How does rising atmospheric CO2 affect marine organisms?

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Aquatic Herbivores in a CO2-Enriched World of the Future
Urabe, J. and Waki, N. 2009. Mitigation of adverse effects of rising CO2 on a planktonic herbivore by mixed algal diets. Global Change Biology 15: 523-531.

The authors write that "the ongoing increase in atmospheric CO2 level is expected to increase primary production in both terrestrial and aquatic ecosystems," and that this increase is generally thought to be favorable to herbivores because it increases food availability. However, as they continue, elevated CO2 "often decreases the contents of key elements such as nitrogen (N) and phosphorus (P) relative to carbon (C)," and they say that this phenomenon "may have a negative affect on herbivore growth."

What was done
To explore this possibility, Urabe and Waki grew three algal species -- Scenedesmus obliquus (green algae), Cyclotella sp. (diatoms) and Synechococcus sp. (cyanobacteria) -- in mono- and mixed cultures at ambient (360 ppm) and high (2000 ppm) CO2 levels, and allowed a planktonic herbivore (Daphnia) to feed on the different algal populations thereby produced either individually or in various mixtures.

What was learned
The two researchers report that, as expected, "both in the mono- and mixed cultures, algal steady state abundance increased but algal P:C and N:C ratios decreased when they were grown at high CO2." Also as expected, they found that "Daphnia fed monospecific algae cultured at high CO2 had decreased growth rates despite increased algal abundance." But "when fed mixed algae cultured at high CO2, especially consisting of diatoms and cyanobacteria or the three algal species," they found that "Daphnia maintained high growth rates despite lowered P and N contents relative to C in the algal diets."

What it means
Urabe and Waki say their results imply that "algal diets composed of multiple species can mitigate the adverse effects of elevated CO2 on herbivore performance," and that "in environments with high CO2, herbivores may find a new diet producer or a combination of producer species to best meet their nutritional demands."

Interestingly, the advice that nutritionists often give to people -- to eat a wide variety of foods -- would appear to apply to aquatic herbivores as well (and maybe even terrestrial herbivores), especially in a high-CO2 world of the future. And maybe we should retain that wisdom too, for variety, it would seem, may be more than just the spice of life.

Reviewed 6 May 2009