The past and future of phytoplankton in the UK's largest lake, Lough Neagh

Lough Neagh is the largest lake in the UK and has been extensively monitored since 1974. It has suffered from considerable eutrophication and toxic algal blooms. The lake continues to endure many of the symptoms of nutrient enrichment despite improvements in nutrient management throughout the catchment, in particular a permanently dominant crop of the cyanobacterium Planktothrix agardhii. This study examines the historical changes in the Lough, and uses the PROTECH lake model to predict how the phytoplankton community may adapt in response to potential future changes in air temperature and nutrient load. PROTECH was calibrated against 2008 observations, with a restriction on the maximum simulated mixed depth to reflect the shallow nature of the lake and the addition of sediment released phosphorus throughout the mixed water column between 1 May and 1 October (with an equivalent in-lake concentration of 2.0 mg m−3). The historical analysis showed that phytoplankton biomass (total chlorophyll a) experienced a steady decline since the mid-1990s. During the same period the key nutrients for phytoplankton growth in the lake have shown contrasting trends, with increases in phosphorus concentrations and declines in nitrate concentrations. The modelled future scenarios which simulated a temperature increase of up to 3 °C showed a continuation of those trends, i.e. total chlorophyll a and nitrate concentrations declined in the surface water, while phosphorus concentrations increased and P. agardhii dominated. However, scenarios which simulated a 4 °C increase in air temperature showed a switch in dominance to the cyanobacteria, Dolichospermum spp. (formerly Anabaena spp.). This change was caused by a temperature related increase in growth driving nutrient consumption to a point where nitrate was limiting, allowing the nitrogen-fixing Dolichospermum spp. to gain sufficient advantage. These results suggest that in the long term, one nuisance cyanobacteria bloom may only be replaced by another unless the in-lake phosphorus concentration can be greatly reduced.