Whilst a diverse array of phosphorus (P)-adsorbent materials is currently available for application to freshwater aquatic systems, selection of the most appropriate P-adsorbents remains problematic. In particular, there has to be a close correspondence between attributes of the P-adsorbent, its field performance and the management goals for treatment. These management goals may vary from a rapid reduction in dissolved P to address seasonal enrichments from internal loading, targeting external fluxes due to anthropogenic sources, or long-term inactivation of internal P inventories contained within bottom sediments. It also remains a challenge to develop new methods and materials that are ecologically benign and cost-effective. We draw on evidence in the literature and the authors’ personal experiences in the field, to summarise the attributes of a range of P-adsorbent materials. We offer ‘guiding principles’ to support practical use of existing materials and outline key development needs for new materials.

Climatic change is shaping our planet's ecosystems yet our capacity to predict the consequences and prepare for the future remains rudimentary. Changes to the hydrological cycle mean that large regions of the planet are experiencing changes in precipitation. Responses by phytoplankton were assessed in three regions: 1) globally, 2) in regions that are wet and getting wetter, 3) in regions that are dry and getting drier. Using long-term time-series data the temporal variation in precipitation was compared with variation in chlorophyll a, diatoms, dinoflagellates, chlorophytes, chrysophytes and euglenophytes from 106 sites worldwide. The results demonstrate that phytoplankton responses to precipitation depend upon the season and region. In general phytoplankton responded more positively to increased precipitation during summer rather than winter. Increased precipitation during winter was likely to reduce chlorophyll a, diatoms and chrysophytes, whereas increasing precipitation in summer was likely to increase chlorophyll a and favor chlorophytes. Within regions that are wet and getting wetter chlorophyll a increased and dinoflagellate abundances were reduced in wet autumns; while diatom abundances were reduced in wet springs. In dry and drying ecosystems the abundances of chlorophytes decreased during dry springs and summers. The existence of these widespread patterns of phytoplankton abundance associated with inter annual variability in precipitation improves our capacity to predict the future composition of phytoplankton communities in estuarine and coastal water bodies.

The understanding and management of eutrophication leading to troublesome algal blooms is pivotal to the health of aquatic systems. There are also considerable economic implications. Algal blooms in Australia cost between A$180 and 240 million dollars per annum (Atech 2000). Initial strategies to control phosphorus (P) supply as a limiting nutrient for algal growth centred on the reduction of point sources and the manipulation of water-column oxygen. In the past decade, however, mineral-based amendments have been recognised for their potential to bind P under a range of physico-chemical conditions (e.g. Douglas et al. 2004). Akhurst et al. (2004) examined the performance of two mineral-based materials to treat P-enriched water and/or sediment. The first was Bauxsol, a red mud–seawater derivative, presumably similar to the seawater amendment of red mud originally described in Thornber and Hughes (1987). The second material was a lanthanum (La)-modified bentonite. We address two major issues:

Phoslock™ is a specially modified clay designed to permanently bind phosphorus in those situations where phosphorus (P) release from sediments is a main driver of algal bloom formation. Extensive laboratory and mesocosm trials have demonstrated the effectiveness of Phoslock™ in binding sediment released P using less than a millimetre thickness of clay. Two full-scale applications were undertaken in the summer of 2001/2002 in the impounded riverine section of two estuaries along the coastal plain of south west Western Australia. Both rivers are subject to blue-green algal blooms in the summer months. Phoslock™ applied in a slurry from a small boat reduced dissolved P in the water column to below detection limit in the few hours it took for the clay to settle and substantially reduced P efflux from the sediments during the course of the trial. The effect of P reduction on phytoplankton growth was clearly evident in the phytoplankton dominated Vasse River but was less clear in the alternating phytoplankton to aquatic plant dominated Canning River which is also subject to surface nutrient inputs.

A trial to artificially destratify part of the water column of the Swan-Canning Estuary took place over four weeks in 1997. Destratification was attempted with bubble plumes created by pumping compressed air through a porous pipe near the bed at a location in the upper estuary. The purpose of the trial was to determine whether the resultant bubble 'curtain' could mix the water column, thereby altering the extent of upstream and downstream mixing and the vertical structures of salinity, temperature, dissolved oxygen and turbidity. The destratification trial was monitored using three techniques: vertical profiles taken on several days at selected stations along the estuary, fine scale profiles in the vicinity of the bubble curtain on one day, and continuous water column profiles taken I km upstream of the curtain. The seasonal upstream propagation of brackish water underneath a residual freshwater discharge was affected by the magnitude of freshwater discharge and tidal elevations. Superimposed upon diurnal and semi-diurnal tides were non-tidal water level changes induced mostly by variations in barometric pressure. These caused large changes in the salt wedge position. Destratification by the bubble curtain was compromised by these large oscillations, which limited the exposure of the salt wedge region to the mixing action of the bubble curtain. Complete vertical mixing of the water column was observed up to 30 m either side of the curtain on 30 October. Disruptions to the density stratification were not evident beyond 350 m of the curtain. In the immediate vicinity (similar to 30 m) of the curtain, deficits of dissolved oxygen in bottom waters were generally reduced or obliterated and a naturally turbid plume of water near the bottom of the estuary was mixed through the water column. The effects of the bubble curtain on dissolved oxygen, turbidity and temperature were, however, similar to those for salinity, with mixing confined to around 30 m from the curtain and no effect observed further than 350 m away. The upper Swan River estuary, although relatively narrow and strongly vertically stratified, is unsuited to destratification using bubble plumes. Cycling of spring and neap tides and non-tidal water level changes, together with freshwater inflows to the estuary, strongly limit the longitudinal extent of mixing by bubble curtains. Copyright (C) 2001 John Wiley & Sons, Ltd.