Abstract
Abstract. Various methods have been used to remove organisms from sediments to investigate structure and function of faunal assemblages in intertidal habitats. Nevertheless, little is known about how these treatments affect properties of the sediments themselves, although changing these properties may cause changes in the assemblages, independently of other hypotheses being tested. This study assesses the efficacy of defaunation and effect on selected biogeochemical properties of five different methods of defaunating soft muddy sediments in an estuary. The methods were removal and freezing of sediment, removal and oven-heating, freezing in situ with liquid N2, spraying with formalin and spraying with hydrogen peroxide. The first four of these methods have been used in previous studies, whilst the fifth was considered to be a potentially useful defaunator because it does not leave toxic residues. The first two methods required sediment to be brought back to the lab, disrupting the natural structure of the sediment; the last three were done in situ, with much less disturbance. Variables measured to assess effects of the treatments on the sediment were amount of water, grain size, total carbohydrate, suspension index (relative erosion rate), erosion threshold, chlorophyll a and b, colloidal carbohydrate, Fo (minimal fluorescence) and Fv / Fm (photosynthetic yield). There were no significant effects of any treatment on the first four variables. For the others, effects of defaunation varied from treatment to treatment and with time after treatment. Generally, the greatest disturbance was to the microphytobenthos (MPB, measured by chlorophyll and fluorescence) and related variables. For most treatments, recovery was rapid, but the effects of formalin and H2O2 persisted for a few days. Effects on physical properties of the sediment were mostly minor and insignificant. Removal and freezing or heating, however, caused major changes to the sediments because of the disturbances involved. Choosing the appropriate method of defaunation is very important if interpretations are not to be confounded between the effects of defaunation per se and any effects of changes to other biota (such as microphytobenthos) and/or the properties of sediments caused by the method used to defaunate experimental areas.
Highlights
Intertidal areas, such as salt marshes, mangrove forests and mudflats, support diverse assemblages of organisms, many of which are intimately associated with the sediments
The mean proportions of live animals in plots treated with liquid N2 or H2O2 were 0.48 (SE 0.10; n = 6) and 0.63 (SE 0.01), respectively
Formalin had significant persistent effects on the sediment and microphytobenthos, but, because it was more effective at killing the fauna than was H2O2, it would be preferable if complete defaunation was required
Summary
Intertidal areas, such as salt marshes, mangrove forests and mudflats, support diverse assemblages of organisms, many of which are intimately associated with the sediments (reviewed by Lopez and Levington, 1987). Biota alter the properties of and processes in sediments (Black et al, 2002), which have important consequences for associated fauna. Erosion on mudflats is mediated by the interplay between biological and physical processes (de Brouwer et al, 2000; Defew et al, 2002; Tolhurst et al, 2006b) and can be significantly altered by fauna (Austen et al, 1999; Widdows et al, 2000; Andersen et al, 2002) and flora (Friend et al, 2003; Tolhurst et al, 2006a). An organism can have multiple synergistic or antagonistic influences on properties and processes that may vary temporally and/or spatially (e.g. Fernandes et al, 2006)
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