Vertical structure in chlorophyll profiles: influence on primary production in the Arctic Ocean

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'.

A Conceptual Approach to Partitioning a Vertical Profile of Phytoplankton Biomass Into Contributions From Two Communities

“…Consequently, the fluxes of carbon, driven by the productivity of community 1, will be important in controlling exchanges of inorganic carbon between the ocean and the atmosphere. Community 2, situated below the mixed‐layer, likely supported by new nutrients, may contribute significantly to new production (Bouman et al., 2020 ). Trophic pathways within the marine ecosystem may vary between the two communities of phytoplankton, with implications for secondary production, trophic energy transfer, and even fisheries (Gittings et al., 2021 ).…”

Bio-optical evidence for increasing Phaeocystis dominance in the Barents Sea

“…As there is currently much debate in the literature as to the effects of neglecting the SCM on production estimates (e.g. [57–59]), Arctic shelf SCMs clearly warrant further investigation.…”

The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning

“…[21,22]). Bouman et al [23] use a spectrally resolved model of primary production to identify the set of conditions under which subsurface chlorophyll maxima contribute to water column productivity, a key feature that escapes detection by satellites. They conclude that the uneven distribution and sparsity of chlorophyll measurements in the Arctic Ocean mean that the common practice of spatial and temporal averaging of profile data underestimates the importance of subsurface chlorophyll maxima.…”