Partial recovery of a eutrophic reservoir through managed phosphorus limitation and unmanaged macrophyte growth

Recovery from an algal to a macrophyte-dominated state in a freshwater system can require both nutrient reduction and a switch to initiate the change. Despite ferric dosing of pumped inputs since 1983 to reduce external phosphorus loading and forced mixing of the main basin by helices since 1985, Alton Water reservoir, Suffolk, UK suffered from increasing planktonic chlorophyll a concentrations in the main basin, from construction in 1981 until 1992. From 1987 blooms of cyanobacteria were increasingly frequent, with a major bloom of Woronichinia ( originally Coelosphaerium) in 1992, resulting in water supply problems. Since 1993 a partial recovery from a eutrophic to mesotrophic state occurred. Studies of chlorophyll a, orthophosphate (OP), total phosphorus (TP), Alkaline Phosphatase Activity (APA) and response of phytoplankton to in situ phosphate addition experiments between 1995 and 1997, revealed gradients indicative of increasing phosphorus limitation of phytoplankton along the length of the reservoir, away from the pumped input. From 1993 to 1997 no algal blooms occurred in the main basin of the reservoir and extensive macrophyte growth was observed around the reservoir periphery, principally Elodea spp. This corresponded with an increase in weed-eating birds, such as coot (Fulica atra) and tufted duck (Aythya fuligula). The partial recovery of the main basin was attributed to bottom-up control through phosphorus limitation as a result of management in the form of ferric dosing, as well as competition for nutrients and light by the extensive macrophyte beds. Increased macrophyte growth was probably induced by changes in the reservoir ecology following the cyanobacterial bloom and associated fish kill in 1992.