Bottle incubations were conducted to examine how exposure to seawater containing 8000 ppm carbon dioxide (CO 2 ; pH 6.95) influenced the growth and reproduction of the keystone copepod Calanus finmarchicus. The chosen concentration of CO 2 is expected to occur over 100s of cubic kilometres of seawater as a result of marine CO 2 storage/disposal, and is also representative of the predicted 'worst-case' atmospheric CO 2 scenario in the year 2300. Growth (egg production and biomass loss) in adult female copepods was not affected by the simulated ocean acidification. In contrast, a maximum of only 4% of the eggs successfully yielded nauplii after 72 h in the experimental treatment. Our results demonstrate that environmental risk assessments for marine CO 2 storage/disposal must look beyond adult mortality as an endpoint. Furthermore, if CO 2 is to be disposed of in the deep sea, the location and timing of such activities must take into consideration the overwintering populations of C. finmarchicus.

Gelatinous plankton are critical components of marine ecosystems. Recent studies are providing evidence of increased population outbursts of such species. Jellyfish seem to respond when an ecosystem is over-fished, and their ecology is under-researched.

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Deep-sea sediments cover B70% of Earth's surface and represent the largest interface between the biological and geological cycles of carbon. Diatoms and zooplankton faecal pellets naturally transport organic material from the upper ocean down to the deep seabed, but how these qualitatively different substrates affect the fate of carbon in this permanently cold environment remains unknown. We added equal quantities of 13 C-labelled diatoms and faecal pellets to a cold water (À0.7 1C) sediment community retrieved from 1080 m in the Faroe-Shetland Channel, Northeast Atlantic, and quantified carbon mineralization and uptake by the resident bacteria and macrofauna over a 6-day period. High-quality, diatom-derived carbon was mineralized 4300% faster than that from low-quality faecal pellets, demonstrating that qualitative differences in organic matter drive major changes in the residence time of carbon at the deep seabed. Benthic bacteria dominated biological carbon processing in our experiments, yet showed no evidence of resource qualitylimited growth; they displayed lower growth efficiencies when respiring diatoms. These effects were consistent in contrasting months. We contend that respiration and growth in the resident sediment microbial communities were substrate and temperature limited, respectively. Our study has important implications for how future changes in the biochemical makeup of exported organic matter will affect the balance between mineralization and sequestration of organic carbon in the largest ecosystem on Earth.

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