Big in the benthos: Future change of seafloor community biomass in a global, body size‐resolved model

Abstract

Deep-water benthic communities in the ocean are almost wholly dependent on near-surface pelagic ecosystems for their supply of energy and material resources. Primary production in sunlit surface waters is channelled through complex food webs that extensively recycle organic material, but lose a fraction as particulate organic carbon (POC) that sinks into the ocean interior. This exported production is further rarefied by microbial breakdown in the abyssal ocean, but a residual ultimately drives diverse assemblages of seafloor heterotrophs. Advances have led to an understanding of the importance of size (body mass) in structuring these communities. Here we force a size-resolved benthic biomass model, BORIS, using seafloor POC flux from a coupled ocean-biogeochemistry model, NEMO-MEDUSA, to investigate global patterns in benthic biomass. BORIS resolves 16 size classes of metazoans, successively doubling in mass from approximately 1 μg to 28 mg. Simulations find a wide range of seasonal responses to differing patterns of POC forcing, with both a decline in seasonal variability, and an increase in peak lag times with increasing body size. However, the dominant factor for modelled benthic communities is the integrated magnitude of POC reaching the seafloor rather than its seasonal pattern. Scenarios of POC forcing under climate change and ocean acidification are then applied to investigate how benthic communities may change under different future conditions. Against a backdrop of falling surface primary production (-6.1%), and driven by changes in pelagic remineralization with depth, results show that while benthic communities in shallow seas generally show higher biomass in a warmed world (+3.2%), deep-sea communities experience a substantial decline (-32%) under a high greenhouse gas emissions scenario. Our results underscore the importance for benthic ecology of reducing uncertainty in the magnitude and seasonality of seafloor POC fluxes, as well as the importance of studying a broader range of seafloor environments for future model development.

The meagre future of benthic fauna in a coastal sea—Benthic responses to recovery from eutrophication in a changing climate

“…These results are in line with both observations and experimental work suggesting that higher temperatures increase zooplankton growth rates, metabolic demands and grazing rates on phytoplankton, which leads to decreased export production (Tamelander, Spilling, & Winder, and references therein). Also Maar and Hansen () found negative effects of warming on sedimentation and deposit‐feeder biomass in a simulation study of the SW Baltic Sea, whereas Yool et al () predicted a global decrease in benthic biomass due to decreased POC input to the sediments in a warmer climate, although not in shallow (<100 m) areas. The different result for shallow areas can probably be attributed to the different scales of the model systems, with a photic zone in the Baltic Sea which is an order of magnitude shallower than in the global oceans.…”

“…it-feeder biomass in a simulation study of the SW Baltic Sea, whereasYool et al (2017) predicted a global decrease in benthic biomass due to decreased POC input to the sediments in a warmer climate, although not in shallow (<100 m) areas. The different result for shallow areas can probably be attributed to the different scales of the model systems, with a photic zone in the Baltic Sea which is an order of magnitude shallower than in the global oceans.…”

Has Phytodetritus Processing by an Abyssal Soft-Sediment Community Recovered 26 Years after an Experimental Disturbance?

“…In contrast, a single mining operation will likely remove polymetallic nodules over an area of 300-800 km 2 per year (Smith et al, 2008) and last for 15-30 years (Levin et al, 2016). Moreover, we did not take cumulative effects of deep-sea mining into account like the overlap of sediment plumes from close-by mining operations, changes in POC export fluxes due to climate-change or biodiversity loss due to deep-sea mining (Levin et al, 2016;Sweetman et al, 2017;Van Dover et al, 2017;Yool et al, 2017).…”

Recovery of Holothuroidea population density, community composition, and respiration activity after a deep‐sea disturbance experiment

“…The type of disturbance examined here (sediment and nodules ploughing with little removal of the upper sediment) is also not representative of the actual disturbances, which will be associated with nodule and surface sediment removal (Thiel und Tiefsee‐Umweltschutz 2001). Furthermore, the CCZ is a key target area for deep‐sea mining and spans across a range of trophic settings, which may delay recovery, and potentially cumulative effects, such as overlapping mining plumes from nearby mining operations or climate change‐related shifts in POC export fluxes (Levin et al 2016; Sweetman et al ; Yool et al ) were not considered in this current study. Ultimately, to gain more knowledge about potential recovery rates of fauna after industrial‐scale mining, a scientifically supported industrial test‐mining operation in the CCZ is required, and all species of megafauna as well as all size classes of fauna should be monitored for several decades after resource extraction.…”