Modelling the Karenia mikimotoi bloom that occurred in the western English Channel during summer 2003
Introduction
Harmful algal blooms have occured for many centuries in many pelagic ecosystems, before the possible influence of significant human activity, however, during the last decades these events have increased in number, areal distribution and biomass Anderson, 1997, Chrétiennot-Dinet, 1998, Riegman, 1998, Granéli, 2004. They have many forms and effects (Anderson, 1997), impressive primary production by non-toxic algae can provoke a temporary oxygen deficiency when decaying and strongly disturb the ecosystem. This primary production can often occur during a short period. A number of other species, for example Dinophysis, are poisonous at low concentrations. Estimates of the number of phytoplanktonic species that contain toxins varies from 60–78 (Sournia, 1995) to 100 (Granéli, 2004). The majority of these species belong to the Dinophyceae class (Sournia, 1995). Among them, the dinoflagellate Karenia mikimotoi is observed in all of the oceans and particularly in the coastal waters of northern Europe. This species was previously described as Gyrodinium aureolum, Gymnodinium cf. n agasakiense and Gymnodinium mikimotoi. Karenia G. Hansen and Moestrup is a new genus defined by Daugbjerg et al. (2000). It is responsible for the red to dark-brown discoloring waters when the density reaches over 1 million cells l−1. Effects on marine fauna are measurable above a few million cells per litre (Gentien, 1998). The production of high rates of viscous extracellular polysaccharides can cause asphyxiation in fish, a property called “rheotoxicity” Jenkinson and Arzul, 1998, Jenkinson and Arzul, 2001. Widespread mortality events of wild fishes and benthic invertebrates were observed along the English south coast in 1978 and off the southwest Ireland in 1976 and 1978 (review of Jones et al., 1982). The economic consequences of fish kills due to red tides can be significant, e.g. fish farms in Scottish lochs in September 1980 (review of Jones et al., 1982), 3546 tonnes of caged fish were killed in Hong-Kong Bay in 1998 Hodgkiss and Yang, 2001, Yang and Hodgkiss, 2001. In 1985, the occurence of a bloom at 800,000 cells l−1of K. mikimotoi in the Bay of Brest caused a loss of 4,000,000 individuals in scalop nurseries and culture trays (Erard-Le Denn et al., 2001). Along the French Atlantic coast, a mortality of 800–900 tonnes of the mussel M ytilus edulis (Gentien, 1998) and many fish in 1995 coincided with an exceptional bloom of 48 million cells l−1(Arzul et al., 1995). Exotoxin production can also affect the growth of other algae, this allelopathic effect has been demonstrated in many phytoplankton groups, for example on the diatom Chaetoceros gracile in the Ushant Front (Arzul et al., 1993) and on a natural population of dinoflagellates (Fistarol et al., 2004).
In the western English Channel, blooms of this species have often been observed in the Ushant Front Pingree, 1975, Holligan, 1979, Holligan et al., 1984, Garcia and Purdie, 1994 and in the seasonally stratified region which extends from the central western English Channel to the coast of Cornwall Le Corre et al., 1993, Rodrìguez et al., 2000. These monospecific blooms can reach many millions of cells per litre and represent up to 100 mg m−3of chl-a (Holligan, 1979).
During summer 2003, spectacular sea surface chlorophyll a (SS-Chla) concentrations were observed from space by ocean color images. Observations from the R.V. Corystes Cruise 8/03 from 26 June to 9 July 2003 in the western English Channel highlighted that these high chlorophyll concentrations were due to a monospecific bloom of K. mikimotoi. The exceptional characteristic of this event is visible in Fig. 1 (SS-Chla derived from SeaWiFS), while blooms are common in this area, the 2003 bloom started much earlier and reached a much greater cell density (up to 1,000,000 cells l−1).
A modelling study of K. mikimotoi blooms in the context of the Bay of Biscay was first developed by Loyer (2001) and Loyer et al. (2001). The goal of this work is to introduce the Loyer’s K. mikimotoi submodel in a regional ecosystem model of the Channel and southern North Sea, in order to test the robustness of this model in another ecosystem which is hydrodynamically and hydrologically different from the Bay of Biscay. The model is also used to evaluate the importance of each process in the K arenias’s dynamics and the sensitivity of the model to some parameters.
Section snippets
Description of the study site
The English Channel is part of the Northwest European continental shelf, it connects the Atlantic Ocean to the North Sea (Fig. 2). Its boundaries are normally defined as the Dover Strait in the east, with the western end marked by the Isles of Scilly (UK) to Ushant (France). This system receives significant freshwater and nutrient inputs, in the east, from the river Seine (mean flow about 600 m3 s−1). However, the rivers that discharge into the western English Channel contribute very little to
The model
During the last decades, the enhanced computer capacities allowed to build sophisticated models in terms of spatial refinement and biochemical complexity. 3D models are useful for the study of harmful algae events which are associated to local imbalance of nutrients or/and to local specific hydrodynamical structure. Ecological models are developped to study the dynamics of macroalgae (e.g. Zostera marina in Mediterranean lagoons, Zharova et al., 2001, Plus et al., 2003, Pastres et al., 2004;
Validation data
A large data set was available for the validation of the model. The stations used are displayed in Fig. 2. The CEFAS research vessel Corystes (Cruise 8/03) provides an extensive source of validation with numerous CTD profiles, surface samplings and scanfish sections collected during from the 26th June to 9th July. Samples for phytoplankton analysis were taken from discrete depths at each CTD station by preserving 55 ml sub-samples withdrawn from the CTD rosette sampling bottles, in acidified
Results
The performance of the model was statistically evaluated by linear regressions between observed versus computed data. For each parameter, the slope (a) and the ordinate at origin (b) of the regression are indicated. The outputs are saved every 4 d at midday.
Global characteristics of the model
Compared to other recents ecosystem models developped in this area, ECO-MARS3D appears as a compromise between biogeochemical and hydrodynamical complexity. The phytoplanktonic compartment is described by three functionnal groups and one dinoflagellate species. Some marine ecosystem models still simulate one aggregated state variable for phytoplankton (e.g. Tuchkovenko and Lonin, 2003). The top-down control is also quite finely represented with two class of size of zooplankton. According to the
Conclusion
A species specific model for K. mikimotoi has been developped and inserted in a 3D model of primary production. Such refined ecosystem models focused on a species of interest are few and mainly focused on microalgae (e.g. Phaeocystis, Lacroix et al., 2007) or macroalgae (e.g. Ulva, Ménesguen et al., 2006) responsible for eutrophication disturbance.
The sensitivity of K. mikimotoi to agitation was taken into account through the shear rate () in the mortality calculation as first suggested by
Acknowledgements
This study was part of a project managed by Alain Lefevre (IFREMER/LER, Boulogne-sur-mer) and funded by IFREMER, the Région Nord-Pas de Calais and the national program ‘PNEC-Chantier Manche orientale’. We thank Franck Dumas for the improvement of the circulation model MARS3D for this study. We are indebted also to Sophie Loyer for her advice concerning the K. mikimotoi submodel and her comments on the paper. The authors would like to thank Dr Matthew Frost from the Marine Environmental Change
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