Decomposition of Macrophyte Detritus in Seawater: Effects of Grazing by Amphipods

Abstract

In laboratory experiments, amphipods collected from eelgrass detritus in shallow water grazed on untreated leaves of Zostera marina L. but did not consume leaves from which the epibionts had been removed. The action of two adult Gammarus oceanicus SegerstrAle increased the rate of decomposition of 100 mg of Zostera leaf particles by 32 % (50C) and 35 % (210C) in 24 d and markedly reduced the average size of the leaf particles remaining. Experiments with homogeneously '4C-labelled hay as the source of detritus gave similar results but revealed details of the.daily rates of decomposition; e.g., the presence of an amphipod caused an immediate rise in the daily rate of loss of carbon to a maximum of 1.44 % after 21 d compared to the control system which had a maximum of 0.55 % per day. The addition of inorganic nitrogen and phosphorus also increased the rate of decomposition but the maximum daily rate was only 1.16 % and was attained after 28 d.