The fate of diazotrophic nitrogen (ND) fixed by planktonic cyanobacteria in pelagic food webs remains unresolved, particularly for toxic cyanophytes that are selectively avoided by most herbivorous zooplankton. Current theory suggests that ND fixed during cyanobacterial blooms can enter planktonic food webs contemporaneously with peak bloom biomass via direct grazing of zooplankton on cyanobacteria or via the uptake of bioavailable ND (exuded from viable cyanobacterial cells) by palatable phytoplankton or microbial consortia. Alternatively, ND can enter planktonic food webs post-bloom following the remineralization of bloom detritus. Although the relative contribution of these processes to planktonic nutrient cycles is unknown, we hypothesized that assimilation of bioavailable ND (e.g., nitrate, ammonium) by palatable phytoplankton and subsequent grazing by zooplankton (either during or after the cyanobacterial bloom) would be the primary pathway by which ND was incorporated into the planktonic food web. Instead, in situ stable isotope measurements and grazing experiments clearly documented that the assimilation of ND by zooplankton outpaced assimilation by palatable phytoplankton during a bloom of toxic Nodularia spumigena Mertens. We identified two distinct temporal phases in the trophic transfer of ND from N. spumigena to the plankton community. The first phase was a highly dynamic transfer of ND to zooplankton with rates that covaried with bloom biomass while bypassing other phytoplankton taxa; a trophic transfer that we infer was routed through bloom-associated bacteria. The second phase was a slowly accelerating assimilation of the dissolved-ND pool by phytoplankton that was decoupled from contemporaneous variability in N. spumigena concentrations. These findings provide empirical evidence that ND can be assimilated and transferred rapidly throughout natural plankton communities and yield insights into the specific processes underlying the propagation of ND through pelagic food webs.

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 lmol g 21 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 22 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.

Environmental context.Effective mitigation of algal blooms, and their associated detrimental impacts on flora and fauna, requires an understanding of the factors leading to bloom development, including nutrients, light and hydrodynamics. We investigated a shallow, freshwater lake and demonstrate that there is sufficient bioavailable phosphorus to annually generate a large algal biomass. Extensive, seasonal phosphorus release from sediments is controlled by the interactions of the biogeochemical cycles of nitrogen, carbon, oxygen, iron and sulfur. Abstract.The shallow, polymictic Ornamental Lake in the Royal Botanic Gardens, Melbourne, Australia, has suffered significant blooms of toxic Anabaena then Microcystis species every summer over the last decade. Although the hydrodynamic conditions of the water column are conducive for algal growth, the prolific growth is controlled by the bioavailable phosphorus concentration. Springtime phosphorus fluxes of 0.1–0.2 mmol m–2 day–1 from the sediment contribute to bloom development. These rates are also observed in anoxic sediment core incubations. Diel stratification, combined with high oxygen consumption associated with organic carbon loading, favour P release. Release rates may be amplified by the effects of sulfate reduction on P sorption onto FeIII (oxyhydroxide) surfaces. Sulfate concentrations are at the threshold where methanogenesis is inhibited in anoxic conditions. Effective bloom mitigation will require a >100-fold reduction in P concentrations, which may be achieved through macrophyte planting and inducing greater water flow through the lake system.

† Electronic supplementary information (ESI) available: Experimental procedures and characterisation data for [Co(H 2 L)Cl 2 ], [Co(H 2 L) 2 ][PF 6 ] 2 , and [{Co(H 2 L) 2 }(H 2 L) 4 ][PF 6 ] 2 . CCDC reference numbers 756326 (Co[(H 2 L) 2 ][PF 6 ] 2 ) and 756327 ([{Co(H 2 L) 2 }(H 2 L) 4 ][PF 6 ] 2 ) contain the supplementary crystallographic data for this paper. For ESI and crystallographic data in CIF or other electronic format see

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