Citation:

Paulsen, R., C. Smith, AND D. O'Rourke. PECONIC ESTUARY: A PRELIMINARY ANALYSIS OF THE RELATIONSHIP BETWEEN SUBMARINE GROUNDWATER DISCHARGE (SDG) AND SUBMERGED AQUATIC VEGETATION IN THE PECONIC ESTUARY. U.S. Environmental Protection Agency, Washington, D.C.

Description:

Seagrass meadows represent an important ecosystem in tropic and temperate waters around the globe. The most common species in the shallow, coastal regions of the North Atlantic Ocean is Zostera marina, also referred to as eelgrass. In general, these marine angiosperms act as food source, habitat, nursery ground and shelter for various fish and invertebrate populations. They contribute a significant part to the primary production of the ocean, stabilize coastlines, recycle nutrients, and are widely valued as sensitive indicators of human pollution and perturbations (Bortone, 2000). Scientific studies on seagrass, and eelgrass in particular, are conducted by various research and commercial institutions, to gain more understanding of the ecology and distribution of these marine plants. In addition, the tremendous decline of eelgrass on the Atlantic coast in the 1930's due to the socalled "wasting disease" gave great motivation for more intensive studies on seagrasses (Larkum et al., 1989). Eelgrass distribution follows natural fluctuation and depends on a number of interdependent factors. Temperature, light availability and water motion are, among others, important abiotic factors. The phytoplankton and algae composition of the ocean water, as well as anthropogenic factors, such as pollution, harvesting, and coastal development, can also influence eelgrass distribution patterns (Thayer et al., 1984). Since eelgrass meadows are of vital importance to regional fisheries, recreation and coastal stabilization, the Peconic Estuary Program in cooperation with Cornell Cooperative Extension (CCE) Marine Program in Riverhead, N.Y., initiated pilot research projects to learn more about the factors that influence eelgrass (Zostera marina) distribution, so that eelgrass populations can be restored and managed more effectively. The main focus of this pilot study is on the influence of abiotic factors, particularly submarine groundwater discharge and the characteristics of the substrate, on the distribution of eelgrass in the Peconic Estuary, Long Island. Submarine groundwater discharge (SGD) is believed to contribute a significant freshwater input to the estuary and can also be a carrier of anthropogenic pollution from inland sources (Rutkowski et al., 1999). A preliminary analysis on the relationship between SGD sediment characteristics, pore water chemistry and eelgrass beds was undertaken with a modest budget. Four potential study sites were initially chosen. Preliminary information on the magnitude of SGD, as well as the presence of eelgrass was attained. From this preliminary analysis two sites were chosen for more rigorous evaluation. The Northwest Harbor sites I and II were chosen largely due to the fact that significant differences in SGD and in eelgrass density were found. Figure 1 illustrates the differences in eelgrass density between the two sites; the red line indicates the boundary of the eelgrass beds in the area. The yellow lines indicate the transect locations. Water and soil samples were taken along two transects at Northwest Harbor, during fall 2000 and spring 2001, in order to acquire information about the chemical composition of the water, the grain-size distribution of the sediment at these sites and the magnitude of SGD in the study areas. Differences in eelgrass density is observable along the transects in Northwest Harbor. Any observable differences in the abiotic parameters between the transects could potentially provide preliminary explanations for the absence or presence of eelgrass. Submarine groundwater discharge at Orient Harbor, Northwest Harbor and Flanders Bay was measured, as was electric resistivity of the pore fluid offshore. From this information ultrasonic seepage meters were placed at suspected seepage zones. The resistivity measurements provide constraints on the spatial extent of the submarine groundwater discharge offshore and its possible affect on the distribution of eelgrass.

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