No abstract

A warmer ocean will change plankton physiological rates, alter plankton community composition, and in turn affect ecosystem functions, such as primary production, recycling, and carbon export. To predict how temperature changes affect plankton community dynamics and function, we developed a mechanistic trait-based model of unicellular plankton (auto-hetero-mixotrophic protists and bacteria). Temperature dependencies are specifically implemented on cellular process rather than at the species level. As the uptake of resources and metabolic processes have different temperature dependencies, changes in the thermal environment will favor organisms with different investments in processes such as photosynthesis and biosynthesis. The precise level of investments, however, is conditional on the limiting process and is ultimately determined dynamically by competition and predation within the emergent community of the water column. We show how an increase in temperature can intensify nutrient limitation by altering organisms' interactions, and reduce relative cell-size in the community. Further, we anticipate that a combination of temperature and resource limitation reduces ecosystem efficiency at capturing carbon due to strengthening of the microbial loop. By explicitly representing the effects of temperature on traits responsible for growth, we demonstrate how changes on the individual level can be scaled up to trends at the ecosystem level, helping to discern direct from indirect effects of temperature on natural plankton communities.

Abstract. The daily vertical migrations of fish and other metazoans actively transport organic carbon from the ocean surface to depth, contributing to the biological carbon pump. We use an oxygen-constrained, game-theoretic food-web model to simulate diel vertical migrations and estimate near-global (global ocean minus coastal areas and high latitudes) carbon fluxes and sequestration by fish and zooplankton due to respiration, fecal pellets, and deadfalls. Our model provides estimates of the carbon export and sequestration potential for a range of pelagic functional groups, despite uncertain biomass estimates of some functional groups. While the export production of metazoans and fish is modest (∼20 % of global total), we estimate that their contribution to carbon sequestered by the biological pump (∼800 PgC) is conservatively more than 50 % of the estimated global total (∼1300 PgC) and that they have a significantly longer sequestration timescale (∼250 years) than previously reported for other components of the biological pump. Fish and multicellular zooplankton contribute about equally to this sequestered carbon pool. This essential ecosystem service could be at risk from both unregulated fishing on the high seas and ocean deoxygenation due to climate change.

Diel vertical migration of fish and other metazoans actively transports organic carbon from the ocean surface to depth, contributing to the biological carbon pump. Here, we use a global vertical migration model to estimate global carbon fluxes and sequestration by fish and metazoans due to respiration, fecal pellets, and deadfalls. We estimate that fish and metazoans contribute 5.2 PgC/yr (2.1-8.8PgC/yr) to passive export out of the euphotic zone. Together with active transport, we estimate that fish are responsible for 20% (9-29%) of global carbon export, and 32% (18-43%) of oceanic carbon sequestration, with forage and deep-dwelling mesopelagic fish contributing the most. This essential ecosystem service could be at risk from unregulated fishing on the high seas.

Plankton contribute to the removal of atmospheric CO 2 by photosynthesizing in the surface ocean and sinking into the deep ocean, where remineralized carbon may remain sequestered for hundreds of years (Ducklow et al., 2001;Longhurst & Harrison, 1989). The amount of carbon exported and carbon export efficiency (the fraction of net primary production (NPP) being exported at) emerge from intricate processes that result in either carbon being respired in the surface ocean -and therefore not sequestered -or exported and respired in the deep ocean. Where and how much carbon is respired depends on the community composition and interactions between organisms who eat, respire, and excrete this carbon several times as energy flows across the food-web. However, due to the large amount of players and processes that alter carbon export, global estimates of the flux

The biological carbon pump transports photosynthetically fixed carbon from surface waters to depths. It removes carbon from the atmosphere and sequesters it in the deep ocean, playing an important role in global climate regulation. As the biological carbon pump is directly related to biological processes, it is heavily influenced by the biomass and trophic interactions between populations in the ecosystem. However, behavioral responses and adaptations to predation risk change trophic interactions, potentially having larger impacts than direct effects on trophic interactions and population abundances. Thus, predation risk may play an important role in shaping the biological carbon pump's strength (how much carbon leaves the euphotic zone) and efficiency (what fraction of detritus reaches a certain depth without being degraded). Except in the case of active carbon transport by vertically migrating organisms, this role of risk is not generally recognized. Here, we synthesize the existing knowledge on the consequences of anti‐predation responses on the biological carbon pump. First, we consider a generic anti‐predation response and investigate the different direct, indirect, and cascading effects that the response can induce. Then, we focus on pelagic anti‐predation responses and detail how they can specifically alter the different components of the pump. Finally, we discuss points to consider in biological carbon pump studies and highlight directions for future research. In particular, there is a need for more quantitative research to evaluate the importance of anti‐predation responses in shaping the biological carbon pump.

The flux of detrital particles produced by plankton is an important component of the biological carbon pump. We investigate how food web structure and organisms' size regulate particulate carbon export efficiency (the fraction of primary production that is exported via detrital particles at a given depth). We use the Nutrient-Unicellular-Multicellular (NUM) mechanistic size-spectrum model of the planktonic community (unicellular protists and copepods), embedded within a 3D model representation of the global ocean circulation. The ecosystem model generates emergent food webs and size distributions of all organisms and detrital particles. Model outputs are compared to field data. We find that strong predation by copepods increases export efficiency, while protist predation reduces it. We find no clear relation between primary production and export efficiency. Temperature indirectly drives carbon export efficiency by affecting the biomass of copepods. High temperatures, combined with nutrient limitation, result in low growth efficiency, smaller trophic transfer to higher trophic levels, and decreased carbon export efficiency. Even though copepods consume a large fraction of the detritus produced, they do not markedly attenuate the particle flux. Our simulations illustrate the complex relation between the planktonic food web and export efficiency, and highlights the central role of zooplankton and their size structure.

Abstract. The daily vertical migrations of fish and other metazoans actively transport organic carbon from the ocean surface to depth, contributing to the biological carbon pump. We use an oxygen-constrained, game-theoretic food-web model to simulate diel vertical migrations and estimate global carbon fluxes and sequestration by fish and zooplankton due to respiration, fecal pellets, and deadfalls. Our model provides estimates of the carbon export and sequestration potential for a range of pelagic functional groups, despite uncertain biomass estimates of some functional groups. While the export production of metazoans and fish is modest (∼20 % of global total), we estimate that their contribution to carbon sequestered by the biological pump (∼ 800 PgC) is conservatively more than 50 % of the estimated global total (∼1300 PgC) and have a significantly longer sequestration time scale (∼250 years) than previously reported for other components of the biological pump. Fish and multicellular zooplankton contribute about equally to this sequestered carbon pool. This essential ecosystem service could be at risk from both unregulated fishing on the high seas and ocean deoxygenation due to climate change.