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Twenty-one studies on the effects of pH on marine phytoplankton were found and are herein reviewed. Under laboratory conditions, the optimum pH for growth is between pH 6.3 and 10. Some species can grow well at a wide range of pH, while others have growth rates that vary greatly over a 0.5 to 1 pH unit change. Different clones of the same species were found to have slightly to strikingly different relationships between pH and growth rate. The pH in typical coastal environments may vary by 1 or more pH units, with over 10% of observations being more than 0.5 units above or below equilibrium pH. This range is great enough, relative to the observed pH effect on growth rate for many species, for seawater pH to affect the growth rate, and hence the timing and abundance of coastal marine phytoplankton species. Effects of pH are not limited to extreme pH conditions. The growth rates of some species are influenced significantly by changes in pH near the equilibrium pH of coastal seawater. Care must be taken in growth experiments with phytoplankton to avoid effects due to pH of the culture media. Eutrophication of coastal waters may amplify the range of pH found in coastal environments.