Abyssal plain faunal carbon flows remain depressed 26 years

after a simulated deep-sea mining disturbance


By Tanja Stratmann, Lidia Lins, Autun Purser, Yann Marcon, Clara F. Rodrigues, Ascensão Ravara, Marina R. Cunha, Erik Simon-Lledó, Daniel O. B. Jones, Andrew K. Sweetman, Kevin Köser, and Dick van Oevelen / Biogeosciences


Abstract


Future deep-sea mining for polymetallic nodules in abyssal plains will negatively impact the benthic ecosystem, but it is largely unclear whether this ecosystem will be able to recover from mining disturbance and if so, to what extent and at what timescale. During the “DISturbance and reCOLonization” (DISCOL) experiment, a total of 22 % of the seafloor within a 10.8 km2 circular area of the nodule-rich seafloor in the Peru Basin (SE Pacific) was ploughed in 1989 to bury nodules and mix the surface sediment. This area was revisited 0.1, 0.5, 3, 7, and 26 years after the disturbance to assess macrofauna, invertebrate megafauna and fish density and diversity. We used this unique abyssal faunal time series to develop carbon-based food web models for each point in the time series using the linear inverse modeling approach for sediments subjected to two disturbance levels: (1) outside the plough tracks; not directly disturbed by plough, but probably suffered from additional sedimentation; and (2) inside the plough tracks. Total faunal carbon stock was always higher outside plough tracks compared with inside plough tracks. After 26 years, the carbon stock inside the plough tracks was 54 % of the carbon stock outside plough tracks. Deposit feeders were least affected by the disturbance, with modeled respiration, external predation, and excretion rates being reduced by only 2.6 % inside plough tracks compared with outside plough tracks after 26 years. In contrast, the respiration rate of filter and suspension feeders was 79.5 % lower in the plough tracks after 26 years. The “total system throughput” (T..), i.e., the total sum of modeled carbon flows in the food web, was higher throughout the time series outside plough tracks compared with the corresponding inside plough tracks area and was lowest inside plough tracks directly after the disturbance (8.63 × 10−3 ± 1.58 × 10−5 mmol C m−2 d−1). Even 26 years after the DISCOL disturbance, the discrepancy of T.. between outside and inside plough tracks was still 56 %. Hence, C cycling within the faunal compartments of an abyssal plain ecosystem remains reduced 26 years after physical disturbance, and a longer period is required for the system to recover from such a small-scale sediment disturbance experiment.



Figure 1 (a) Location of the DISCOL experimental area (DEA) in the Peru Basin (SE Pacific; red square) (b) detailed map of the DEA indicated by the white circle, (c) location of all plough tracks (black lines) that were observed by the "AUV Abyss" (Geomar Kiel) after 26 year during the R/V Sonne cruise SO242-1


1 Introduction


Abyssal plains cover approximately 50 % of the world's surface and 75 % of the seafloor (Ramírez-Llodrà et al., 2010). The abyssal seafloor is primarily composed of soft sediments consisting of fine-grained erosional detritus and biogenic particles (Smith et al., 2008). Occasionally, hard substrate occurs in the form of clinker from steam ships, glacial drop stones, outcrops of basaltic rock, whale carcasses, and marine litter (Amon et al., 2017; Kidd and Huggett, 1981; Radziejewska, 2014; Ramírez-Llodrà et al., 2011; Ruhl et al., 2008). In some soft-sediment regions, islands of hard substrate are provided by polymetallic nodules, authigenically formed deposits of metals, that grow at approximately 2 to 20 mm per million years (Guichard et al., 1978; Kuhn et al., 2017). These nodules have shapes and sizes of cauliflower florets, cannon balls, or potatoes, and are found on the sediment surface and in the sediment at water depths between 4000 and 6000 m in areas of the Pacific, Atlantic, and Indian Ocean (Devey et al., 2018; Kuhn et al., 2017).


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