Date of Award

Summer 2009

Document Type

Legacy Thesis

Degree Name

Master of Science in Coastal Marine and Wetland Studies

Department

Coastal and Marine Systems Science

College

College of Science

First Advisor

Robert F. Young

Second Advisor

Eric T. Koepfler

Third Advisor

Brent L. Lewis

Abstract

Nekton (fish, shrimp and crabs) thrive in marsh-estuarine systems and contribute to the cycling of inorganic nutrients by excretion and bioturbation (the mixing of sediments by living organisms). The objectives of this study were to quantify the nutrient contributions to the water column via bioturbation and excretion by common nekton species of southeastern salt marshes, including the Atlantic blue crab (Callinectes sapidus), grass shrimp (Palaemonetes pugio), spot (Leiostomus xanthurus), mullet (Mugilidae spp.), pinfish (Lagodon rhomboids) and white shrimp (Panaeus setiferus). It was hypothesized that the magnitude of changes in water column nutrient concentrations and the relative importance of excretion and bioturbation processes would vary between species. Field and laboratory studies were conducted in a closed system, mimicking the pools of the intertidal creek environment. Experiments followed a factorial design of nekton presence and/or vexar (3mm plastic mesh screen) presence. The vexar screen was placed above the sediment in the mesocosms in order to prevent access to the sediment and hence prevent bioturbation. The results showed considerable variability in the system. Blue crabs and pinfish had the largest increases of ammonium in the overlying water through excretion and bioturbation compared to all the species studied. Grass shrimp caused the largest increase of ammonium from just bioturbation. Phosphate results did not show significant trends between treatments. White shrimp and spot showed the largest changes in suspended sediments in the water column when allowed to bioturbate. The depth of disturbance was examined and blue crabs were also found to dig to a deeper depth (5.34±0.6cm), at least double the disturbance depth of all the other nekton. The deepest depth of disturbance also caused the highest change in ammonium (3.68±0.33 µmol/L) in controlled disturbance experiments. However, disturbance depth by each species did not correlate with suspended sediment results for each species or with grass shrimp being the most significant bioturbater. Other factors beside depth of sediment disturbance, such as extent and duration of disturbance, likely contributed to the species-specific differences in nutrient contributions. Haertel-Borer et al. (2004) estimated the total contribution of nekton to the ammonium flux in the North Inlet estuary. Using the bioturbation and excretion contributions to changes in ammonium found in this study, as well as average biomass and species composition estimates for the North Inlet intertidal creek nekton community, percent contributions of each species to the total nekton nutrient flux were calculated. In this model, grass shrimp were responsible for 52% of the total nekton ammonium production from bioturbation alone, while blue crabs and pinfish combined contributed 93% of the total nekton ammonium production from excretion alone. This study is important in that it characterized the role of these organisms in processing and transporting nutrients within the marsh-estuarine system.

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