Description: Subseabed CO2 storage is considered a future climate change mitigation technology. We investigated the ecological consequences of CO2 leakage for a marine benthic ecosystem. For the first time with a multidisciplinary integrated study, we tested hypotheses derived from a meta-analysis of previous experimental and in situ high-CO2 impact studies. For this, we compared ecological functions of naturally CO2-vented seafloor off the Mediterranean island Panarea (Tyrrhenian Sea, Italy) to those of nonvented sands, with a focus on biogeochemical processes and microbial and faunal community composition. High CO2 fluxes (up to 4 to 7 mol CO2 m−2 hour−1) dissolved all sedimentary carbonate, and comigration of silicate and iron led to local increases of microphytobenthos productivity (+450%) and standing stocks (+300%). Despite the higher food availability, faunal biomass (−80%) and trophic diversity were substantially lower compared to those at the reference site. Bacterial communities were also structurally and functionally affected, most notably in the composition of heterotrophs and microbial sulfate reduction rates (−90%). The observed ecological effects of CO2 leakage on submarine sands were reproduced with medium-term transplant experiments. This study assesses indicators of environmental impact by CO2 leakage and finds that community compositions and important ecological functions are permanently altered under high CO2.

Description: Sandy communities were exposed to six different seawater pCO2 regimes for a total of three months (17.12.2011–06.03.2012) in a climate - controlled room. Six header tanks were continuously supplied with filtered seawater from Kiel Fjord, each one connected to six experimental units (EU) ensuring continuous seawater supply. Each EU consisted of a round plastic container with a volume of 12.5 L containing ca. 9.5 L of sediment and an overlying water column of ca. 3 L. The lower 10 cm of the sediment consisted of sieved sand taken from a local beach (Kiel, Falckenstein: 54°23,66 N; 10°11.56 E) while the upper 10 cm consisted of surface sediment from the station at which the experimental animals were sampled to resemble natural conditions as well as to provide naturally occurring microbial and meiofauna communities. Bivalves and sediment were sampled in Kiel Fjord at Falckenstein with a Van Veen grab in 1–2 m depth and kept in holding basins at 9 °C before being placed in EUs. Numbers per EU simulated a natural size distribution: 5 M. arenaria (size classes: 0.5–1 cm: 2 animals; 1–1.5 cm: 2 animals; 2–2.5 cm: 1 animal), 1 M. balthica, and 40 C. edule (size classes: 0–0.5 cm: 3 animals; 0.5–1 cm: 18 animals; 1–1.5 cm: 11 animals; 1.5–2 cm: 7 animals; 2–2.5 cm: 1 animal). Small gastropods (exclusively Hydrobia spp.) were abundant with ~10 individuals per EU. Due to their small size (〈 0.5 mm) they were randomly distributed within all EUs with the sieved sediment. Due to the natural low diversity of the Baltic, the density of other macrofauna individuals was 〈 1 individuals per m². These low abundant species (e.g. nereid polychaetes, pharid bivalve species) were excluded from the experiment. The EUs were kept in a seawater flow-through system for two weeks under control conditions prior to the experiment to allow proper acclimatization of biogeochemistry and the faunal community. Seawater pH was maintained in the header tanks using a pH feedback system (IKS Aquastar, iksComput- ersysteme GmbH, Karlsbad, Germany). Treatment levels were achieved through continuous addition of acidified water from the header tanks into the overlaying seawater of each EU and included levels of 900 µatm (control, pH 7.8 NBS scale), 1,500 µatm (pH 7.7), 2,900 µatm (pH 7.4), 6,600 µatm (pH 7.0), 12,800 µatm (pH 6.7), and 24,400 µatm (pH 6.4). 900 µatm was used as a control due to the high background pCO2 in Kiel Fjord. To support the bivalve nutritional needs unicellular algae (Rhodomonas sp.) were cultured and added continuously into the header tanks via a peristaltic pump, thus maintaining a stable concentration of 3,500–4,000 cells ml−1 within header tanks. A flow rate of 100 ml min−1 was provided to each EU from the respective header tank via gravity feed. Throughout the experiment, pH, salinity, temperature, and flow rate were measured daily in each replicate. Salinity and temperature fluctuated in accordance with naturally occurring changes in Kiel Fjord seawater (14.6–20.5 psu and 4.3–8.9 °C, respectively). Light conditions were similar for all EUs. Dead animals were removed daily and behaviour of bivalves (presence/absence on the sediment surface) was noted every other day starting in the third experimental week. Carbonate chemistry and algae concentration in the EUs were measured weekly. Dissolved inorganic carbon (CT) was measured using an Automated Infrared Inorganic Carbon Analyzer (AIRICA, Marianda, Kiel, Germany). Seawater chemistry (pCO2 and calcium carbonate saturation state) was then calculated according to the guide to best practices for ocean CO2 measurements, using CO2SYS57 with pH (NBS scale) and CT, temperature, salinity, and first and second dissociation constants of carbonic acid in seawater.

Description: Between September 2016 and August 2017, we conducted year-long reciprocal transplantation experiments using the cold-water coral Desmophyllum dianthus along natural oceanographic horizontal and vertical gradients (vertically: 20 m to 300 m depth and horizontally: head to mouth of fjord) in Comau Fjord to study seasonal changes and the acclimatisation potential of its biochemical composition. Seasonal energy reserves (proteins, carbohydrates and lipids) and the C:N ratio of native and novel (cross-transplanted) corals were measured at six shallow (A-F, 20 m) and one deep station (Ed, 300 m) during autral summer (January), autumn (May) and winter (August).