Responses of eelgrass Zostera marina seedlings to reduced light

We subjected seedlings of Zostera marina L. to High (72 %), Medium (23 %), and Low (10 %) daily irradiance (mean daily PAR of 24.4, 7.9, and 3.3 17 m(-2) d(-1) respectively) over 12 wk. We measured plant response in terms of survivorship, lateral shoot production, morphology, growth rate, photosynthesis and respiration, and leaf pigment concentration. Decreasing the light available to eel-grass seedlings from 72 to 23 % resulted in a reduction of lateral shoot formation, lower plant biomass, and longer and wider leaves. Shoot area, growth rate, and pigment concentrations remained similar. A reduction of incident light to 10 % decreased survival to 74 % and had a negative effect on shoot growth, size, and above- and belowground biomass. Pigment concentrations increased with respect to seedlings raised at medium light. In general, the responses of seedlings to reduced light are similar to those reported for mature Z. marina. Rapid expansion of seedling patches can only occur at irradiance levels greater than 7.9 E m(-2) d(-1). Morphological changes resulting from exposure! to mean daily PAR levels of less than 8 E m(-2) d(-1) such as thinner leaves and low belowground biomass, have serious implications for decreased seedling survival in the field.