Is Alfacs Bay a phytoplankton bloom incubator? In situ measurements and modeling approaches, open questions

Abstract

Alfacs Bay is a shallow microtidal estuary (Ebre Delta, NW Mediterranean) which receives freshwater inputs from adjacent rice field irrigation channels discharging from its northern coast and from groundwater seepage. The semi-enclosed geography of the bay along with the constant nutrient influx, favor a high productivity that has allowed for the development of valuable aquaculture activities. Unfortunately, recurrent harmful phytoplankton outbreaks (caused mainly by Alexandrium minutum, Dinophysis spp., Pseudo-nitzschia spp. and Karlodinium spp.) threaten this industry. The present study was aimed to improve our knowledge of the spatio-temporal variability of phytoplankton in the bay. Data have been collected for 6 years using synoptic cruises, time series of physical parameters, and combining them with 3D modeling. The cruises showed the existence of a preferential phytoplankton accumulation area in the NE interior of the bay. We hypothesis that the accumulation may be caused by the constant influx of nutrients from the rice fields in combination with the hydrodynamic regime which facilitates long residence times in this part of the bay. In this contribution we examine these issues using new modeling approach featuring a 3D hydrodynamic model in combination with a Lagrangian particle-tracking module. The results suggest that the flow regime may play an important part in facilitating bloom development in the bay's interior.