A direct method for assessing sediment load in epilithic algal communities

Abstract

S. W. Purcell Department of Marine Biology, James Cook University, Townsville Qld. 4811, Australia Accepted: 14 March 1996 Mobile sediments within coral reef environments can be deleterious to coral (Rogers 1990), and their deposition onto reef surfaces may directly affect other benthic biota. Epi- lithic algal communities (EACs), which cover significant proportions of coral reef surfaces, can trap sediments which may deleteriously affect neighbouring biota such as corals (Walker and Ormond 1982). Furthermore, sediments can remain in coral reef systems for long periods (Chansang et al. 1992) and via resuspension events, may be transported through other biotopes. The ability to measure sediment loads on hard reef surfaces, such as EAC-covered substra- ta, where they are directly influencing reef biota, is therefore important to assessing sediment disturbance on coral reefs. The principal methodology used to assess sedimenta- tion on coral reefs has been the deployment of sediment traps (Rogers 1990). Sediment traps collect suspended sediments over a given time; these values are interpreted as the gross downward flux rates of particles in the water column. Within this context, sediment traps have been invaluable as research tools for documenting gross sedi- ment inputs to reef systems. Problems can arise, however, when sediment trap data are extended to infer accrual rates of sediments to reef substrata (Cort6s and Risk 1985), since a sediment trap does not approximate the nature in which particules are bound to the benthos (Gardner 1980). More- over, the technique does not provide any information about the amount of sediment deposited on natural sub- strata, in contact with sessile biota. Ideally, studies should incorporate direct sampling of accumulated sediments on reef substrata to provide base- line assessments of natural loads and supplement informa- tion on sedimentation rates during disturbances. I outline here a portable apparatus and sampling technique which allows a diver to collect sediments on reef surfaces, and a test of its efficiency at removing sediments held within EACs of natural coral reef substrata. This rapid method permits the measurement of the instantaneous (net) load, rather than the sedimentation rate, of terrigenous and reef-derived sediments on algae-covered substrata. The sediment sampler (Fig. 1) is a submersible appar- atus which uses a bilge pump to vacuum particulate material from substrata. It can be viewed as four connected components; a brush to loosen bound sediments from algae, a filter to retain coarse sediments, a pump which produces an intake current, and a collection bag to retain water and fine sediments. The electric rotary bilge pump (Whale ® Superline 99) is powered by a 12V dryfit battery and associated electronics contained within a waterproof housing. An adjustable timer and current regulator circuit, which maintains a constant voltage (10.5V), standardises the water flow speed and duration for each sample. A voltmeter mounted within the housing can be checked periodically, to indicate when the battery power is insuf- ficient to run the current regulator. The sediment filter used for sampling is contained within a 250 ml plastic sample vial, which has two tapered fittings attached to the lid. The filter houses an intake hose which is fitted through the centre of five perforated PVC baffles stacked 10 mm apart within the vial, with a 200 gm nylon mesh screen below the uppermost baffle. The central hose carries water and particulate material to the bottom of the vial, and as they passes through the filter, the perforated baffles impede turbulence, and retain the coarse sediments. A brush (25mm diameter) with soft, nylon bristles around the perimeter, is attached to the intake hose. The brush attachment has a central opening (20 mm diameter) which results in an intake current speed of about 28 cm s- 1 at the opening. A circular sampling ring of 6 mm thick PVC is pinned to the substrate to outline a 100cm 2 sampling area and partition the enclosed sediments from those on neighbouring surfaces. The brush is applied to the substratum within the sampling unit and, once the pump is switched on, it is used to brush/dab the algae (or other epilithic biota) to release bound sediments which are im- mediately vacuumed through the filter. Fine sediment grains and water (5.4 I) pass through the bilge pump and fill a plastic collection bag (530 x 300mm, 100 gauge). Upon