Karenia mikimotoi is one of the most common red-tide dinoflagellates proliferating in the eastern North Atlantic and around Japan. Kills of marine fauna are associated with its blooms. In mixed water columns it migrates vertically, while in stratified water columns, the population remains confined within pycnocline layers. Wind events, increasing mixing and agitation initiate declines in its populations. This paper is focused on the formulation of mortality rate relative to shear rate. Autotoxicity is demonstrated by the use of a synthetic toxin. Bioconvection observed in cultures allows the establishment of a trade-off between phototropism, which leads to the local accumulation of cells, and their autotoxicity, which would prevent cell concentration. The combination of these processes allows diffusion of the toxin into the underlying water, where it subsequently degrades. Confinement of the population in the pycnocline layer results also from another trade-off between growth conditions and shear-rate-modulated mortality. A simplified encounter kernel was introduced into the population dynamics equation to account for a mortality factor. Under realistic forcing conditions with a small number of parameters, this model reproduced the confinement of the population in the pycnocline layer, the proper timing and the duration of the recurrent K. mikimotoi bloom on the Ushant front (France).

International audienceThe occurrence of alkaline phosphatase activity (APA) that hydrolyses organic phosphorus into phosphate (PO4) is commonly related to PO4 deficiency of oceanic, coastal and fresh waters. APA is almost never investigated in PO4-rich estuaries, since very low activities are expected to occur. As a consequence, microbial mineralization of organic phosphorus into PO4 has often been ignored in estuaries. In this study, we examined the importance of potential APA and the associated microbial dynamics in two estuaries, the Aulne and the Elorn (Northwestern France), presenting two different levels of PO4 concentrations. Unexpected high potential APA was observed in both estuaries. Values ranged from 50 to 506 nmol L−1 h−1, which range is usually found in very phosphorus-limited environments. High potential APA values were observed in the oligohaline zone (salinity 5–15) in spring and summer, corresponding to a PO4 peak and a maximum bacterial production of particle-attached bacteria. In all cases, high potential APA was associated with high suspended particulate matter and total particulate phosphorus. The low contribution of the 0.2–1 μm fraction to total APA, the strong correlation between particulate APA and bacterial biomass, and the close relationship between the production of particle-attached bacteria and APA, suggested that high potential APA is mainly due to particle-attached bacteria. These results suggest that the microbial mineralization of organic phosphorus may contribute to an estuarine PO4 production in spring and summer besides physicochemical processes. © 2016 Association for the Sciences of Limnology and Oceanography

A new Fine Scale Sampler (FSS) was designed and used in the Loire River plume (Bay of Biscay, France) in order to study phytoplankton distribution in highly stratified water columns. The FSS consists of a linear array of 15 sampling bottles, set horizontally at 20 cm intervals. Data acquired from the CTD and fluorescence probe of the FSS enable it to be precisely positioned at the depth of the highest density gradient or at the chlorophyll a maximum. The FSS made it possible to investigate fine scale vertical distribution of phytoplankton and relationships with water density, nutrient concentrations or light availability, for spring and summer conditions. Under spring conditions, the entire upper layer exhibited a dominant diatom population (Chaetoceros sociale, Thalassiosira rotula) above 15 meters with evident uptake in dissolved inorganic nutrients and the maximum concentration of dinoflagellates (mainly small forms of Gymnodinium spp. and Gyrodinium spp.) was located 16 meters deep, where light energy began to be a limiting factor for phytoplankton growth. During summer conditions, the maximum phytoplankton biomass (10 µg l-1 of chlorophyll a) was only concentrated in the 2 m thick pycnocline layer, located at a depth of 12 m. The diatom Chaetoceros sociale was dominant in the phytoplankton peak. Dinoflagellates (Dinophysis acuminata and Gymnodinium spp.) were also concentrated around the density gradient. Low inorganic nitrogen concentration (< 0.2 µmol.l-1) was responsible for the limitation of phytoplankton production in the surface layer above the pycnocline, and the phytoplankton growth was irradiance-limited underneath the pycnocline.

The macrotidal estuary of Penzé (Brittany, Western part of the Channel, France) has been subjected to recurrent annual toxic blooms of Alexandrium minutum since 1988. This study aims to specify the phosphorus dynamics and bioavailability in sediments in order to improve our understanding of Alexandrium occurrences. Sediment-P pools and diffusive phosphate fluxes were studied under similar hydrodynamic conditions, in the intermediate estuary in . The results highlight a decrease in bioavailable phosphorus (iron and organic bound) from the inner part of the estuary seaward. The ratio of iron-bound phosphorus to ironoxyhydroxides is lower in the inner and intermediate estuaries (5-8) than in the outer site (15), suggesting a saturation of sorption sites and greater phosphorus bioavailability in this area. Pools of bioavailable phosphorus in surficial sediments are about eight times higher than the annual net-export of P (7 ton year -1 ). Phosphate releases from sediments are always lower than 5 lmol m -2 d -1 in March. The highest supplies occur in June and August in the intermediate area (up to 400 lmol m -2 d -1 ) where they represent up to 50% of river loadings. These results further suggest that phosphate pulses coincide with occurrences of Alexandrium reported in June.

a b s t r a c tThe speciation of particulate phosphorus (TPP) into inorganic (PIP) and organic phosphorus (POP), has been scarcely investigated in suspended particulate matter (SPM), contrary to numerous sediment studies. This work evaluates the actual capacity of methods for TPP, POP and PIP determination in SPM to measure efficiently and specifically each pool. Methods were tested on a range of plankton-associated organic and inorganic P compounds, on microalgal cultures and estuarine samples. Then results are specifically discussed reviewing P speciation in phytoplankton. For TPP analysis, the method of Solórzano and Sharp (1980) is recommended compared with that of Aspila et al. (1976), originally dedicated to sediment, as there is one step less, the protocol is more explicit (use of MgSO 4 ) and it provides the best results. The application of the original protocol of the Aspila method for TPP (no MgSO 4 ) results in underestimated and highly variable values. For POP, the CWO (Chemical Wet Oxidation) method tested here succeeds in providing sufficient oxidation of organic P, but, inorganic polyphosphate is yet too significantly hydrolysed, and intracellular phosphate, an underestimated PIP fraction, contributes to the POP pool with this method. Finally the Aspila method for PIP is the best compromise in estimating the contribution of PIP to TPP and POP by difference. Even if particulate polyphosphate is only partially hydrolysed (41-49%), intracellular phosphate is included and only a very low percentage of organic P, essentially nucleotidic P, is attacked. In any case, our findings show clear evidence of the necessity to fully describe the procedure when P speciation methods are used.