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Investigating the effects of nutrient concentration and light intensity on benthic biofilm development and phytoplankton growth in UK rivers.

Student thesis: Doctoral Thesis

  • Stephanie McCall
Phosphorus is often considered the limiting nutrient in rivers, and reducing concentrations has long been a key policy focus. However, experience has shown that many phosphorus mitigation schemes fail to achieve any improvements in ecological status. The primary aim of this thesis is to identify target phosphorus concentrations that need to be attained to improve river ecology.

In-stream flume mesocosms were deployed on three UK rivers of varying levels of nutrient enrichment. Phosphorus concentrations were simultaneously increased and decreased (by iron-dosing) to provide a concentration gradient over which periphyton accrual was examined. This identified whether rivers were phosphorus limited, and allowed a phosphorus-limiting threshold to be quantified. The effects of nitrogen, and combined phosphorus and nitrogen addition were also examined.

The River Lambourn study simultaneously manipulated light intensity and nutrient concentrations, demonstrating shading to have benefits in improving periphyton quality as a food resource and in reducing periphyton accrual. The near-pristine River Rede showed that increase in river phosphorus concentration had no effect on periphyton growth rate, but that there was a 3.5-fold increase in growth rate when phosphorus and nitrogen were added simultaneously, demonstrating the presence of sequential nutrient co-limitation. By repeating a 2005 study on the River Frome, the 2012 study proved that phosphorus-limiting thresholds change in response to changing river nutrient concentrations. Examination of changes in community structure by the trophic diatom index and flow cytometry provide evidence for a lower ecological threshold in rivers of ca. 30 μg l-1.

A fast repetition rate fluorometer was used to assess phytoplankton stress across the Thames catchment throughout an algal bloom and data was examined alongside water quality data. Bloom development and collapse was primarily controlled by residence time and, secondarily, phosphorus concentration. This thesis has challenged traditional beliefs that phosphorus is the limiting nutrient in freshwater ecosystems and has provided insights on how to best meet the requirements of the Water Framework Directive and improve ecological status.
Original languageEnglish
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Award dateApr 2014

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