Invertebrate Response to Nutrient-Induced Changes in Macrophyte Assemblages in Waquoit Bay.

Abstract

Coastal marine systems such as Waquoit Bay, Massachusetts, have experienced a major increase in nutrient loads through groundwater in recent decades (1). Five estuaries of Waquoit Bay (Childs River, Hamblin Pond, Jehu Pond, Quashnet River, and Sage Lot Pond) are subject to different nutrient loads due to different land uses in their watersheds (1). Nutrient loading prompts replacement of eelgrass (Zosteru marina) by macroalgae-dominated communities (2). In this paper we address whether the vegetation changes caused by nutrient loading result in parallel changes in the benthic fauna of the affected areas. In each of the five estuaries, the macrophyte and invertebrate communities were sampled at 10 sites (randomly selected within representative depth strata) with an Eckman dredge (0.15 m’), June-August 1995. Samples were rinsed through a l-mm sieve. Macrophytes were sorted by species, dried, and weighed (aboveground biomass only for eelgrass). Macroinvertebrates were categorized into taxonomic groupings and counted. The estuaries varied widely in eelgrass and macroalgal biomass (Fig. I). Benthic invertebrate density did not change markedly with the decrease in eelgrass biomass from Sage Lot Pond and Jehu Pond to the other estuaries (Fig. 1, top). In contrast, invertebrate density showed a clear inverse relationship with macroalgal biomass (? = 0.5, P = 0.003) (Fig. 1, bottom). Childs River, the most nutrient-loaded site, had a consistently higher macroalgal biomass and a lower invertebrate density (Fig. 1, bottom). The other sites (shown without initials in Fig. 1) had much lower macroalgal biomass than Childs River, but still show a strong negative relationship between invertebrate density and macroalgal biomass (? = 0.3, P = 0.07) when Childs River points are excluded. The other estuaries did not, however, have consistently different macroalgal biomass from each other. Although the decrease in eelgrass biomass did not alter invertebrate density, it did change fauna1 composition. In eelgrassdominated communities (Jehu Pond, Sage Lot Pond), polychaetes composed 26% of the invertebrate abundance, whereas in macroalgae-dominated communities polychaetes increased to 4 1% of benthic invertebrates. Changes in the abundance of a single species in response to eelgrass loss have been documented for shellfish species of Waquoit Bay (1). Many factors may influence invertebrate abundance in estuaries. Low-oxygen conditions associated with accumulations of macroalgae (3) may be responsible for the decrease in invertebrate density with increasing macroalgae biomass; this oxygen