The size spectrum of phytoplankton: a key parameter for water quality and ecosystem function.

The diverse group of organisms forming the phytoplankton represents just 2% of global photosynthetic biomass, but due to their high growth rates they contribute almost a half of annual global carbon fixation. The phytoplankton can be divided into different functional groups according to their size, taxonomic composition and biochemical properties. Size is an important parameter in terms of carbon turnover, nutrient uptake and efficiency of trophic transfer in the marine food web. Three main size categories have been adopted for the phytoplankton: picoplankton (smaller than 2 microns), nanoplankton (between 2 and 20 microns) and net plankton (larger than 20 microns). Generally in routine monitoring programmes, only the larger nanoplankton and net plankton are taken into account. Thus, the existing paradigm is that the annual production cycle of phytoplankton in U.K. coastal waters commences with a spring bloom of diatoms, followed by summer dominance of dinoflagellates. However, advances in optical and molecular techniques in the last few years have demonstrated that pico- and nanoplankton can be very abundant, representing for example 98% of the productive biomass in the Southern Ocean. Recent cruises to the central North Sea in 2007 have used pigment analysis to show that phytoplankton functional types (PFT) such as chrysophytes, prymesiophytes, diatoms and picocyanobacteria can all be dominant at different times (CEFAS, unpublished). Today, the improvements in commercially available technology allow us to quickly count and identify, from division to species level, a large numbers of pico and nanoplankton samples. In this proposal we will use cytometric analysis together with pigment fingerprinting and molecular tools to advance our knowledge of phytoplankton ecology in coastal waters. For this PhD studentship our aims are to assess the natural variability in phytoplankton functional types at a range of spatial and temporal scales, to relate PFT distribution to environmental parameters and to test whether the observed patterns of distribution can be explained by existing theories. In the first part of the study the student will be familiarised with the techniques. Flow cytometry is now a well-established method for counting and analysing phytoplankton according to their shape, size and pigment composition. Instruments are available at both UEA and Cefas. The primary instrument to be used for this work is the CEFAS CytoSense (Cytobuoy, BV.) which is a versatile, portable bench-top flow cytometer capable of measuring cells from 1 to over 500 microns in length. Phytoplankton cultures will be used to establish a library of cytograms to be used as a reference for natural samples. Fluorescence hybridisation techniques (COD-FISH, TSA-FISH) will be developed to determine the proportion of the main functional groups in the pico and nanoplankton in the different water bodies of the study. Additionally, the measurement of accessory pigments will reinforce the information available for the identification of the different functional groups. These combined techniques will allow the student to pursue the second part of the study which will involve classifying the naturally occurring phytoplankton communities. This will necessitate appropriate training in multivariate statistics. The work will be done across large spatial scales in U.K. shelf seas from coastal waters to the open ocean, and with a high level of replication. Several opportunities will be available to the student to access different sampling areas from the North Sea, Irish Sea and Celtic Sea (e.g. the CEFAS RV Endeavour Ground Fish Surveys), to receive samples from localities along the East coast of England every month (from the Environment Agency's Water Framework Directive monitoring programme). The geographical variation will be coupled by a temporal variation of measurements from daily water sampling with an automatic system (SmartBuoy network) maintained by CEFAS.