Physico-chemical factors alone cannot simulate phytoplankton behaviour in a lowland river

•A river quality model (QUESTOR) was applied in the upper R. Thames (UK) in 2009–11.•Three alternative assumptions representing phytoplankton were tested.•A mixed group performed better than if diatom or green algal dominance was assumed.•Nutrients appear to be in excess; flow is the main control on phytoplankton blooms.•There is evidence of between-year differences in grazing loss rates.

SummaryThere are still a number of gaps in our understanding regarding phytoplankton behaviour in rivers. Given predicted future changes in climate, which appear superficially at least, to favour larger phytoplankton blooms, this study was initiated to assess how well we can currently simulate this behaviour with a river water quality model. The river quality model (QUESTOR) was run for a 45 km stretch of the upper Thames for 2009–2011 (UK). To identify the most suitable model representation, phytoplankton was simulated and compared to actual observed data under three alternative assumptions. The first of these was of a Mixed Phytoplankton population and the other two being that there was domination by either of two groups (Green Algae, or cool water diatoms such as Stephanodiscus hantzschii) known to be abundant in the river. The factors for controlling the phytoplankton populations were found to be flow, temperature and radiation. Of these controlling factors, river flow has the larger effect on depletion or build-up of phytoplankton, based on residence time. The nutrient concentrations (phosphate and nitrate) seem to be in excess and not limiting or controlling of the phytoplankton behaviour. The data highlighted two main blooms in late spring and summer, which were successfully modelled with a Mixed Phytoplankton population (which explained 16–35% of the weekly variability throughout 2009–10). On a year-to-year time frame there is clear evidence of between-year differences in grazing loss rates. This can be accounted for by a combination of benthic filter feeders and zooplankton, both having been observed in sufficient numbers in the Thames.