DECADAL SHIFTS IN A SALT-MARSH MACROINFAUNAL COMMUNITY IN RESPONSE TOSUSTAINED LONG-TERM EXPERIMENTAL NUTRIENT ENRICHMENT

The effect of nutrient enrichment on benthic macroinfauna was examined in a long-term experiment that increased food supply in a macroinfaun al community inhabiting salt marsh muddy tidal creeks in Great Sippewi ssett Marsh (MA, USA). After 15 years of nutrient addition, macrofauna l density and production were higher in fertilized compared to control creeks, primarily because of the response of oligochaete species. Alt hough mean annual total secondary production was higher in fertilized creeks (9.4 g dry weight . m(-2). y(-1)) than in control creeks (6 g d ry weight . m(-2). y(-1)), P:B ratios for the key species were similar in both creeks, ranging between 2 and 3. Sustained enrichment shifted species dominance in fertilized creeks. Two oligochaete species, the naidid Paranais litoralis (Muller, 1784), and the tubificid Monopyleph orus evertus Baker and Brinkhurst, 1981, became the dominant species r esponding to the sustained enhancement of food resources by the fertil izer addition. In non-fertilized creeks, the polychaete Streblospio be nedicti Webster, 1879, remained the dominant species throughout the 15 year duration of the experiment. The fertilizer has stimulated growth of different oligochaete populations that are able to exploit differe nt resources through the year (bottom-up regulation). During summer, t he bottom-up effect of fertilization disappears, and predation control s invertebrate abundance (top-down regulation).