Response of ocean phytoplankton community structure to climate change over the 21st century: partitioning the effects of nutrients, temperature and light

Abstract

Abstract. The response of ocean phytoplankton community structure to climate change depends, among other factors, upon species competition for nutrients and light, as well as the increase in surface ocean temperature. We propose an analytical framework linking changes in nutrients, temperature and light with changes in phytoplankton growth rates, and we assess our theoretical considerations against model projections (1980-2100) from a global Earth System model. Our proposed "critical nutrient hypothesis" stipulates the existence of a critical nutrient threshold below (above) which a nutrient change will affect small phytoplankton biomass more (less) than diatom biomass, i.e. the phytoplankton with lower half-saturation coefficient K are influenced more strongly in low nutrient environments. This nutrient threshold broadly corresponds to 45 • S and 45 • N, poleward of which high vertical mixing and inefficient biology maintain higher surface nutrient concentrations and equatorward of which reduced vertical mixing and more efficient biology maintain lower surface nutrients. In the 45 • S-45 • N low nutrient region, decreases in limiting nutrientsassociated with increased stratification under climate change -are predicted analytically to decrease more strongly the specific growth of small phytoplankton than the growth of diatoms. In high latitudes, the impact of nutrient decrease on phytoplankton biomass is more significant for diatoms than small phytoplankton, and contributes to diatom declines in the northern marginal sea ice and subpolar biomes. In the context of our model, climate driven increases in surCorrespondence to: I. Marinov (imarinov@sas.upenn.edu) face temperature and changes in light are predicted to have a stronger impact on small phytoplankton than on diatom biomass in all ocean domains. Our analytical predictions explain reasonably well the shifts in community structure under a modeled climate-warming scenario. Climate driven changes in nutrients, temperature and light have regionally varying and sometimes counterbalancing impacts on phytoplankton biomass and structure, with nutrients and temperature dominant in the 45 • S-45 • N band and light-temperature effects dominant in the marginal sea-ice and subpolar regions. As predicted, decreases in nutrients inside the 45 • S-45 • N "critical nutrient" band result in diatom biomass decreasing more than small phytoplankton biomass. Further stratification from global warming could result in geographical shifts in the "critical nutrient" threshold and additional changes in ecology.

Southern Ocean phytoplankton physiology in a changing climate

“…The predicted reduction in MLD means an increase in the integrated daily irradiance delivered to phytoplankton in the surface waters (Marinov et al, 2010). This trapping of phytoplankton in surface waters exposes them to higher irradiances of photosynthetic active radiation (PAR) and damaging, short wavelength ultraviolet radiation (UVR) (Marinov et al, 2010;Caron and Hutchins, 2013 and refs therein). High light and UVR are responsible for photoinhibition and cellular photodamage that can reduce rates of photosynthesis, growth and survival by phytoplankton (Davidson, 2006).…”

“…Under these conditions, phytoplankton are trapped in warm, macro-and micronutrient-poor surface waters (Fig. 4a) where they are exposed to high light and damaging UV radiation (Marinov et al, 2010), likely reducing rates of phytoplankton photosynthesis, growth and NPP. These future conditions also favour phytoplankton communities typical of the POOZ, which contribute little to POC export, weakening the biological pump (Fig.…”

Winners and losers: Ecological and biogeochemical changes in a warming ocean

“…[6] Previous studies [e.g., Bopp et al, 2001;Steinacher et al, 2010;Marinov et al, 2010] have suggested an opposite response in productivity between nutrient-rich high and oligotrophic low latitudes. Here we use simple theoretical insights to explore these regional differences further.…”

“…[5] Model studies suggest that reduced rates of nutrient supply in a future ocean will lower productivity [Schmittner et al, 2008;Steinacher et al, 2010;Marinov et al, 2010;Bopp et al, 2013;Bopp et al, 2001]. Taucher and Oschlies [2011], however, suggest that this "indirect" effect might be counteracted by the direct effect of increased growth rates due to increased temperatures.…”

“…[4] Published numerical projections of future oceans suggest very different changes in global integrated primary production in future scenarios [Sarmiento et al, 2004;Schmittner et al, 2008;Steinacher et al, 2010;Marinov et al, 2010;Taucher and Oschlies, 2011;Bopp et al, 2005;Bopp et al, 2013], even disagreeing on the sign of the net change. Locally, there are only select regions of general agreement between models, with other regions of disparate signs of changes [Bopp et al, 2013].…”