Role of Daily Light Period in the Depth Distribution of Zostera marina (Eelgrass)

ABSTRACT: Photosynthesis, biomass and growth characteristics of the temperate seagrass, Zostera marina L. (eelgrass), were examined in a meadow in Great Harbor, Woods Hole, Massachusetts during June and August, 1982. Underwater lamps and light shading screens were placed at shallow (1.3 m) and deep (5.5 m) stations to lengthen and shorten daily light periods. The portion of the day (H,,,) that light intensities saturated Z. marina photosynthesis was lengthened by 4 to 6 h and shortened by 3 to 5 h. Photosynthesis vs irradiance relations, leaf chlorophyll content, photosynthetic unit size and density, shoot size, specific leaf area, leaf biomass, and production rates were determined at the end of 30 d experiments. Cellular photosynthetic adjustment (photosynthesis vs irradiance relationships, chlorophyll and photosynthetic unit characteristics) to the H,,, manipulations occurred only in June, however biomass and growth adjustments occurred both in June and August. Photosynthesis and respiration rates, and H,,, regimes, were used to calculate daily leaf carbon balances. Daily carbon balance was an accurate predictor of plant survival. Plant responses to manipulations indicate that growth is light limited for eelgrass growing near the deep edge of the meadow, and that these plants appear to be living near the minimum light regime for growth and survival (H,,, = 6 to 8 h). Maximum depth distribution of this species appears to be controlled principally by daily light periods.

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