Influence of littoral periphyton on whole-lake metabolism relates to littoral vegetation in humic lakes

The role of littoral habitats in lake metabolism has been underrated, especially in humic lakes, based on an assumption of low benthic primary production (PP) due to low light penetration into water. This assumption has been challenged by recent recognition of littoral epiphyton dominance of whole-lake PP in a small highly humic lake and of epiphyton as an important basal food source for humic lake biota. However, as these studies have mostly concerned single lakes, there is a need to test their wider generality. We studied the whole-lake PP and community respiration (CR) in eight small humic lakes in southern Finland during July 2015 using 14C incorporation to measure pelagic PP and the changes in dissolved inorganic carbon in light and dark in situ incubations to measure CR and littoral PP by epiphyton. Changes in O2 concentration in both pelagic and littoral surface water were measured periodically from each lake and, additionally, continuously with a data logger from one lake during the study period. The results revealed that the littoral dominated whole-lake net primary production (NPP) in five of the eight lakes, which was supported by observed O2 supersaturation in the littoral surface water in most of the lakes. Calculated pelagic:littoral ratios by area correlated negatively with both littoral NPP and littoral contribution to whole-lake NPP. Moreover, there was a significant positive relationship between littoral proportion of whole-lake NPP and the fraction of lake surface area covered by littoral aquatic vegetation. This demonstrates that increased aquatic littoral vegetation cover increases the overall importance of the littoral to whole-lake PP in highly humic lakes. Littoral NPP also correlated strongly with littoral O2 saturation, and the continuously measured O2 revealed substantial temporal variation in O2 saturation, particularly in the littoral zone. Whole-lake gross primary production:community respiration (GPP:CR) ratios revealed that accounting for littoral metabolism produced a marked shift towards lake metabolic balance, although all the eight lakes remained net heterotrophic. This study emphasizes that littoral metabolism needs to be accounted for when estimating whole-lake C fluxes in all lakes, even in highly colored humic waters.