What does chlorophyll variability tell us about export and air‐sea CO₂ flux variability in the North Atlantic?

Abstract

[1] The importance of biology to the ocean carbon sink is often quantified in terms of export, the removal of carbon from the ocean surface layer. Satellite images of sea surface chlorophyll indicate variability in biological production, but how these variations affect export and air-sea carbon fluxes is poorly understood. We investigate this in the North Atlantic using an ocean general circulation model coupled to a medium-complexity ecosystem model. We find that biological CO 2 drawdown is significant on the mean and dominates the seasonal cycle of pCO 2 , but variations in the annual air-sea CO 2 flux and export are not significantly correlated. Large year-to-year variability in summertime pCO 2 occurs, because of changing bloom timing, but integrated bloom strength and associated carbon uptake and export do not vary substantially. The model indicates that small biological variability, quantitatively consistent with SeaWiFS (1998SeaWiFS ( -2006, is not sufficient to be a first-order control on annual subpolar air-sea CO 2 flux variability.

The role of phytoplankton dynamics in the seasonal and interannual variability of carbon in the subpolar North Atlantic – a modeling study

“…The dinoflagellates and coccolithophores peak in July through August, during which drawdown of surfaceocean CO 2 reaches its maximum value. The effect of biological changes in the surface ocean pCO 2 exceeds the temperature effect by a factor of almost 2, a clear indication of the importance of phytoplankton photosynthesis on the uptake of atmospheric CO 2 in the region, a result that is in agreement with previous studies (Takahashi et al, 1993(Takahashi et al, , 2002(Takahashi et al, , 2009Ullman et al, 2009;Bennington et al, 2009;Metzl et al, 2010).…”

“…Note that without the biology (Experiment C), the ocean becomes a source of CO 2 for most of the year, while the reference run (Experiment A), which agrees best with observations (see next section), shows an ocean that is a sink of CO 2 for most of the year and a small source during April-May. This result agrees with previous studies that show that biology is critical to these seasonal changes at this location and that vertical mixing also plays a major role (Takahashi et al, 1993(Takahashi et al, , 2002(Takahashi et al, , 2009Ullman et al, 2009;Bennington et al, 2009;Metzl et al, 2010). The annual mean values of pCO 2 and air-sea CO 2 flux derived from the reference, no relaxation, and abiotic runs (Experiments A, B, and C, respectively) are 337.0, 325.6, 371.0 µatm, and +7.3, +12.5, −1.7 mmol C m −2 d −1 , respectively.…”

Impact of phytoplankton community size on a linked global ocean optical and ecosystem model

“…They play an important role in our understanding of how ocean biogeochemistry and carbon cycling operates as an integrated ecological, physical and chemical system. These models are crucial for understanding observed trends in satellite observations (Antoine et al, 2005;Gregg et al, 2005); can help inform satellite-based primary production algorithms that strive to incorporate ecological information beyond just chlorophyll concentration (Mouw and Yoder, 2005); can be used to link observed variability in bloom functioning to variability in carbon export and air-sea CO 2 fluxes (Bennington et al, 2009); and are the basis for predictions about the future state of marine ecology, biogeochemistry, and ocean carbon storage in a changing climate (Bopp et al, 2001;Orr et al, 2005;Henson et al 2010).…”

Mechanisms of northern North Atlantic biomass variability

“…The biogeochemical model explicitly cycles phosphorus, silica and iron, and complete carbon chemistry is also included. This 110 model is identical to the one presented in Breeden and McKinley [2016], and uses the same biogeochemical code as Bennington et al [2009], Ullman et al [2009] and Koch et al [2009].…”

“…The phosphorus-based ecosystem is parameterized following Dutkiewicz et al [2005], and with modest revisions by Bennington et al [2009]. This ecosystem has one zooplankton class and two phytoplankton classes ("large" diatoms and "small").…”