The importance of nutrient source in determining the influence of retention time on phytoplankton: an explorative modelling study of a naturally well-flushed lake

Two models were used to examine the relationship between hydraulic retention time, nutrient source and total chlorophyll in a shallow lake (Bassenthwaite Lake, UK). The first model was a derivation of the Vollenweider model and the second was the phytoplankton community model, PROTECH. The adapted Vollenweider model produced two different responses to changing retention time that were phosphorus source dependent. If the phosphorus was totally from a point source, then annual mean chlorophyll steadily declined with increasing flushing rate. However, when a diffuse source was used, the chlorophyll changed little and even increased with short retention times (retention time <40 days). The PROTECH model produced some similar responses but they were more season dependent. Winter mean chlorophyll always declined with decreasing retention time, regardless of nutrient source, but the summer mean curves were source dependent and similar to those produced by the adapted Vollenweider model. Further simulations with PROTECH using a standardised flow regime provided strong evidence as to the mechanisms behind these responses. Analysis showed that the decline in chlorophyll with decreasing retention time was the prevalent response of the PROTECH simulations due to flushing loss of both nutrients and algae. Furthermore, the curve formed an asymptote at long retention times because other factors (e.g. light) limited growth; retention times >100 days had little effect on chlorophyll. However, with a diffuse phosphorus source and short retention times, an increase in biomass was observed when the nutrient was limiting for growth.