Biodiversity response to natural gradients of multiple stressors on continental margins

Abstract

Sharp increases in atmospheric CO 2 are resulting in ocean warming, acidification and deoxygenation that threaten marine organisms on continental margins and their ecological functions and resulting ecosystem services. The relative influence of these stressors on biodiversity remains unclear, as well as the threshold levels for change and when secondary stressors become important. One strategy to interpret adaptation potential and predict future faunal change is to examine ecological shifts along natural gradients in the modern ocean. Here, we assess the explanatory power of temperature, oxygen and the carbonate system for macrofaunal diversity and evenness along continental upwelling margins using variance partitioning techniques. Oxygen levels have the strongest explanatory capacity for variation in species diversity. Sharp drops in diversity are seen as O 2 levels decline through the 0.5-0.15 ml l 21 (approx. 22-6 mM; approx. 21-5 matm) range, and as temperature increases through the 7-108C range. pCO 2 is the best explanatory variable in the Arabian Sea, but explains little of the variance in diversity in the eastern Pacific Ocean. By contrast, very little variation in evenness is explained by these three global change variables. The identification of sharp thresholds in ecological response are used here to predict areas of the seafloor where diversity is most at risk to future marine global change, noting that the existence of clear regional differences cautions against applying global thresholds.

Major impacts of climate change on deep-sea benthic ecosystems

“…Already, distinct deep-water masses in the Southern Ocean (Helm et al, 2011), eastern North Atlantic (e.g., Sub-polar Mode Water, the Intermediate Water and the Mediterranean Outflow Water; Stendardo et al, 2015), and in the West Pacific (Levin, 2003;Helly and Levin, 2004). Ecosystems within and on the fringes of OMZs could be particularly affected by the O 2 and warm- ing changes predicted for bathyal environments (Table 3; Figures 2, 3; Keeling et al, 2010;Sperling et al, 2016).…”

“…Sediment-burrowing fauna will probably be increasingly excluded as water column O 2 levels decline leading to a reduction in the mixed layer depth, and altered bioturbation rates and C-sequestration in sediments (Smith et al, 1997;Smith et al, 2000;Sperling et al, 2016; Figure 4B). This cascade of effects is highly likely at depths of 500-1000 m in parts of the Eastern Pacific, where OMZ expansion is projected to exceed thresholds for biodiversity (Sperling et al, 2016; Figures 2, 3).…”

Ecophysiological limits to aerobic metabolism in hypoxia determine epibenthic distributions and energy sequestration in the northeast Pacific ocean

“…Species-specific adaptations and evolutionary history strongly reflect the oxygen characteristics of a region. The O crit 2 we measured for our focal species, the [O 2 ] env level at which they respond to 'hypoxia', and the metabolism-based hypoxia thresholds we calculated for the East Pacific Ocean all indicate that species from this region are adapted to much lower [O 2 ] env (Tunnicliffe 1981;Chu and Tunnicliffe 2015a;Sperling et al 2016) compared with species from the Atlantic Ocean. The marked difference in metabolic thresholds we report between the East Pacific Ocean and both sides of the Atlantic Ocean follows the [O 2 ] env minima characterizing these regions (Kamykowski and Zentara 1990;Karstensen et al 2008).…”

“…Thus, an imperative first step for establishing realistic predictions on the biological consequences of deoxygenation will be to assess oxygen requirements and physiological constraints of the local species. This would explain the oceanscale differences already observed in species responses to deoxygenation (Prince et al 2010;Sperling et al 2016) and will ground projections regarding future aerobic habitat loss and reduced ecosystem function.…”

“…As a result, general thresholds are biased toward Atlantic systems, where a majority of past hypoxia research has been performed, and fail to predict in situ species distributions in areas with historically lower oxygen levels such as the northeast Pacific Ocean (Chu and Tunnicliffe ). In such regions, benthic communities are naturally found in lower [O 2 ] env (Keller et al ; Chu and Tunnicliffe ) and respond to hypoxia at much lower oxygen levels than what general thresholds would suggest (Sperling et al ). Although the metabolic relationship between oxygen demand and [O 2 ] env availability is well‐established, few studies have applied as a tool for assessing ecological response of in situ distributions (Deutsch et al ).…”