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This study aims to evaluate the seasonal and interannual variability in the importance of the microbial food web in Lake Tanganyika. Phytoplankton, bacteria and protozoa (heterotrophic nanoflagellates and ciliates) were monitored at two contrasting pelagic sites (Kigoma and Mpulungu) during 3 consecutive years. In addition, spatial variation was studied along 3 north-south transects during contrasting seasons. The study period covered a wide range of limnological conditions, with mixing depth ranging from 13 to >100 m and euphotic depth from 14 to 65 m. The consistently high bacterial biomass (up to 62 µg C l-1) and the high contribution of small phytoplankton (<5 µm) to total phytoplankton biomass (on average 50 % in Kigoma and 84 % in Mpulungu) point to an important role of the microbial food web in the lake throughout the year. Total phytoplankton biomass increased during periods of low water column stability, with an increased biomass of small eukaryotic phytoplankton (2 to 5 µm) at both stations, together with autotrophic prokaryotic picoplankton at the southern station Mpulungu and diatoms at the northern station Kigoma. Heterotrophic bacteria, heterotrophic nanoflagellates (0.06 to 11.01 µg C l-1) and ciliates (up to 8.16 µm C l-1) did not show this seasonality. The main seasonal and spatial variability in the importance of the microbial food web seems therefore primarily linked to the contribution of the small phytoplankton, which may be better adapted to lower average light intensities and higher N:P ratios during periods of deep mixing
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1. Initation of different phytoplankton groups by inorganic nutrients, nutrient enrichment bioassays using the addition of iron (Fe) and the combined addition of nitrogen and phosphorus (NP) were carried out in the north and the south of Lake Tanganyika during the rainy and dry seasons in 2003 and 2004. 2. Nutrient additions resulted in an increase in phytoplankton growth rate relative to control treatments in all experiments. HPLC pigment data and epifluorescence microscopy counts indicated differential stimulation of the dominant phytoplankton groups. Iron additions mainly stimulated prokaryotic picophytoplankton, while enrichments with nitrogen and phosphorus stimulated green algae and in some cases diatoms. Extended incubation (3 days) indicated co-limitation of Fe and NP, in particular for picocyanobacteria
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To date, little is known about the relative importance of dispersal related versus local factors in shaping microbial metacommunities. A common criticism regarding existing datasets is that the level of taxoromic resolution might be too coarse to reliably assess microbial community structure and study biogeographical patterns. Moreover, few studies have assessed the importance of geographic distance between habitats, which may influence metacommunity dynamics through its effect on dispersal rates. We applied variation partitioning analyses to 15 separate regional datasets on diatoms found in lakes in Eurasia, Africa and Antarctica. These analyses quantified the relative contributions of dispersal related and local factors in determining patterns of taxonomic turnover at the species and at the genus level. In general, results were similar at both taxonomic levels. Local environmental factors accounted for most of the explained variation (median = 21 %), whereas dispersal related factors were much less important (median of significant fractions = 5.5% variation explained) and failed to significantly explain any variation, independent of the environmental variables, in the majority of the datasets. However, the amount of variation explained by dispersal related factors increased with increasing geographic distance and increasing taxonomic resolution. We extrapolated our regional scale observations to the global scale by combining the regional datasets into a global dataset comprising 1039 freshwater lakes fron both hemispheres and spanning a geographic distance of over 19000 km. At this global scale, taxonomic turnover was lowest in highly connected habitats, once environmental factors were partialled out. In common with many other studies of macroorganisms, these analyses showed that both dispersal related and local variables significantly contribute to the structure of global lacustrine diatom communities
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There is a long-standing belief that microbial organisms have unlimited dispersal capabilities, are therefore ubiquitous, and show weak or absent latitudinal diversity gradients. In contrast, using a global freshwater diatom dataset, we show that latitudinal gradients in local and regional genus richness are present and highly asymmetric between both hemispheres. Patterns in regional richness are explained by the degree of isolation of lake districts, while the number of locally coexisting diatom genera is highly constrained by the size of the regional diatom pool, habitat availability, and the connectivity between habitats within lake districts. At regional to global scales, historical factors explain significantly more of the observed geographic patterns in genus richness than do contemporary environmental conditions. Together, these results stress the importance of dispersal and migration in structuring diatom communities at regional to global scales. Our results are consistent with predictions from the theory of island biogeography and metacommunity concepts and likely underlie the strong provinciality and endemism observed in the relatively isolated diatom floras in the Southern Hemisphere
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