Dinoflagellates encystment with emphasis on blooms in Boughrara Lagoon (South-Western Mediterranean): Combined effects of trace metal concentration and environmental context

Abstract

Resting cysts (RCs), vegetative cell abundance, sediment characteristics, nutrient and trace metal concentrations were investigated in fifteen stations at Boughrara Lagoon during October and November 2016. Twelve morphotypes of RC were recorded. The sampling sites showed a similar cyst assemblage trend but differed in total cyst abundance. The cysts of heterotrophic dinoflagellates dominated over the cysts of autotrophic dinoflagellates throughout the study area and were correlated to the abundance of Bacillariophyceae explained by a likely trophic relationship. The abundance of cysts was positively correlated to phosphorus concentrations measured in the sediment. As well, cysts of Polykrikos kofoidii and Scrippsiella trochoidea were significantly correlated to nitrogen concentrations in water column. The trace metal concentrations did not show any effect on cyst abundances in most cases, except for Cr on mixotrophic dinoflagellates, suggesting that metal contamination is not determinant in the encystment of dinoflagellates. Autotrophic dinoflagellates cyst abundance was dependent on sediment characteristics with the highest densities being recorded in muddy sediments. The hydrodynamics of the lagoon, characterized by a weak water circulation and a low water renewal in the South-Western zone, was considered among the environmental factors most affecting RC distribution. The dominance of potentially toxic species cysts highlights the necessity of monitoring these forms suspected to constitute the precursor of toxic blooms in this area.