GROWTH OF THE BIVALVE NUCULA-ANNULATA IN NUTRIENT-ENRICHED ENVIRONMENTS

Measured changes in shell length and in the oxygen isotopic compositio n of shell growth bands were used to determine growth rates and season of maximum growth of the bivalve mollusc Nucula annulata from unstres sed and nutrient-enriched environments in Narragansett Bay (Rhode Isla nd, USA). Clams from unenriched environments grew between 0.55 and 0.9 mm yr(-1). Significant growth was limited to temperatures between 10 and 20 degrees C, in late spring and early fall. Populations in unstre ssed environments were dominated by high abundances of small clams (< 1 mm in length). Populations from nutrient-enriched environments consi sted of fewer, larger clams. Data from a 2.3 yr nutrient gradient expe riment and a 2 mo growth experiment suggest the larger individuals are simply older clams rarely found in unenriched environments. delta(18) O temperature values from shell growth bands indicated a shift in the season of maximum growth from spring to summer for clams in eutrophic environments. Peak spawning activity of N. annulata usually coincides with the summer maxima in temperature (22 degrees C) in Narragansett B ay, The low numbers of juveniles found in enriched environments couple d with warmer shell growth temperatures imply a decline in successful reproduction and/or recruitment in eutrophic areas. Deteriorating envi ronmental conditions associated with nutrient enrichment, i.e. low wat er column oxygen levels, may also enhance the survivorship of older, l arger N. annulata possible through a reduction in predation pressure.