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To test the consequences of decreased diversity and exclusion of keystone species, we compared the planktonic food webs in two acidic (pH <= 3), species-poor mining lakes with those in two species-rich, neutral lakes. The ratio of heterotrophic to autotrophic biomass (HIA) was similar in acidic and neutral lakes with comparable productivity. However, food webs in both acidic lakes were largely restricted to two trophic levels in contrast to the four levels found in neutral lakes. This restriction in food chain length was attributed to the absence of efficient secondary consumers, rather than to productivity or lake size which resulted in unusually low predator-prey weight ratios, with small top predators hardly exceeding their pry in size. In contrast to the neutral lakes, plankton biomass size spectra of acidic lakes were discontinuous due to a lack of major functional groups. The unique size-dependence of feeding modes in pelagic food webs, with bacteria in the smallest size classes followed by autotrophs, herbivores and carnivores, was maintained for bacteria but the other feeding modes strongly overlapped in size. Thus, their characteristic succession along the size gradient was roughly preserved under extreme conditions but the flow of energy along the size gradient was truncated in the acidic lakes. For most but not all attributes studied, differences were larger between acidic and neutral lakes than between neutral lakes of different trophic state
The fate of allochthonous dissolved organic carbon (DOC) in aquatic systems is primarily controlled by the turnover of heterotrophic bacteria. However, the roles that abiotic and biotic factors such as light and DOC release by aquatic primary producers play in the microbial decomposition of allochthonous DOC is not well understood. We therefore tested if light and autochthonous DOC additions would increase allochthonous DOC decomposition rates and change bacterial growth efficiencies and community composition (BCC). We established continuous growth cultures with different inocula of natural bacterial communities and alder leaf leachates (DOCleaf) with and without light exposure before amendment. Furthermore, we incubated DOCleaf together with autochthonous DOC from lysed phytoplankton cultures (DOCphyto). Our results revealed that pretreatments of DOCleaf with light resulted in a doubling of bacterial growth efficiency (BGE), whereas additions of DOCphyto or combined additions of DOCphyto and light had no effect on BGE. The change in BGE was not accompanied by shifts in the phylogenetic structure of the BCC, but BCC was influenced by the DOC source. Our results highlight that a doubling of BGE is not necessarily accompanied by a shift in BCC and that BCC is more strongly affected by resource properties.