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Annual plankton succession has been investigated for many decades with hypotheses ranging from abiotic to biotic mechanisms being proposed to explain these recurrent patterns. Here, using data collected by the Continuous Plankton Recorder (CPR) survey and models originating from the MacroEcological Theory on the Arrangement of Life, we investigate Annual Phytoplankton Succession (APS) in the North Sea at a species level. Our results show that this phenomenon can be predicted well by models combining photosynthetically active radiation, temperature and macro-nutrients. Our findings suggest that APS originates from the interaction between species’ ecological niches and the annual environmental fluctuations at a community level. We discuss our results in the context of traditional hypotheses formulated to explain this recurrent pattern in the marine field.

In this study, we examined the possibility of using the FluoroProbe for monitoring the dynamics of the Haptophyte Phaeocystis globosa in the coastal waters of the eastern English Channel. The FluoroProbe was recalibrated by recording a new fingerprint for P. globosa and the use of this new fingerprint was tested through a series of laboratory and in situ experiments. The annual dynamics of P. globosa estimated using the FluoroProbe and by flow cytometry were similar. A strong relationship was found between the FluoroProbe estimates of P. globosa biomass expressed in terms of chlorophyll a equivalent per litre (eq. mg L 21 ) and flow cytometric cell counts (r ¼ 0.889, P , 0.001, n ¼ 121). The FluoroProbe can be used to detect the flagellated cells as well as the colonial cells of P. globosa but not to distinguish these two cell types in mixed assemblages. The use of the new fingerprint recorded for P. globosa improved the detection of Isochrysis sp. This suggests the possibility of using the FluoroProbe to monitor Haptophytes other than P. globosa by calibrating the device with species representative of the region of interest. However, it is important to note that the detection of P. globosa at the species level was possible in the eastern English Channel because it was the only Haptophyte species present with a biomass sufficient to be detected by the FluoroProbe. In areas where several Haptophyte species are simultaneously present, their discrimination will be impossible and in such situations the FluoroProbe can be used to monitor the dynamics of the combined Haptophyte group.

In theoretical and field pnrnary productlon studies, much interest is currently focused on the influence of apenodic vertical mixing generated at the surface by wlnd speed and/or heat flux. In the present work, a Lagrangian random walk model was used to study the interactions between penodic vertical tidal mixing and both photoadaptation and primary productlon of phytoplankton, in typical shallow coastal waters, such as the eastern English Channel. The model considers a depthdependent d~ffusion coefficient fluctuahng according to the high-low tidal cycles and neap-spring tidal cycles, water columns of different euphotic zone and mixed layer depths, and photoresponse time constants of natural phytoplankton populations collected in the eastern English Channel. Cells were allowed tn light-shade adzpt, accordizg to the vertical mixing tune scdles, by aitenng their photosynthetic properties in response to vanations in light. The simulation results indicate first that vertical tidal mixing could control photoadaptation processes at the scale of the high-low tidal cycles at spring tlde, and at the scale of neap-spring tidal cycles in shallow coastal systems. Secondly, it appears that the decreasing vertical mixing intensity between spnng and neap tide condihons is responsible for a significant increase in daily primary production rates, despite the occurrence of photoinhibition at neap tide. Therefore, primary production in coastal seas would b e a function not only of light and nutrient concentrations, but also of photoadaptation processes In relation with vertical tidal mixing In another way, the Lagrangian model suggests that the theory according to which cells are adapted to the mean light intenslty of a water column in a turbulent regime is valid only from a populational point of view. From the model used, ~t appears also that our present knowledge on photosynthetic dynamic modeling is unsuited to generating pronounced vertical grad~ents of photosynthetic parameters In all water columns.

The distribution of phytoplankton (from pico-to microphytoplankton) was investigated, at single-cell level and at high spatial resolution, during an oceanographic cruise across the eastern English Channel (EEC) between April 27 and 29, 2012. Seawater was continuously collected from surface waters and analysed on board at high frequency (one sample every 10 min), by using a new generation of pulseshape recording scanning flow cytometer (CytoSense, Cytobuoy©). A Bray-Curtis matrix analysis based on phytoplankton composition allowed the discrimination of 4 communities. Within these communities, abundance, cell size as well as single cell and total red fluorescence of 8 phytoplankton groups were measured. Picoeukaryotes and Synechococcus spp. cells dominated the mid Channel and most of the English waters monitored, whereas waters off Eastbourne as well as French coastal waters (under remote and direct estuarine influence) were characterized by the dominance of Phaeocystis globosa haploid and diploid cells. Most of the total red fluorescence signal, which correlated with chlorophyll a concentrations, was attributable to P. globosa and, to a lesser extent, to diatoms. In addition to submesoscale variation within phytoplankton communities, the single-cell features within each phytoplankton group gave information about the physiological status of individual phytoplankton cells.

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