Modulation of nearshore harmful algal blooms by in situ growth rate and water renewal

Species belonging to the genera Alexandrium and Gymnodinium are amongst the dinoflagellates that regularly cause massive coastal phytoplankton blooms along Mediterranean beaches. These episodes encompass a variety of factors favouring bloom development, including near-shore nutrient enrichment, enhanced growth and low water renewal. During the summer of 2003 the development of a bloom was monitored at 2 nearby beaches, Peguera and Santa Ponça, located at the head of Santa Ponça Bay (Mallorca). Both sites are under the influence of the same physical regime — which is mainly wind-forced — and present relatively high inorganic nutrient concentrations for Mediterranean waters during summer (mean dissolved inorganic nitrogen > 1.2 μM and PO4 > 0.18 μM). Total dinoflagellate abundance exhibited a similar trend at both beaches, with remarkable outbursts in late June (>8 × 106 cells l–1). Water exchange calculations, based on 3D numerical modeling, yielded low average renewal rates at both sites (<0.08 d–1), and cell growth estimations suggested a significant increase in the specific growth rates during the blooming season associated with the seasonal temperature variation. We postulate that both the increased growth rates and the low wind-induced water renewal times are complementary factors and are of key relevance to the modulation of these blooms. Diagnostic analyses using a simple phytoplanktonzooplankton (PZ ) model allowed us to observe the effect of growth rate and water renewal on bloom dynamics, and to identify a threshold condition for bloom occurrence.