How does pulsed stratification alter coastal primary and secondary production? A case study in Liverpool Bay

Liverpool Bay is a dynamic coastal sea that receives freshwater from 3 rivers, the Mersey, Dee and Ribble, and saltwater from the Atlantic Ocean. It is an important region for commercial fisheries and is a migratory route for spawning fish. But, Liverpool Bay is a fragile environment that has been exposed to anthropogenic inputs for >150 years. Under recent directives (European Commission), the Environment Agency (EA) and CEFAS monitor nutrient inputs, in an attempt to assess their impact in driving eutrophication and harmful algal blooms in coastal seas. In general, coastal seas are responsible for 25% of the global total primary productivity and are potentially an important and climate-sensitive sink for carbon. To understand the physical dynamics in Liverpool Bay related to tidal forcing and freshwater inputs, Proudman Oceanographic Laboratories, (Liverpool), are directing the Liverpool Bay Coastal Observatory: monthly, a 34-station grid is sampled in the bay and physical parameters, (e.g. temperature, salinity, light) are determined. We know that freshwater inputs and the tides drive oscillations between a mixed and stable water column, which strongly influences the distribution of nutrients and light, key requirements for phytoplankton growth. We now hypothesise that during periods of tidal-induced enhancement in water column stability, there is a short-lived and local increase in phytoplankton growth and fixation of carbon. However, this hypothesis remains untested. This studentship will directly assess the impact of alteration in nutrient and light distributions on primary production in a physical context within Liverpool Bay, thus creating lateral knowledge transfer between the University, the EA, and CEFAS. It will also provide a fundamental understanding of the region, needed to estimate impacts of climate change. Specifically, the student will examine how water column structure influences primary production, and ultimately assess the fate of carbon fixed. We consider two potential pathways for carbon loss. Carbon may enter the 'microbial loop' and be recycled by bacteria and lost as dissolved organic carbon via advection. Alternatively, carbon may accumulate in particulate matter and be grazed by mesozooplankton and lost via sedimentation, or to other pelagic organisms, thus providing a food source for commercially important higher trophic levels. The studentship consists of 3 hypothesis-driven themes, each ultimately leading to peer-reviewed publications. First, the student will participate in seasonal Coastal Observatory cruises to assess spatial coupling between food web structure (i.e. the relative distribution of phytoplankton, bacteria, and micro and mesozooplankton) and physical water column structure. In the second theme, the student will couple high-frequency observations of nutrient concentrations and phytoplankton composition, with real-time physical data generated from 2 fixed mooring sites (Smart Buoys, CEFAS) to assess and potentially develop capabilities to predict the water column properties necessary to stimulate phytoplankton blooms, especially harmful blooms, in Liverpool Bay. The third theme will assess the short-lived, rapid response of phytoplankton, zooplankton, and bacteria to changes in water column stability over a 25-h tidal cycle during stable (neap) and mixed (spring) tidal cycles. Throughout the three themes, there will be specific emphasis on the pathway of carbon transfer and loss. The student will be encouraged to generate a fourth component by inserting observational data into a 1-D model which can be used to further explore physical-biological coupling in the local, regional and international coastal environment. This research has academic and socioeconomic aspects; it will improve our understanding of the degree of coupling between physical and biological processes in Liverpool Bay, which will benefit coastal seas studies in general.