Understanding the spatio-temporal variability of phytoplankton in aquaculture zones is necessary for the prevention and/or prediction of harmful algal bloom events. Synoptic cruises, time series analyses of physical and biological parameters, and 3D modeling were combined to investigate the variability of phytoplankton biomass in Alfacs Bay at basin scale. This microtidal estuary located in the NW Mediterranean is an important area of shellfish and finfish exploitation, which is regularly affected by toxic outbreaks. Observations showed the existence of a preferential phytoplankton accumulation area on the NE interior of the bay. This pattern can be observed throughout the year, and we show that it is directly linked to the physical forcing in the bay, in particular, the interplay between freshwater input and wind-induced turbulence. Both drivers affect the strength of the estuarine circulation, explaining nearly 75% of the variability in phytoplankton biomass. More cells are retained when stratification is weakened and the estuarine circulation reduced, while flushing rates are higher during times of increased stratification and stronger estuarine flow. This has been confirmed by using a 3D hydrodynamic model with Eulerian tracers. Nutrients, while important to support phytoplankton populations, have been found to play only a secondary role in explaining this variability at basin scale

This study was supported by the Spanish funded projects TURECOTOX (CTM2006-13884-C02-00/MAR) and ECOALFACS (CTM2009-09581), endorsed to the SCOR/IOC program GEOHAB (Global Ecology and Oceanography of Harmful Algal Blooms), and by ANERIS (PIF08-015) and the 2009 SGR 588 (Generalitat de Catalunya). M.L.A. and V.R. held FPU fellowships from the Spanish Ministry of Science and Education (SMSE), and N.Z.N, and FPI fellowship from the same Ministry. O.N.R. was supported through a Marie-Curie Intra-European Fellowship for Career Development (Grant agreement no. 255396) with project AQUALIGHT (www.aqualight.info). C.L. was supported by a CSIC fellowship (Beca CSIC Predoctoral I3P-BPD2005), and by a Postgraduate Grant of La Fundación Caja Madrid

Special issue Harmful Algal Blooms in Stratified Systems.-- 13 pages, 9 figures, 2 tables

Peer reviewed