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.