Systems off-balance? Exploring the cumulative impacts of enriched coastal waters, altered nutrient ratios and a warming climate on coastal phytoplankt

Coastal ecosystems are under unprecedented stress through eutrophication which can lead to harmful blooms and hypoxia. Legislative measures to reduce riverine nutrient inputs and preserve coastal ecosystem services have prioritised reductions in phosphorus shifting the N:P ratio and nutrient limitation characteristics. When an inorganic nutrient drops below its useable concentration, productivity is limited by the rate at which that nutrient can be recycled from organic matter or photosynthetic organisms can directly utilise dissolved organic nutrients.

To assess the degree to which increasing N:P is affecting phosphate limitation, dissolved inorganic N, P, Si and dissolved organic phosphorus will be determined in samples collected in the field. Alkaline phosphatase activity rates will also be analysed to determine utilisation of DOP by phytoplankton and bacteria. Combining these with nutrient add-back incubations will help us understand the role of nutrient limitation across the North Sea. Linking the results to associated salinity gradients will allow a detailed assessment of the status and impacts of phosphate limitation in the summer period, making use of the annual Cefas Endeavour summer North Sea survey cruise. Additional shorter sampling trips will be done throughout the year with Cefas, the Environment Agency and Marine Scotland to assess the seasonality of nutrient limitation at 2 contrasting locations - Stonehaven and the Thames.

A comprehensive dataset of riverine nutrient concentrations and offshore nutrient cycles in UK seas will also be compiled using data from the supervisory team and publicly available datasets we have tested previously. This will establish i) the degree to which N:P has changed over the past >5 decades in our rivers and seas and ii) how much this has affected the balance between nitrate and phosphate limitation. The student will also use existing satellite and ocean-based data for phytoplankton abundance and type to explore the possibility of using such data to indicate changes in phytoplankton response to N:P input or P limitation in existing data. Climate-related changes (temperature, circulation and mixing, Spring bloom dynamics etc) will also be considered in this analysis and could be further explored by biogeochemical modelling work.

The project will also involve the use of the Plankton Index, developed by Cefas and others, to assess changes in pelagic biodiversity of relevance to assessments under the MSFD and understanding potential shifts in food webs. This will provide a more holistic assessment of the impact of nutrient management on marine communities in a period of changing climate, and explore the cumulative impacts and interactions of climate and water quality. This will provide information about the potential impact of different nutrient management measures on national/international status assessments, shifting baselines and vital ecosystems goods and services.