How does groundwater impact eelgrass in Long Island? The role of nitrogen and herbicide in reducing eelgrass growth,survival and photosynthetic efficiency.

Abstract

Eelgrass communities in the Peconic estuary of Long Island NY have been in decline for several decades prompting resource managers to reduce nitrogen loads into the estuary. Despite improved water quality in recent years, eelgrass continues to decline. This lack of response has caused the management agencies to suggest that seepage of groundwater contaminated with herbicides such as Diuron and high nitrate from regions with a long history of agriculture may be to blame. We performed two types of manipulative experiments during the summer of 2009 to assess the possible impacts of groundwater on eelgrass growth and survival. We exposed the plants to multiple stressors of decreased light availability and increased water temperatures as well as Diuron exposure to the root and rhizome exclusively. In addition, we conducted a mesocosm experiment that delivered high nitrogen groundwater to the sediment of eelgrass planters via peristaltic pumps to assess the relative stimulation of grass growth versus phytoplankton biomass. In addition to measuring eelgrass survival and productivity, a PAM fluorometer was used to assess the impact that the herbicide had on the photosynthetic efficiency of the plants. Diuron in concentrations 80-200æg/l was found to decrease growth and productivity of eelgrass in the 1st root exposure experiment. Plants subjected to elevated temperature, reduced light and Diuron had the lowest mass and productivity.