From pico- to micro: variability of a spring bloom in an estuarine bay at a high temporal and spatial resolution

Abstract

Predicting the occurrence of Harmful Algal Blooms (HABs) in coastal ecosystems is crucial for their management. We followed up on the results of recent long-term data series and modelling studies of Alfacs Bay (Ebro Delta, Spain), an important aqua-/agricultural site and national park, recurrently threatened by HABs, to validate the previous results on a high spatio-temporal resolution, across a range of trophic levels. In 10 weekly cruises, we investigated the variability of the plankton community, including hetero- and autotrophic bacteria, nanoflagellates, and all sizes of phytoplankton as well as microzooplankton, sampling every meter of the shallow water column at a central station. Data on phytoplankton pigments and nutrients, as well as meteorological and hydrographic (using CTD and deployed sensors) parameters were recorded. Additionally, we conducted a 6-point-transect, taking CTD, nutrient and phytoplankton data. We discuss the high variability of the plankton community, the surprising importance of nanoflagellates, the influence of grazing and mixotrophy, the invasion of Mnemiopsis leydii and the culture of Mytilus edulis as possible factors for bloom development and evaluate how our data relate to the model-generated hypotheses