Effects of land use on the degradability of dissolved organic matter in three watersheds of the Plum Island Sound estuary.

(Valiela et al., unpub. data) and possess different amounts of surrounding salt marsh. Ten sites at each estuary were sampled with an Eckman dredge (0.15 m*) at the beginning of each month in all estuaries and at intervening 2-week intervals in Jehu and Hamblin Ponds. Samples were sorted by species, dried, and weighed. Nitrogen loading and salt marsh area showed no relationship (P for regression >0.05). We therefore regressed nutrient loading versus biomass, and salt marsh area versus biomass. Eelgrass biomass was inversely related to nitrogen loading (Fig. 1, top left; P = 0.000 I), in contrast to macroalgal biomass, which increased with nutrient loading (top right; P = 0.0041). The shapes of the best-fit curves suggest that eelgrass is particularly sensitive to even low rates of additional nutrient loading (Fig. 1, top left), but macroalgae respond linearly over a broader range of nutrient loadings. The correlations (Fig. 1, top), however, have considerable variation; for instance, Jehu Pond has substantially more eelgrass and less algae than would be expected based on nutrient loadings alone. Part of this variability may be associated with the amount of salt marsh in the estuaries. As salt marsh area increases, eelgrass biomass increases (Fig. 1, bottom left; P = 0.000 I) and macroalgal biomass decreases (Fig. 1, bottom right; P = 0.0011). Correlation coefficients for best-fit lines improved for both eelgrass and macroalgal biomass when plotted against salt marsh area (compare r* values in top and bottom panels, Fig. I). This result could be caused by lowered nitrogen loading owing to denitrification in salt marshes. Seagrass growth is inhibited under increased nitrogen loading and macroalgal growth is enhanced. Eelgrass apparently undergoes rapid, near-exponential reduction in biomass over a narrow range of nutrient loadings, whereas macroalgae respond in a linear fashion over a broader range. The relationship between nutrient loading to coastal watersheds and estuarine macrophytes may, however, be strongly influenced by salt marsh fringing the estuaries. Fringing salt marsh might act as a buffer against nutrient loading, and thus might allow eelgrass beds to be maintained even when they are surrounded by relatively urbanized watersheds. This work was supported by REU-NSF (OCE 9300490), NOAA (NA 170R2 110 1), and The Waquoit Bay Fellowship awarded to J. McClelland.