Effects of macroalgal blooms on carbon and nitrogen biogeochemical cycling in photoautotrophic sediments: an experimental mesocosm

Abstract

The effects of floating macroalgae (250 g DW m−2) on photoautotrophic microbenthos were studied in a flow-through mesocosm with a parallel mesocosm without macroalgae serving as Control. Vertical microprofiles of O2 at the sediment–water interface showed a immediate and complete suppression of photosynthetic activity of microphytobenthos (MPB) under the macroalgal canopy, resulting in a shift of benthic metabolism from autotrophic to heterotrophic. MPB abundance and chlorophyll a content decreased and a change from a diatom-dominated to cyanobacteria-dominated community was observed. Inorganic nitrogen nutrients´ concentrations increased in the porewater as a result of the inhibition of MPB nutrient demand, leading to an increase in net ammonification and anaerobic consumption rates. No organic matter transfer from macroalgae to the sediment was detected, resulting in a net consumption of the carbon and nitrogen stored in the sediment. In consequence, sediment was progressively impoverished in nitrogen, reducing sediment nutrient regeneration.