SECONDARY PRODUCTION OF GAMMARUS MUCRONATUS SAY (AMPHIPODA: GAMMARIDAE) IN WARM TEMPERATE ESTUARINE HABITATS, YORK RIVER, VIRGINIA

Abstract

ABSTRACT Secondary production of the amphipod Gammarus mucronatus was studied in a sea-grass (Zostera marina) habitat and a macroalgal fouling community on old pier pilings. Populations of G. mucronatus were present in the macroalgal habitat during only 4 months of the year with a production of 10.2-12.9 g dry wt · m―2 · yr―1 compared with 5.0-6.5 g dry wt · m―2. yr―1 in the sea-grass habitat, where populations were maintained throughout the year. Rapid growth to maximum size results in short cohort production intervals (CPI) with annual production to mean biomass ratios (P/B) ranging from 36.8-76.8. A modified instantaneous growth rate method gave production estimates that were approximately 25% higher than estimates using a size-frequency method. Production estimates using 4 different variations of the size-frequency method produced similar results. The sum of size-frequency estimates when males and females were considered separately results in slightly lower production values than combined calculations since dimorphic size biases are excluded. The results of this study indicate a need to evaluate carefully the productivity of tropical and warm and cool temperate habitats, since rapid growth of organisms such as amphipods could result in extremely high production estimates even where standing stock is low.