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The space-time dynamic of phytoplankton diversity and succession was investigated during the wane of a Phaeocystis globosa spring bloom in four distinct hydrological sub-systems of the eastern English Channel. Nutrients, chlorophyll-a concentrations, and phytoplankton composition, standing stocks, size spectra and diversity were monitored during three key periods in 2003: late spring, early summer and summer. Two consecutive diatom assemblages were observed, respectively dominated by: (i) small colonial species (,100 mm; Melosira sp., Diploneis sp. and Navicula transitans) in April; and (ii) large fine-walled cells (.200 mm; Guinardia striata and Rhizosolenia imbricata) in May and July. This shift in diatom composition appeared to be related to the potentially limitating silicic acid in early summer. Specific phytoplankton assemblages identified in distinct water masses have evolved from a mature/senescent community towards a relatively homogeneous aestival structure of dominant species that might have been triggered by the wane of the P. globosa bloom. Our results also identified a strong heterogeneity in the distribution of secondary species between distinct water masses during the summer period, suggesting that the magnitude of the observed patterns was intrinsically related to the hydrological properties prevailing in each sub-system. The identification of distinct temporal patterns in phytoplankton species diversity and succession following the wane of a spring bloom at relatively small spatial scales (i.e. ,10 km) is discussed in the framework of P. globosa blooms in particular and phytoplankton blooms in general and is suggested to have potentially strong consequences on food web dynamics and the carbon cycle in coastal ecosystems.

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International audienceHigh-frequency temperature fluctuations recorded in the English Channel are compared using two long-term autonomous underwater monitoring stations at less than 20-min time resolution. Measurements were taken from 2005 to 2011 from two systems 460 km apart in the western and eastern parts of the English Channel. Spectral analysis reveals similar statistical behaviors, with approximate 5/3 spectra and several forcing frequencies in relation to tidal and daily cycles. A co-spectra study reveals a transition scale of 11 days. The influence of this scale is also visible though Time-Dependent Intrinsic Correlation analysis (TDIC)–a recently introduced cross-correlation analysis based on Empirical mode decomposition. This helps to spatialize high-frequency temporal records at a fixed location

A new automated procedure for nanomolar nitrate analysis was applied to the study of uptake of low nitrate concentrations (< 100 ngat l-') by phytoplankton. The precision of this analytical method (+3ngatl-') ma d 't e 1 possible to monitor the absorption of very low quantities of nitrate over short term periods by a low cell-density culture of the marine diatom Phaeodactylum tricornutum, where the levels of particulate nitrogen and chlorophyll were equivalent to those found in oligotrophic areas (0.5 pgat N I-* and 0.4 pg l-' respectively). By continuous monitoring of nitrate disappearance from the culture medium, we are able to describe accurately the transient uptake responses of the diatom after a spike addition of trace quantities of nitrate and thus to provide new information on the still largely unknown small-scale phenomenon of pulsed nitrate supply in the upper layer of stratified oceans and rapid uptake of these nitrate patches by phytoplankton.The results show that a N-limited culture of Phaeodactylum tricornutum is immediately capable of taking up trace quantities of nitrate (-c 100 ngat l-') at high rates (0.10-o. 14 h-'). These initial rates are one order of magnitude higher than the theoretical rates calculated from the Michaelis-Menten equation and are close to the level of V,,, (0.15 h -') obtained when cells are exposed to saturating nitrate concentrations. This rapid initial uptake would be a considerable advantage in oligotrophic areas where nanomolar nitrate supply is thought to be episodic. The present results suggest that phytoplankton evolve adaptations to utilize the available nitrate at the spatial and temporal scales at which it occurs. On the other hand, we can consider this physiological adaptation as evidence of nitrate pulses in the field which would invalidate the steady-state approach to the oligotrophic ecosystems.

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