Abstract
AbstractNumerous observations demonstrate that considerable spatial variability exists in components of the marine planktonic ecosystem at the mesoscale and submesoscale (100 km–1 km). The causes and consequences of physical processes at these scales (“eddy advection”) influencing biogeochemistry have received much attention. Less studied, the nonlinear nature of most ecological and biogeochemical interactions means that such spatial variability has consequences for regional estimates of processes including primary production and grazing, independent of the physical processes. This effect has been termed “eddy reactions.” Models remain our most powerful tools for extrapolating hypotheses for biogeochemistry to global scales and to permit future projections. The spatial resolution of most climate and global biogeochemical models means that processes at the mesoscale and submesoscale are poorly resolved. Modeling work has previously suggested that the neglected eddy reactions may be almost as large as the mean field estimates in some cases. This study seeks to quantify the relative size of eddy and mean reactions observationally, using in situ and satellite data. For primary production, grazing, and zooplankton mortality the eddy reactions are between 7% and 15% of the mean reactions. These should be regarded as preliminary estimates to encourage further observational estimates and not taken as a justification for ignoring eddy reactions. Compared to modeling estimates, there are inconsistencies in the relative magnitude of eddy reactions and in correlations which are a major control on their magnitude. One possibility is that models exhibit much stronger spatial correlations than are found in reality, effectively amplifying the magnitude of eddy reactions.
Highlights
Oceanic plankton play a significant role in the Earth’s biogeochemical cycles despite the disparity in size between organism and environment being up to 12 orders of magnitude, from ~1 m cyanobacteria to ~1000km ocean basins
As previously stated (Section 2.1.4), nitrate was not limiting during the cruise but the upper limit for kN was chosen as a simple proxy to explore the effect on eddy reaction (ER)/mean reaction (MR) if it had been
The reason why there is a maximum in absolute value is because at larger kN the magnitude of the eddy reaction decreases as the correlation between P and the nitrate limitation term decreases, whilst as kN tends to zero the nutrient limitation term becomes increasingly close to a value of one resulting in primary production becoming increasingly linear and, in the eddy reaction decreasing towards zero
Summary
Oceanic plankton play a significant role in the Earth’s biogeochemical cycles despite the disparity in size between organism and environment being up to 12 orders of magnitude, from ~1 m cyanobacteria to ~1000km ocean basins. Ecological interactions are inherently non-linear, with linear interactions being the exception rather than the rule This leaves those seeking to quantify the global role of plankton with two choices: they can either directly estimate key processes, such as primary production, at each scale, or they can find a way to infer the estimate at a given scale indirectly using an empirical relationship or parameterisation. Increases in computing power mean that the resolution of models is always improving, Earth system models still poorly resolve features at scales of 100km and smaller This is unfortunate, as this regime is one where timescales of the physical circulation - that transport, mix and disperse nutrients and plankton - are close to those of the ecological interactions within them, and the literature is increasingly well-stocked with evidence for the significant ways in which eddies, fronts, filaments and their ilk can influence biogeochemistry from local to global scales There are widely used techniques for representing the influence of sub-gridscale physical processes on the ocean circulation and tracers carried by it (hereafter ‘eddy transports’) but only preliminary studies (e.g. Wallhead et al, 2013) dealing with what we will call here the ‘eddy reaction’ terms
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