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

Abstract

151. 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. 20Many lakes have an extensive ring of aquatic vegetation 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 Mekkojärvi, a small, highly humic headwater lake, in southern Finland throughout the open water season in 2012. We used a 14 C incorporation technique to measure pelagic PP, while littoral PP was 25 determined using changes in dissolved inorganic carbon (DIC) concentrations during in situ incubations. We then estimated whole-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 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. 30For both habitats 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 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, 35 especially in the boreal zone, and at least for similar lakes, evaluating lake metabolism from pelagic measurements alone is likely to be misleading.3

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

“…This assumption has led to the prevalent notion that littoral pri-mary producers are of minor importance in the energy budgets of humic lakes. However, we recently demonstrated that PP by littoral periphyton dominated whole-lake PP in small, highly humic Lake Mekkojärvi (Vesterinen et al 2016), contradicting predictions for the benthic fraction of whole-lake PP in such lakes according to the model presented by Vadeboncoeur et al (2008). In Mekkojärvi, the floating littoral vegetation mat fringe supports highly productive periphyton communities just under the water surface under well-illuminated conditions.…”

“…Mekkojärvi becomes totally anoxic during winter under ice and, therefore, cannot sustain natural fish populations. Details of the lake's physical and chemical characteristics are presented elsewhere (e.g., Devlin et al 2015, Vesterinen et al 2016. The absence of planktivorous fish normally leads to the development of dense populations of the large bodied cladoceran Daphnia longispina in summer.…”

“…The absence of planktivorous fish normally leads to the development of dense populations of the large bodied cladoceran Daphnia longispina in summer. Mekkojärvi has a depth ratio (DR = z=z max , where z = depth) of 0.47, so the lake is steep-sided and lacks illuminated benthic surfaces because of the highly colored water and very low light penetration (light-attenuation coefficient, K d , ranges from 4.5 to 7.5; Vesterinen et al 2016). The littoral zone is not clearly defined, but most of the lake's perimeter is covered by a floating vegetation mat consisting mainly of Sphagnum and Warnstorfia moss species with some fallen terrestrial sedges (Carex spp.…”

“…associated with the moss. This surrounding littoral vegetation does not extend >1 m from the lake edge and not deeper than ∼0.5 m, but provides habitat for macroinvertebrate populations and supports highly productive periphyton assemblages (Vesterinen et al 2016) that develop rapidly in spring and achieve their highest biomass in summer. Daphnia longispina is the predominant invertebrate in the pelagic zone, which is relatively poor in other invertebrate taxa, but the littoral vegetation mat supports a more diverse invertebrate community (Table S1).…”

“…In Mekkojärvi, the floating littoral vegetation mat fringe supports highly productive periphyton communities just under the water surface under well-illuminated conditions. Gross PP by periphyton exceeded community respiration in the littoral zone and balanced whole-lake metabolism or made it net autotrophic (Vesterinen et al 2016). The relative importance of allochthonous and autochthonous resources for secondary production in humic lakes continues to be debated, but the need to study the importance of such highly productive periphyton communities in humic lakes is obvious.…”