Climate, carbon cycling, and deep-ocean ecosystems

Abstract

Climate variation affects surface ocean processes and the production of organic carbon, which ultimately comprises the primary food supply to the deep-sea ecosystems that occupy Ϸ60% of the Earth's surface. Warming trends in atmospheric and upper ocean temperatures, attributed to anthropogenic influence, have occurred over the past four decades. Changes in upper ocean temperature influence stratification and can affect the availability of nutrients for phytoplankton production. Global warming has been predicted to intensify stratification and reduce vertical mixing. Research also suggests that such reduced mixing will enhance variability in primary production and carbon export flux to the deep sea. The dependence of deep-sea communities on surface water production has raised important questions about how climate change will affect carbon cycling and deep-ocean ecosystem function. Recently, unprecedented time-series studies conducted over the past two decades in the North Pacific and the North Atlantic at >4,000-m depth have revealed unexpectedly large changes in deep-ocean ecosystems significantly correlated to climate-driven changes in the surface ocean that can impact the global carbon cycle. Climate-driven variation affects oceanic communities from surface waters to the much-overlooked deep sea and will have impacts on the global carbon cycle. Data from these two widely separated areas of the deep ocean provide compelling evidence that changes in climate can readily influence deep-sea processes. However, the limited geographic coverage of these existing time-series studies stresses the importance of developing a more global effort to monitor deepsea ecosystems under modern conditions of rapidly changing climate.

Major impacts of climate change on deep-sea benthic ecosystems

“…Holothurian community dynamics have been examined in detail at long-term time-series sites in relation to shifts in surface phytodetritus input linked to the North Atlantic Oscillation (Porcupine Abyssal Plain, NE Atlantic; Billett et al, 2001) and to the Northern Oscillation Index and Bakun Upwelling Index (Station M, NE Pacific; Ruhl et al, 2014). At both sites, variation in these climate indices were correlated with increased pulses of POC, which resulted in significant increases in abundance of holothurian species, particularly those species able to rapidly use phytodetrital material and successfully reproduce and recruit (Billett et al, 2001;Smith et al, 2006Smith et al, , 2009Huffard et al, 2016). These studies confirm the predictions of basic macro-ecology and spatial-gradient studies that climate change fluctuations can cause temporal changes in food inputs leading to changes in overall macro-and megafaunal biomass and community structure in terms of size distributions and dominance (Ruhl et al, 2008.…”

“…Recent studies have suggested that the NE Atlantic Ocean could also undergo similar reductions in POC flux . Because the quantity and quality of POC flux is an important ecological forcing factor in the abyss, abyssal ecosystems will be highly sensitive to such changes (Smith et al , 2009Tittensor et al, 2011). For example, 3-fold reductions in POC flux (e.g., from 1.5 to 0.5 g C m -2 yr -1 ), which might occur in the equatorial Pacific (Laws et al, 2000), are predicted to halve benthic microbial and nematode biomass ( Figure 4D).…”

Inter-annual species-level variations in an abyssal polychaete assemblage (Sta. M, NE Pacific, 4000 m)

“…M megafauna during both periods, (i.e. at both locations, 1991-2005[1989-2004Ruhl et al, 2014Ruhl et al, ] and 2009Ruhl et al, -2011Ruhl et al, [2006Ruhl et al, -2012Kuhnz et al, 2014]). It is conceivable that small active surface deposit feeding megafauna may have intercepted much of the incoming organic matter flux, reducing the magnitude of temporal variation experienced by the infaunal benthos .…”

“…Billett et al, 2010;Ruhl and Smith, 2004;Smith et al, 2009). However, the mechanisms by which environmentally-driven resource variations impact the structure and function of abyssal faunas are challenging to disentangle.…”

“…However, the mechanisms by which environmentally-driven resource variations impact the structure and function of abyssal faunas are challenging to disentangle. A better understanding of the ecological and biogeochemical processes behind these dynamics is necessary to assess: (a) how deep-sea systems will respond to climate change (Jones et al, 2014;Mora et al, 2013), (b) increasing seabed resource exploitation (Ramirez-Llodra et al, 2011), and (c) how long-term carbon cycles such as remineralisation, bioturbation, and burial (Smith et al, 2009), are influenced by benthic community dynamics. Significant inter-annual changes have been observed in abyssal macrofaunal communities (North East Pacific, Ruhl et al, 2008;North East Atlantic, Laguionie-Marchais et al, 2013;Soto et al, 2010) as exemplified by polychaetes, typically the dominant macrofaunal taxon (Paterson et al, 2009).…”