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URI Chemical Oceanographer Analyzes the Effects of pH on Coastal Marine Phytoplankton


A largely overlooked but significant factor in marine ecology concerns the effects of variable pH on the growth rate and abundance of coastal marine phytoplankton, the base of the marine food chain in productive coastal waters. The pH of the open ocean varies very little. This has led to the common, but faulty, assumption that the pH of coastal waters also varies little and is unimportant.

In an article in a recent issue of the scientific journal Marine Ecology Progress Series, URI Graduate School of Oceanography marine scientist Dr. Kenneth Hinga analyzed all the available, albeit limited and scattered, studies conducted in the laboratory and in the field on the effect of pH on marine phytoplankton in coastal waters. pH is a measure of acidity or alkalinity of water.

The normal pH of open ocean seawater is about 8.1, or slightly alkaline. Hinga found that the pH of seawater in typical estuaries and coastal waters routinely varies from pH 7.5 to 8.5 with occasional occurrences of pH greater than 9 or less than 7. This is significant because pH levels routinely found in our coastal waters affect the growth rates of many species. At the more extreme pH values, some species cannot grow at all and only species with a tolerance for high or low pH would grow, eventually dominating the community.

Where a coastal ecosystem has a regular pH cycle, pH may play a role in determining the timing of occurrence of species and the total abundance of blooms and the amount of plant growth. Phytoplankton communities were found to fix carbon only half as fast at pH 9 compared to those at pH 8. In some experiments, the abundance of certain species of dinoflagellates (the phytoplankton group responsible for red tides) correlated strongly with high pH. The significance of pH variability may also be increasing as pH variability increases with increased loadings of plant nutrients to the coastal environments. Increased nutrient loading (from sewage treatment plants, the use of fertilizers, and from atmospheric deposition) to many coastal environments over the last century is well documented.

"The existing data suggest that scientists and coastal managers should not exclude pH as a factor in coastal marine phytoplankton ecology," said Hinga. "It seems probable that upon further study, pH will prove to be a non-trivial factor in phytoplankton dynamics in coastal environments."

The study was supported by the National Oceanic and Atmospheric Administration (NOAA) Coastal Ocean Program.

The URI Graduate School of Oceanography is one of the country’s largest marine science education programs, and one of the world’s foremost marine research institutions. Founded in 1961 in Narragansett, RI, GSO serves a community of scientists who are researching the causes of and solutions to such problems as acid rain, harmful algal blooms, global warming, air and water pollution, oil spills, overfishing, and coastal erosion. GSO is home to the Coastal Institute, the Coastal Resources Center, Rhode Island Sea Grant, the Institute for Archaeological Oceanography, and the National Sea Grant Library.

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File last updated: Thursday, February 6, 2003

Lisa Cugini | University of Rhode Island
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