Gradients of chlorophyll a and water chemistry along an eutrophic reservoir with determination of the limiting nutrient by in situ nutrient addition

Alton Water, Suffolk, UK, is a linear reservoir, with input water pumped into the northwest end and drawoff tower in the main basin at the southeast end. The reservoir had an eutrophic to mesotrophic gradient in 1995 and 1996, primarily due to differences in summer. In 1995 mean summer chlorophyll a concentration at the northwest end was 95±34 μg l−1, 24 times that in the main basin. Total phosphorus, orthophosphate, nitrate and ammonium concentrations significantly decreased along the length of the reservoir, whilst Secchi depth increased. The first two components of principal component analysis (PCA) explained 37.4% of the variation in the data set, with ordination of sample sites along opposing gradients of TP and Secchi depth. The northwest end of the reservoir had high suspended solids and algal biomass whilst the main basin was a clear water site with high Secchi depth. Phosphorus was more limiting than nitrogen (DIN:OP>16:1), however Si:TP fell below 16:1 during spring (March–May) and summer (July–September) indicating possible silica limitation to diatoms. In situ nutrient addition experiments (September 1996–August 1997) indicated phosphorus limitation, with response to phosphate addition significantly increasing along the length of the reservoir. Co-limitation did not occur as addition of phosphate, ammonium and silicate together produced the same response as phosphate addition alone. Alkaline phosphatase activity assay supported the pattern of phosphorus limitation with highest activity in the main basin. Manipulation of zooplankton grazing did not control the algal biomass response to phosphate addition. Ferric dosing of the pumped input coupled with the topography of the reservoir limited algal biomass in the main basin through bottom-up nutrient limitation.