Changes in the annual harmful algal blooms of Alexandrium minutum: effects of environmental conditions and drainage basin inputs in the Rance estuary (Brittany, France)

¹ IFREMER, Laboratoire Environnement littoral et Ressources aquacoles, Bretagne Occidentale, Place de la Croix, 29900 Concarneau, France
² IFREMER, Laboratoire Environnement littoral et Ressources aquacoles, Bretagne Nord, rue Port Blanc, 35800 Dinard, France
³ IFREMER, Laboratoire Environnement littoral et Ressources aquacoles, Languedoc-Roussillon, av. Jean Monnet, 34200 Sète, France

^a Corresponding author: clebec@ifremer.fr

Received: 22 April 2015
Accepted: 28 January 2016

Abstract

Time series of physico-chemical data and concentrations (cell L^-1) of the toxic dinoflagellate Alexandrium minutum collected in the Rance macrotidal estuary (Brittany, France) were analyzed to understand the physico-chemical processes of the estuary and their relation to changes in bloom development from 1996 to 2009. The construction of the tidal power plant in the north and the presence of a lock in the south have greatly altered hydrodynamics, blocking the zone of maximum turbidity upstream, in the narrowest part of the estuary. Alexandrium minutum occurs in the middle part of the estuary. Most physical and chemical parameters of the Rance estuary are similar to those observed elsewhere in Brittany with water temperatures between 15–18 °C, slightly lowered salinities (31.8–33.1 PSU), low river flow rates upstream and significant solar radiation (8 h day^-1). A notable exception is phosphate input from the drainage basin which seems to limit bloom development: in recent years, bloom decline can be significantly correlated with the decrease in phosphate input. On the other hand, the chemical processes occurring in the freshwater-saltwater interface do not seem to have an influence on these occurrences. The other hypotheses for bloom declines are discussed, including the prevalence of parasitism, but remain to be verified in further studies.