Projected expansion of <i>Trichodesmium</i>’s geographical distribution and increase in growth potential in response to climate change
Tobias G. Boatman, Graham Upton, Tracy Lawson, Richard J. Geider
Abstract
Abstract Estimates of marine N 2 fixation range from 52 to 73 Tg N/year, of which we calculate up to 84% is from Trichodesmium based on previous measurements of nif H 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 p CO 2 , temperature, light and Fe. We conclude that growth rate was severely limited by low p CO 2 at the LGM, that current p CO 2 levels do not significantly limit Trichodesmium growth and thus, the potential for enhanced growth from future increases in CO 2 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 N 2 fixation and thus pelagic ecology, biogeochemistry and CO 2 sequestration, the projected increase in the geographical range for optimal growth could provide a negative feedback to increasing atmospheric CO 2 concentrations.