Abstract
Estimates of marine N2 fixation range from 52 to 73TgN/year, of which we calculate up to 84% is from Trichodesmium based on previous measurements of nifH gene abundance and our new model of Trichodesmium growth. Here, we assess the likely effects of four major climate change-related abiotic factors on the spatiotemporal distribution and growth potential of Trichodesmium for the last glacial maximum (LGM), the present (2006-2015) and the end of this century (2100) by mapping our model of Trichodesmium growth onto inferred global surface ocean fields of pCO2 , temperature, light and Fe. We conclude that growth rate was severely limited by low pCO2 at the LGM, that current pCO2 levels do not significantly limit Trichodesmium growth and thus, the potential for enhanced growth from future increases in CO2 is small. We also found that the area of the ocean where sea surface temperatures (SST) are within Trichodesmium's thermal niche increased by 32% from the LGM to present, but further increases in SST due to continued global warming will reduce this area by 9%. However, the range reduction at the equator is likely to be offset by enhanced growth associated with expansion of regions with optimal or near optimal Fe and light availability. Between now and 2100, the ocean area of optimal SST and irradiance is projected to increase by 7%, and the ocean area of optimal SST, irradiance and iron is projected to increase by 173%. Given the major contribution of this keystone species to annual N2 fixation and thus pelagic ecology, biogeochemistry and CO2 sequestration, the projected increase in the geographical range for optimal growth could provide a negative feedback to increasing atmospheric CO2 concentrations.
Highlights
Marine phytoplankton account for about 45% of global net primary production (Field et al, 1998), and as such play an important role in the global carbon cycle (Arrigo, 2007)
We found that the area of the ocean where sea surface temperatures (SST) are within Trichodesmium’s thermal niche increased by 32% from the last glacial maximum (LGM) to present, but further increases in SST due to continued global warming will reduce this area by 9%
A representative model for the Trichodesmium genus A notable consideration of our Trichodesmium growth model (Equation 1) is that it is based on data for Trichodesmium erythraeum, but the Trichodesmium genus includes six species assigned to four clades (Lundgren et al, 2005): the lower and upper temperature limits that we measured for T. erythraaeum IMS101 are nearly identical to the temperature limits currently observed for Trichodesmium spp. in nature from shipboard samples (Fig. 1f)
Summary
Marine phytoplankton account for about 45% of global net primary production (Field et al, 1998), and as such play an important role in the global carbon cycle (Arrigo, 2007). 20% of the annual marine net primary production is exported from the surface to the deep via sinking particles (Buesseler & Boyd, 2009). This export production contributes to the draw-down of CO2 from the atmosphere and its sequestration for hundreds or thousands of years in the deep ocean. Changes in export production could significantly affect the ocean’s ability to sequester CO2 from the atmosphere and store it in the deep ocean. When operating over geological timescales, for example, between glacial and interglacial periods, even small changes in the balance between N2 fixation and the loss of fixed N due to denitrification can significantly affect the amount of CO2 that can be stored in the ocean (Falkowski & Raven, 1997, Kohfeld & Ridgwell, 2009)
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