Role of daily light period in the depth distribution of Zostera marina (eelgrass)

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Abstract
Photosynthesis, biomass and growth characteristics of the temperate seagrass, Z. marina L. (eelgrass), were examined in a meadow in Great Harbor, Woods Hole, Massachusetts [USA] during June and Aug. 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 (Hsat) that light intensities saturated Z. marina photosynthesis was lengthened by 4-6 h and shortened by 3-5 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 day experiments. Cellular photosynthetic adjustment (photosynthesis vs. irradiance relationships, chlorophyll and photosynthetic unit characteristics) to the Hsat manipulations occurred only in June, biomass and growth adjustments occurred both in June and Aug. Photosynthesis and respiration rates, and Hsat regimes, were used to calculate daily leaf C balances. Daily C 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 these plants appear to be living near the minimum light regime for growth and survival (Hsat = 6-8 h). Maximum depth distribution of this species appears to be controlled principally by daily light periods.