Increasing importance of small phytoplankton in a warmer ocean

Abstract

The macroecological relationships among marine phytoplankton total cell density, community size structure and temperature have lacked a theoretical explanation. The tiniest members of this planktonic group comprise cyanobacteria and eukaryotic algae smaller than 2 lm in diameter, collectively known as picophytoplankton. We combine here two ecological rules, the temperature-size relationship with the allometric sizescaling of population abundance to explain a remarkably consistent pattern of increasing picophytoplankton biomass with temperature over the À0.6 to 22 1C range in a merged dataset obtained in the eastern and western temperate North Atlantic Ocean across a diverse range of environmental conditions. Our results show that temperature alone was able to explain 73% of the variance in the relative contribution of small cells to total phytoplankton biomass regardless of differences in trophic status or inorganic nutrient loading. Our analysis predicts a gradual shift toward smaller primary producers in a warmer ocean. Because the fate of photosynthesized organic carbon largely depends on phytoplankton size, we anticipate future alterations in the functioning of oceanic ecosystems.

Novel communities from climate change

“…Similarly, diatom cell size has decreased with increasing temperatures through the fossil record, suggesting that impending warming will shift phytoplankton communities towards smaller cells, where diatoms would be at a competitive disadvantage [24]. In another study, picophytoplankton (which comprise cyanobacteria and eukaryotic algae smaller than 2 mm) increased with temperature along a natural temperature gradient in the ocean, regardless of differences in trophic status or in inorganic nutrient loading [25]. Finally, studies in one of the areas experiencing the most extreme climatic warming on Earth, the Western Antarctic Peninsula, have shown that over the last 30 years there has been an increasing fraction of the largest components of phytoplankton, including diatoms and other large cells, in the southern (colder) region than in the northern (warmer) one [26].…”

Major impacts of climate change on deep-sea benthic ecosystems

“…Ocean acidification is also predicted to reduce microbial production of nitrate from ammonium (Beman et al, 2011), which could have major consequences for oceanic primary production because a significant fraction of the nitrate used by phytoplankton is generated by nitrification at the ocean surface (Yool et al, 2007). Major consequences of such changes over regional scales will probably include (1) reductions in primary production combined with (2) shifts from diatom-dominated (low SA:V ratio) phytoplankton assemblages with high POC-export efficiencies to picoplankton communities (high SA:V ratio) characterized by low export efficiencies Morán et al, 2010;Morán et al, 2015). In addition, reductions in calcification from lowered pH in surface waters could reduce phytoplankton sinking rates through loss of ballast (Hofmann and Schellnhuber, 2009), though this effect will depend on the ratio of the fraction of ballasted vs. un-ballasted fractions of the sinking POC.…”

“…Enhanced warming of the upper ocean is predicted to enhance stratification, reducing nutrient input to the upper euphotic zone and causing a shift in phytoplankton assemblages from large, fast-sinking diatoms (with low surface area:volume [SA:V] ratios) to slow-sinking picoplankton (with high SA:V ratios; Bopp et al, 2005). This shift is likely to reduce export flux to the seafloor, as well as transfer efficiency (Buesseler et al, 2007;Morán et al, 2010Morán et al, , 2015Steinacher et al, 2010). Furthermore, freshening of Arctic regions by sea-ice meltwater and episodic input of large river runoff have been shown to reduce phytoplankton size and, by inference, export flux, a trend that has been projected to continue into the future (Li et al, 2009(Li et al, , 2013.…”

“…While few continental margin systems have been investigated directly in terms of the consequences of climate change, their strong gradients and regional variations have allowed significant understanding of the effects of temperature, pH, O 2 and POC flux on deep-sea benthic ecosystems. Warming of surface waters along continental margins, and increased thermal stratification and reduced nutrient supply to the surface are likely to reduce both productivity and phytoplankton type and size Morán et al, 2010Morán et al, , 2015, yielding reduced phytodetrital flux to the seabed Buesseler et al, 2008;. Model outputs suggest that bathyal areas particularly prone to declining POC flux lie in the Norwegian and Caribbean Seas, NW and NE Atlantic, the eastern tropical Pacific, and bathyal Indian and Southern Oceans, which could experience as much as a 55% decline in POC flux by 2100 (Tables 2, 3; Figures 2, 3).…”

Scaling of growth rate and mortality with size and its consequence on size spectra of natural microphytoplankton assemblages in the East China Sea

“…Worth noting is that, since nutrient supply often covaries with temperature in natural environments , it is not easy to distinguish the effects of nutrients from temperature. These studies, however, focus on the size-structure variations with respect to environmental factors instead of directly measuring the phytoplankton growth rate and grazing mortality (Moran et al, 2010). While other studies focused on the selective grazing behaviour of microzooplankton and inferred the potential effect on the phytoplankton size structure, they did not measure the phytoplankton size structure together with feeding experiments Teixeira et al, 2011).…”