The Trophic Fate of Shrimp Farm Effluent in Mangrove Creeks of North Queensland, Australia

Abstract

Zooplankton and fish communities in mangrove creeks removed materials originating from the discharge of effluent from ponds used for shrimp aquaculture at two commercial farms in North Queensland, Australia. Undisturbed mangrove creeks were compared to creeks receiving effluent from shrimp farms. Shrimp farm effluent was rich in chlorophyll a (56 μg l −1) and bacteria (1·9×10 6 cells ml −1). The potential grazing impact of ciliates was higher than that of copepod nauplii or copepodids upstream. In contrast, copepods were more important downstream. Carbon removal by ciliates and copepods accounted for as much as 85% of primary production during non-discharge periods, but was less important during discharge periods. Direct measurement of microzooplankton grazing with the dilution method indicated that growth and grazing were usually in balance, but during pond discharge periods microzooplankton grazing removed >120% of primary production and 117–266% of bacterioplankton production in the mixed lower reaches of the creeks and immediately offshore. Grazing by bacterivores was saturated in the upper reaches of the creeks, but was very high near the creek mouths, where the range of specific grazing rates was 5·2–11·8 d −1. Baitfish juveniles were abundant in the creek systems, and fed either directly on macro-particulates by indiscriminate filter feeding, or by selective feeding on microfauna. Trophic processes and their associated respiratory losses are instrumental in the assimilation and dissipation of effluent materials within the creek system, and are responsible for returning concentrations of bio-available materials to ambient levels. The sustainable use of coastal environments depends to a large degree on understanding and regulating the impacts from activities within the catchment. This research provides environmental managers with direct evidence that, under certain conditions, perturbations in creek water quality and biota originating from shrimp farm discharge are temporary.