Strong effects of elevated CO₂ on freshwater microalgae and ecosystem chemistry

Abstract

The carbonate chemistry of freshwater systems can range from inorganic carbon‐limited to supersaturated with respect to the atmosphere, and the pH of these systems can vary temporally and spatially from alkaline to acidic. Determining how these heterogeneous systems respond to increases in atmospheric CO2 is critical to understanding global impacts of these changes. Here, we synthesize 22 studies from a variety of systems to explore the effects of elevated CO2 on freshwater chemistry and microalgae, which form the base of autotrophic food webs. Across the variability in freshwater systems, elevated CO2 significantly affected water chemistry by decreasing pH and increasing dissolved inorganic carbon. Microalgae were also affected by elevated CO2 with measured increases in (1) nutrient acquisition through microalgal carbon‐to‐nutrient ratios, (2) photosynthetic activity, and (3) growth. While these effects were measured from controlled experiments, the results indicate a wide range of potential freshwater ecosystem effects from elevated atmospheric CO2. Our synthesis also identified several knowledge gaps. Generally, larger sample sizes and studies of longer duration are needed for more robust analyses and conclusions. Additionally, more field experiments across a range of freshwater ecosystem types and studies involving benthic species and multiple trophic levels are needed to strengthen global predictions across the broad variability found within and among freshwater systems.

An experimental test of climate change effects in northern lakes: Increasing allochthonous organic matter and warming alters autumn primary production

“…Increased cDOM, by contrast, has multiple impacts on lake characteristics, including reduced light availability, but also increased macronutrients like nitrogen (N) and phosphorous (P) bonded to cDOM and increased carbon dioxide (CO 2 ) concentrations from cDOM mineralisation (Carpenter et al., 1998; Creed et al., 2018; Nydahl et al., 2019). Generally, elevated nutrient and CO 2 levels enhance GPP, mainly in the pelagic habitat, while reduced light penetration from brownification decreases GPP, mainly in benthic habitats (Ask et al., 2012; Brown et al, 2020; Genkai‐Kato et al., 2012).…”

An ecological and methodological assessment of benthic gross primary production in northern lakes

“…Possibly GPP z rates and DIC were not correlated because DIC was negatively correlated to temperature and positively to depth, which is the opposite for GPP z rates (Appendix S1: Table S5a). Effects of CO 2 on benthic GPP are rarely studied in freshwater systems (Brown et al, 2019; Hamdan et al, n.d.), but positive effects have been convincingly demonstrated in marine coastal experimental studies (Cartaxana et al, 2015; Johnson et al, 2013; Vieira et al, 2016). It is therefore possible that a subsidizing effect of CO 2 only occurs in unconstraining light, temperature, and nutrient conditions.…”

Photosynthesis, carbon acquisition and primary productivity of phytoplankton: a review dedicated to Colin Reynolds

“…The pCO 2 might increase photosynthesis of plankton and macrophytes and alter community structure (Hasler et al, 2016). A meta-analysis of 22 microalgal studies by Brown et al (2019) indicates substantial alterations of water chemistry, nutrient acquisition, photosynthesis, carbon uptake and growth (see Fig. 1 in Brown et al, 2019).…”

“…A meta-analysis of 22 microalgal studies by Brown et al (2019) indicates substantial alterations of water chemistry, nutrient acquisition, photosynthesis, carbon uptake and growth (see Fig. 1 in Brown et al, 2019). Since these results are all based on controlled experiments, long-term field measurements in various types of freshwater ecosystems are needed to increase predictability.…”