A large eddy simulation study of the formation of deep chlorophyll/biological maxima in un‐stratified mixed layers: The roles of turbulent mixing and predation pressure

Abstract

Recent experimental measurements of fluorescence values and turbulent energy dissipation rates, recorded in weakly stratified boundary layers in the open ocean, have highlighted a significant correlation between the formation of deep chlorophyll maxima (DCM) and turbulent mixing. Specifically, the depth of many DCM are observed to lie below, but within about one standard deviation, of the point at which the energy dissipation rate profile reaches its maximum. This correlation of DCM and turbulent mixing is both exciting and curious, as conventional thinking tends to see the latter as a destructive rather than a constructive agent in regards to the formation of deep biological maxima (DBM), for which DCM data is usually interpreted as a proxy. In order to investigate this phenomenon, a three-dimensional large eddy simulation (LES) of the ocean boundary layer was combined with a generic nutrient-phytoplankton-zooplankton (NPZ) type biological model, in order establish what mechanisms might be driving the experimental observations. Simulations of the LES-NPZ model, based upon various sets of generic biological parameters, demonstrate DCM/DBM formation occurs at normalized depths close to those seen in the experimental observations. The simulations support the hypothesis that the DBM are generated by a combination of zooplankton predation pressure curtailing phytoplankton growth near the surface, and a decline in the strength of the vertical mixing processes advecting nutrient through the boundary layer. In tandem, these produce a region of the water column in which predation pressure is relatively low and nutrient aggregation relatively high, suitable conditions for DBM formation.The presence of deep (or sub-surface) chlorophyll/biological maxima (DCM/DBM) observed in vertical fluorescence profiles is one of the most ubiquitous features of the world's oceans (Macıas et al. 2008), and much research effort has been devoted to understanding the mechanisms behind their formation and dynamics (see Cullen 2015 for a comprehensive review of the subject). Observations of DCM are not just confined to the ocean boundary layer (e.g., Cullen 1982;Estrada et al. 1993;Letelier et al. 2004; Macıas et al. 2013), but are a pervasive feature of the limnology of lakes too (Hamilton et al. 2010;White and Matsumoto 2012; Simmonds et al. 2015). Here, the terminology DCM specifies a broad (10-20 m) region of relatively high chlorophyll concentrations (but weak concentration gradients), usually to be found at or somewhat below the mixed layer depth. This should not be confused with the concept of a rapidly varying biological "thin layer," confined to vertical scales of just a few meters (e.g., Dekshenieks et al. 2001;McManus et al. 2003McManus et al. , 2005Benoit-Bird et al. 2009;Durham et al. 2009;Johnston et al. 2009), which are usually found residing within the top 10 m or so of the water column.A number of different postulates have been advanced to attempt to explain both the formation of DCM, and the reasons as to why they ar...

Physical flow effects can dictate plankton population dynamics

“…One criticism of the current approach is that the population dynamics depend only on local concentrations and not fluxes. Clearly, higher contact rates may increase grazing of phytoplankton by zooplankton, an effect that could be considered in future investigations, in line with Lewis et al [30].…”

Deep chlorophyll maxima across a trophic state gradient: A case study in the Laurentian Great Lakes

“…Of course, several of these additional processes, as well as in situ growth and photoacclimation, may influence DCM dynamics across lakes. For example, phytoplankton migration for nutrient acquisition (Heaney and Eppley 1981;Ralston et al 2007;Baek et al 2009;Ryan et al 2010), variable phytoplankton sinking rates (Steele and Yentsch 1960), and physical processes (even in the absence of stable stratification, Lewis et al 2017) can be important factors affecting DCM formation. Vertical migrations of mobile plankton follow a variety of patterns that could accentuate DCM or cause additional peaks (Lande et al 1989;Cullen and MacIntyre 1998;Prairie et al 2011), and these features can vary with time of day.…”

Material Transport in the Ocean Mixed Layer: Recent Developments Enabled by Large Eddy Simulations

“…From a transport perspective, all three concentration fields are passively transported by the flow (i.e., they behave as tracer particles), and the model does not account for zooplankton swimming. Lewis et al () and Brereton et al () employed the same model to investigate the formation of peaks in biological activity in the middle of the OML and the conditions leading to horizontal patchiness in plankton populations, respectively. Both studies considered Langmuir turbulence with fixed La t =0.3, but varying wind conditions.…”