Carbon and nitrogen cycling on intertidal mudflats of a temperate Australian estuary. IV. Inverse model analysis and synthesis

Microphytobenthos (MPB) are recognised as exerting an important controlling influence over C and N flows in euphotic sediments; however, the coupling between these flows remains poorly studied. We undertook an inverse model analysis of C and N fluxes through the microbial compartment on intertidal flats in a temperate estuary. The analysis showed that the trophic balance of the sediment exerted a strong influence on the relative fluxes of C and N through the sediment microbial community. Under increasingly autotrophic conditions (production:respiration >1), the assimilation of C relative to N rose above the cellular C:N ratio of MPB, resulting in increased excretion rates of organic matter by MPB. The C:N ratio of the organic matter excreted was also highly variable, ranging from similar to 20 (mol:mol) under heterotrophic conditions, and increasing to >50 under autotrophic conditions. The relative fluxes of C and N through bacteria were also significantly affected by the trophic balance of the sediment and the ratio of C:N mineralized by bacteria was significantly higher under autotrophic conditions. Dissolved organic N release by bacteria and uptake by MPB also predominated over inorganic N forms under autotrophic conditions. We conclude that C and N fluxes through shallow euphotic sediments may become significantly decoupled and well above the commonly assumed Redfield ratio and measured cellular C:N ratios of MPB.