Phytoplankton distribution in a temperate floodplain lake and river system. I. Hydrology, nutrient sources and phytoplankton biomass

A floodplain system of streams, a main river (the R.Bure) and 10 man-made lakes (Broads) is described. The system is hypereutrophic and tidal. Retention times of water in the river and Broads have been estimated by dye-tracing (using rhodaxnine WT), flow and water level gauging, and chloride dilution following tidal surges. Retention times varied from <1 day to >8 weeks in different parts of the system. There was no correlation between measured retention time and mean phytoplankton chlorophyll a concentration. This was probably because mixing in even the most frequently flushed Broads was incomplete. Water was retained in parts of them (nursery areas) far longer than the average retention time, so that algal crops, with which the river was inoculated, could build up. This also reduced the seasonal variability in size of standing crop, which was also uncorrelated with retention time. There was a very close linear relationship between mean growing-season chlorophyll a concentration and total phosphorus concentration (chlorophyll:P ratio=0.56), suggesting that crop levels are set by phosphorus availability. N:P ratios in incoming water were very high. There was evidence that much phosphorus was lost to the sediments but that in only one area (South Walsham Broad) was internal loading of phosphorus from the sediment likely to be significant. Overall, the results ran counter to conventional wisdom about the development of river phytoplankton.