Macrofaunal contributions to benthic nutrient fluxes revealed by radium disequilibrium

Abstract

The benthic flux of nutrients underpins marine food webs, influences conditions for life in the oceans, and contributes to climatic feedback. Estimates of these fluxes’ contributions to macro- and micronutrient cycles are, however, highly variable, generating significant uncertainty in biogeochemical models. Traditional benthic flux methodologies have potentially significant limitations, and in geochemically framed studies there is a notable omission of contributions from macrofaunal activity. 224Ra/228Th disequilibrium is a cutting-edge geochemical technique for quantifying benthic flux, however its capacity to incorporate the influence of benthic fauna has not been assessed. Here we present first results using 224Ra/228Th disequilibrium to examine the influence of four macrobenthic infaunal invertebrate species on the flux of macronutrients (NH4+, NO3-, NO2-, PO43-) between the sediment and water column. Overall, estimates from 224Ra/228Th disequilibrium were up to 15 times greater than those derived from commonly used methods, and both 224Ra and nutrient fluxes differed in the presence of different macrofauna. Greatest efflux (NH4+, PO43-) and influx (NO2-, NO3-) occurred in the presence of the burrowing actinarian Edwardsia claparedii, and 224Ra flux patterns suggested different mechanisms of influence between species. Notable variability in flux enhancement was also present between individuals of the same species, highlighting the complex relationship between biodiversity and ecosystem functioning. We conclude that 224Ra/228Th disequilibrium is a powerful tool for interdisciplinary biogeochemical and ecological studies, and if applied widely enough may more holistically represent benthic flux dynamics in the natural environment than standard approaches