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) and10 man-made lakes (Broads) is described. The system is hypereutrophicand tidal. Retention times of water in the river and Broadshave been estimated by dye-tracing (using rhodaxnine WT), flowand water level gauging, and chloride dilution following tidalsurges. Retention times varied from <1 day to >8 weeksin different parts of the system. There was no correlation betweenmeasured retention time and mean phytoplankton chlorophyll aconcentration. This was probably because mixing in even themost frequently flushed Broads was incomplete. Water was retainedin parts of them (nursery areas) far longer than the averageretention time, so that algal crops, with which the river wasinoculated, could build up. This also reduced the seasonal variabilityin size of standing crop, which was also uncorrelated with retentiontime. There was a very close linear relationship between meangrowing-season chlorophyll a concentration and total phosphorusconcentration (chlorophyll:P ratio=0.56), suggesting that croplevels are set by phosphorus availability. N:P ratios in incomingwater were very high. There was evidence that much phosphoruswas lost to the sediments but that in only one area (South WalshamBroad) was internal loading of phosphorus from the sedimentlikely to be significant. Overall, the results ran counter toconventional wisdom about the development of river phytoplankton.