Photosynthesis, respiration, and nitrogen uptake by different compartments of a Zostera marina community

Abstract

Gross and net photosynthesis, respiration, and net uptake of dissolved inorganic and organic nitrogen were studied in two seasons in different compartments of an eelgrass ( Zostera marina) community: the water column, the eelgrass leaves, and the sediment plus benthic microalgae. Community gross photosynthesis (eelgrass + benthic microalgae) was three times lower in April than in August. Benthic microalgae accounted for a major fraction of gross and net photosynthesis in April, whereas eelgrass was responsible for the largest fraction of gross photosynthesis in August. This change was mainly due to a 5.5-fold increase in the eelgrass leaf biomass from April to August. The eelgrass community was a sink for inorganic nitrogen, urea, and dissolved free amino acids supplied from the water column in both April and August. Most of the nitrogen uptake (80%) was inorganic-N. The uptake of inorganic-N from the water column was controlled mainly by plant activity as the uptake was higher in the light than in the dark. Most of the community uptake of inorganic-N was related to benthic microalgae in April, whereas the uptake of nitrogen from the water column by eelgrass leaves and benthic microalgae were comparable in August. The increased importance of the eelgrass leaves for the community nitrogen uptake in August was mainly due to the increase in the leaf biomass from April to August. We suggest that the water column was the main N-source for benthic algae in April, and that the sediment was the dominant N-source for eelgrass in August.