• European Marine Science
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• Limnology and Oceanography close

We studied the methane (CH4) budget of Lake Baikal, the most voluminous lake in the world and the only freshwater body with known occurrences of methane hydrates in the sediments. CH4 concentrations were measured in water samples taken during six expeditions between October 2002 and June 2004; these expeditions covered the entire lake volume. A one‐dimensional model was applied to (1) estimate the large‐scale vertical CH4 fluxes within the South Basin of Lake Baikal, (2) determine the exchange with the atmosphere, and (3) constrain the CH4 inputs from seeps and mud volcanoes to the deep water. Fluxes were generally several orders of magnitude below previous estimates. The annual internal source of CH4 to the pelagic surface mixed layer was roughly estimated to be 40 Mg CH4. A large part of this input diffuses downwards and is consumed in the water column, with a CH4 residence time of about 4 yr. The input of CH4 from deep gas seeps and mud volcanoes is less than a few 10 Mg CH4 yr−1, most of which is oxidized before reaching the surface. The net CH4 flux between the atmosphere and the main waterbody distant from shallow areas is negligible and not significantly different from zero. However, occasional high CH4 concentrations, both in the surface water and in the atmosphere, indicate that the region near the Selenga delta is a local CH4 source to the atmosphere. CH4 fluxes in the Central Basin are very similar to those in the South Basin, whereas in the North Basin, the shallow CH4 sources are weaker.

AbstractAnthropogenic atmospheric loading of CO2 raises concerns about combined effects of increasing ocean temperature and acidification, on biological processes. In particular, the response of appendicularian zooplankton to climate change may have significant ecosystem implications as they can alter biogeochemical cycling compared to classical copepod dominated food webs. However, the response of appendicularians to multiple climate drivers and effect on carbon cycling are still not well understood. Here, we investigated how gelatinous zooplankton (appendicularians) affect carbon cycling of marine food webs under conditions predicted by future climate scenarios. Appendicularians performed well in warmer conditions and benefited from low pH levels, which in turn altered the direction of carbon flow. Increased appendicularians removed particles from the water column that might otherwise nourish copepods by increasing carbon transport to depth from continuous discarding of filtration houses and fecal pellets. This helps to remove CO2 from the atmosphere, and may also have fisheries implications.

AbstractMining polymetallic nodules on abyssal plains will have adverse impacts on deep‐sea ecosystems, but it is largely unknown whether the impacted ecosystem will recover, and if so at what rate. In 1989 the “DISturbance and reCOLonization” (DISCOL) experiment was conducted in the Peru Basin where the seafloor was disturbed with a plough harrow construction to explore the effect of small‐scale sediment disturbance from deep‐sea mining. Densities of Holothuroidea in the region were last investigated 7 yr post‐disturbance, before 19 yr later, the DISCOL site was re‐visited in 2015. An “ocean floor observatory system” was used to photograph the seabed across ploughed and unploughed seafloor and at reference sites. The images were analyzed to determine the Holothuroidea population density and community composition, which were combined with in situ respiration measurements of individual Holothuroidea to generate a respiration budget of the study area. For the first time since the experimental disturbance, similar Holothuroidea densities were observed at the DISCOL site and at reference sites. The Holothuroidea assemblage was dominated by Amperima sp., Mesothuria sp., and Benthodytes typica, together contributing 46% to the Holothuroidea population density. Biomass and respiration were similar among sites, with a Holothuroidea community respiration of 5.84 10−4 ± 8.74 10−5 mmol C m−2 d−1 at reference sites. Although these results indicate recovery of Holothuroidea, extrapolations regarding recovery from deep‐sea mining activities must be made with caution: results presented here are based on a relatively small‐scale disturbance experiment as compared to industrial‐scale nodule mining, and also only represent one taxonomic class of the megafauna.