Periphyton support for littoral secondary production in a highly humic boreal lake

Steep stratification and poor light penetration in highly humic lakes typically restrict oxygenated littoral areas to narrow lake margins. However, in some instances, surrounding floating vegetation mats can sustain highly productive periphyton and more diverse invertebrate communities than pelagic areas. Little is known about how these littoral food webs function or the extent to which the pelagic and littoral food webs are coupled. We added ¹⁵N-labeled ammonium nitrate to the floating moss mat surrounding the littoral zone of Mekkojärvi, a small highly humic and fishless lake in southern Finland. Our goal was to increase the δ¹⁵N values of periphyton to investigate the diets of littoral invertebrates and possible pelagic-littoral coupling in the lake. We divided the lake in 2 with a plastic curtain and added European Perch (Perca fluviatilis) to 1 basin while the other remained fishless. δ¹⁵N of periphyton and most littoral invertebrates increased well above the natural abundance levels. δ¹⁵N of pelagic Daphnia generally did not increase, except for a sudden and transitory increase in the basin where fish were introduced. Only one perch of the 33 recaptured following their introduction showed clearly increased δ¹⁵N. The lowest δ¹³C values were found in pelagic invertebrates. Most littoral invertebrates had values closer to those of periphyton, which clearly contributed significantly to the diets of most littoral invertebrate groups and was an important basal resource in the littoral food web. Chironomids and ephemeropterans had surprisingly low δ¹³C values, which may reflect inclusion in their diets of highly ¹³C-depleted methane-oxidizing bacteria, which were known to contribute to the diets of Daphnia in the lake. Our results indicate that the pelagic and littoral habitats are not strongly coupled in the absence of fish but that zooplanktivorous fish may increase coupling by driving zooplankton into the littoral zone to seek refuge from predation.