A review of zooplankton and deep carbon fixation contributions to carbon cycling in the dark ocean

Abstract

Models of the marine carbon cycle assume that virtually all heterotrophic production in the open ocean is derived from near-surface carbon fixation (primary production) by phytoplankton. However, current carbon budget estimates show that respiration throughout the ocean far exceeds surface primary production. This disconnect can be grouped into two categories: Inaccurate estimates of water column respiration and carbon transport from metazoans; and missing primary production sources and. heterotrophic processing in the dark ocean. In this review, we examine the contribution to the ocean carbon cycle of chemoautotrophic production, as well as secondary production and respiration from meso-zooplankton and micro-nekton below 400 m depth. About one-third of epipelagic biomass in the ocean migrates diurnally, distributing dissolved organic carbon (DOC) and total nitrogen (TN), along with about 30–80% of the particulate organic carbon (POC) flux, from the upper ocean. Although mostly this occurs in the upper 400 m, migration depths can extend to 3000 m. In addition, up to 80% of the biomass of secondary consumers in the open ocean live part of their life cycle at depths up to 2000 m, contributing significantly to deep-sea respiration and particle flux, particularly over fall/winter in temperate-subarctic oceans, submarine canyons, and deep seas such as the Mediterranean. This active flux provides fresh organic input to the deep ocean at a time of year when surface primary productivity, and thus organic carbon (OC) flux to the deep ocean, is low. The complex spatial, temporal and depth scales of horizontal and vertical migration make modelling of the global oceanic carbon cycle extremely complex, requiring consideration of biomass movements throughout the entire water column over diurnal, lunar and seasonal cycles over broad geographic regions. An additional 10 to 50% of surface primary production occurs within mid-depth oxygen minimum zones (OMZs), fuelled by ammonia excreted from vertically migrating zooplankton concentrated near OMZ boundaries. Crustal sources such as gas and methane seeps, hydrothermal vents and submarine volcanoes support active deep-sea food webs, as well as contributing to upper ocean productivity. Crustal sources are conservatively estimated to provide >30%, and probably up to 50%, of oceanic OC flux to the dark ocean. These estimates are still poorly constrained but can no longer be ignored in global oceanic carbon cycles.