How does rising atmospheric CO2 affect marine organisms?

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Sediment Deposition on the Great Barrier Reef
Umar, M.J., McCook, L.J. and Price, I.R. 1998. Effects of sediment deposition on the seaweed Sargassum on a fringing coral reef. Coral Reefs 17: 169-177.

The authors begin their paper with a brief review on the concern about coral reef degradation due to anthropogenic changes in water quality. In particular, they note that several human activities - such as deforestation, agricultural practices, coastal development, construction, mining, drilling, dredging and tourism - have been linked to increasing sediment load, which has in turn been found to cause noticeable degradation of coral reefs. Over one region of Australia's Great Barrier Reef, there is concern that such activities have led to increased sediment loads on inshore coral reefs. In this same region, the authors note a high degree of macroalgae, which they hypothesize may have accumulated in response to the higher sediment load on the inshore coral reefs, resulting in a decreased abundance of corals.

What was done
The authors investigated the quantitative effects of sediment load or thickness on the recruitment, growth, and survival/mortality rates of the macroalga S. microphyllum C. Agardh on the fringing coral reef in Geoffrey Bay, Magnetic Island, Australia (199'S, 14651'E).

What was learned
For three sampling periods (3, 9 and 15 months) the recruitment, growth, and survival rates of S. microphyllum were significantly lowered as a result of an increased sediment load that was twice that of normal conditions. Sediment removal, however, did not significantly increase these rates, suggesting that "the abundance of S. microphyllum is not significantly affected" by the sediment levels experienced at the study site.

What it means
According to the authors, the results of this study suggest that the abundance of S. microphyllum in areas with high sediment load "is not due to direct effects of sediments, but must be caused either by other factors correlated with sediments (i.e., nutrients), or indirectly by the effects of sediments on other organisms." The microalgae may therefore be merely "opportunistic beneficiaries" of the detrimental effects of sediments on other organisms, rather than being favored by high sediment loads.

Reviewed 1 June 1999