Climate change is expected to alter water quality in rivers, but where and when this may happen is uncertain. This report describes a study of projected response in the amount of algal plant growth (phytoplankton biomass). Increasing algal growth is one of the ecological manifestations of eutrophication in slow flowing rivers, where the water starts to resemble a green soup. Eutrophication is a process in which too much nutrient in water causes algae and higher plants to grow excessively. Eutrophication alters the quality of the water and how it can be used. Phytoplankton (suspended algae) is considered to be a useful indicator of eutrophication in standing freshwaters and can also be useful as one measure of impacts in rivers, particularly slow flowing rivers. Excess algal growth can result in blooms that eventually die off. The disruption of dissolved oxygen dynamics in the water column may, in turn, have adverse impacts on fish and macroinvertebrates. The onset and decline of algal blooms is measured by the concentration of chlorophyll (a green pigment in algae) in the water. In this context, algal bloom risk – and the risk of negative eutrophication impacts in the lower reaches of rivers – is identified through observations of threshold chlorophyll concentrations.
Excedence of a chlorophyll concentration threshold is not by itself used in the diagnosis of river eutrophication but can be used as a proxy for algal blooms for understanding and modelling risk. The future risk of eutrophication impact, including algal blooms, is affected by changes in the concentration of nutrients from altered river flow and changes in phosphorus inputs from a range of sources. An earlier study (Phase 1 of this project) demonstrated that climate change impacts on river flow would increase phosphorus concentrations by 2050 and beyond. However, climate-driven changes in river temperature regime and light, and plant responses to these, are also important in altering the future risk of excess algal growth. This report considers these aspects. Click here to read more