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

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 NOx- 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.

► Floating macroalgae inhibit microphytobenthic photosynthesis. ► Macroalgae induce a dominance of cyanobacteria over diatoms. ► Healthy macroalgae are not a source of organic matter to the sediment. ► Nutrient regeneration in the sediment is increased transiently under macroalgae. ► Permanence of healthy macroalgae leads to sediment N and C impoverishment.