Modelling the Karenia mikimotoi bloom that occurred in the western English Channel during summer 2003

Abstract

Observations from space and in situ from the R.V. Corystes 8/03 Cruise show that a massive Karenia mikimotoi bloom occured during summer 2003 in the western English Channel. Due to exceptional climatoligical conditions that occured in June 2003, the installation of a very strong thermocline enhanced the development of a massive bloom over 1 million cells l⁻¹ in the Central English Channel. This paper presents the application of a mathematical model of this species, previously developed in for the Bay of Biscay, into a general 3D model of the primary production of the English Channel and southern Bight of the North Sea. Allelopathic interactions exerted by K. mikimotoi on other phytoplankton species and the role of agitation in the mortality of this species are taken into account. The model includes the dynamics of the bloom and consequently reproduces with good agreement the geographical distribution of the K. mikimotoi bloom both surface and subsurface. The model suggests that the apparent transport of the bloom towards the French coasts as inferred from the satellite observation was not due to advection but was only caused by the establishment of suitable conditions. The sensivity of the K. mikimotoi distribution to boundary conditions, initialization and the role of turbulence is discussed.

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.