Benthic metabolism and nitrogen cycling in a sub-tropical coastal embayment: spatial and seasonal variation and controlling factors

Abstract

Spatial and seasonal variation in sediment properties, benthic metabolism (O2 and TCO2 fluxes) and nitrogen (N) cycling (dinitrogen gas (N2), dissolved inorganic N (DIN) and organic N (DON) fluxes) in western Moreton Bay, Queensland, Australia, were investigated using spatially intensive sediment core incubations. Principal component analysis (PCA) showed that variation in the dataset was best explained by 2 components representing heterotrophic and autotrophic pro- cesses respectively. Heterotrophic respiration was most closely associated with sediment chlorophyll c, indicating that benthic labile organic carbon (OC) was dominated by phytodetritus (diatoms) and viable phytoplankton cells. The supply of this OC to the sediments was controlled by a combination of phytoplankton biomass and hydrodynamic circulation patterns. Autotrophic benthic production across the study area was closely associated with light attenuation, which varied according to depth and turbidity. DIN fluxes were most closely associated with the autotrophic component, indicating the importance of N assimilation by benthic microalgae. N2 fluxes were negatively associated with the heterotrophic component, most likely due to the increasing inhibition of coupled nitrification- denitrification and/or an increase in N fixation as OC loading increases. DON fluxes dominated N fluxes overall and were controlled by autotrophic/heterotrophic DON production and heterotrophic DON consumption. Cluster analysis identified 3 basic states that describe the spatial and temporal variation in benthic processes across western Moreton Bay: reactive nearshore sediments underlying phytoplankton bloom centres (Cluster 1), less reactive sediments further offshore which receive a lesser supply of phytodetritus (Cluster 2) and a new suite of sediment properties, metabolism and N fluxes at the nearshore sites following a post-flood phytoplankton bloom (Cluster 3).