Reddish-brown bloom of the dinoflagellate Prorocentrum cordatum in the brackish lake “Lago delle Nazioni” (Comacchio, FE)

Transitional Waters (TWs) include lagoons, saline lakes, river estuaries and deltas, and are highly dynamic and heterogeneous systems, widely distributed in the Italian Peninsula and strongly used for human exploitation. The monitoring program proposed by the European Water Framework Directive (WFD, 2000/60/EEC) represents an important assessment of the ecological status, and an increased knowledge of the existing variability is a key point for the conservation and the management of these aquatic systems. The TWs investigated in this study are the Sacca di Goro and the Lago delle Nazioni (Ferrara, Emilia-Romagna). The Lago delle Nazioni (44°46′10.74″N, 12°14′30.39″E) is a semi-artificial big brackish lake, located besides the seaside town of Lido delle Nazioni. In the ‘60s, the basin was exploited for touristic and sporting purposes; to date it hosts experimental breedings of juveniles of the clam Ruditapes philippinarum (Mollusca, Bivalvia, Veneridae). The lake’s water monitoring raised outstanding interest along the study, especially in terms of trophic index and quali-quantitive composition of phytoplankton assemblages. During the last winter (December 2019-January 2020) an exponential reddish-brown discoloration of the water caused by an intense bloom of the dinoflagellate Prorocentrum cordatum was observed. The cells reached a density of 4.7 x 107 cells L-1, which amounted to 99% of the phytoplankton community, and corresponded to a Chl a peak of 251 μg L-1. As shown by water analysis, the bloom occurred with low concentrations of N and P (avg. 5.5 μg L-1 and 5.7 μg L-1, respectively), resulting in a N:P ratio far lower than the Redfield one, ranging from 0.3 to 5. The evident lack of dissolved nitrogen led to hypothesize that mixotrophy and, presumably, organic matter inputs from the sediment or the land, could play an important role in driving the bloom. Since the last decades of the 20th century, P. cordatum has been linked to harmful effects on bivalves (Leibovitz et al., 1984); moreover, recent studies have reported the presence of tetrodotoxins (TTXs) in European bivalve molluscs in correlation with blooms of Prorocentrum (Vlamis et al., 2015). Therefore, this strain was isolated in the laboratory and thereafter LC-MS analysed by the Marine Research Centre of Cesenatico, with the result that no TTXs were detected. In addition, no toxic effects and mortality for juvenile shellfish were reported; on the contrary, a positive empirical correlation between the presence of P. cordatum and the health of R. philippinarum was noted during the winter bloom. Further studies are in progress to evaluate the algal life cycle and the potential relationship with other microorganisms; at the same time the lake’s monitoring program should be pursued to better understand the phytoplankton community’s dynamics and occurrence in time.