Organic matter and benthic metabolism in euphotic sediments along shallow sub-tropical estuaries, northern New South Wales, Australia

Abstract

Sediment organic matter (OM), chloropigments and benthic metabolism (oxygen, carbon dioxide and alkalinity fluxes) were investigated along 3 sub-tropical Australian estuaries during different seasons. Significant rates of benthic productivity occurred throughout the year in the study estuaries, especially in the lower reaches where sediments were commonly net autotrophic. Benthic microalgae (BMA) contributed up to 20% of the total carbon content in the lower estuary sands, while organic matter was dominated by phytodetritus and allochthonous material in the middle to upper estuaries. Sediment oxygen demand (SOD) incubations were carried out on surface sediments to investigate labile carbon contents. SOD normalised to sediment carbon content (SOD mol C -1 ) confirmed a much higher proportion of labile OM in the lower estuary sediments due to a predominance of living BMA, while the OM pool in the upper and middle estuaries was relatively more degraded. SOD mol C -1 correlated well with both the C:N and the chlorophyll:pheophytin ratios of surface sediments, providing 3 independent measures of sediment OM quality. Comparisons between SOD and benthic community respiration rates (O 2 fluxes) measured in sediment core incubations suggested that oxygen demand due to the oxidation of reduced sulphur compounds formed a significant fraction of the O 2 flux, and that fluxes may be enhanced by bioturbation. O 2 and TCO 2 respiration rates were overall poorly correlated and suggested that benthic metabolism is characterised by periods of anaerobic mineralisation (TCO 2 :O 2 > 2) during periods of high OM supply, followed by aerobic respiration and reoxidation (TCO 2 :O 2 1 to 2) as OM supply waned. At many sites throughout the study TCO 2 :O 2 flux ratios were less than 1 and coupled with alkalinity uptakes, indicating that either sulphide oxidation or nitrification may be significant influences on the benthic fluxes. These processes cause a net drawdown on O 2 effluxes that can lead to significant underestimations of benthic productivity. As such, the use of CO 2 and O 2 fluxes yield distinctly different measures of benthic metabolism.