Drivers of trophic amplification of ocean productivity trends in a changing climate

Abstract

Abstract. Pronounced projected 21st century trends in regional oceanic net primary production (NPP) raise the prospect of significant redistributions of marine resources. Recent results further suggest that NPP changes may be amplified at higher trophic levels. Here, we elucidate the role of planktonic food web dynamics in driving projected changes in mesozooplankton production (MESOZP) found to be, on average, twice as large as projected changes in NPP by the latter half of the 21st century under a high emissions scenario in the Geophysical Fluid Dynamics Laboratory's ESM2M-COBALT (Carbon, Ocean Biogeochemistry and Lower Trophics) earth system model. Globally, MESOZP was projected to decline by 7.9 % but regional MESOZP changes sometimes exceeded 50 %. Changes in three planktonic food web properties -zooplankton growth efficiency (ZGE), the trophic level of mesozooplankton (MESOTL), and the fraction of NPP consumed by zooplankton (zooplankton-phytoplankton coupling, ZPC), explain the projected amplification. Zooplankton growth efficiencies (ZGE) changed with NPP, amplifying both NPP increases and decreases. Negative amplification (i.e., exacerbation) of projected subtropical NPP declines via this mechanism was particularly strong since consumers in the subtropics have limited surplus energy above basal metabolic costs. Increased mesozooplankton trophic level (MESOTL) resulted from projected declines in large phytoplankton production. This further amplified negative subtropical NPP declines but was secondary to ZGE and, at higher latitudes, was often offset by increased ZPC. Marked ZPC increases were projected for high-latitude regions experiencing shoaling of deep winter mixing or decreased winter sea iceboth tending to increase winter zooplankton biomass and enhance grazer control of spring blooms. Increased ZPC amplified projected NPP increases in the Arctic and damped projected NPP declines in the northwestern Atlantic and Southern Ocean. Improved understanding of the physical and biological interactions governing ZGE, MESOTL and ZPC is needed to further refine estimates of climate-driven productivity changes across trophic levels.

Projecting Marine Mammal Distribution in a Changing Climate

“…While progress has been made in projecting large-scale prey resource changes (e.g., Stock et al, 2014;Lefort et al, 2015), marine mammal species such as blue whales (Balaenoptera musculus) show tight coupling to smaller scale oceanographic features (Fiedler et al, 1998;Moore et al, 2002;Croll et al, 2005) associated with high euphausiid (krill) abundance (Santora et al, 2011). Similar relations have been exhibited by bowhead whales (Balaena mysticetus) across the Arctic (Laidre et al, 2007;Citta et al, 2015;George et al, 2015) and North Atlantic right whales (Baumgartner et al, 2003;Baumgartner and Mate, 2005).…”

“…Recent studies suggest robust amplification of productivity changes at higher trophic levels (Chust et al, 2014;Stock et al, 2014Stock et al, , 2017Lefort et al, 2015). This potential sensitivity is further heightened by mid-trophic level fishing (Forcada et al, 2012), motivating work to analyze marine mammal responses in a food web context.…”

“…Large intermodel differences in regional climate change projections limit confidence in scenarios for the future states of drivers of marine resource distribution and abundance (Hawkins and Sutton, 2009;Frölicher et al, 2016). Limited resolution of marine food webs in most earth system models may underestimate the potential amplification of climate change effects for higher trophic levels, particularly at regional scales (Chust et al, 2014;Stock et al, 2014Stock et al, , 2017Lefort et al, 2015). As a result of these limitations, confidence in predicting the magnitude and direction of climate change trends that are capable of altering marine resource distributions is generally greatest at ocean-basin spatial scales and multi-decadal to century timescales.…”

Response of O<sub>2</sub> and pH to ENSO in the California Current System in a high-resolution global climate model

“…Observed climatologies of O 2 , nitrate, phosphate, and silicate were taken from the World Ocean Atlas 2005 (WOA05; Garcia et al, 2006a, b), and modeled DIC and ALK were initialized from the Global Data Analysis Project (GLODAP; Key et al, 2004). The ocean biogeochemistry in the previous ESM2.6 development version was started from a spun-up ESM2M-COBALT 1860 control simulation (Stock et al, 2014a).…”

Net primary productivity estimates and environmental variables in the Arctic Ocean: An assessment of coupled physical-biogeochemical models

“…COBALT derives many elements of its biogeochemical formulation from the TOPAZ model (see Model 6). However, COBALT enhances the resolution of planktonic food web dynamics to better understand the climate impacts on the flow of energy from phytoplankton to fish [Stock et al, 2014b]. The model includes three phytoplankton groups (small, large, and diazotrophs) and uses the variable chlorophyll:C formulation of Geider et al, [1997] to estimate NPP.…”