Diversity of Growth Rates Maximizes Phytoplankton Productivity in an Eddying Ocean

Abstract

The diverse community of phytoplankton and the heterotrophic ecosystem that it supplies affects the depth and efficacy of ocean primary production, as well as the cycling of carbon and other elements. Most of the world's ocean is depleted of nutrients (oligotrophic) in the sunlit (euphotic) layer where nutrients are taken up by phytoplankton, but nutrient concentration increases with depth below the euphotic layer. In such regions, the production of phytoplankton relies on the physical transport of nutrient-enriched water from depth to the euphotic zone where light enables photosynthesis (McGillicuddy et al., 1998). Over long spatial and temporal scales, the system is balanced such that the rate of export of organic matter is determined by the rate of nutrient input that contributes to photosynthetic carbon fixation (Ducklow et al., 2001;Falkowski et al., 1998).The upward transport of nutrient-rich water occurs via a range of mechanisms, including surface boundary layer turbulence, wind-driven upwelling, coastal upwelling, eddy uplift, and frontal instabilities (Denman & Gargett, 1983;Lipschultz et al., 2002). In the pelagic ocean, the physical supply of nutrients induced by vertical advection associated with fronts and eddies (Jenkins & Goldman, 1985) is thought to limit the rate of new production, which is the rate of phytoplankton production fueled by a fresh supply of macronutrients from outside the euphotic layer. Vertical velocities are typically 10 −3 -10 −4 times smaller than the horizontal velocities associated with ocean currents and eddies on scales of 1-100 km. But, submesoscale dynamics, associated with strong vertical vorticity (of the order of the planetary vorticity f) can result in vertical velocities of 𝐴𝐴  (100) m/day on spatial scales 𝐴𝐴  (1 km). These rapid vertical motions are thought to be particularly influential for phytoplankton growth (Mahadevan, 2016).Besides physical processes, the gradient in the mean nutrient distribution and the anomalies in concentration from the mean distribution also affect nutrient transport. Physical transport, as well as biological and chemical processes that create sinks (or sources) for nutrients, affect nutrient distribution. The spatial heterogeneity of

Oceanic Frontal Divergence Alters Phytoplankton Competition and Distribution

“…In our observations, it may be reasonable to model populations as restricted to a particular depth range because there is a strong depth partitioning of the community structure with reduced abundance of Synechococcus IV below the shallow mixed layer (Figure S10 from the Supporting Information S1). Of course, there are also many other important scenarios in which the assumption of a population restricted to a surface does not hold (Freilich et al, 2022).…”

“…Despite recent progress in the field of submesoscale dynamics (McWilliams, 2016), the ways that these dynamics influence ecological interactions are only beginning to be understood. For example, submesoscale flows can lead to higher productivity and alter community composition on timescales of days by facilitating the exchange of nutrients and organisms between the dark ocean interior and the sunlit surface layer (Freilich et al., 2022; Kessouri et al., 2020; Lévy et al., 2018; Uchida et al., 2020). The increased horizontal velocity divergence displayed by submesoscale flows may be especially relevant to phytoplankton ecology, as recent theoretical work in population genetics has shown that even weak horizontal velocity divergence can affect long‐term competition outcomes between organisms (Guccione et al., 2019; Plummer et al., 2019).…”

Mixed layer eddies supply nutrients to enhance the spring phytoplankton bloom

“…Compared to the SUB, the time-integrated new production is 10-13.5% and 32.2-39.7% lower in the MLE and NVF, respectively, with smaller differences in the extreme m values (0.5 and 1.25 day -1 ). This dependence on the growth rate might be due to the timescale coupling between nutrient uptake by the phytoplankton and nutrient vertical fluxes (Freilich et al, 2022). We focus the rest of our analysis using m= 0.75 day -1 .…”

Characterizing the role of non-linear interactions in the transition to submesoscale dynamics at a dense filament

“…Submesoscale dynamics, the dynamics that operate at the spatial scales between the nearly geostrophically balanced mesoscale eddies (∼100 km scales in mid-latitudes) and three-dimensional turbulence (smaller than 100 m), are particularly important for these boundary layer processes (McWilliams, 2016). Submesoscales influence ocean biogeochemistry by modulating vertical transport (Mahadevan, 2016;Freilich et al, 2022) and influence air-sea interactions by modulating buoyancy and momentum transfer (Strobach et al, 2022). Submesoscale dynamics are hypothesized to facilitate a forward cascade of kinetic energy resulting in dissipation of eddy kinetic energy in the surface ocean (Capet et al, 2008b;Barkan et al, 2015;.…”