Abstract/Summary

This thesis examines biological controls on phytoplankton in temperate lowland rivers on microscopic scale using the River Thames, a major UK river, as an example. The central part of the study explores river zooplankton-phytoplankton interactions in relation to physical environment, water chemistry and spatial patterns in the catchment. The zooplankton were surveyed weekly from 12 sites within the catchment (the Thames, its tributaries and Farmoor Reservoir) during spring-summer months in 2015, a year representative of the long-term seasonal low flow conditions. Six microcosm experiments were conducted to assess zooplankton grazing effect on phytoplankton diversity and abundance. They were supplemented with six laboratory experiments where the zooplankton were adjusted to replicate pre-bloom termination communities. The final part of the study looks at the relationships of phytoplankton, bacteria and chytrids* through experimental work involving incubation in thermal and low nutrients stress. Evidence was found that apart from water temperature, river flow and travel distance, zooplankton in the Thames are regulated by phytoplankton. In particular the presence of centric diatoms. It was also proposed that plankton may originate in certain tributaries of the Thames, especially those connected to canals, therefore the mixing of waters from these tributaries may be the key control on phytoplankton and consequently on zooplankton, rather than site-specific flow or water quality conditions. Microcosm experiments showed that zooplankton exert seasonal, site specific grazing effect on phytoplankton composition and abundance. Laboratory experiments reinforced the microcosms findings that physical environment is a stronger regulator of phytoplankton dynamics than zooplankton. Phytoplankton-bacteria-chytrids experiments revealed that both diatom metabolism and presence of attaching bacteria play an important role in diatom bloom termination and recycling. These results indicate a complex interplay between physical and biological environments in terms of nutrient availability and bacteria-diatom interactions. Further investigation is needed to unpick these complex relationships.