Eutrophication symptoms: the French Celtic Sea and the western English Channel

The French Celtic Sea and the western English Channel have seen the nitrate fluxes from Brittany to coastal waters increase ten times during the last four decades. Eutrophication has become obvious along beaches invaded by mass strandings of Ulva sp. (Ménesguen and Piriou, 1995).

Ulva sp. stranding and specimen

Ulva sp. Left: massive stranding (credit: A.Ménesguen). Right: individual specimen (credit: Wikimedia Commons).

Karenia Mikimotoi

Karenia Mikimotoi (credit: E.Nezan)

Also on the continental shelf, episodic massive blooms of phytoplankton (diatoms or sometimes dinoflagellates as the green Lepidodinium (e.g. Gymnodinium) chlorophorum; ICES 2000) occur in the plumes of the Loire and Vilaine rivers (Ménesguen, 1990), where they create massive organic settlements in bottom waters, which sometimes result in lethal anoxia events (Chapelle et al., 1994). The nitrogen enrichment of the coastal zone is also suspected to have triggered the toxicity of some phytoplankton species as the production of the amnesic shellfish poisoning (ASP) toxin by diatoms belonging to the Pseudo-Nitzschia genus. Karenia mikimotoï is another toxic dinoflagellate, which can produce red to dark-brown waters and induce widespread mortality events of wild fishes and benthic invertebrates (Jones et al. 1982). Blooms of this species may occur in eutrophication-free places (Holligan, 1979; Rodriguez et al., 2000), but man-made coastal enrichment of river plumes in nitrogen may also trigger coastal blooms of this species that turn out to be nuisance. In the Bay of Brest, they caused a loss of 4,000,000 individuals in scallop nurseries and culture trays (Erard-Le Denn et al., 2001), and along the French Atlantic coast, the mortality of 800-900 tons of mussels and many fishes coincided with an exceptional bloom of 48 million cells l−1 (Arzul et al., 1995), following very high flow rates of Loire river.

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Arzul G., Erard-Le Denn E., Belin C., N´ezan E., 1995. Ichtyotoxic events associated with Gymonodinium cf. nagasakiense on the Atlantic coast of France. Harmful Algal News, 2–3:8–9.

Chapelle A., Lazure P., Ménesguen A., 1994. Modelling eutrophication events in a coastal ecosystem. Sensitivity analysis. Estuar., Coast. and Shelf Sci., 39, 529-548.

Erard-Le Denn E., Belin C., Billard C., 2001. Various cases of ichtyotoxic blooms in France. In : Arzul G., (Ed.), Aquaculture environement and marine phytoplankton. 21-23 May 1992, Brest (France).

Jones K. J., Ayres P., Bullock A. M., Roberts R. J., Tett P., 1982. A red tide of Gyrodinium aureolum in sea lochs of the Firth of Clyde and associated mortality of pond-reared salmon. J. Mar. Biol. Assoc. U.K., 62: 771–782.

Holligan P. M., 1979. Dinoflagellate blooms associated with tidal fronts around the Brittish Iles. In : Taylor D. L., Seliger Howard H., (Ed.), International conference on Toxic Dinoflagellate blooms, 31 Oct. - 5 Nov. 1978, Key Biscayne (Florida), pp. 249–256. Elsevier, New York.

Ménesguen A., 1990. Eutrophication along the French coasts. in : "Eutrophication-related phenomena in the Adriatic Sea and in other Mediterranean coastal zones", Bart H., Fegan L. (eds), Proc. Conf. 28-30 May 1990, Rome (Italie), C.E.C. Water Pollution Research Report, 16, 63-82.

Ménesguen A., Piriou J.-Y., 1995. Nitrogen loadings and macroalgal (Ulva sp.) mass accumulation in Brittany (France). Ophelia, 42: 227-237.

Rodriguez F., Fernandez E., Head R., Harbour D., Bratak G., Heldal M., Harris R., 2000. Temporal variability of viruses, bacteria, phytoplankton and zooplankton in the western English Channel off Plymouth. J. Mar. Biol. Ass. U. K., 80: 575–586.