Abstract

Phosphorus (P) often limits the biomass of primary producers in freshwater lakes. However, in unproductive northern lakes, where anthropogenic nitrogen (N) deposition is low, N instead of P can limit primary producers. In addition, light can be limiting to primary producers at high concentrations of coloured dissolved organic matter (cDOM), as cDOM is the major determinant of light penetration in these lakes. To address resource limitation of epilithic algal biomass, we repeatedly sampled epilithon (periphyton on stony substrata) in 20 lakes covering a large, correlated cDOM and N-deposition gradient across boreal and subarctic Sweden. Across these lakes, pelagic total N (TN) and total P (TP) were positively correlated, and benthic light supply was negatively correlated, with cDOM. Microscopically determined algal biovolume and epilithic carbon (C), N and P were subsequently regressed against benthic light supply and pelagic TN and TP. Patterns in epilithic biovolume were driven by N-2-fixing cyanobacteria, which accounted for 2%-90% of total epilithic biovolume. Averaged over the growing season, epilithic algal biovolume, C and N were negatively related to TP and positively to TN, and were highest in the clearest, most phosphorus-poor lakes, where epilithon was heavily dominated by potentially N-2-fixing cyanobacteria. A structural equationmodel supports the hypothesis that cDOM had two counteracting effects on total epilithic algal biovolume: a positive one by providing N to algae that depend on dissolved N for growth, and a negative one by shading N-2-fixing cyanobacteria, with the negative effect being somewhat stronger. Together, these findings suggest that (1) light and N are the main resources limiting epilithic algal biomass in boreal to subarctic Swedish lakes, (2) epilithic cyanobacteria are more competitive in high-light and low-nitrogen environments, where their N-2-fixing ability allows them to reach high biomass, and (3) epilithic N increases with N-2 fixer biomass and isseemingly paradoxicallyhighest in the most oligotrophic lakes.

Keywords

dissolved organic matter; light; nitrogen-fixing cyanobacteria; nutrients; periphyton

Published in

Freshwater Biology
2018, Volume: 63, number: 7, pages: 662-675

SLU Authors

• Kahlert, Maria

  • Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Environmental Sciences


Publication identifier

DOI: https://doi.org/10.1111/fwb.13103

Permanent link to this page (URI)

https://res.slu.se/id/publ/94487