Carbon and nitrogen cycling in the southern Laguna Madre

Ecosystem metabolism, nutrient and dissolved organic matter (DOM) cycling, and the effect of sunlight on microbial processes were investigated in the seagrass-dominated southern Laguna Madre, Texas from February 1996 to June 1997. Ecosystem gross primary production and respiration rates were high and nearly balanced. The water column was net heterotrophic (annual P:R = 0.3) and the benthos was net autotrophic (annual P:R = 1.2) most of the year. Water column heterotrophic activity was fueled by the benthic release of DOM. The benthic fluxes of dissolved organic carbon (DOC) occurred primarily in the light and from seagrass-dominated sediments. Most of the release of DOC was due to seagrass exudation and represented about 10% of benthic net primary production (NPP). Seagrass exudation was more important as a source of carbon to the water column than previously thought and appeared to be as significant as the leaching of senescent seagrass tissues on an annual basis. Water column respiration was responsible for the remineralization of most of the DOC released from the benthos, and represented a large fraction of the benthic NPP. Water column nutrient concentrations were low throughout the year (...