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 ( < l pM 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 pM NO3--N d -', 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 w e maintained identical treatments except that the former HIGH regime was used to test the response of eelgrass to residual sediment N, accumulation (RES) wlthout additional enrichments. Eelgrass survival was significantly higher in controls and the RES treatment than under MOD enrichment. Nitrate-enriched plants maintained higher N uptakektorage In the aboveground tissue, but lower carbon storage In the rhizomes. The data indicate that water-column nltrate enrichment causes decline of eelgrass especially under increasing/high temperatures, as a d~r e c t phys~ological effect unrelated to algal l~g h t attenuation. The mechanism likely involves Internal Imbalances in nutrient supply ratios resulting from sustamed nltrate uptake through the leaf tissue.

ABSTRACT. In an experimental mesocosm system during late summer-fall, we examined shoot production by eelgrass Zostera marina without nitrate additions (generally with ambient water-column concentrations c 2 pM N03--N) versus production by eelgrass that previously had been exposed to low nitrate enrichment (pulsed additions of 5 pM NO,--N d-' to the water for 12 wk during an unusually cool spring season). During late summer-fall, the previously enriched plants were subjected to higher nitrate loading (10 pM NO<-N d-' for 14 wk), while control plants were maintained without nitrate additions as in spring. We also compared shoot production in fall by recent field transplants of Z. manna, Halodule wrightii, and Ruppia rnarit~rna with and without additions of 10 pM water-column NO3--N d-'. Low water exchange (10% d-l) was used to s~mulate conditions in sheltered embayments or lagoons, and light reduct~on from high tide was simulated by covering the mesocosnls with neutraldensity screens that reduced incident light by 30 % for 3 h d.' on a rotating schedule. Shoot production by both enriched and unenriched Z. marina was comparable dunng the spring low-level NO3--N exposure. However, eelgrass enriched with nitrate in both spring and fall attained significantly lower shoot production than control plants without enrichment. This decrease, unrelated to light reduction from algal growth, suggests a direct adverse effect of long-term water-column nitrate exposure on Z. marina. The more recent transplants of eelgrass without prior enrichment history also showed a trend for decreased lateral growth under moderately elevated nitrate. In contrast, H. wrightii was slightly stimulated and R. rnaritima was highly stimulated by water-column nitrate relative to growth of controls. By the end of the fall experiment, Z. marina (f NO3--N), H. wrightii (k NO3--N), and unenriched R. maritima had increased shoot densities by <50%, whereas nitrate-enriched R. maritima increased shoot production by >300%. The data indicate that H. wrightii or R. maritima could be established successfully by transplanting efforts as a management strategy in nitrate-enriched waters where eelgrass meadows have disappeared. Unlike 2. marina, these species apparently have developed physiological mechanisms to more effectively control nitrate uptake and metabolism.