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1. Global patterns in marine organic matter stoichiometry driven by phytoplankton ecophysiology

   https://doi.org/10.1038/s41561-022-01066-2  

   Inomura, Keisuke; Deutsch, Curtis; Jahn, Oliver; Dutkiewicz, Stephanie; Follows, Michael J. (December 2022, Nature Geoscience)

   Abstract The proportion of major elements in marine organic matter links cellular processes to global nutrient, oxygen and carbon cycles. Differences in the C:N:P ratios of organic matter have been observed between ocean biomes, but these patterns have yet to be quantified from the underlying small-scale physiological and ecological processes. Here we use an ecosystem model that includes adaptive resource allocation within and between ecologically distinct plankton size classes to attribute the causes of global patterns in the C:N:P ratios. We find that patterns of N:C variation are largely driven by common physiological adjustment strategies across all phytoplankton, while patterns of N:P are driven by ecological selection for taxonomic groups with different phosphorus storage capacities. Although N:C varies widely due to cellular adjustment to light and nutrients, its latitudinal gradient is modest because of depth-dependent trade-offs between nutrient and light availability. Strong latitudinal variation in N:P reflects an ecological balance favouring small plankton with lower P storage capacity in the subtropics, and larger eukaryotes with a higher cellular P storage capacity in nutrient-rich high latitudes. A weaker N:P difference between southern and northern hemispheres, and between the Atlantic and Pacific oceans, reflects differences in phosphate available for cellular storage. Despite simulating only two phytoplankton size classes, the emergent global variability of elemental ratios resembles that of all measured species, suggesting that the range of growth conditions and ecological selection sustain the observed diversity of stoichiometry among phytoplankton. 

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2. Diel light cycles affect phytoplankton competition in the global ocean

   https://doi.org/10.1111/geb.13562  

   Tsakalakis, Ioannis; Follows, Michael J.; Dutkiewicz, Stephanie; Follett, Christopher L.; Vallino, Joseph J. (July 2022, Global Ecology and Biogeography)

   Abstract AimLight, essential for photosynthesis, is present in two periodic cycles in nature: seasonal and diel. Although seasonality of light is typically resolved in ocean biogeochemical–ecosystem models because of its significance for seasonal succession and biogeography of phytoplankton, the diel light cycle is generally not resolved. The goal of this study is to demonstrate the impact of diel light cycles on phytoplankton competition and biogeography in the global ocean. LocationGlobal ocean. Major taxa studiedPhytoplankton. MethodsWe use a three‐dimensional global ocean model and compare simulations of high temporal resolution with and without diel light cycles. The model simulates 15 phytoplankton types with different cell sizes, encompassing two broad ecological strategies: small cells with high nutrient affinity (gleaners) and larger cells with high maximal growth rate (opportunists). Both are grazed by zooplankton and limited by nitrogen, phosphorus and iron. ResultsSimulations show that diel cycles of light induce diel cycles in limiting nutrients in the global ocean. Diel nutrient cycles are associated with higher concentrations of limiting nutrients, by 100% at low latitudes (−40° to 40°), a process that increases the relative abundance of opportunists over gleaners. Size classes with the highest maximal growth rates from both gleaner and opportunist groups are favoured by diel light cycles. This mechanism weakens as latitude increases, because the effects of the seasonal cycle dominate over those of the diel cycle. Main conclusionsUnderstanding the mechanisms that govern phytoplankton biogeography is crucial for predicting ocean ecosystem functioning and biogeochemical cycles. We show that the diel light cycle has a significant impact on phytoplankton competition and biogeography, indicating the need for understanding the role of diel processes in shaping macroecological patterns in the global ocean. 

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3. MESMO 3: Flexible phytoplankton stoichiometry and refractory dissolved organic matter

   https://doi.org/10.5194/gmd-14-2265-2021  

   Matsumoto, Katsumi; Tanioka, Tatsuro; Zahn, Jacob (January 2021, Geoscientific Model Development)

   null (Ed.)

   Abstract. We describe the third version of Minnesota Earth System Model for Oceanbiogeochemistry (MESMO 3), an Earth system model of intermediate complexity,with a dynamical ocean, dynamic–thermodynamic sea ice, and an energy–moisture-balanced atmosphere. A major feature of version 3 is the flexibleC:N:P ratio for the three phytoplankton functional types represented in themodel. The flexible stoichiometry is based on the power law formulation withenvironmental dependence on phosphate, nitrate, temperature, and light.Other new features include nitrogen fixation, water column denitrification,oxygen and temperature-dependent organic matter remineralization, andCaCO3 production based on the concept of the residual nitrate potentialgrowth. In addition, we describe the semi-labile and refractory dissolved organicpools of C, N, P, and Fe that can be enabled in MESMO 3 as an optionalfeature. The refractory dissolved organic matter can be degraded byphotodegradation at the surface and hydrothermal vent degradation at thebottom. These improvements provide a basis for using MESMO 3 in furtherinvestigations of the global marine carbon cycle to changes in theenvironmental conditions of the past, present, and future. 

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4. Killing the predator: impacts of highest-predator mortality on the global-ocean ecosystem structure

   https://doi.org/10.5194/bg-21-2493-2024  

   Talmy, David; Carr, Eric; Rajakaruna, Harshana; Våge, Selina; Willem_Omta, Anne (January 2024, Biogeosciences)

   Abstract. Recent meta-analyses suggest that microzooplankton biomass density scales linearly with phytoplankton biomass density, suggesting a simple, general rule may underpin trophic structure in the global ocean. Here, we use a set of highly simplified food web models, solved within a global general circulation model, to examine the core drivers of linear predator–prey scaling. We examine a parallel food chain model which assumes microzooplankton grazers feed on distinct size classes of phytoplankton and contrast this with a diamond food web model allowing shared microzooplankton predation on a range of phytoplankton size classes. Within these two contrasting model structures, we also evaluate the impact of fixed vs. density-dependent microzooplankton mortality. We find that the observed relationship between microzooplankton predators and prey can be reproduced with density-dependent mortality on the highest predator, regardless of choices made about plankton food web structure. Our findings point to the importance of parameterizing mortality of the highest predator for simple food web models to recapitulate trophic structure in the global ocean. 

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5. What Controls the Large‐Scale Efficiency of Carbon Transfer Through the Ocean's Mesopelagic Zone? Insights From a New, Mechanistic Model (MSPACMAM)

   https://doi.org/10.1029/2021GB007131  

   Dinauer, Ashley; Laufkötter, Charlotte; Doney, Scott C; Joos, Fortunat (October 2022, Global Biogeochemical Cycles)

   Abstract A key challenge for current‐generation Earth system models (ESMs) is the simulation of the penetration of sinking particulate organic carbon (POC) into the ocean interior, which has implications for projections of future oceanic carbon sequestration in a warming climate. This paper presents a new, cost‐efficient, mechanistic 1D model that prognostically calculates POC fluxes by carrying four component particles in two different size classes. Gravitational settling and removal/transformation processes are represented explicitly through parameterizations that incorporate the effects of particle size and density, dissolved oxygen, calcite and aragonite saturation states, and seawater temperature, density, and viscosity. The model reproduces the observed POC flux attenuation at 22 locations in the North Atlantic and North Pacific. The model is applied over a global ocean domain with seawater properties prescribed from observation‐based climatologies in order to address an important scientific question: What controls the spatial pattern of mesopelagic POC transfer efficiency? The simulated vertical POC transfer is more efficient at high latitudes than at low latitudes with the exception of oxygen minimum zones, which is consistent with recent inverse modeling and neutrally buoyant sediment trap studies. Here, model experiments show that the relative abundance of large‐sized, rapidly sinking particles and the slower rate of remineralization at high latitudes compensate for the region's lack of calcium carbonate ballast and the cold‐water viscous resistance, leading to higher transfer efficiencies compared to low‐latitude regions. The model could be deployed in ESMs in order to diagnose the impacts of climate change on oceanic carbon sequestration and vice versa. 

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