Effects of increased nitrate concentrations on Zostera marina health and prevalence of Labyrinthula zosterae

Abstract

Seagrasses are facing a global population decline and bottom cover has grown sparser in many locations. Garrison Bay, in the San Juan Archipelago, was the site of a complete loss of eelgrass (Zostera marina) in 2003, and no recovery has occurred. Although the exact cause of the disappearance is unknown, one possible factor is nitrate loading caused by submarine groundwater discharge (SGD). This study was divided into two parts in order to investigate the causative factors behind the disappearance. First a field component focused on identifying SGD sites and determining the concentration of nitrate in the discharge. Second, in a mesocosm study Z. marina was exposed to ambient, 2X ambient, or 5X ambient concentrations of nitrate. Three SGD sites were identified, with the site in a highly modified area (the site of a hotel and marina established in 1886) containing elevated levels of nitrate. Eelgrass grown in mesocosms was exposed to increased nitrate concentrations (2X and 5X) introduced into the water column. Although shoot growth differences were not significant, there was a significant difference in the total number of lesions, associated with the marine pathogen Labyrinthula zosterae, found in each treatment with the fewest lesions in the 5X ambient nitrate concentration and 180% more lesios in the 2X ambient nitrate concentration. This finding may indicate that while growth differences between treatments were not statistically significant, the 5X nitrate treatment may have decreseased the spread of Labyrinthula zosterae along shoots.