MEPS 205:61-83 (2000)  -  doi:10.3354/meps205061

ABSTRACT: Nutrient transformation and phytoplankton growth were examined in 9 river-dominated sub-tropical east Australian estuaries in 1996 using modified mixing diagrams. The sampling program was rapid and strictly controlled, using 2 or 3 boats simultaneously, so that all 9 estuaries were usually sampled within 4 to 5 d of each other. River samples were collected at the head of each estuary on a flow-weighted basis, and variations in river concentrations and flushing times were used to calculate conservative mixing lines. This was to avoid Œapparent¹ non-linear distributions in the mixing diagrams associated with river-source variations on a time scale less or equal to the flushing time of the estuary. A number of general patterns of biogeochemical behaviour were observed across most, or all, of the estuaries. The 4 northern, and most likely the 5 southern, estuaries flushed fresh to the mouth during a flood in May, allowing most of the flood-borne material to escape from the system. Phytoplankton appear to exert the dominant control on nutrient transformation in the 9 estuaries, with non-biological processes only playing a minor role, if any. Eight of the 9 estuaries are potentially nutrient-limited, with nutrient concentrations falling below the upper half-saturation constants required for phytoplankton growth. The estuaries became potentially more P-limited, and less N-limited, as the wastewater loading to each system increased. During most sampling runs, maximum and mean concentrations of phytoplankton biomass (chlorophyll a) in the Tweed, Brunswick, Bellinger, Nambucca, Macleay and Hastings estuaries were significantly correlated with the wastewater DIN (dissolved inorganic nitrogen) loading index (daily wastewater load per m³ of estuary volume multiplied by the flushing time of the estuary in days). In contrast, the diffuse DIN loading index appeared more important for controlling phytoplankton biomass (chlorophyll a) in the 3 estuaries (Richmond, Clarence and Manning) that received a low wastewater DIN load. Sub-tropical Australian estuaries are characterised by a high degree of variability in nutrient delivery and phytoplankton growth. The timing and magnitude of hydrological factors appears to be the major feature that determines the differences in the temporal patterns of phytoplankton growth between sub-tropical and temperate regions. Nutrient-loading and phytoplankton growth in the 9 estuaries appears to be in phase, suggesting that stored and recycled nutrients may play a smaller role in maintaining phytoplankton growth in these systems compared to the larger partially mixed temperate systems. Management efforts in the 9 estuaries should be first directed towards reducing the wastewater DIN loading index to <1, followed by management strategies focused on controlling diffuse loadings. There may, however, be a trade-off associated with reducing the wastewater loading index with a proportional reduction in fisheries production.

KEY WORDS: Sub-tropical · Estuary · Comparison · Mixing plot · Nutrients · Phytoplankton growth