Toward quantifying the response of the oceans' biological pump to climate change

Philip W. Boyd

doi:10.3389/fmars.2015.00077

Abstract

The biological pump makes a major global contribution to the sequestration of carbon-rich particles in the oceans’ interior. This pump has many component parts from physics to ecology that together control its efficiency in exporting particles. Hence, the influence of climate change on the functioning and magnitude of the pump is likely to be complex and non-linear. Here, I employ a published 1-D coupled surface-subsurface Particulate Organic Carbon (POC) export flux model to systematically explore the potential influence of changing oceanic conditions on each of the pump’s ‘moving parts’, in both surface and subsurface waters. These simulations were run for typical high (High Nutrient Low Chlorophyll, HNLC) and low (Low Nutrient Low Chlorophyll, LNLC) latitude sites. Next, I couple pump components that have common drivers, such as temperature, to investigate more complex scenarios involving concurrent climate-change mediated alteration of multiple ‘moving parts’ of the pump. Model simulations reveal that in the surface ocean, changes to algal community structure (i.e., a shift towards small cells) has the greatest individual influence (decreased flux) on downward POC flux in the coming decades. In subsurface waters, a shift in zooplankton community structure has the greatest single effect on POC flux (decreased) in a future ocean. More complex treatments, in which up to ten individual factors (across both surface and subsurface processes) were concurrently altered, ~ halved the POC flux at both high and low latitudes. In general climate-mediated changes to surface ocean processes had a greater effect on the magnitude of POC flux than alteration of subsurface processes, some of which negated one another. This relatively simple 1-D model provides initial insights into the most influential processes that may alter the future performance of this pump, and more importantly reveals many knowledge gaps that require urgent attention before we can accurately quantify future changes to the biological pump.

Highlights

Two ocean pumps play key roles in removing carbon from the surface ocean—the solubility pump is physico-chemically mediated whereas the biological pump is driven primarily by the interactions of marine biota from microbes to metazoa (Volk and Hoffert, 1985)
Influence of Alteration of Individual Factors Controlling Downward Particulate Organic Carbon (POC) Flux Simulations from the 1-D model are presented as vertical plots of downward POC flux vs. depth, expressed as a systematic alteration of each of the climate-change mediated factors influencing POC export (Tables 3, 5), relative to the control, for the euphotic zone (Ez) (e.g., Figure 4A) and the subsurface Twilight Zone (e.g., Figure 4B)
For the high latitude simulations, the export flux from the base of the 50 m deep Ez ranged from 109 mg C m−2 d−1 to ∼160 mg C m−2 d−1 relative to the control run (146 mg C m−2 d−1, Figure 4A)

Summary

Introduction

Two ocean pumps play key roles in removing carbon from the surface ocean—the solubility pump is physico-chemically mediated whereas the biological pump is driven primarily by the interactions of marine biota from microbes to metazoa (Volk and Hoffert, 1985). Climate-change and the biological pump and attenuated by a wide range of grazing activities which transform most of the phytoplankton and bacterial carbon into heterogeneous particles which eventually settle out of the surface ocean after a residence time of days to weeks (Boyd and Stevens, 2002). Further transformations of these settling particles, by heterotrophic bacteria and grazers, further diminish this POC flux in subsurface waters (Steinberg et al, 2008). An increasing number of studies have focused on individual processes that will influence how the sign and magnitude of the pump is likely to change in a future ocean (Boyd and Trull, 2007; Riebesell et al, 2009; Passow and Carlson, 2012)

Methods

Results

Conclusion