Seasonal variations in eelgrass (Zostera marina L.) responses to nutrient enrichment and reduced light availability in experimental ecosystems

The single and interactive effects of altered water column nutrient concentrations and light availability on the growth of the seagrass Zostera marina L. (eelgrass) and its attached epiphytes were investigated in 110 liter microcosms. Experiments lasting 4 to 6 weeks were conducted seasonally during spring, summer and fall in a greenhouse equipped with flow-through seawater from the adjacent York River estuary of the Chesapeake Bay. Nutrient treatments consisted of inflow seawater with ambient or enriched (2 3 to 3 3 ) concentrations of dissolved inorganic 21 nitrogen and phosphorus and with rapid turnover (16 d ). Enrichment levels were chosen to evaluate conditions found in regions of the Chesapeake Bay where Z. marina has declined. Light reductions were accomplished by shading individual microcosms with neutral density screening so that mean scalar irradiance was 42, 28, or 9% of solar PAR. These levels were chosen to simulate light reductions observed along gradients of turbidity which characterize present and former Z. marina habitats in the region. Epiphytic grazers consisted of gastropods (Bittium varium and 22 Mitrella lunata) which were applied at consistent densities (5200 m ) for all experiments. Growth of both the seagrasses and their associated epiphytes decreased with increased shading. There was little additional response to nutrient enrichment except at highest light levels during the spring when macroepiphytes increased to over 10 3 the seagrass mass and seagrass growth decreased. The results suggest that it is principally light availability which governs seagrass growth in moderately nutrient enriched regions of the bay. In systems such as the York River, given adequate grazer densities, observed levels of nutrient enrichment are unlikely to cause excessive epiphyte loads and subsequent seagrass declines. Although Z. marina tissue levels of nitrogen and phosphorus increased significantly with enrichment and with shading no direct effects of nitrate toxicity were observed.  2000 Elsevier Science B.V. All rights reserved.