Bloom-Forming Microalgae in High-Species Phytoplankton Assemblages Under Light-Fluctuating and Low Phosphate Conditions

Wide changes of key environmental conditions prevented competitive exclusion in the phytoplankton assemblage of a turbid shallow wetland complex artificially managed for aquaculture production (Doñana Natural Park, SW Spain), and some genera showed specific adaptations to exploit those changes. The seasonal pattern of light, temperature and conductivity was frequently overridden by irregular events of sediment resuspension. Consequently, these shallow aquaculture ponds exhibited wide fluctuations of underwater light climate with clear-water, inorganic-turbid and phytoplankton-turbid phases that accommodated a variety of primary producers, such as submerged macrophytes, a soft-mud bottom phytobenthos and a phytoplankton assemblage of 72 recorded taxa. The algal biomass in the benthos was about 10 times higher than that found in the plankton (as estimated by chlorophyll a concentration) though the phytoplankton bloomed (>10⁶ cell ml⁻¹) on 10 occasions with four microalgae: Isochrysis sp. (Haptophyta), Diacronema sp. (Haptophyta), Nannochloris sp. (Chlorophyta) and Nephroselmis pyriformis (Chlorophyta). Several environmental conditions influenced microalgae both concurrently and in anticipation (with a time lag of 8–18 days), and this influence was different for both haptophytes with regard to Nannochloris sp. which significantly thrived when the water conductivity was above 12 mS cm⁻¹ (about 6.85 psu of salinity) and the water dissolved inorganic phosphate (DIP) concentration was >0.40 μM. In contrast, the density of Isochrysis sp. was significantly higher at a water DIP concentration