Accounting for littoral primary production by periphyton shifts a highly humic boreal lake towards net autotrophy

Abstract

Abstract The prevailing view that many humic lakes are net heterotrophic is commonly based on pelagic measurements alone. Poor light conditions in humic lakes are assumed to constrain littoral primary production (PP), such that the littoral zone has been considered an insignificant contributor to whole‐lake PP. However, that assumption is based on models and inferences from pelagic processes which do not take littoral zone structure into account. Many lakes have an extensive ring of aquatic vegetation lying near the water surface, which provides substratum for epiphytic algae under well‐illuminated conditions. We measured both pelagic and littoral PP and community respiration (CR) in Mekkojärvi, a small, highly humic headwater lake, in southern Finland throughout the open water season in 2012. We used a 14C incorporation technique to measure pelagic PP, while littoral PP was determined using changes in dissolved inorganic carbon concentrations during in situ incubations. We then estimated whole‐lake PP and CR for both the littoral and pelagic zones. We found that littoral PP usually dominated whole‐lake PP, contributing >90% to total PP in summer. A mean pelagic production to respiration ratio (GPP : CR) of 0.4 clearly indicated strong net heterotrophy, but a value of 2.2 for the littoral zone indicated strong autotrophy. For both habitats combined, the mean whole lake GPP : CR was 1.6, indicating net autotrophy. We suggest that littoral PP can contribute significantly to whole‐lake PP even in highly humic lakes, and that the littoral contribution can shift some lakes which have been considered net heterotrophic to metabolic balance or even net autotrophy during the ice‐free period. Small humic lakes like Mekkojärvi with rings of littoral vegetation are widespread, especially in the boreal zone, and at least for similar lakes, evaluating lake metabolism from pelagic measurements alone is likely to be misleading.