We report surface observations of a mesoscale coccolithophore bloom at the shelf break of the Patagonian Shelf during December 2008, representing the densest coccolithophore population in the Southern Ocean. The bloom was most intense within the Falklands Current, northeast of the Falkland Islands. Emiliania huxleyi dominated bloom waters, with a mixed E. huxleyi and Prorocentrum sp. dinoflagellate bloom to the west and mixed assemblage of diatoms, dinoflagellates, and flagellates to the east. Optical measurements of coccolith light scattering, analytical measurements of their calcite, and microscopic counts all showed this to be an intense coccolithophore bloom. Average particulate inorganic carbon per coccolith in the bloom was low, typical of the B coccolith morphotype and in agreement with independent measurements made by scanning electron microscopy. Highest particulate inorganic carbon (measured optically and chemically) was observed when residual nitrate (defined as the difference, [NO { 3 ] 2 [Si(OH) 4 ]) was 10-17 mmol L 21 and nitrate to phosphate ratios were close to Redfield values. Elevated particle backscattering was observed in the E. huxleyi bloom, whereas the highest particle scattering occurred in the adjoining Prorocentrum sp. bloom. Backscattering from coccolithophores represented up to 50% of the total backscattering (from organic and inorganic particles) along the main axis of the E. huxleyi bloom. Chlorophyll-specific absorption in the coccolithophore bloom was typical of marine phytoplankton. Residual nitrate plotted vs. temperature showed that the E. huxleyi bloom was associated with waters between 5uC and 15uC, with depleted silicate. Results suggest that previous drawdown of silicate by diatoms occurred prior to the densest E. huxleyi blooms over the Patagonian Shelf. We speculate that such conditions might also be important for annual development of the broader Great Calcite Belt and other coccolithophore blooms.

Coccolithophores are a critical component of global biogeochemistry, export fluxes, and seawater optical properties. We derive globally significant relationships to estimate integrated coccolithophore and coccolith concentrations as well as integrated concentrations of particulate inorganic carbon (PIC) from their respective surface concentration. We also examine surface versus integral relationships for other biogeochemical variables contributed by all phytoplankton (e.g., chlorophyll a and particulate organic carbon) or diatoms (biogenic silica). Integrals are calculated using both 100 m integrals and euphotic zone integrals (depth of 1% surface photosynthetically available radiation). Surface concentrations are parameterized in either volumetric units (e.g., m−3) or values integrated over the top optical depth. Various relationships between surface concentrations and integrated values demonstrate that when surface concentrations are above a specific threshold, the vertical distribution of the property is biased to the surface layer, and when surface concentrations are below a specific threshold, the vertical distributions of the properties are biased to subsurface maxima. Results also show a highly predictable decrease in explained‐variance as vertical distributions become more vertically heterogeneous. These relationships have fundamental utility for extrapolating surface ocean color remote sensing measurements to 100 m depth or to the base of the euphotic zone, well beyond the depths of detection for passive ocean color remote sensors. Greatest integrated concentrations of PIC, coccoliths, and coccolithophores are found when there is moderate stratification at the base of the euphotic zone.

Allelopathic interactions among phytoplankton are well documented. The potency of allelopathic species and responses of target species to allelochemicals are quite variable, however, limiting full understanding of the role these interactions may play in nature. One trait that may influence the sensitivity of an individual to allelochemicals is cell size. The few studies that have examined relationships between cell size and susceptibility to allelochemicals have compared different species and thus could not distinguish between the role of size and species-specific physiological differences. Culturing an actively sexually reproducing diatom allowed us to focus on the influence of target cell size within a single species. We studied growth and nutrient acquisition by the chain-forming Thalassiosira cf. gravida Clever in the presence and absence of allelochemicals released by Alexandrium fundyense Balech as a function of T. cf. gravida cell size. Upon exposure to filtrate of A. fundyense, T. cf. gravida cultures "bleached" and both growth and nutrient utilization ceased for up to 4 d. The magnitude of the effect was dependent on filtrate concentration and T. cf. gravida cell surface area:volume ratio. The greatest inhibition was observed on the smallest cells, while T. cf. gravida cultures that had undergone cell enlargement via sexual reproduction were least sensitive to A. fundyense filtrate. These results demonstrate that competitor cell size, independent from taxonomy, may influence the outcome of allelopathic interactions. The findings presented here suggest a potential ecological impact of diatom cell size reduction and sexual reproduction that has not yet been described and that may be important in determining diatom survival and success.

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