Potential increasing dominance of heterotrophy in the global ocean

Abstract

Autotrophy is largely resource-limited in the modern ocean. Paleo evidence indicates this was not necessarily the case in warmer climates, and modern observations as well as standard metabolic theory suggest continued ocean warming could shift global ecology towards heterotrophy, thereby reducing autotrophic nutrient limitation. Such a shift would entail strong nutrient recycling in the upper ocean and high rates of net primary production (NPP), yet low carbon export to the deep ocean and sediments. We demonstrate transition towards such a state in the early 22nd century as a response to business-asusual representative concentration pathway forcing (RCP8.5) in an intermediate complexity Earth system model in three configurations; with and without an explicit calcifier phytoplankton class and calcite ballast model. In all models nutrient regeneration in the near-surface becomes an increasingly important driver of primary production. The near-linear relationship between changes in NPP and global sea surface temperature (SST) found over the 21st century becomes exponential above a 2-4 C • global mean SST change. This transition to a more heterotrophic ocean agrees roughly with metabolic theory.

Primary production sensitivity to phytoplankton light attenuation parameter increases with transient forcing

“…gration. The physical response is the same across all simulations and closely follows that described in Kvale et al (2015). Zonally averaged ocean surface temperatures rise by as much as 10 • C, North Atlantic maximum meridional overturning reduces from 20 to 9 Sverdrups (not shown), and widespread near-surface stratification occurs (Fig.…”

“…Figure 6 (left plot) shows some decline between years 1800 and 2100 in tropical depth-integrated NPP between 10 and 20 • for all simulations, with the declines generally increasing with decreasing k c . Declines in low-latitude NPP in simulations K1-K6 are not fully offset by increasing NPP in the Southern Ocean, which is driven by regional increasing temperature, wind-driven overturning, and nutrient remineralisation (Kvale et al, 2015). K1 and K2 demonstrate a particularly strong increase in NPP in the Southern Ocean around 60 • S, for reasons discussed below.…”

“…5). The phytoplankton and zooplankton respond to surface warming by increasing metabolic rates, and microbial fast recycling in the near surface increases (Kvale et al, 2015). Stratification reduces the availability of nutrients in the near surface.…”

“…Before the year 2100, physical limitation of nutrients is the dominant driver of changes in global NPP (Kvale et al, 2015). Over this time, choice of k c parameter value affects the magnitude and direction of how NPP in different regions responds.…”

“…The drivers of net primary production are of scientific interest as they may respond to anthropogenic climate change (e.g. Kvale et al, 2015;Laufkötter et al, 2015). To our knowledge, such a simple assessment has not appeared in the peer-reviewed domain despite there being a wide range of phytoplankton light attenuation parameter values currently in use (described in more detail below) and a demonstrated sensitivity of primary production, export, and nutrients in OGCMs and ESMs to how the underwater light field is modelled (described above).…”