The effects of warming oceans on algae

Increased temperature has both direct and indirect effects on marine algae, and has been found to affect marine photosynthesisers both positively and negatively. Warmer waters will enhance metabolic rates, but will also lead to nutrient limitation due to increased stratification (Beardall and Raven 2004, Behrenfeld et al. 2006). Higher temperatures change the availability of inorganic carbon to phytoplankton cells by lowering the solubility of CO₂; they also alter seasonal triggers in plankton communities (Bowling and Baker 1996, Suikkanen et al. 2006), and may have other disruptive effects. 

Behrenfeld et al. (2006) used remote sensing to detect the wavelength of light radiating from the oceans, and hence to quantify the amount of chlorophyll in the water, and tested for correlation with sea surface temperature (SST) and a measure of stratification. This novel technique allowed vast areas of ocean to be surveyed quickly and repeatedly, and over almost a decade of observations the workers found that higher SST correlated strongly with decreased algal biomass. According to this study, average global ocean carbon assimilation fell by 190 Tg C yr^-1 between 1999 and 2005, after an annual increase prior to this period of ten times this amount  (Behrenfeld et al. 2006; 1Tg = 10¹²g = 1 megatonne; Fig. 2). This was a period of warming climatic conditions associated with a La Niña cycle. This represents a reduction of 0.01 Tg yr^-1 in global chlorophyll concentrations, and therefore a very significant decrease in the biomass of photosynthetic microorganisms available as energy to higher trophic levels. These results were not, however, distributed evenly across the world’s oceans (Behrenfeld et al. 2006).

 Fig 1: Mechanism of ocean water stratification, driven by increasing water temperature, and its effects on phytoplankton (from Doney 2006).

Fig. 2:     Net Primary production (NPP) anomalies are highly correlated with periods of climate variability MEI (Fig 2a), and changes in ocean stratification (Fig 2b). The scales for MEI and stratification are inverted for clarity (from Behrenfeld et al. 2006).

Higher summer surface temperatures have instead been correlated with increased algal biomass in the Gulf of Finland, by statistical analysis of historical records (Suikkanen et al. 2006). Levels of dissolved inorganic nitrogen were shown to contribute to this effect but other environmental factors detract from the clarity of this retrospective study. Higher temperatures were associated with increased cyanobacterial and chlorophyte biomass but with a reduction in diatom succession. These workers also noted a likely effect on communities lower in the water column due to 0₂ depletion resulting from a post-bloom increase in net respiration (Suikkanen et al. 2006). This outcome has been verified in other studies (Bowling and Baker 1996, Wohlers et al. 2009). Wohlers et al. (2009) further demonstrated that artificially elevated temperatures promoted early seasonal blooming in populations of pelagic phytoplankton, but did not increase net photosynthesis. Four tanks of 1400L each were filled with seawater replete with its natural plankton community, and with natural levels of zooplankton added. Autotrophic assimilation of inorganic nutrients was measured along with the uptake of dissolved inorganic carbon and accumulation of biomass, dissolved organic carbon and other indicators. Respiration rates were also measured, and observed to spike after phytoplankton blooms, with faster rates at higher temperatures. Their results did not reveal significantly greater net photosynthetic rates, but showed increased respiration relative to photosynthesis (Wohlers et al 2009). This represents a very significant effect when extrapolated to ecosystem scales. 

Some microalgae rely on diffusion for CO₂ supply and on ribulose bisphosphate carboxylase-oxygenase (RUBISCO) to catalyse their carbon fixation, and hence operate at sub-optimal efficiency (28-31% of their maximum) at today’s atmospheric CO₂ concentrations (Beardall and Raven 2004). Most microalgal taxa have carbon-concentrating mechanisms (CCMs) to overcome this problem. Some species exhibit significantly increased growth while others show negligible gains in response to experimentally increased CO₂ concentrations. Cells with CCMs show little impact of elevated CO₂; cells without CCMs are stimulated (Beardall and Raven 2004). Efficiency of resource use may be greater under CO₂ fertilisation, especially among taxa lacking CCMs, but other resources would need to be present at appropriate levels for this to translate into a net increase in photosynthesis by oceanic algae. Nutrient limitation will be explored in later sections of this paper.