Interaction of benthic microalgae and macrofauna in the control of benthic metabolism, nutrient fluxes and denitrification in a shallow sub-tropical coastal embayment (western Moreton Bay, Australia)

Benthic biogeochemistry and macrofauna were investigated six times over 1 year in a shallow sub-tropical embayment. Benthic fluxes of oxygen (annual mean −918 μmol O2 m−2 h−1), ammonium (NH4 +), nitrate (NO3 −), dissolved organic nitrogen, dinitrogen gas (N2), and dissolved inorganic phosphorus were positively related to OM supply (N mineralisation) and inversely related to benthic light (N assimilation). Ammonium (NH4 +), NO3 − and N2 fluxes (annual means +14.6, +15.9 and 44.6 μmol N m−2 h−1) accounted for 14, 16 and 53 % of the annual benthic N remineralisation respectively. Denitrification was dominated by coupled nitrification–denitrification throughout the study. Potential assimilation of nitrogen by benthic microalgae (BMA) accounted for between 1 and 30 % of remineralised N, and was greatest during winter when bottom light was higher. Macrofauna biomass tended to be highest at intermediate benthic respiration rates (−1,000 μmol O2 m−2 h−1), and appeared to become limited as respiration increased above this point. While bioturbation did not significantly affect net fluxes, macrofauna biomass was correlated with increased light rates of NH4 + flux which may have masked reductions in NH4 + flux associated with BMA assimilation during the light. Peaks in net N2 fluxes at intermediate respiration rates are suggested to be associated with the stimulation of potential denitrification sites due to bioturbation by burrowing macrofauna. NO3 − fluxes suggest that nitrification was not significantly limited within respiration range measured during this study, however comparisons with other parts of Moreton Bay suggest that limitation of coupled nitrification–denitrification may occur in sub-tropical systems at respiration rates exceeding −1,500 μmol O2 m−2 h−1

Carbon and nitrogen cycling in a shallow productive sub-tropical coastal embayment (western Moreton Bay, Australia)

Climatic variables, water quality, benthic fluxes, sediment properties, and infauna were measured six times over an annual cycle in a shallow sub-tropical embayment to characterize carbon and nutrient cycling, and elucidate the role of pelagic–benthic coupling. Organic carbon (OC) inputs to the bay are dominated by phytoplankton (mean 74%), followed by catchment inputs (15%), and benthic microalgae (BMA; 9%). The importance of catchment inputs was highly variable and dependent on antecedent rainfall, with significant storage of allochthonous OC in sediments following high flow events and remineralization of this material supporting productivity during the subsequent period. Outputs were dominated by benthic mineralization (mean 59% of total inputs), followed by pelagic mineralization (16%), burial (1%), and assimilation in macrofaunal biomass (2%). The net ecosystem metabolism (NEM = production minus respiration) varied between −4 and 33% (mean 9%) of total primary production, whereas the productivity/respiration (p/r) ranged between 0.96 and 1.5 (mean 1.13). Up to 100% of the NEM is potentially removed via the demersal detritivore pathway. Dissolved inorganic nitrogen (DIN) inputs from the catchment contributed less than 1% of the total phytoplankton demand, implicating internal DIN recycling (pelagic 23% and benthic 19%) and potentially benthic dissolved organic nitrogen (DON) fluxes (27%) or N fixation (up to 47%) as important processes sustaining productivity. Although phytoplankton dominated OC inputs in this system, BMA exerted strong seasonal controls over benthic DIN fluxes, limiting pelagic productivity when mixing/photic depth approached 1.3. The results of this study suggest low DIN:TOC and net autotrophic NEM may be a significant feature of shallow sub-tropical systems where the mixing/photic depth is consistently less than 4

Metabolism of different benthic habitats and their contribution to the carbon budget of a shallow oligotrophic sub-tropical coastal system (southern Moreton Bay, Australia)

The major benthic habitats in a shallow oligotrophic sub-tropical coastal system were mapped, benthic productivity and respiration were measured seasonally (summer, winter) in each open water habitat, and an annual carbon budget was constructed using measured, modelled and literature fluxes to estimate the functional importance of each major benthic habitat to the whole ecosystem. Stable Zostera Seagrass Communities covered 16% of the open water system but made little contribution to whole system metabolism. In contrast, ephemeral Halophila Seagrass Communities covered only 8% of the open water system but contributed 46% of the net productivity (p). The less ‘iconic’ Inter- and Sub-tidal Pimpama Shoals also only had a small areal extent (10%) but accounted for 50% of the net benthic production. Similarly, Yabby Shoals only covered 27% of the open water system but accounted for 89% of the net respiration (r). Budget estimates suggest that lateral import of organic matter, most likely tidally transported phytoplankton trapped in seagrass beds, across the Broadwater boundaries was required to balance the carbon budget if any reasonable estimate of burial was invoked. However, budget errors make it difficult to distinguish this import from zero. This study demonstrated that shallow subtropical coastal systems have a complex mosaic of benthic habitats, and that some of the less ‘iconic’ habitats (i.e. non-seagrass, non-mangrove) also make an important functional contribution that controls the flow of energy and nutrients through the whole ecosystem and determines the net ecosystem metabolism and possible exchanges with adjacent systems