Abstract
Subsurface chlorophyll maximum (SCM) layers are prevalent throughout the Arctic Ocean under stratified conditions and are observed both in the wake of retreating sea ice and in thermally stratified waters. The importance of these layers on the overall productivity of Arctic pelagic ecosystems has been a source of debate. In this study, we consider the three principal factors that govern productivity within SCMs: the shape of the chlorophyll profile, the photophysiological characteristics of phytoplankton and the availability of light in the layer. Using the information on the biological and optical parameters describing the vertical structure of chlorophyll, phytoplankton absorption and photosynthesis–irradiance response curves, a spectrally resolved model of primary production is used to identify the set of conditions under which SCMs are important contributors to water-column productivity. Sensitivity analysis revealed systematic errors in the estimation of primary production when the vertical distribution of chlorophyll was not taken into account, with estimates of water-column production using a non-uniform profile being up to 97% higher than those computed using a uniform one. The relative errors were shown to be functions of the parameters describing the shape of the biomass profile and the light available at the SCM to support photosynthesis. Given that SCM productivity is believed to be largely supported by new nutrients, it is likely that the relative contribution of SCMs to new production would be significantly higher than that to gross primary production. We discuss the biogeochemical and ecological implications of these findings and the potential role of new ocean sensors and autonomous underwater vehicles in furthering the study of SCMs in such highly heterogeneous and remote marine ecosystems.This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.
Highlights
The influence of melting sea ice on the density structure in the upper ocean strongly impacts the availability of the two major factors influencing phytoplankton growth: the access to light and the supply of nutrients
There was a weak negative relationship between the surface chlorophyll concentration (Bsurf) and zm (R2 = 0.14, p < 0.001), which underscores the difficulty of using surface biomass as a predictor of the vertical structure of chlorophyll concentration in Arctic marine ecosystems [3,4]
We show that the importance of the relative contribution of subsurface chlorophyll maximum (SCM) to water-column productivity is strongly controlled by the shape of the chlorophyll profile and that a failure to account for the presence of SCMs can lead to a significant underestimate in water-column primary production, with relative errors approaching 100%
Summary
The influence of melting sea ice on the density structure in the upper ocean strongly impacts the availability of the two major factors influencing phytoplankton growth: the access to light and the supply of nutrients. The complex vertical structure of SCMs in Arctic seas is difficult to predict using surface ocean observables such as chlorophyll concentration, regional [6] and pan-Arctic [4] algorithms have been proposed based on in situ datasets. As a direct result of the sparsity of chlorophyll data in the Arctic Ocean, our estimates of the contribution of SCMs to water-column primary production often involve using compilations of in situ chlorophyll measurements and partitioning them according to ecological provinces [5]; or the surface chlorophyll concentration and bloom phase [4]; or region and degree of ice cover [3]; or geographical sector [2]. Sensitivity analysis has demonstrated that the impact of chlorophyll layers on estimates of water-column primary production depends on both the sharpness and the location of the chlorophyll peak in relation to the photic depth [8]
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