Ecosystem metabolism in a subtropical, seagrass-dominated lagoon

Abstract

Measurements of ecosystem gross primary production (GPP) and respiration (R) were made for 2 to 3 consecutive days on 10 occasions from February 1996 through June 1997 in the seagrass-dominated Laguna Madre (Texas, USA). Ecosystem GPP and R were quantified using 3 independent measurements including the open-water technique using dissolved oxygen (DO) and dissolved inorganic carbon (DIC) and the summation of discrete measurements of the water column and benthos. Die1 curves of DIC were often influenced by water movement while the impact of water movement on DO was not apparent. This was attributed to the longer turnover time of D1C relative to DO in the water column. The ecosystem GPP and R estimated using the open-water method with DO ranged from 56 to 366 and 103 to 381 mm01 C m-Z d-l, respectively. The discrete estimates of GPP and R were usually lower and ranged from 81 to 233 and 54 to 218 mm01 C m-' d-l, respectively. The results of these 2 approaches indicated that Laguna Madre is a very productive system where GPP and R are nearly balanced. Rates of GPP and R for this system were similar to those measured in other studies of temperate and tropical seagrass systems. Discrete measurements of GPP and R indicated that the water colun~n was net heterotrophic most of the year (annu~illy weighted average P:R = 0.27). Measurements of benthic GPP and R indicated that it was net autotrophic during the entire year (annual P:R = 1.16), suggesting that the benthos was responsible for fueling heterotrophic activity in the water column.