COMPARATIVE EFFECTS OF WATER-COLUMN NITRATE ENRICHMENT ON EELGRASS ZOSTERA-MARINA, SHOALGRASS HALODULE-WRIGHTII, AND WIDGEONGRASS RUPPIA-MARITIMA

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 < 2 muM NO3--N) versus production by eelgrass that previously had been exposed to low nitrate enrichment (pulsed additions of 5 muM NO3--N d-1 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 muM NO3--N d-1 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. marina, Halodule wrightii, and Ruppia maritima with and without additions of 10 muM water-column NO3--N d-1. Low water exchange (10 % d-1) was used to simulate conditions in sheltered embayments or lagoons, and light reduction from high tide was simulated by covering the mesocosms with neutral-density screens that reduced incident light by 30 % for 3 h d-1 on a rotating schedule. Shoot production by both enriched and unenriched Z marina was comparable during 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. maritima was highly stimulated by water-column nitrate relative to growth of controls. By the end of the fall experiment, Z marina (+/- NO3--N), H. wrightii (+/- NO3--N), and unenriched R. maritima had increased shoot densities by less-than-or-equal-to 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 Z marina, these species apparently have developed physiological mechanisms to more effectively control nitrate uptake and metabolism.