composition. Bird abundance and species richness remain relatively stable as macrophyte abundance increases, but birds that use open-water habitats (e.g., double-crested cormorant, Phalacrocorax auritus) are replaced by species that use macrophyte communities (e.g., ring-necked duck, Aythya collaris).     

A Century of Change in Macrophyte Abundance and Composition in Response to Agricultural Eutrophication

Clear Lake, Iowa, USA is a shallow, agriculturally eutrophic lake that has changed drastically over the past century. Eight macrophyte surveys since 1896 were pooled and examined to characterize long-term impacts of eutrophication on macrophyte community composition and relative abundance. Surveys in 1981 and 2000 revealed few submergent and floating-leaved species and a dominance in emergent species (Scirpus, Typha). Over the past century, however, species richness has declined from a high of 30 species in 1951 to 12 found today, while the community composition has shifted from submergent-(99%) to emergent-dominated floras (84%). Potamogeton praelongus was the first emergent species to disappear but was followed by several other clear water Potamogeton species. Several floating leaved and emergent genera increased in relative abundance with eutrophication, notably Nuphar, Nymphaea, Phragmites, Polygonum, Sagittaria, Scirpus, and Typha. P. pectinatus was present over the entire century due to its tolerance of eutrophic conditions. Macrophyte growth was generally light-limited, with 93% of the variance in relative abundance of submergent species explained by changes in water transparency. Clear Lake exhibits signs of alternative stable states, oscillating between clear and turbid water, coupled with high and low submerged species relative abundance. The maximum macrophyte richness occurred as the lake oscillated between submergent- and emergent-dominated states. Changes in the water level have also impacted macrophyte growth since the area of the lake occupied by emergent macrophytes was negatively correlated with water level. Strongest correlations indicated that macrophytes respond to water level variations with a 2-year time-lag.     

Paleolimnological reconstruction of the trophic state in Lake Balaton (Hungary) using Cladocera remains

The sediment of Lake Balaton (Hungary) provides important information about the lake’s history, particularly with regard to eutrophication. In this study, we used fossil pigment analysis and subfossil Cladocera remains preserved in a dated sediment core to identify trophic stages from ~250 bc to present. Dates of the most recent eutrophic events are in good agreement with previously published data. In general, the abundance and diversity of the Cladocera community increased with eutrophication and decreased with oligotrophication. The sediments of Lake Balaton were characterised by Chydoridae remains, of which Alona species were the most abundant. Of these, Alona quadrangularis and Alona affinis accounted for 40 and 20% of the total Cladocera remains, respectively. The trophic state of Lake Balaton varied between mesotrophic and eutrophic regimes. Seven different trophic periods were identified in Lake Balaton on the basis of Sedimentary Pigment Degradation Unit (SPDU) content of the sediment. Eutrophic states were (1) from ~250 to ~30 bc, (3) between ~300 and ~590 ad, (5) between 1834 and 1944 and (7) from the 1960s until present. Mesotrophic states were (2) ~30 bc to ~300 ad, (4) 590–1834, (6) 1944–1960s. Discriminant analysis of the cladoceran data confirmed these historic events, except for the short mesotrophic episode between 1944 and 1960. The first stage of eutrophication of Lake Balaton (~250 to ~30 bc) was characterised by extensive macrophyte vegetation, as indicated by the increasing abundance of vegetation-associated Cladocera species (Eurycercus lamellatus, Sida crystallina, Pleuroxus sp.). Intensification of eutrophication was identified since the 1980s, reflected by a high abundance of Bosmina species. The most significant planktivorous fish of Lake Balaton was the Sabre carp (Pelecus cultratus), and when its number decreased, the abundance of Bosmina species increased. This study shows that Cladocera are responsive to trophic state changes, underlining their importance as a tool for the assessment of lake eutrophication.     

Response of a shallow Mediterranean lake to nutrient diversion: does it follow similar patterns as in northern shallow lakes?

1. In view of the paucity of data on the response of warm shallow lakes to reductions in nutrient loading, this paper presents a long‐term limnological data set to document changes in the food‐web of a shallow Mediterranean lake (Lake Albufera, Valencia, Spain) that has experienced reductions in phosphorus (P) (77%) and nitrogen (N) (24%) loading following sewage diversion. 2. Nine years after sewage diversion, P concentration in the lake was reduced by 30% but remained high (TP = 0.34 mg L^?1), although the mean water retention time in the lake was only 0.1 years. Nitrate concentrations did not significantly change, probably because the lake continued to receive untreated effluents from ricefields. 3. Chlorophyll a concentration was reduced by half (annual mean of 180 μg L^?1). Cyanobacteria abundance remained high but its composition changed towards smaller species, both filamentous and chroococcal forms. 4. Cladocera abundance increased and reached peaks twice a year (December to March and July to September). After nutrient reduction, short‐term clear‐water phases (up to 5 weeks) occurred during February to March in several years, concomitant with annual flushing of the lake and lower fish densities. The abundance of Cladocera in winter contrasted with the spring peaks observed in northern restored shallow lakes. The zooplankton to phytoplankton biomass ratio remained lower than in northern temperate shallow lakes, probably because of fish predation on zooplankton. 5. Improvement of the water quality of Lake Albufera remained insufficient to counteract littoral reed regression or improve underwater light allowing submerged plants re‐colonise the lake. 6. Sewage diversion from Lake Albufera impacted the food web through the plankton, but higher trophic levels, such as fish and waterfowl, were affected to a lesser degree. Although the fish species present in the lake are mainly omnivorous, long‐term data on commercial fish captures indicated that fish communities changed in response to nutrient level and trophic structure as has been observed in restored shallow lakes at northern latitudes. 7. Phosphorus concentrations produced similar phytoplankton biomass in Lake Albufera as in more northern shallow lakes with abundant planktivorous fish and small zooplankton. However, in Lake Albufera, high average concentrations were maintained throughout the year. Overall, results suggest that nutrient control may be a greater priority in eutrophicated warm shallow lakes than in similar lakes at higher latitudes.     

Occurrence of filtering crustacea in the near-bottom and pelagial waters of the Mikolajskie Lake

Numbers and species composition of filter-feeding Cladocera and Calanoida occurring in the near-bottom water layer in different seasons of the year and at different depths were compared with their occurrence in the pelagial. Specific species for the near-bottom water layer were not recorded, but numbers of crustaceans were frequently higher in this layer than numbers noted in the pelagial. The food availability may be one of the reasons for the abundance of filter-feeders close to the bottom. Two species:Bosmina longirostris andEudiaptomus graciloides formed aggregations in particular parts of the lake at various seasons: in autumn the highest densities of these species were in the near-bottom layer and closer to the shore, in winter — in close to the bottom in the deepest part of the lake, in spring and summer     

Zooplankton (Cladocera, Ostracoda), Chironomidae and other benthic faunal remains in sediment cores from nine North African wetland lakes: the CASSARINA Project

Palaeolimnological studies were carried out on sediment cores from nine North African wetland lakes. The lakes represented a variety of habitats ranging from freshwater to mixohaline conditions and with and without direct connections to the sea. Sediment cores were examined for records of recent environmental change during the 20th century period. Faunal remains analysed in the sediments included those of Cladocera, Ostracoda, Chironomidae, Mollusca and, at the sites with marine connections, Foraminifera. Cyprideis torosa littoralis was the most common ostracod, occurring in sediments from the seven sites linked with the sea and also in brackish water Sidi Bou Rhaba. In acid Megene Chitane ostracods were scarce, being represented by a single species (Cypria ophtalmica). Candona neglecta completely disappeared at two sites (Sidi Bou Rhaba and Bokka) during the 20th century. Of the Cladocera, Chydorus sphaericus was common in the moderately mixohaline sites and but Heterocypris salina occurred only where marine salinities were occasionally achieved (in Zerga, Ichkeul and Korba). Microinvertebrate assemblages in the Nile Delta lake cores and to a less extent in Zerga showed a clear response indicative of more fresher conditions occurring during the latter part of the 20th century. However the freshening trend began prior to construction of the Aswan High Dam (mid 1960s).With the exception of the most saline site (Korba), microinvertebrate communiuties at all the CASSARINA sites have experienced major disturbances during the 20th century. Changes in freshwater availability associated with increased human usage of water resources is perceived as a major factor regulating the abundance and occurrence of aquatic microinvertebrate species at these sites     

Changes in relative abundance of zooplankton in northern Lake Victoria,East Africa

The zooplankton community of Lake Victoria was studied between March 1990 and February 1991 with the aim of identifying its constituent groups/species, their distribution, abundance, and long term changes in community structure. Zooplankton samples were taken from four stations using plankton nets (75–300 m mesh size) hauled vertically through the water column. The plankton comprised mainly Crustacea and to a lesser extent early stages of aquatic insects. Cyclopoid copepods, their nauplii and copepodites constituted the most frequent group at all stations. Calanoid copepods, Cladocera and Caridina nilotica (Roux), though widely distributed, contributed a small proportion. Chaoborus larvae and pupae and mites were occasionally present. A comparison of the relative proportions of the main zooplanktonic groups from three sources revealed remarkable changes in community structure since 1931. These changes are discussed with respect to predation, eutrophication and other changes in the food web structure of the lake.     

Stratigraphy of Cladocera (Crustacea) and Chironomidae (Insecta: Diptera) in Three Sediment Cores from the Central Baltic Sea as Related to Paleo-salinity

In three sediment cores from the Central Baltic Sea (Bornholm Basin, Gdansk Bay) corresponding horizons with maximum abundance of remains of a limnetic fauna (Cladocera, Chironomidae) were found. These associations indicate the existence of an oligotrophic freshwater lake. However, this horizon does not coincide with the Ancylus stage but is located in sediment layers from the very end of that period, or from the Mastogloia stage, which was a transitional brackish water phase.     

Pelagic cladocera in upper volga water reservoirs

The results of cladoceran crustaceans studies in the pelagial of the Ivankovo and Uglich water reservoirs were generalized. In the period of 1973–1995, both waterbodies were similar in terms of Cladocera species composition and the dominating complex. The list of species composition of pelagic cladocerans has increased since the 1950s. The highest abundance of cladocerans was observed in the Ivankovo water reservoir. Daphnia cucullata G. Sars has been stably dominant in zooplankton summary biomass, while Chydorus sphaericus (O.F. Müller) and Bosmina longirostris (O.F. Müller) prevailed in terms of abundance. Changes in the pelagic cladoceran complex composition is evidence of the waterbodies’ eutrophication. The share of Cladocera in the zooplankton’s total summer biomass was 68 and 53% in the Ivankovski and Shoshinski stretches of the Ivankovo water reservoir, correspondingly, and 60% in the Uglich water reservoir (on average for 1970s–1990s).     

Impact of predation by Mysis relicta on zooplankton in Flathead Lake, Montana, USA

We documented major changes in the zooplankton community of Flathead Lake following the appearance of Mysis relicta. The three common cladocerans found in the lake decreased in abundance, most notably Daphnia longiremis which disappeared from the lake. Copepods were also affected by M.relicta, especially Diacyclops thomasi which decreased in abundance by an order of magnitude. The only macrozooplankton species which seemed to benefit from the presence of M.relicta was Leptodiaptomus ashlandi. The varied responses of dominant zooplankton species appeared to be the result of a combination of factors. Laboratory feeding studies indicate that M.relicta preferred Cladocera over Copepoda, with the following order of feeding preference: Bosmina longirostris > D.longiremis >, Daphnia thorata > L.ashlandi > D.thomasi. The two zooplankton species which declined most following the appearance of M.relicta showed the greatest degree of habitat overlap with M.relicta. Daphnia longiremis and D.thomasi, together with M.relicta, are cold stenotherms and were concentrated in the hypolimnion during the summer. Finally, another trait shared by D.longiremis and D.thomasi was the absence of a diapause stage, which made them vulnerable to predation by M.relicta throughout the year.      

The Cladocera of Sri Lanka (Ceylon), with remarks on some species

The freshwater Cladocera of Sri Lanka has been revised based on the study of over 700 zooplankton samples collected from all habitat types during 1965–1980. The cladoceran fauna is represented by six families; members of the families Polyphemidae, Leptodoridae and Holopedidae are absent. The common temperate genus Daphnia is rare.Sixty-two species have been recorded from Sri Lanka. Of these, five are new records. Remarks on a few species are given with illustrations. The distribution of Cladocera in different types of habitats is discussed. The greatest species diversity was found in ponds. The Sri Lankan fauna is numerically and in species diversity typical of tropical cladoceran fauna. It resembles the southern Indian fauna very closely except for the absence in Sri Lanka of the genera Acroperus and Camptocercus.     

Impact of a Protective Limestone Treatment on the Water Chemistry and Zooplankton Community of Thrush Lake, Minnesota

A limestone slurry was sprayed on the surface of Thrush Lake, a small headwater lake in northeastern Minnesota, to test a treatment designed to protect acid-sensitive waters from anthropogenic acidification. The 6-year study, consisting of pretreatment, transition, and post-treatment phases, was part of the four-state Acid Precipitation Mitigation Program directed by the U.S. Fish and Wildlife Service. Measured water-chemistry parameters, including acid-neutralizing capacity, pH, dissolved calcium, and dissolved inoroganic carbon, increased following treatment, although local climatic conditions influenced the magnitude and duration of the chemical changes. Physical changes to the lake, other than an increase in conductivity and a short-term alteration of water clarity subsequent to treatment, were not documented. The composition of the zooplankton community was altered, with the proportion of rotifers increasing after treatment. Individual zooplankton species showed a variety of changes in abundance that were associated with treatment over both seasonal and multi-year intervals. For example, Holopedium gibbemm was absent from lake samples immediately following treatment and recovered within a season, whereas Diaptomus minutus and Keratella taurocephala populations were reduced after treatment and had not recovered by the end of the study. Alternately, Asplanchna priodonta increased in abundance after treatment. These observed abundance patterns were generally consistent with previzous acidification or base-addition studies. In contrast, the changes in community composition of zooplankton did not consistently fit patterns developed from regional studies across water-chemistry gradients. These differences emphasize the importance of biotic as well as abiotic factors in controlling zooplankton community composition.     

A technique for examining microspatial distribution of Cladocera associated with shallow water macrophytes

Cladocera living in close association with shallow water macrophytes were collected from specific locations on plants using a device similar to an aspirator bottle. The proposed technique did not differ significantly from plastic bag or cylindrical tube enclosure techniques in sampling Cladocera living on Chara stems. Shaking the plants followed by collection of the surrounding waters seriously underestimated the abundance of plant associated organisms.The method successfully demonstrated diel changes in the microdistribution of Chydorus brevilabris living on stems of the emergent plant Hydrolea ovata. C. brevilabris was most abundant at the bases of vertical stems at midday and appeared to move up the stems and into the water column at night.     

Cladocera paleocommunity to disentangle the impact of anthropogenic and climatic stressors on a deep subalpine lake ecosystem (Lake Iseo,Italy)

In big lakes with strong anthropogenic pressure, it is usually difficult to disentangle the impacts of climate variability from those driven by eutrophication. The present work aimed at the reconstruction of change in the species distribution and density of subfossil Cladocera in Lake Iseo (Italy) in relation to climate and anthropogenic pressure. We related subfossil Cladocera species composition and density in an 80-cm sediment core collected in the pelagic zone of Lake Iseo to long-term temperature trends and phosphorus concentration inferred by diatoms frustules. The Cladocera remains detected in Lake Iseo sediment reflected the species composition and density of modern pelagic Cladocera assemblages. Cladocera rapidly respond to environmental change, and that climate change combined with eutrophication can induce changes in community composition and species density. At the beginning of twentieth century, when global warming was not yet so accentuated, the nutrient increase in water resulted as the principal driver in determining the long-term development of plankton communities and pelagic food web structure. Moreover, catchment-related processes may decisively affect both species composition and density of the lake planktonic communities due to the decrease of lake water transparency induced by input of inorganic material from the catchment area to the lake. The paleolimnological investigation, through the combined study of biotic and abiotic factor, allowed clarifying the synergic effects of the most important drivers of change in lake ecosystems, suggesting that climatic factors should be considered with nutrient availability as determinant element in controlling the temporal development of plankton communities and pelagic food web structure.

     

Diatom associations in the aufwuchs of inundated trees and underwater leaves of Salvinia,drowned Mwenda River,Lake Kariba,Zimbabwe

Diatom associations from the aufwuchs of inundated trees and under-water leaves of Salvinia communities in open and in enclosed transitional and semi-riverine water of the drowned upper Mwenda river area, lake Kariba were examined in relation to seasonal changes in lake and river, to species of host plant and to decreasing light quality to a depth of 3 m. The flora remained fairly stable in the summer months, changes in the relative percentage composition of the major and minor associations occurred from scouring floodwaters to stagnation in the semi-riverine areas from limited flow and extensive growth of Salvinia. The percentage frequency of occurrence remained high. The flora was less stable in the winter; some species were slow to recolonise tree areas exposed during down-draw of lake water in summer, especially at the lake face. A few species increased in relative percentage value in winter but the percentage frequency of occurrence in the minor association was poor. Achnanthes minutissima dominated particularly in the enclosed water where it was the cause of serious space competition especially to the larger species of diatom. Highest values throughout were obtained from the area of greatest change viz. the transitional water between riverine and lake conditions.Local environment was of greater significance in relative percentage abundance of diatoms than was the species of host plant. A few species showed ambivalency, becoming planktonic or epiphytic with changes in the environment. Epiphytes taken at increasing depth of water showed that 50% of them occurred in the first 1.5 m, some increased with depth, notably Rhopalodia rhopala and a few appeared to be tolerant of light changes.