The Yarra River estuary is a salt-wedge estuary prone to periods of stratification-induced anoxia and hypoxia (O 2 , 100 mmol L 21 ) during low-flow events. Nitrate reduction pathways were examined using the 15 N isotope pairing technique in intact sediment cores, emulating in situ conditions, to evaluate the fate of NO Whole-system estimates using deviations from conservative behavior and core incubations were in good agreement and showed that NH z 4 was regenerated more efficiently relative to DIC under hypoxic conditions. For the whole system, mean DDIC : DNH z 4 ratios under oxic (85 6 33) and hypoxic (20 6 3) conditions were significantly different. The more-efficient NH z 4 regeneration during hypoxia was attributed to rapid mineralization rates and cessation of nitrification; dissimilatory nitrate reduction to ammonium (DNRA) was not a significant contributor. Unexpectedly, the denitrification : DNRA ratio was significantly higher under hypoxic conditions, with denitrification contributing 99.1% 6 0.3% of total nitrate reduction. DNRA rates were significantly higher during oxic conditions (123.5 6 30.7 mmol m 22 h 21 ) when compared with rates during hypoxia (0.6 6 0.1 mmol m 22 h 21 ). The increase in DNRA in the presence of oxygen was attributed to the alleviation of NO { 3 limitation during these conditions.

In episodically driven estuaries, nutrient delivery is typically confined to sporadic flow events, followed by intense biogeochemical recycling of nutrients. We document the response of a temperate lagoon system with episodic winter-or spring-dominated catchment inflows (Gippsland Lakes, southeast Australia) to a flood event. High winter inflows resulted in high surface-water concentrations of inorganic nitrogen (IN; NO { 2 + NO { 3 + NH z 4 ) relative to reactive phosphorus (RP) with an IN : RP ratio . 100. This resulted in a rapid diatom and dinoflagellate bloom, which collapsed as water column nutrients were exhausted. Increased carbon delivery to the sediment stimulated benthic respiration and depleted bottom-water oxygen, which led to a large release of RP from the sediments, decreasing the IN : RP ratio in the bottom water to , 6. Strong stratification of the water column allowed the accumulation of RP in the bottom water over spring and into summer. Mixing of RP from the bottom water over summer triggered the development of a Nodularia spumigena Mertens bloom. As such, the nutrient supply for the Nodularia bloom was through a biogeochemical filter that strongly favored N 2 -fixing cyanobacterial blooms by reducing IN : RP ratios through a combination of sediment denitrification and release of stored phosphorus from within the sediment. These observations lead us to the paradoxical conclusion that high nitrogen loading over winter contributed to severe nitrogen limitation over summer, and highlight the importance of controlling both nitrogen and phosphorus loading to estuaries.

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While rare globally, blooms of the toxic cyanobacteria Nodularia spumigena are a recurring problem in a few estuaries, such as the Baltic Sea and several southern Australian estuaries. Here, we document recurring Nodularia spumigena Mertens blooms in the Gippsland Lakes, S.E. Australia; a temperate lagoon system with episodic, winter-spring dominated catchment inflows. Physico-chemical conditions exerted a strong influence over bloom development, with blooms consistently occurring at surface water salinities between 9 and 20 (average = 15), inorganic nitrogen concentrations\0.4 lM, and inorganic nitrogen to reactive phosphorus ratios \5. There was a positive correlation between average annual chlorophyll a and total phosphorus (TP) load in years when there was no Nodularia bloom, but this relationship broke down in Nodularia bloom years, even though there was a strong correlation between in-lake TP and chlorophyll a during these years; this highlights the importance of internal sources of phosphorus to bloom development. Large catchment derived nitrate and nitrite (NO x ) inputs following wildfires and floods in 2007, led to high concentrations of NO x within the surface waters of the Gippsland Lakes through the second half of 2007 and the start of 2008. We hypothesise that these high NO x concentrations were a key factor leading to an unprecedented Synechococcus sp. bloom that developed in the austral summer of 2007-2008, despite conditions that would otherwise favour a Nodularia bloom.

This investigation examines the occurrence of carbon concentrating mechanisms (CCMs) in eight species of the acellular green marine macroalgal genus Caulerpa. The measurements made were of the ␦ 13 C of organic matter, extracellular carbonic anhydrase activities, pH compensation values, and the inorganic C dependence of light-saturated photosynthesis rates. The data suggest that the pyrenoid-containing C. cactoides and C. geminata, and probably C. scalpelliformis (which lacks pyrenoids) have CCMs. Net diffusive influx of CO 2 fulfills the inorganic carbon requirements of the other species for which pH-drift data are available, i.e. C. flexilis, C. longifolia, C. obscura and C. brownii. No pH drift data are available for C. trifaria and no information is available as to whether it has pyrenoids, although ␦ 13 C data suggest the absence of a CCM in this species. The three species showing evidence of CCMs have the lowest affinities for inorganic C of the eight species tested. This apparently paradoxical finding has precedence for marine red-macroalgae, and requires that the selective significance of the CCMs in these organisms is not that of increased inorganic C affinity, but is perhaps associated with the ability to both suppress photoinhibition and to photosynthesize at higher seawater pH values.