Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


In order to assess the secondary production potential of the amphipod Gammarus mucronatus its life history was examined with laboratory experimentation and field sampling in two warm temperate estuarine habitats in the York River, Virginia. A seagrass (Zostera marina) bed and a macroalgae (Ulva spp., Enteromorpha spp.) fouling community on old pier pilings. Variations in amphipod abundance between the habitats were not similar. G. mucronatus was present in the seagrass habitat during most of the year attaining maximum densities of 1200(.)m('-2) in the spring. Presence of G. mucronatus in the macroalgae habitat was almost totally restricted to late spring and early summer, but maximum densities as high as 6800(.)m('-2) occurred. Estimates from field and laboratory evidence suggest that G. mucronatus produced 6-9 generations over a one year period. Rapid turnover resulted from increased spring and summer growth rates and maturation at smaller sizes during these months. Reduction in size of summer adults is hypothesized to be a co-evolutionary response to predation by which the amphipod population can increase its intrinsic rate of growth (r). Production calculations were made using four different approaches. The size-frequency method was calculated for 26 fortnightly sample dates, two alternate subsets of the data set (13 sample dates each), and separate calculations for the sexes. The fourth approach utilized a modified instantaneous growth (IGR) equation. The IGR method produced results that were more than 25% greater than the size-frequency estimates which all agreed fairly well. Monthly sampling is sufficient to characterize the population for production estimates, but because of rapid spring and summer growth it is incapable of detecting voltinism. Production in the algal habitat was 10.2 g m('-2)(.)yr('-1) with a P/B of 60.8, greater than the 5.0 g m('-2)(.)yr('-1) production and 40.0 P/B of the seagrass habitat. The higher algal values are the result of the greater maximum abundance of this habitat coinciding with rapid spring and summer growth. of three different predictive models tested, one proposed by Robertson (1979) provides the best agreement with the P/B estimates calculated from empirical data.



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