A field test of the effects of megafaunal burrows on benthic chamber measurements of sediment-water solute fluxes

Benthic chambers are widely used in biogeochemical studies of marine sediments, but it is not known whether sediment-water solute fluxes recorded using such apparatus will be affected by the presence of a megafaunal burrow opening in the enclosed area of sea floor. We investigated this question in a shallow sea loch using simple benthic chambers deployed by divers over 2 contrasting burrow types, those of the mud-shrimp Callianassa subterranea and of the echiuran worm Maxmuelleria lankesteri. Fluxes of dissolved oxygen, manganese (II) and nutrients were measured over an 18 to 25 h incubation period and compared with those recorded over nearby sediment lacking megafaunal burrow openings. Chambers enclosing C. subterranea burrows recorded a significantly higher efflux of phosphate than controls, and efflux of ammonium was also consistently (though non-significantly) greater over burrows. C. subterranea burrows had no detectable effect on any other measured flux, Chambers enclosing M. lankesteri burrow openings differed from controls only in showing a significantly higher efflux of nitrate. This nitrate effect was particularly marked over M. lankesteri ejecta mounds, suggesting that these features may be localized sites of enhanced nitrification. Ranges of variation were high for all measured parameters, indicating that at this site, data from a single chamber incubation would be unlikely to unambiguously signify the presence of an identifiable megafaunal burrow opening. Spatial heterogeneity in solute flux rates illustrates the need to maximize replication in field studies using benthic chambers.