Decrease in diatom palatability contributes to bloom formation in the Western English Channel

The aim of this paper is to investigate the role of phytoplankton nutritional status in the formation of the spring bloom regularly observed at the station L4 in the Western English Channel. Using a modelling approach, we tested the hypothesis that the increase in light from winter to spring induces a decrease in diatom nutritional status (i.e., an increase in the C:N and C:P ratios), thereby reducing their palatability and allowing them to bloom. To this end, a formulation describing the Stoichiometric Modulation of Predation (SMP) has been implemented in a simplified version of the European Regional Seas Ecosystem Model (ERSEM). The model was coupled with the General Ocean Turbulence Model (GOTM), implemented at the station L4 and run for 10 years (2000-.2009). Simulated carbon to nutrient ratios in diatoms were analysed in relation to microzooplankton biomass, grazing and assimilation efficiency. The model reproduced in situ data evolutions and showed the importance of microzooplankton grazing in controlling the early onset of the bloom. Simulation results supported our hypothesis and provided a conceptual model explaining the formation of the diatom spring bloom in the investigated area. However, additional data describing the microzooplankton grazing impact and the variation of carbon to nutrient ratios inside phytoplanktonic cells are required to further validate the proposed mechanisms.

Modelling the Stoichiometric Regulation of C-Rich Toxins in Marine Dinoflagellates

Toxin production in marine microalgae was previously shown to be tightly coupled with cellular stoichiometry. The highest values of cellular toxin are in fact mainly associated with a high carbon to nutrient cellular ratio. In particular, the cellular accumulation of C-rich toxins (i.e., with C:N > 6.6) can be stimulated by both N and P deficiency. Dinoflagellates are the main producers of C-rich toxins and may represent a serious threat for human health and the marine ecosystem. As such, the development of a numerical model able to predict how toxin production is stimulated by nutrient supply/deficiency is of primary utility for both scientific and management purposes. In this work we have developed a mechanistic model describing the stoichiometric regulation of C-rich toxins in marine dinoflagellates. To this purpose, a new formulation describing toxin production and fate was embedded in the European Regional Seas Ecosystem Model (ERSEM), here simplified to describe a monospecific batch culture. Toxin production was assumed to be composed by two distinct additive terms; the first is a constant fraction of algal production and is assumed to take place at any physiological conditions. The second term is assumed to be dependent on algal biomass and to be stimulated by internal nutrient deficiency. By using these assumptions, the model reproduced the concentrations and temporal evolution of toxins observed in cultures of Ostreopsis cf. ovata, a benthic/epiphytic dinoflagellate producing C-rich toxins named ovatoxins. The analysis of simulations and their comparison with experimental data provided a conceptual model linking toxin production and nutritional status in this species. The model was also qualitatively validated by using independent literature data, and the results indicate that our formulation can be also used to simulate toxin dynamics in other dinoflagellates. Our model represents an important step towards the simulation and prediction of marine algal toxicity.