Influence of small-scale patchiness on resilience of nutrient cycling to extended hypoxia in estuarine sediments

Although much work has been done to predict the effects of hypoxia (dissolved oxygen < 2 mg l−1) at regional scales, individual estuaries consist of a patchwork of micro-environments that can have different responses. We followed the effects of extended dissolved oxygen (DO) depletion on benthic fluxes of CO2, O2, NO3 −, NH4 +, N2, PO4 3− and Fe from estuarine sediments from 3 shallow sites with different macrofauna communities and levels of organic enrichment. DO depletion was achieved by a prolonged (40 d) dark incubation of sealed sediment cores. There were no discernible differences in NO3 − and N2 fluxes between sites, but the effects of hypoxia on sediment metabolism, and on bioavailable nutrient release, NH4 + and PO4 3−, were modified by the initial macrofauna communities. The DO in cores containing sediments from a site dominated by small epifauna declined significantly faster than in cores containing a greater portion of burrowing infauna; burrows may provide an oxic reservoir within the sediments. Macrofauna mortality led to a more rapid efflux of mineralization products (CO2, NH4 + and PO4 3−) in the epifaunadominated sites, as the small surface-dwelling animals decomposed more quickly. However, the cores were sealed, preventing migration of mobile epifauna away from the hypoxic conditions. The site with the lowest abundance of macrofauna also contained a large amount of refractory organic matter. Decomposition of this material was slow, with little release of nutrients. The study highlights the fact that environmental patchiness can modify the effects of hypoxia and stresses the importance of deeper burrowing fauna as a buffer against declining DO conditions.