Tolerance to high irradiance levels as a determinant of the bloom-forming Heterosigma akashiwo success in estuarine waters in summer

The effect of irradiance, temperature and salinity was studied on the growth rates of two strains of the bloom-forming Heterosigma akashiwo isolated from the Nervión-Ibaizabal estuary (Bay of Biscay). Growth rates (μ) were measured at varying conditions of salinity (5–35), irradiance (90–1200 μE m−2 s−1), temperature (10–25 °C) and photoperiod (10L:14D–14L:10D). The response to these factors was compared with the observed distribution of the algae in the estuary. Irradiance was the factor that induced more variations on H. akashiwo growth rates, showing a positive correlation with the presence and abundance of the algae in the estuary. At irradiances higher than 350 μE m−2 s−1 H. akashiwo adjusted the concentration of its photoprotective pigments (mainly violaxanthin). In addition, these strains are able to grow at irradiances as high as 1200 μE m−2 s−1 without experiencing photoinhibition. H. akashiwo also seems to be efficient at low irradiances as it was deduced from Ic values (∼90 μE m−2 s−1). This response is in agreement with the summer H. akashiwo presence in the estuary. Highest temperature would affect positively cell survival at low, suboptimal salinities (5, 10). However, there was no correlation between temperature and H. akashiwo presence. H. akashiwo bloomed at salinities 30–35 in protected areas of the Nervión-Ibaizabal estuary, while culture salinity optima was found between 20 and 25. Thus, during summer, salinity would be a growing determinant for H. akashiwo only at suboptimum levels and other factors would be responsible for blooms at salinities 30 and 35. Photoperiod would not to have any significant effect on its growth rates.