M. J. Follows

A Holistic Approach to Marine Eco-Systems Biology

The structure, robustness, and dynamics of ocean plankton ecosystems remain poorly understood due to sampling, analysis, and computational limitations. The Tara Oceans consortium organizes expeditions to help fill this gap at the global level.

The role of nutricline depth in regulating the ocean carbon cycle

Carbon uptake by marine phytoplankton, and its export as organic matter to the ocean interior (i.e., the “biological pump”), lowers the partial pressure of carbon dioxide (pCO2) in the upper ocean and facilitates the diffusive drawdown of atmospheric CO2. Conversely, precipitation of calcium carbonate by marine planktonic calcifiers such as coccolithophorids increases pCO2 and promotes its outgassing (i.e., the “alkalinity pump”). Over the past ≈100 million years, these two carbon fluxes have been modulated by the relative abundance of diatoms and coccolithophores, resulting in biological feedback on atmospheric CO2 and Earths climate; yet, the processes determining the relative distribution of these two phytoplankton taxa remain poorly understood. We analyzed phytoplankton community composition in the Atlantic Ocean and show that the distribution of diatoms and coccolithophorids is correlated with the nutricline depth, a proxy of nutrient supply to the upper mixed layer of the ocean. Using this analysis in conjunction with a coupled atmosphere–ocean intermediate complexity model, we predict a dramatic reduction in the nutrient supply to the euphotic layer in the coming century as a result of increased thermal stratification. Our findings indicate that, by altering phytoplankton community composition, this causal relationship may lead to a decreased efficiency of the biological pump in sequestering atmospheric CO2, implying a positive feedback in the climate system. These results provide a mechanistic basis for understanding the connection between upper ocean dynamics, the calcium carbonate-to-organic C production ratio and atmospheric pCO2 variations on time scales ranging from seasonal cycles to geological transitions.

Plankton networks driving carbon export in the oligotrophic ocean.

The biological carbon pump is the process by which CO2 is transformed to organic carbon via photosynthesis, exported through sinking particles, and finally sequestered in the deep ocean. While the intensity of the pump correlates with plankton community composition, the underlying ecosystem structure driving the process remains largely uncharacterized. Here we use environmental and metagenomic data gathered during the Tara Oceans expedition to improve our understanding of carbon export in the oligotrophic ocean. We show that specific plankton communities, from the surface and deep chlorophyll maximum, correlate with carbon export at 150 m and highlight unexpected taxa such as Radiolaria and alveolate parasites, as well as Synechococcus and their phages, as lineages most strongly associated with carbon export in the subtropical, nutrient-depleted, oligotrophic ocean. Additionally, we show that the relative abundance of a few bacterial and viral genes can predict a significant fraction of the variability in carbon export in these regions.

Interconnection of nitrogen fixers and iron in the Pacific Ocean: Theory and numerical simulations

[1] We examine the interplay between iron supply, iron concentrations and phytoplankton communities in the Pacific Ocean. We present a theoretical framework which considers the competition for iron and nitrogen resources between phytoplankton to explain where nitrogen fixing autotrophs (diazotrophs, which require higher iron quotas, and have slower maximum growth) can co-exist with other phytoplankton. The framework also indicates that iron and fixed nitrogen concentrations can be strongly controlled by the local phytoplankton community. Together with results from a three-dimensional numerical model, we characterize three distinct biogeochemical provinces: 1) where iron supply is very low diazotrophs are excluded, and iron-limited nondiazotrophic phytoplankton control the iron concentrations; 2) a transition region where nondiazotrophic phytoplankton are nitrogen limited and control the nitrogen concentrations, but the iron supply is still too low relative to nitrate to support diazotrophy; 3) where iron supplies increase further relative to the nitrogen source, diazotrophs and other phytoplankton coexist; nitrogen concentrations are controlled by nondiazotrophs and iron concentrations are controlled by diazotrophs. The boundaries of these three provinces are defined by the rate of supply of iron relative to the supply of fixed nitrogen. The numerical model and theory provide a useful tool to understand the state of, links between, and response to changes in iron supply and phytoplankton community structure that have been suggested by observations.

Evaluating carbon sequestration efficiency in an ocean circulation model by adjoint sensitivity analysis

[1] We demonstrate the application of the adjoint method to develop three-dimensional maps of carbon sequestration efficiency and mean residence time in an ocean general circulation model. In contrast to perturbation sensitivity experiments, the adjoint approach provides a computationally efficient way to characterize both temporal and spatial variations of sequestration efficiency and residence time for a complete global model domain. Sequestration efficiency (the percentage of carbon injected at a continuous point source that remains in the ocean after an elapsed time), for injections at the base of the main thermocline (� 900 m), is initially lowest in the North Atlantic basin (except for regions of deep convection in the Labrador Sea) relative to the North Pacific. For injection periods of the order of a century or more, however, the model suggests that Pacific injection sites are generally less efficient for a constant rate injection source. The mean residence time (defined as the average period that impulsively injected carbon from a particular point source remains within the ocean) is also evaluated and mapped. This measure also suggests that Atlantic sequestration is more efficient in the long term. Our calculations draw out the dual role of convective mixing, both exposing shallow sequestration sources to the atmosphere and also, in the subpolar Atlantic and Labrador Sea, feeding carbon from shallow injection sources into the deep circulation away from the atmosphere. INDEX TERMS: 1635 Global Change: Oceans (4203); 4532 Oceanography: Physical: General circulation; 4806 Oceanography: Biological and Chemical: Carbon cycling; 3210 Mathematical Geophysics: Modeling; KEYWORDS: adjoint, ocean circulation, carbon sequestration

Mechanisms Controlling the Air-Sea Flux of CO2 in the North Atlantic

The air-sea flux of carbon is controlled by the disequilibrium in partial pressure of carbon dioxide between the atmosphere and surface ocean. This disequilibrium is a consequence of the interactions of physical, chemical and biological processes in the ocean and, today, includes a response to the anthropogenic increase of atmospheric pCO 2. Fig. 1 illustrates the annual mean airsea flux of carbon, F, estimated from a knowledge of the atmospheric partial pressure, pCO 2 at and compilation of surface pCO 2 observations by Takahashi et al. (1999). The air-sea flux of carbon is determined by

Applying “‐omics” Data in Marine Microbial Oceanography

F = - {K_g}{K_0}(pC{O_2} - pCO_2^{at}

Mesoscale Iron Enrichment Experiments 1993-2005: Synthesis and Future Directions

(1) where K 0 is the solubility of CO 2 at local temperature and salinity. K g is the air-sea gas transfer coefficient, which is dependent on local environmental conditions and is usually parameterized as a function of wind speed, sea-surface temperature and sea-surface salinity (Wanninkhof, 1992). The major global scale features in Fig. 1 are the outgassing of CO 2 from the tropical oceans, and the influx at mid and high latitudes. In this chapter we focus on understanding what sets the basin wide, and regional patterns of air-sea carbon flux in the North Atlantic basin. While we focus on the North Atlantic, some of the concepts and discussions are also relevent to other regions of the ocean.