The ecosystem services of the ocean's twilight zone

Abstract

The mesopelagic, or the “ocean’s twilight zone” (OTZ), occurring at depths between 200-1000m, is renowned for its unusual life forms, including 13 species of bristlemouths, which are thought to be the most numerous vertebrates on earth. Irigoien et al. (2014) have raised the median estimates of OTZ fish biomass by an order of magnitude to ~11-16 Bt, although much uncertainty surrounds this estimate (7-214 Bt). OTZ fish help support stocks of apex predators, including marine mammals and commercially important fish and cephalopods, and some observers have suggested that they could constitute an enormous potential source of protein for human consumption per se. Diel vertical migrations of zooplankton, fish, squids, jellies, salps, and other organisms comprise a “biological carbon pump” that may facilitate the very long-term sequestration of carbon in deep waters or on the seabed. The net amounts of carbon sequestered in the ocean are also highly uncertain, with estimates ranging between 4-12 Bt annually. The OTZ can be conceptualized as a stock of differentiated natural capital, subject to capital gains or losses, that may yield flows of benefits, termed “ecosystem services.” Here, approaches developed through the Millennium Ecosystem Assessment are utilized as an organizing framework for beginning to assess and evaluate OTZ ecosystem services. Understanding the benefits associated with these services is central to gauging the opportunity costs of activities that might diminish those benefits, such as unregulated fishery exploitation or the effects associated with climate changes in the ocean, including warming, acidification, decreased dissolved oxygen levels, or shifts in biological diversity. Such an understanding is foundational for conserving the OTZ in a sustainable way. The potential relevance of OTZ scientific research to contemporary developments in the international law of the high seas is discussed as well.