Stability of marine phytoplankton communities facing stress related to global change: Interactive effects of heat waves and turbidity

According to climate models, coastal ecosystems will face an increased frequency of heat waves and increased turbidity due to terrestrial sediment run-off induced by increasing precipitation. Several studies have examined the effects of heat waves and turbidity separately, whereas this study analysed the individual effects of both stressors as well as their interaction, because stressors affect communities differently when acting in combination. Using a factorial experimental design, we simulated heat waves (22 degrees C and 26 degrees C compared to an 18 degrees C control) and turbidity (sediment addition). The response of the phytoplankton community was analysed for the aggregate parameters biovolume and diversity index (H'), as well as for community composition. Heat waves had a significant negative effect on biovolume, whereas turbidity tended to affect biovolume positively. Repeated measures ANOVA revealed significant interactions of heat waves and turbidity for H' and community composition. Strong heat waves (26 degrees C) alleviated the otherwise positive effect of turbidity on H', i.e. highest diversity remained in the turbid control. Diatoms gained dominance in the control and the 22 degrees C heat wave treatment with Cylindrotheca closterium being the successful competitor. At 26 degrees C this species was lost and small flagellates dominated the experimental communities. Future increases in heat wave intensity and frequency may thus induce major changes in phytoplankton community structure whereas algae might profit from increased turbidity as an additional source of nutrients.