16.4 The Responses of Photosymbiotic Organisms
to Climate Change Will Be a Key Determinant
of the Future Diversity and Distribution of Life
In addition to their presence in numerous animals, photosymbioses also occur in
seaweed and phytoplankton, the latter including diatoms and dinoflagellates, as well
as nonsymbiotic cyanobacteria. As noted above, photosymbiotic corals are
bleaching in response to anthropogenic stresses with negative effects on the communities they support. But production by corals pales relative to that of phytoplankton, which globally, represent roughly 45% of net primary production, despite
representing less than 1% of biomass (Field et al. 1998).
Plastid-bearing phytoplankton occupy shallow waters (<200 m) and have high
turnover rates across a massive surface area. As such, they exhibit the potential for
rapid responses to climate variation on a global scale, which is expected to impact
energy transfer throughout food webs (Winder and Sommer 2012). And as major
sinks for carbon, changes in their abundance and species composition affect biogeochemical cycling that further impacts climate (Sabine et al. 2004). Hence,
understanding the fate of phytoplankton in our warming and acidifying oceans is
critical.
Phytoplankton abundance and distribution are determined largely by ocean
upwelling and circulation with growth rates depending on sea surface temperature,
nutrients, light availability, and species interactions. While predictions are complicated, surveys using satellite-based ocean color sensors show consistent, climatetracking patterns, including shifts in the magnitude, timing, and length of seasonal
blooms (Winder and Sommer 2012; Racault et al. 2012; Friedland et al. 2018;
Behrenfeld et al. 2006). While some studies report a decline in global phytoplankton
since the 1950s (Boyce et al. 2010), this is not a consensus view (McQuatters-Gollop
et al. 2011). Warming enhances ocean stratification, which in turn reduces the supply
of limiting nutrients, resulting in regional biomass reductions. In tropical waters, for
example, warming produces delayed winter blooms that occur over shorter durations
and with reduced biomass (Gittings et al. 2018). Marine heat waves, and associated
stratification, can decrease local production by phytoplankton and result in widespread losses of productive habitats, such as kelp forests (Smale et al. 2019). Another
factor that may decrease phytoplankton abundance in warm waters is that consumers
(e.g., viruses and grazers) often increase in biomass relative to producers and hence
can exert top-down control on the phytoplankton population (O’Connor et al. 2009).
In addition to warming, the oceans have absorbed about 31% of anthropogenic
CO 2 , causing ocean acidification; pH is down 0.1 unit since the Industrial Revolution, and predicted to drop 0.3–0.4 more units by 2100 (Hurd et al. 2018; Gattuso
et al. 2015). Rising CO 2 levels in the oceans intuitively lead to increases in primary
production via phytoplankton. However, this capacity is variable within and among
taxonomic groups, and overall, increases in photosynthesis appear slight at best,
constrained by limiting nutrients and other stresses (Mackey et al. 2015). Furthermore, some phytoplankton, including diatoms, are negatively impacted by
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K. M. Oliver and C. H. V. Higashi
to Climate Change Will Be a Key Determinant
of the Future Diversity and Distribution of Life
In addition to their presence in numerous animals, photosymbioses also occur in
seaweed and phytoplankton, the latter including diatoms and dinoflagellates, as well
as nonsymbiotic cyanobacteria. As noted above, photosymbiotic corals are
bleaching in response to anthropogenic stresses with negative effects on the communities they support. But production by corals pales relative to that of phytoplankton, which globally, represent roughly 45% of net primary production, despite
representing less than 1% of biomass (Field et al. 1998).
Plastid-bearing phytoplankton occupy shallow waters (<200 m) and have high
turnover rates across a massive surface area. As such, they exhibit the potential for
rapid responses to climate variation on a global scale, which is expected to impact
energy transfer throughout food webs (Winder and Sommer 2012). And as major
sinks for carbon, changes in their abundance and species composition affect biogeochemical cycling that further impacts climate (Sabine et al. 2004). Hence,
understanding the fate of phytoplankton in our warming and acidifying oceans is
critical.
Phytoplankton abundance and distribution are determined largely by ocean
upwelling and circulation with growth rates depending on sea surface temperature,
nutrients, light availability, and species interactions. While predictions are complicated, surveys using satellite-based ocean color sensors show consistent, climatetracking patterns, including shifts in the magnitude, timing, and length of seasonal
blooms (Winder and Sommer 2012; Racault et al. 2012; Friedland et al. 2018;
Behrenfeld et al. 2006). While some studies report a decline in global phytoplankton
since the 1950s (Boyce et al. 2010), this is not a consensus view (McQuatters-Gollop
et al. 2011). Warming enhances ocean stratification, which in turn reduces the supply
of limiting nutrients, resulting in regional biomass reductions. In tropical waters, for
example, warming produces delayed winter blooms that occur over shorter durations
and with reduced biomass (Gittings et al. 2018). Marine heat waves, and associated
stratification, can decrease local production by phytoplankton and result in widespread losses of productive habitats, such as kelp forests (Smale et al. 2019). Another
factor that may decrease phytoplankton abundance in warm waters is that consumers
(e.g., viruses and grazers) often increase in biomass relative to producers and hence
can exert top-down control on the phytoplankton population (O’Connor et al. 2009).
In addition to warming, the oceans have absorbed about 31% of anthropogenic
CO 2 , causing ocean acidification; pH is down 0.1 unit since the Industrial Revolution, and predicted to drop 0.3–0.4 more units by 2100 (Hurd et al. 2018; Gattuso
et al. 2015). Rising CO 2 levels in the oceans intuitively lead to increases in primary
production via phytoplankton. However, this capacity is variable within and among
taxonomic groups, and overall, increases in photosynthesis appear slight at best,
constrained by limiting nutrients and other stresses (Mackey et al. 2015). Furthermore, some phytoplankton, including diatoms, are negatively impacted by
278
K. M. Oliver and C. H. V. Higashi