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

Abstract

1. The prevailing view that many humic lakes are net heterotrophic is commonly based on pelagicmeasurements alone. Poor light conditions in humic lakes are assumed to constrain littoral primaryproduction (PP), such that the littoral zone has been considered an insignificant contributor towhole-lake PP. However, that assumption is based on models and inferences from pelagic processeswhich do not take littoral zone structure into account. Many lakes have an extensive ring of aquaticvegetation lying near the water surface, which provides substratum for epiphytic algae under well-illuminated conditions.2. We measured both pelagic and littoral PP and community respiration (CR) in Mekkoj€arvi, a small,highly humic headwater lake, in southern Finland throughout the open water season in 2012. Weused a14C incorporation technique to measure pelagic PP, while littoral PP was determined usingchanges in dissolved inorganic carbon concentrations during in situ incubations. We then estimatedwhole-lake PP and CR for both the littoral and pelagic zones.3. We found that littoral PP usually dominated whole-lake PP, contributing >90% to total PP insummer. A mean pelagic production to respiration ratio (GPP : CR) of 0.4 clearly indicated strongnet heterotrophy, but a value of 2.2 for the littoral zone indicated strong autotrophy. For bothhabitats combined, the mean whole lake GPP : CR was 1.6, indicating net autotrophy.4. We suggest that littoral PP can contribute significantly to whole-lake PP even in highly humiclakes, and that the littoral contribution can shift some lakes which have been considered netheterotrophic to metabolic balance or even net autotrophy during the ice-free period. Small humiclakes like Mekkoj€arvi 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 likelyto be misleading.