Climate impact on diatom size and distribution

Abstract

Diatoms are one of the ocean’s key primary producers and are responsible for a large proportion of carbon export from the surface layer. With this, diatoms also play a vital role in the cycling of many key nutrient’s, such as phosphate and nitrogen, and are the key driver of the silica cycle. However, changes to the Earth’s climate can alter the distribution, abundance, and size classes of this organism, which in turn will impact global carbon export.
This study investigates the effect of two differing climates - simulated using EcoGENIE – on diatom distribution and size. Firstly, diatoms were incorporated into the ecological component of the model by parameterising their defining features (high growth rate, silica usage and protection from grazers). Opal export was also added to the model, where mortality and messy feeding on diatoms resulted in opal export, allowing it to be reintroduced into the biological model and redistributed as biogenic silica. Once included in the model, I ran experiments to look at how different climates influenced diatoms. The first of these was a cooler environment with an atmospheric CO2 concentration of 190 ppm, in line with that of the LGM. The second, a warmer environment, with a pCO2 concentration of 425 ppm. These two climates were run to reach a steady state, where the impact on diatom distribution and size was compared to that of a pre-industrial control (278 ppm). This study found that at 190 ppm, primary production (PP) and carbon export increased by 4.9% and 1.4% respectively, primarily driven by increased nutrients in the surface waters from increased upwelling (472% global increase), fuelling diatom growth. At 425 ppm, both PP and carbon export decreased with biomass decreasing by almost 11% and consequentially POC export decreases by 4.6%. This decrease was driven by a reduction in the nutrients in the surface waters, particularly in the North Atlantic, due to a reduction in mixing and stratification of the water column. From this it is clear to see that diatoms play an important role in the export of carbon in this model, with the changes to nutrient supply at different climate being the key driver of the change. One of the key findings from this study, was the impact on diatom size class. These did not respond as expected, with a cooler environment leading to an increase in smaller diatoms and the warmer climate seeing an increase in larger sized diatoms. The compliance of diatoms to the general temperature size rule suggested by Bergmann (1847), James (1970) and Atkinson (1995) has been subject to numerous studies (Li et al., 2009; Morán et al., 2010; Yvon-Durocher et al., 2011; Rüger and Sommer, 2012; Adams et al., 2013) with mixed results. I concluded that in this study however, being of a smaller size was not more advantageous – as suggested by Adams et al (2013) - therefore alterations to size with temperature change was not seen.
An additional experiment incorporating ballasting was also conducted on the pre-industrial time period. With this integrated into the model, it showed increased carbon export (by up to 175%) from smaller diatom size class in equatorial regions, highlighting their importance, and the importance of including multiple size classes when modelling diatoms in future studies.

Date of Award	23 Jan 2020
Original language	English
Awarding Institution	University of Bristol
Supervisor	Fanny M Monteiro (Supervisor) & Katharine Hendry (Supervisor)