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

摘要

ABSTRACT: 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 phytoplankton-zooplankton (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.