How droughts can encourage algal blooms

Posted on December 15, 2011

Although phosphorus is a key nutrient in aquatic production, much of it is bound to sediments and is unavailable for biological uptake.  However, when sediments are exposed and dry out, phosphorus is released and is free to be assimilated by aquatic plants the next time the sediments are wetted.  Marsh and lake studies have shown how drying affects phosphorus binding and the forms of phosphorus that occur in sediments, but little is known about how the wide variation in riverbed wetness and composition (e.g., in particle size, total phosphorus, organic content) affects the release of phosphorus.  Working in the upper Brisbane River, Australia during a drought period,  researchers compared sediment samples  from submerged, partly dry and desiccated locations .  The way that sediment surface layers responded to drying wasn’t much affected by their composition, but it was strongly influenced by the length of the drying period.   Partly dry and desiccated sediments contained a lot more loosely-held phosphorus (NH4Cl-P ), but less organic phosphorus ( NaOH-nrP), than submerged sediments.  This suggested that drying encouraged  the conversion of organic phosphorus to a more readily available inorganic form, probably through microbial activity under conditions of heightened oxygen and temperature.  Measures of the binding properties of the sediments showed that desiccation, but not partial drying, markedly increased the capacity for sediments to release phosphate during the next flow event.  Therefore, climatic changes that promote riverbed drying may boost phosphorus levels and increase  the chance of algal blooms in downstream waters.

Reference:  Kerr, J.G., Burford, M., Olley, J. & Udy, J.  2010.  The effects of drying on phosphorus sorption and speciation in subtropical river sediments.  Marine and Freshwater Research 61, 928–935.