Phytoplankton dynamics of the River Nene, England
The distribution of suspended algae was investigated in a 69-km length of a small lowland river in the UK, the Nene (annual median discharge at km 91.7 =6 m(^3) s(^-1). Variations in chlorophyll a data collected between 1975 and 1998 by water management organisations at km 91.7 were evaluated against a range of physical and chemical variables. Interpretation was aided by additional sampling between 1993 and 1997.The latter half of the 24-year period had significantly higher temperatures and sunshine-hours and significantly lower ammonium concentrations. Discharge, temperature and sunshine-hours were significant predictors of chlorophyll concentration, particularly between January and June, and spring chlorophyll maxima ranged from 106 to 276 µg L(^-1).Centric diatoms were the most abundant taxa in the main-river and, in the absence of other limiting factors, appeared to be restricted by the availability of silica. There was also evidence that the centric diatoms suffered from severe parasitism. Inter-year phytoplankton abundance was most variable in the summer, and years with abundant submerged macrophytes had particularly low phytoplankton numbers. Spring phytoplankton peaks occurred earlier and had smaller amplitude at downstream sites than those further upstream. Average spring chlorophyll concentrations (April - June) increased significantly between km 22.4 and km 43.9, thereafter remaining high to km 91.7. Spatial trends were attributed to changes in channel morphology, retention time, dead zones, longitudinal variations in current velocity, temperature and silica limitation. An appraisal of the Utermöhl method of counting phytoplankton was made and a new technique proposed, called 'spaced fields'. The spaced fields method accurately identified small changes in phytoplankton abundance and was used to identify short-term temporal and small-scale spatial trends in the Nene.