Effect of high pH on the growth and survival of marine phytoplankton: implications for species succession

Ten years of pH measurements (1990 to 1999) in the surface waters of the eutrophic Mariager Fjord, Denmark, revealed profound seasonal variation. Typically, pH was relatively constant around 8 from January to March, increased during spring, reached maximum levels in July to August (9 to 9.7), and declined during autumn to about 8 in October. The influence of pH on the growth rate of phytoplankton was tested on 3 species (Ceratium lineatum, Heterocapsa triquetra and Prorocentrum minimum) in laboratory experiments. The growth rate was highest at pH 3.5 to 8.0 in all species. The growth rate of C. lineatum declined by similar to20% at pH 8.3 to 8.5, while a similar reduction in the growth rate in H. triquetra and P. minimum was observed at pH 8.8 to 8.9. C. lineatum stopped growing above pH 8.8, while growth ceased at about pH 9.45 in H. triquetra and 9.6 in P. minimum. Compilation of literature data on pH and phytoplankton growth suggested that while some species cannot grow at pH 8.4, others are able to grow up to pH 10. However, none of the species studied can attain their maximum growth rate above pH 9. Competition experiments using a mixture of C. lineatum, H. triquetra and P. minimum always resulted in the species with the highest pH tolerance (P. minimum) outcompeting the other species, irrespective of the initial pH value. The role of high pH in the succession of marine phytoplankton in nature is discussed.