Scott C. Doney

and 9 more

This study characterized ocean biological carbon pump metrics in the second iteration of the REgional Carbon Cycle Assessment and Processes (RECCAP2) project, a coordinated, international effort to constrain contemporary ocean carbon air-sea fluxes and interior carbon storage trends using a combination of observation-based estimates, inverse models, and global ocean biogeochemical models. The analysis here focused on comparisons of global and biome-scale regional patterns in particulate organic carbon production and sinking flux from the RECCAP2 model ensemble against observational products derived from satellite remote sensing, sediment traps, and geochemical methods. There was generally encouraging model-data agreement in large-scale spatial patterns, though with substantial spread across the model ensemble and observational products. The global-integrated, model ensemble-mean export production, taken as the sinking particulate organic carbon flux at 100 m (6.41 ± 1.52 Pg C yr–1), and export ratio defined as sinking flux divided by net primary production (0.154 ± 0.026) both fell at the lower end of observational estimates. Comparison with observational constraints also suggested that the model ensemble may have underestimated regional biological CO2 drawdown and air-sea CO2 flux in high productivity regions. Reasonable model-data agreement was found for global-integrated, ensemble-mean sinking particulate organic carbon flux into the deep ocean at 1000 m (0.95 ± 0.64 Pg C yr–1) and the transfer efficiency defined as flux at 1000m divided by flux at 100m (0.121 ± 0.035), with both variables exhibiting considerable regional variability. Future modeling studies are needed to improve system-level simulation of interaction between model ocean physics and biogeochemical response.

emma cavan

and 2 more

1. Detritivores need to up-cycle their food to increase its nutritional value. One method is to fragment detritus promoting the colonisation of nutrient-rich microbes, which consumers then ingest. This is known as microbial gardening. Observations and numerical models of the detritus-dominated ocean mesopelagic zone have suggested microbial gardening by zooplankton is fundamental process in the ocean organic carbon cycle, as it leads to increased respiration of carbon-rich detritus. However, no experimental evidence exists to prove microbial respiration is higher on smaller, fragmented detrital particles. 2. Using aquaria-reared Antarctic krill faecal pellets we showed fragmentation increased microbial particulate organic carbon (POC) turnover by 70 %, but only on brown faecal pellets of low nutritional value. Microbial POC turnover on un-and fragmented green faecal pellets of higher nutritional value was equal. Thus we find particle size alone is not enough to determine microbial activity, and the nutritional value and age of the particle are important. 3. We estimate mesopelagic zooplankton can potentially increase the proportion of essential nutrients (e.g. unsaturated fatty acids) in their food by at least 11 %. In addition we propose ‘communal gardening’ may occur whereby other mesopelagic organisms consume the particle and microbes gardened by a neighbouring detritivore. 4. Increases in microbial turnover of detrital POC reduces the sink of organic carbon in the ocean. Thus microbial gardening should be represented in models forecasting the future carbon cycle. Model parameterisations will require further understanding of the energetic gains to zooplankton communities, how microbial gardening influences other sinking particles such as detrital aggregates, and the relative importance of biological (i.e. particle lability, size and age) vs. physical (i.e. temperature and oxygen) constraints on gardening.