Abstract
A new individual-based plankton model is used to test the hypothesis that the timescale of photoacclimation of phytoplankton within the surface mixing layer of the ocean is slow relative to mixing, in which case the chlorophyll to carbon (Chl:C) ratio of individual cells shows little adjustment in response to changes in light environment driven by vertical displacement. Rates of photoacclimation are shown to be a strongly non-linear function of light intensity that depends on the balance of intrinsic chlorophyll synthesis at low irradiance versus increasing growth rate at high irradiance. Predicted photoacclimation was negligible for cells experiencing rates of turbulent mixing typical of the open ocean surface boundary layer (10−3 to 10-1 m2 s-1), in which case Chl:C is set by mean light intensity. The model was extended to incorporate a simple ecosystem of nutrient, phytoplankton, zooplankton and detritus and, using two-layer slab physics, used to study photoacclimation in a more realistic setting, the seasonal cycle of plankton dynamics at Ocean Weather Station India in the North Atlantic (59 °N, 20 °W). Results were remarkably similar when compared with an equivalent ecosystem model that used an Eulerian representation of phytoplankton, reinforcing our conclusion that mixing rates within the surface mixed layer of the ocean are typically too fast to permit photoacclimation by phytoplankton to ambient light.
Highlights
Primary production by phytoplankton drives the ocean food web, leading to export and sequestration of carbon in deep waters (Lutz et al, 2007)
The model was first run for Ocean Weather Station India (OWSI) to examine photoacclimation in a realistic setting, comparing results for Lagrangian and Eulerian representations of phytoplankton
This analysis involved tuning the value of parameter θmax, providing a value of 0.02 g Chl g C−1 that is representative of open ocean phytoplankton and which is subsequently used in the theoretical analysis of the interplay between acclimation and the timescales of mixing (Section 3.2)
Summary
Primary production by phytoplankton drives the ocean food web, leading to export and sequestration of carbon in deep waters (Lutz et al, 2007). The extent to which cells become fully acclimated depends on the relative timescales of mixing and chlorophyll synthesis and it may be the case that phytoplankton do not undergo significant photoacclimation in the surface layer of the ocean given the high mixing rates. Previous studies, both observational (Lewis et al, 1984; Anning et al, 2000) and modelling (Falkowski and Wirick, 1981; Franks and Marra, 1994; McGillicuddy, 1995), have differed in their conclusions on this matter, which has important consequences for the calculation of depth-integrated photosynthesis within the mixed layer of the ocean. Individual-based (Lagrangian) modelling approaches should be most appropriate for calculating primary production if photoacclimation is prevalent, in which the phytoplankton assemblage is represented as the sum of individual cells, each with its own unique position in the water column and associated life history, including the photophysiological response
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