Harmful cyanobacterial blooms dominated by Microcystis, producer of hepatotoxins, are increasing globally because of eutrophication and climate warming. Although both conditions promote cyanobacterial growth, their combined effects on natural multispecies phytoplankton communities remain poorly understood.
In this study, we investigated their combined effects in a bloom-prone area of Lagoa de Vela, Portugal. Communities were incubated under semi-continuous conditions for 30 days at two temperatures (18°C, 21°C) and two nitrogen phosphorus concentrations: balanced 8Redfield ratio (16:1) and 10 × nitrogen enrichment. Phytoplankton communities was assessed using microscopy, and community structure was analysed over time.
Higher relative abundance of Microcystis was observed only at 18°C in the balanced nutrient condition, although differences were not significant. No significant differences were found in phytoplankton community composition over time and treatments. However, toxic cyanobacteria dominate under initial high-nutrient conditions, whereas final equilibrium communities were more balanced, with lower cyanobacteria and greater chlorophytes. This likely reflects nutrient competition driving community structure. Further, Communities showed significantly lower species richness but significantly higher evenness (Pielou index). Therefore, Shannon diversity index was higher (although not significantly) for the final communities.
In conclusion, long-term cyanobacterial dominance may depend on factors beyond warming and eutrophication, including light availability and water-column stability.