Comparison of carbon production and decomposition, benthic nutrient fluxes and denitrification in seagrass, phytoplankton, benthic microalgae- and macroalgae-dominated warm-temperate Australian lagoons

The influence of marine plants representing different stages of eutrophication on carbon decomposition and production, benthic nutrient fluxes and denitrification was examined in 4 shallow warm-temperate Australian lagoons. Differences in carbon production and decomposition across the lagoons were the main regulators of the quantity and quality of benthic nutrient fluxes and the relative proportion of nitrogen lost through denitrification. For example, the efficiency with which the lagoon sediments recycled nitrogen as N-2, (i.e. denitrification efficiency: N-2-N/(N-2-N+DIN), decreased as carbon decomposition rates increased, C:N ratios of the remineralised organic matter in some of the plant-sediment systems were much higher than expected from the stoichiometry of the dominant carbon supply. Dark DON fluxes were also very high in all the plant-sediment systems (30 to 80% of the total nitrogen flux). We offer 2 alternative explanations for the observed sediment and benthic flux characteristics: (1) The low dark C:N ratios of the remineralised organic matter may have been due to dark uptake by benthic microalgae and possibly other plants. The large DON effluxes were either the hydrolysis product of freshly produced in situ organic material or/and associated with the grazing of benthic microalgae. This explanation has important implications regarding the importance of benthic microalage as a sink for nitrogen. (2) Alternatively, the high C:N ratios of the remineralised organic matter may have been directly related to the large dissolved organic nitrogen (DON) effluxes; large DON effluxes with a low C:N ratio increase the C:N ratio of organic matter in the surface sediments, which in turn causes an uptake and accumulation of nitrogen by bacteria due to N-limitation of the microbial decomposition. Production by all the plant groups had a significant influence on benthic nutrient fluxes, with a typical pattern of an efflux during the dark cycle and an uptake during the light cycle. As such, the sediment productivity/respiration (p/r) ratio was one of the major controls on (best indicators of) net benthic inorganic and organic nutrient fluxes and appears to be one of the key changes which occur in shallow coastal lagoons as these become eutrophic. This has important management implications, demonstrating the need to maintain the balance of benthic autotrophy and heterotrophy. The robustness of the denitrification efficiency and sediment p/r relationships across such a diverse range of plant-sediment systems that represent the different stages of eutrophication suggests that these may be useful in synthesising denitrification and benthic flux data across shallow coastal systems and in defining suitable carbon loading rates.