Eutrophication and sedimentation affect estuaries globally as catchments are developed by humans and sea levels rise. Estuaries are complex open systems with lag times between environmental cause and effect that may be longer than is identifiable using available instrumental data series which span fewer than 50 years. Managers are often faced with short term datasets and modelling studies to develop strategies to govern these systems which potentially do not capture the long term complex processes of the system. Longer term datasets which monitor environmental change may improve understanding of estuary biogeochemistry and ecosystem responses. This thesis uses palaeoenvironmental techniques to reconstruct the last ca. 150 years of nutrient and ecological development of Poole Harbour, southern England. There is established concern regarding eutrophication due to a range of human activities. Managers are challenged with setting limits to macronutrient supply that will not adversely affect the estuarine ecosystem, though unambiguous data supporting the process of eutrophication has yet to be presented. Results showed that agricultural practices post WWII increased sediment and nutrient delivery to the estuary, consequently increasing sediment accumulation rates and promoting algal growth. Pigment and geochemical evidence indicates eutrophication of the estuary post ca. 2000 AD. A water quality indicator proxy was developed which demonstrated a lag time of ca. 20-30 years between catchment drivers and water quality decline. The importance of lag times and step-change behaviour within the Poole Harbour system are stressed as being critical in the formulation of future management interventions.