Littoral energy pathways in highly humic boreal lakes

Abstract

Littoral zones in lakes are among the most productive habitats in the world, but have been seriously understudied, as most limnological studies have concerned only pelagic habitats. The likely importance of littoral zones in clear lakes is widely acknowledged, but in contrast their role in dystrophic humic lakes has generally been assumed to be minor. In this thesis, littoral importance in whole-lake metabolism was studied in small and highly humic lakes in Southern Finland, where Lake Mekkojärvi was the principal study lake. Primary production (PP) measurements revealed that littoral periphyton, growing on surrounding aquatic vegetation, can dominate the whole-lake PP, and the highly autotrophic littoral can help balance the whole-lake metabolism or even shift these lakes to occasional net autotrophy. Measurements of bacterial production (BP) within the littoral epiphytic biofilms and in the pelagic zone revealed overall pelagic dominance of BP in Mekkojärvi. However, epiphytic bacteria made a strong contribution, and their highest production rates were almost equal to those of pelagic bacterioplankton. Stable isotope analysis revealed that periphyton can be an important food resource for the consumers in humic lakes. The naturally fishless Mekkojärvi was divided into two basins by a plastic curtain and perch (Perca fluviatilis) were introduced to one basin while the other remained fishless. The results indicate that the pelagic zooplankton do not appreciably utilize littoral prey under the fishless state, but that fish can promote coupling of the pelagic and littoral food webs by directly utilizing littoral prey or by inducing predation pressure on zooplankton, which drives them to the littoral to seek refuge from predation. This study emphasizes the important role of the littoral in whole-lake processes in humic lakes, and suggests that when the littoral is taken into account many small humic lakes are actually more productive and not as heterotrophic as is commonly supposed.