WATER-COLUMN NITRATE ENRICHMENT PROMOTES DECLINE OF EELGRASS ZOSTERA-MARINA - EVIDENCE FROM SEASONAL MESOCOSM EXPERIMENTS

We imposed a replicated gradient of water-column nitrate enrichment in large-scale experimental mesocosms to test the response of eelgrass to nitrate loading during the spring and fall growing seasons. In spring, controls without nitrate additions (< 1-mu-M ambient NO3--N) were compared to treatments with pulsed daily additions of low (LOW), moderate (MOD), and high (HIGH) enrichment (ca 3.5, 7.0, and 35.0-mu-M NO3--N d-1, respectively). Under low water exchange simulating quiet embayments, eelgrass growth and survival significantly decreased at all enrichment levels, with most rapid decline at the highest nitrate loadings. Plant death was preceded by loss of structural integrity in the aboveground meristems. The adverse effects of nitrate enrichment in spring apparently were exacerbated by increasing/high temperatures and by macroalgal growth which reduced light comparably over time in all mesocosms, During fall we maintained identical treatments except that the former HIGH regime was used to test the response of eelgrass to residual sediment Ni accumulation (RES) without additional enrichments. Eelgrass survival Was significantly higher in controls and the RES treatment than under MOD enrichment. Nitrate-enriched plants maintained higher N uptake/storage in the aboveground tissue, but lower carbon storage in the rhizomes. The data indicate that water-column nitrate enrichment causes decline of eelgrass especially under increasing/high temperatures, as a direct physiological effect unrelated to algal light attenuation. The mechanism likely involves internal imbalances in nutrient supply ratios resulting from sustained nitrate uptake through the leaf tissue.