Beschreibung: 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 10.8km2 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 × 10−5mmolCm−2d−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.

Beschreibung: Corals and macroalgae release large quantities of dissolved organic matter (DOM), one of the largest sources of organic matter produced on coral reefs. By rapidly taking up DOM and transforming it into particulate detritus, coral reef sponges are proposed to play a key role in transferring the energy and nutrients in DOM to higher trophic levels via the recently discovered sponge loop. DOM released by corals and algae differs in quality and composition, but the influence of these different DOM sources on recycling by the sponge loop has not been investigated. Here, we used stable isotope pulse-chase experiments to compare the processing of naturally sourced coral- and algal-derived DOM by three Red Sea coral reef sponge species: Chondrilla sacciformis, Hemimycale arabica and Mycale fistulifera. Incubation experiments were conducted to trace 13C- and 15N-enriched coral- and algal-derived DOM into the sponge tissue and detritus. Incorporation of 13C into specific phospholipid-derived fatty acids (PLFAs) was used to differentiate DOM assimilation within the sponge holobiont (i.e. the sponge host vs. its associated bacteria). All sponges assimilated both coral- and algal-derived DOM, but incorporation rates were significantly higher for algal-derived DOM. The two DOM sources were also processed differently by the sponge holobiont. Algal-derived DOM was incorporated into bacteria-specific PLFAs at a higher rate while coral-derived DOM was more readily incorporated into sponge-specific PLFAs. A substantial fraction of the dissolved organic carbon (C) and nitrogen (N) assimilated by the sponges was subsequently converted into and released as particulate detritus (15–24% C and 27–49% N). However, algal-derived DOM was released as detritus at a higher rate. The higher uptake and transformation rates of algal- compared with coral-derived DOM suggest that reef community phase shifts from coral to algal dominance may stimulate DOM cycling through the sponge loop with potential consequences for coral reef biogeochemical cycles and food webs.