Physiological ecology of the seagrass Zostera marina
The adaptive physiology of Zostera marina, L. (eelgrass) was investigated along a water depth gradient in order to assess the interactions of this prolific temperate seagrass with its environment. In situ and controlled-environment manipulations of light and sediment availability along with transplants beyond and within the eelgrass meadow were conducted. Eelgrass photosynthesis, biomass and growth was monitored to test for the effect of manipulations at shallow (1.3 m) and deep (5.5 m) stations near the shallow and deep edges of an eelgrass meadow in Great Harbor near Woods Hole, MA (USA). In situ manipulations of light intensity and daily light periods indicated that eelgrass growth was responsive to the daily periods of light intensities exceeding the light saturation or compensation point for eelgrass photosynthesis rather than light intensity per se. Eelgrass growth near the deep edge of the meadow appeared to be controlled by daily light period. Sediment NH4+ availability was sufficient to not become limiting for eelgrass growth throughout the study site meadow, as indicated by the response of eelgrass growth to fertilizations and depletions of interstitial NH4+ levels. Eelgrass transplanted inshore of the natural distribution survived and grew rapidly, indicating periodic disturbances controlling the minimum water depth for eelgrass growth at the study site. Zostera marina transplanted offshore the natural distribution did not survive and a net carbon loss was calculated for leaf tissue. Along with results from the manipulations of daily light period, these data support the hypothesis that the maximum water depth penetration is controlled by light availability. Reciprocal transplants between shallow and deep stations demonstrated phenotypic plasticity rather than genetic differentiation of photosynthesis, morphology and growth characteristics. These results indicate that different environmental factors can control Zostera marina growth and depth distribution within a single, continuous eelgrass plant stand.