Estuaries Vol. 27, No. 1, p. 1-17 February 2004 Nutrient Cycling in the Sub-tropical Brunswick Estuary, Australia ANGUS FERGUSON*, BRADLEY EYRE, and JENNITA GAY Cent~'efor Coastal Management, Southern C~'oss University, P. O. Box 157, East Lismare, New So'uth Wales 2480, Attst'ralia ABSTRACT: A combination of mixing plots, one-dimensional salt balance modelling, nutrient loading budgets, and benthic flux measnrenlents were used to a.~se~s nutrient cycling pathways in the enriched sub-tropical Brunswick estuary during different freshwater flows. A simple model accounting for freshwater residence times and nutrient availability was found to be a good predictor of pbytoplankton biomaxs along the estuary, and suggested that biomaxs accumulation may become nutrient-limited during low flows and that recycling widfin die water column is important during blooms. DLssolved inorganic nitrogen (DIN) cycling budgets were constructed for the estuary during different freshwater flows accounting for all major inputs (catchment, sewage, and urban) to tbe estuary. Internal cycling due to phytoplankton uptake (based on measured biomass) aald sediment-water fluxes (based on measured rates in each estuarine reach) was considered.. Fonr different nntrient cycling states were identified dnring the study. In high flow, freshwater residence times are le~s than 1 d, internal cycling processes are bypassed and. virtnaily all d.i~soh,ed, and most particulate, nutrients are delivered, to the continental shelf. During the growth phase of a phytoplankton bloom enhanced recycling occnrs as residence times increase snfficienfly to allow biomass accnmnlation. Remineralization of phytoplankton detritus during this phase can supply up to 50% of phytoplankton DIN demands. In post-bloom conditions, DIN uptake byphytoplankton decreases in the autumn wet season when biomass doubling times begin to exceed residence times. OM supply to the sediments diminishes and the benthos becomes nutrient-limited~ resulting in DIN uptake by the sediments, eks flows decrease further in the dry season, there is tig'bt recycling and phytoplankton blooms, and uptake by the sediments can account for the entire DIN loading to the estuary resulting in complete removal of DIN from the water column. The ocean is a potentially important source of DIN to the estnary at this time. The results of the DIN cycling budgets compared favorably with mixing plots of DIN at each time. The results suggest that a combination of different approaches may be useful in developing a more comprehensive understanding of nutrient cycling behavior and the effects of nutrient enrichment in estnaries. Introduction Estuaries can act as dynamic filters, or transform- ers, of water-borne nutrients as they pass from the land to the ocean (Kemp and goynton 1984; Nix- on and Pilson 1984; Eyre 2000). Nutrient transfor- mation is associated with high rates of both auto- trophic and heterotrophic productivity observed in estuaries relative to both fi-eshwater and oceanic systems (Odum 1971), as well as the unique hydro- dynamic and chemical regimes created along the estuarine salinity gradient (Turner and Rabalais 1999). The complex nature of nutrient cycling may be perturbed by excessive nutrient enrichment leading to changes in trophic status and ultimately impacting on the ability of the system to support higher orders of life (Hagerman et al. 1996). An increase in point and diffuse nutrient loadings to estuaries worldwide has seen a general increase in the occurrence of eutrophication, prompting the need to manage nutrient pollution (goynton et al. 1982; Harding 1994; Nixon et al. 1995). Central to this aim is an understanding of nutrient inputs and their recycling within the estuary throughout the year. This allows management efforts to focus on * Corresponding author; e-mail: aferguso@scu.edu.au minimizing the key problematic nutrient sources }~4th a view to maintaining, or enhancing, desirable ecosystenl processes. A key first step is gauging the relative impor- tance, and seasonal variation, of different nutrient inputs. First-order nutrient budgets accounting for external nutrient inputs (e.g., catchment, urban, sewage effluent, and atmospheric) to estuarine sys- tems are reasonably straightforward (e.g., Correll 1981; Eyre 1995; Nixon et al. 1995); however, es- timations of nutrient exports to the ocean are no- toriously difficult due to uncertainties in quantify- ing exchanges at the estuarine-ocean boundary (Nixon 1987). Detailed surveys of nutrient ex- changes between estuaries and the ocean have been carried out (McKee et al. 2000), however there is typically a low signal-to-noise ratio that in- creases the potential error involved in estimations. The main problems with direct measurelnents in- volve high spatial and temporal variability in veloc- ities and nutrient concentrations observed in the estuary-ocean boundary cross-section, and the small net exchanges relatwe to peak ebb and flood discharges (Kjerfve et al. 1981; Jay et al. 1997). Ocean exchanges are more commonly estimated as the difference between more readily measured 9 2004 Estuarine Research Federation 1