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Elevated CO2 Enhances Phytoplankton NPP in a Freshwater Microcosm Experiment

Paper Reviewed
Shi, X., Li, S., Wei, L., Qin, B. and Brookes, J.D. 2017. CO2 alters community composition of freshwater phytoplankton: A microcosm experiment. Science of the Total Environment 607/608: 69-77.

The impact of rising atmospheric CO2 on freshwater ecosystems has been much less studied than for marine ecosystems. Nevertheless, concern still exists with regard to the potential impacts of freshwater acidification, which is the topic of investigation in a recent study by Shi et al. (2017).

In a microcosm in situ experiment carried out at the East Lake Taihu Ecosystem Station (31.07°N, 120.43°E) at the third largest freshwater lake in China, the five researchers examined the seasonal (spring and summer) effects of three CO2 levels (270, 380 and 750 ppm, designed to mimic preindustrial, current and future predicted values) on phytoplankton growth. CO2 depletion/enrichment during both seasons was applied for a period of 27 days.

Results indicated that pCO2 decreased rapidly with the growth of phytoplankton in both the spring and summer, which decline, according to the authors, "indicated that CO2 was limiting during the phytoplankton bloom, under all treatments, and was not sufficient to meet the need for phytoplankton growth." The authors found this observation somewhat surprising, given that prevailing wisdom posits that CO2 levels in lakes are generally supersaturated and therefore CO2 has been thought to play only a minor role as a limiting factor in primary production compared to nitrogen and phosphorus. Nevertheless, despite this consensus of thought, CO2 enrichment was observed by Shi et al. to have significantly increased net primary productivity (NPP) in the summer. At the end of the 27-day summer experiment, for example, NPP in the high CO2 (750 ppm) treatment was 35 % greater than that in the current (380 ppm) treatment and a whopping 130% greater than that found in the low (270 ppm) preindustrial CO2 treatment. In contrast, NPP did not show any significant difference among treatments in the spring experiment, although the authors report that the "growth of phytoplankton was slightly stimulated by an increase in CO2 until the end of the [spring] experiment."

Given the above findings, it would appear that freshwater phytoplankton, contrary to conventional wisdom, are presently carbon-limited (at least for this lake setting) and will likely benefit from so-called freshwater acidification by enhancing their NPP as atmospheric CO2 concentrations continue to rise in the years and decades ahead.

Posted 2 March 2018