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pCO2 effects on species composition and growth of an estuarine phytoplankton community.
Grear, J., T. Rynearson, A. Montalbano, B. Govenar, AND S. Menden-Deuer. pCO2 effects on species composition and growth of an estuarine phytoplankton community. ESTUARINE, COASTAL AND SHELF SCIENCE. Elsevier Science Ltd, New York, NY, 190:40-49, (2017).
Estuarine water quality models being developed and parameterized by ORD typically attempt to capture seasonal and daily dynamics of phytoplankton. This is because photosynthetic and metabolic activity of these phytoplankton drive the response of water quality parameters like dissolved oxygen and pH to external loadings of nutrients and organic and inorganic carbon. However, there is now concern that phytoplankton dynamics, particularly as they are represented in these models, are undergoing change due to ocean and coastal acidification. This manuscript reports results from experimental manipulations of pH in natural seawater and observations of impacts on cell size abundance, growth, and coarse composition of the phytoplankton community. These characteristics are represented in phytoplankton compartments of EPA water quality models, so our results will impact the use and interpretation of water quality models to support nutrient abatement decisions.
The effects of ongoing changes in ocean carbonate chemistry on plankton ecology have important implications for food webs and biogeochemical cycling. However, conflicting results have emerged regarding species-specific responses to pCO2 enrichment and thus community responses have been difficult to predict. To assess community level effects (e.g., production) of altered carbonate chemistry, studies are needed that capitalize on the benefits of controlled experiments but also retain features of intact ecosystems that may exacerbate or ameliorate the effects observed in single-species or single cohort experiments. We performed incubations of natural plankton communities from Narragansett Bay, RI, USA in winter at ambient bay temperatures (5–13 °C), light and nutrient concentrations under three levels of controlled and constant CO2 concentrations, simulating past, present and future conditions at mean pCO2 levels of 224, 361, and 724 μatm respectively. Samples for carbonate analysis, chlorophyll a, plankton size-abundance, and plankton species composition were collected daily and phytoplankton growth rates in three different size fractions (20 μm) were measured at the end of the 7-day incubation period. Community composition changed during the incubation period with major increases in relative diatom abundance, which were similar across pCO2 treatments. At the end of the experiment, 24-hr growth responses to pCO2 levels varied as a function of cell size. The smallest size fraction (20 μm size fraction. Cell size distribution shifted toward smaller cells in both the Past and Future treatments but remained unchanged in the Present treatment. Similarity in Past and Future treatments for cell size distribution and growth rate (5–20 μm size fraction) illustrate non-monotonic effects of increasing pCO2 on ecological indicators and may be related to opposing physiological effects of high CO2 and low pH both within and among species. Interaction of these effects with other factors (e.g., nutrients, light, temperature, grazing, initial species composition) may explain variability among published studies. The absence of clear treatment-specific effects at the community level suggest that extrapolation of species-specific responses or experiments with only present day and future pCO2 treatments levels would produce misleading predictions of ocean acidification impacts on plankton production.