Impact of a flood event on benthic and pelagic coupling in a subtropical east Australian estuary (Brunswick)

We examined the impact of a 1:3 year return period flood on benthic and pelagic coupling in the river-dominated sub-tropical Brunswick Estuary. The flood had a significant impact on the study site flushing it with freshwater, reducing the flushing time 0.6 days, increasing nutrient concentrations in the water column and scouring the sediment surface. In the three weeks post-flood the benthic and pelagic systems alternated between being coupled and un-coupled via dissolved, particulate and living material pathways. Immediately post-flood benthic and pelagic coupling via the deposition of phyto-detritus and viable algal cells was reduced due to the scouring of the top sediment layers, and benthic respiration and productivity and NH₄⁺ effluxes all decreased correspondingly. In contrast, benthic and pelagic coupling was enhanced via the uptake and denitrification of NO₃^- due to elevated NO₃^- concentrations in the water column. Some of the NO₃^- consumed by the sediments may have also been converted to DON. Two weeks post-flood benthic and pelagic coupling was significantly enhanced via the deposition of phyto-detritus and viable algal cells associated with a phytoplankton bloom in the water column. This increased supply of phyto-detritus and viable algal cells rapidly increased benthic respiration and productivity and NH₄⁺ efflux. The depletion of water column DIN by the phytoplankton bloom resulted in a de-coupling of the benthic and pelagic systems via the uptake and denitrification of NO₃^-. However, benthic and pelagic coupling was enhanced via the uptake of NH₄⁺ by benthic microalgae. Three weeks post-flood the phytoplankton bloom had collapsed and the coupling between the benthic and pelagic systems via the deposition of phyto-detritus and living algal cells had diminished. Again benthic and pelagic coupling was enhanced via the uptake and denitrification of NO₃^- due to elevated NO₃^- concentrations in the water column associated with the recycling of bloom material. Overall the sediments became less heterotrophic (increasing benthic productivity/respiration ratio) following the flood. Floods can cause rapid and complex changes in the coupling between benthic and pelagic systems in sub-tropical estuaries.