Ecosystem metabolism along a colonization gradient of eelgrass (Zostera marina) measured by eddy correlation

Abstract

The Virginia coastal bays experienced local extinction of eelgrass (Zostera marina) during the early 1930s, and restoration beginning in 2001 has generated an ecosystem state change from bare to vegetated sediments. Oxygen fluxes were measured seasonally using the eddy correlation technique at three sites representing different stages of seagrass colonization: unvegetated (bare), 5 yr, and 11 yr since seeding. Derived seasonal ecosystem respiration (R) and gross primary production (GPP) increased up to 10‐fold and 25‐fold, respectively, with meadow age. Although hourly oxygen (O2) fluxes were highly correlated with light at the vegetated sites, no identifiable trends with light were observed at the bare site. The light compensation point where O2 production and respiration are in balance increased from 46 µmol photons m−2 s−1 to 257 µmol photons m−2 s−1 and 63 µmol photons m−2 s−1 to 472 µmol photons m−2 s−1 at the 5 yr and 11 yr seagrass sites, respectively, with increasing seasonal temperatures from 12.3°C to 27.9°C and 9.3°C to 30.5°C, respectively. This suggests that more light, and thus more O2 production, is required to offset increasing respiration with both temperature and meadow age. Photosynthesis—irradiance curves generated from hourly O2 fluxes throughout the seasons were used to estimate annual net ecosystem metabolism (NEM). Annual NEM rates at the bare, 5 yr, and 11 yr sites were −7.6, 8.6, and −7.0 mol O2 m−2 yr−1, respectively. Although the system went through a period of net autotrophy during early stages of colonization, the ecosystem state change from unvegetated sediments to dense seagrass meadows changed the magnitude of both GPP and R, but not the overall metabolic balance of the system.