Kirsi Vakkilainen

Effect of nutrient loading on bacterioplankton community composition in lake mesocosms.

Changes in bacterioplankton community composition were followed in mesocosms set up in the littoral of Lake Vesijärvi, southern Finland, over two summers. Increasing nitrogen and phosphorus concentrations in the mesocosms represented different trophic states, from mesotrophic to hypertrophic. In 1998, the mesocosms were in a turbid state with a high biomass of phytoplankton, whereas in 1999, macrophytes proliferated and a clear-water state prevailed. The bacterial communities in the mesocosms also developed differently, as shown by denaturing gradient gel electrophoresis profiling of partial 16S rRNA gene fragments and by nonmetric multidimensional scaling analysis. In 1998, nutrient treatments affected the diversity and clustering of bacterial communities strongly, but in 1999, the bacterial communities were less diversified and not clearly affected by treatments. Canonical correspondence analysis indicated that bacterioplankton communities in the mesocosms were influenced by environmental physicochemical variables linked to the increasing level of eutrophication. Nitrogen concentration correlated directly with the bacterioplankton composition. In addition, the high nutrient levels had indirect effects through changes in the biomass and composition of phyto- and zooplankton. Sequencing analysis showed that the dominant bacterial divisions remained the same, but the dominant phylotypes changed during the 2-year period. The occurrence of Verrucomicrobia correlated with more eutrophic conditions, whereas the occurrence of Actinobacteria correlated with less eutrophic conditions.

Rotifer resting eggs in the sediment indicate trophic changes in Lake Vesijärvi

Most planktonic rotifers are monogononts that reproduce mainly by ameiotic, diploid parthenogenesis. Periodically, however, they reproduce sexually, producing resistant, thickwalled resting eggs. Resting eggs enable dormancy, facilitate dispersal, and enhance genetic variation (PouRRIOT & SNELL 1983). Rotifer resting eggs occur regularly in lake deposits (FREY 1964) but are only rarely used in paleolimnological studies (vAN GEEL 2001). Identification is difficult, in spite of diverse morphologies an d specific external characters o f resting eggs (MERKT & MüLLER 1999, VAN GEEL 2001). Drawings in the general systematic literature of rotifers (KosTE 1978, PONTIN 1978) are often insufficient for detailed identification (MERKT & MüLLER 1999), and photographs, such as in VAN GEEL (2001), are scarce. In addition, the interpretation of results based on rotifer resting eggs is still uncertain (MERKT & MüLLER 1999). Most commonly, rotifer resting eggs have been incorporated in multi-proxy sediment studies as a group (MANCA et al. 1996, GmLIZZONI et al. 2006). The tota1 abundance ofrotifers can increase severa1 orders of magnitude with lake eutrophication (WALLACE & SNELL 1991), which also should be reflected in the production of resting eggs. Furthermore, size-selective predation by fish, which generally intensifies with 1ake eutrophication, often shifts the zooplankton community from the dominance of large crustacean species toward small cladocerans and the prevalence of rotifers (BRooKs & DoosoN 1965, VAKKILAINEN et al. 2004). JEPPESEN et al. (1996) included rotifer resting eggs along with cladoceran remains in the transfer function, which they have used to infer past changes in planktivorous fish density and trophic structure (JEPPESEN et al. 2001). Inc1usion ofrotifer resting eggs in the ana1ysis of other zoop1ankton remains (i. e., the observation of several functiona1 groups within a trophic level) may provide better understanding of ecosystem response to environmental change (HuLOT et al. 2000). We examined, on year-to-year basis, how well the total abundance of rotifer resting eggs in the sediment reflected known changes in the trophic state and fish density in Lake Vesijãrvi, southern Finland. The study period included reduction in tota1 phosphorus and chlorophyll-a concentrations, as well as a drop in fish density resulting from the efficient fish trawling. High sedimentation rate (mean 2 cm/yr) within the deep ofthe Enonselkã basin allowed annual, and even seasonal, resolution in sediment analysis. We studied in detail the uppermost sediment varve to record seasonal appearance of rotifer resting eggs, and to learn whether the concentration of sexually produced resting eggs in the sediment reflected the actual densities o f rotifer populations in the lake plankton.