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The storage and release of phosphorus by sediments can act as an important control on the formation of noxious blooms of cyanobacteria in lakes and estuaries. Here we studied the uptake and release of phosphorus associated with iron oxides within sediments of a lagoon system affected by recurring summer blooms of the cyanobacterium Nodularia spumigena. Using an ascorbate extraction, we observed deep pools of iron oxide associated phosphorus (P) at concentrations of ∼5 μmol g−1 dry sed to a depth of ∼20 cm. This pool rapidly decreased with the onset of water column anoxia, leading to an integrated release of ∼300 mmol of phosphorus m−2 of sediment over 3.5 months. Scaling this flux over the periodically anoxic area of the lake and over the period of an N. spumigena bloom (November-December 2011) gave a broad mass balance agreement with the increase in total phosphorus within the water column and exported from the lagoon. Over this period we estimated that P release from the sediment would have allowed ∼130-187 tonnes of nitrogen to be fixed which agrees with previous estimates of nitrogen fixation. Upon reoxygenation of the water column, a regeneration of the deep iron oxide associated phosphorus pool was observed. This deep dynamic pool of phosphorus was most likely mediated by the burrowing activity of the polychaete Capitella capitata which was observed at the study site. This study underscores the potential importance of sediments colonized by deeply irrigating fauna to become a significant source of phosphorus during water column anoxia. [ABSTRACT FROM AUTHOR]

We used δ15N-nitrate () and δ18O-to unravel the provenance and fate of in a groundwater-fed estuary. A total of 13 monthly and two time series surveys were undertaken in the Werribee River estuary near Melbourne, Australia. The different survey timescales provided a comprehensive evaluation of the hydrological effects on the biogeochemistry of, which accounted for tidal variability and episodic runoff events. The distribution of δ15N and δ18O of the estuarine along a mixing line between shallow and deep groundwater provided strong evidence for the predominance of groundwater-derived in the estuary. During dry periods when water residence time in the estuary was extended, shallow groundwater contributed 60% to 76% of the (calculated from δ15N- and δ18O-) to the estuary, and assimilation removed ~70% of this groundwater-derived . During wet periods, deep groundwater provided more (62%) than shallow groundwater, and there was no indication of consumption. Occasional sources of, which were also reflected by their δ15N- and δ18O- values, included from nitrification and sewage-derived particularly at the bay entrance to the estuary. Greater emphasis should be placed upon the role of groundwater as a substantial end member when assessing biogeochemistry using methods relying on the dual isotopic composition of. [ABSTRACT FROM AUTHOR]

We monitored submarine groundwater discharge (SGD) into the Werribee Estuary, Australia, using both chemical and physical methods. SGD occurred at hotspots where 222 Rn persisted through a 12 month survey period. A significant correlation between Rn222 and (r² = 0.8, p < 0.01), as well as between Rn222 and N2O (r² = 0.6, p < 0.01) at a 222Rn hotspot, and much higher and N2O concentrations in groundwater relative to surface water suggest that elevated and N2O concentrations in the estuary were derived from SGD. Two sampling campaigns over 24 h revealed that variations of 222Rn, and N²O were controlled by tide-induced hydraulic-head gradient fluctuations and, possibly to a much lesser extent, by tidal pumping and density-driven convection. A two-box 222 Rn mass-balance model was used to calculate the rate of SGD into two different layers across the pycnocline of the estuary. A higher total groundwater discharge rate of 0.12 ± 0.09 m d-1 was observed in the surface layer during ebb tide compared with 0.10 ± 0.08 m d-1 during flood tide. Fluxes of groundwater-derived and N O were higher than the riverine flux at baseflow by more than 30 fold and 20 fold, respectively. SGD derived fluxes exceeded the mean annual riverine and TN fluxes by a factor of ~ 5 and ~ 3 respectively. SGD-derived N2O fluxes were 170 µmol-2 m d-1, which are among the highest N2O fluxes observed in estuaries. [ABSTRACT FROM AUTHOR]