Scales of population dynamics, ecology and diversity of planktonic foraminifera and their relationship to particle flux in the eastern tropical Atlantic
Planktonic foraminifera are microscopic zooplankton. They live in the surface layer of the ocean, but produce calcite shells, which accumulate in large quantities on the sea floor, where they produce a large reservoir of carbonate and an archive of past climate change. When empty shells of foraminifera sink, they stick to organic particles and act as ballast for the resulting aggregates. Mineral dust transported from the continent by wind acts in the same way, but its size and production pattern are different. Expedition M140 will combine sampling in the water column with recovery of data and samples from long-term observational platforms (sediment traps and dust collecting buoys) to understand how production of marine particle mineral ballast varies in space and time and how important it is for the marine biological pump, transporting carbon from ocean surface to the sea floor. We will work along a transect from low productive offshore regions to high-productivity coastal upwelling. Observations on biodiversity, species ecology and shell chemical composition of planktonic foraminifera will be used to better decipher their fossil record in the sediment as an archive of past ocean and climate change.
Planktonic foraminifera shells in marine sediments are the principle source of information on the state of past oceans. To unlock the signals preserved in these shells, the ecology of the organisms that produced them have to be constrained in detail. Combining observations from sediment traps, plankton samples and on-board experiments, the cruise aims to investigate the extent and scale of population patchiness, ontogenetic and diel vertical migration, synchronisation of reproduction, symbiont presence and physiology and the extent of genetic diversity in the group.
Together with mineral dust and other mineralised plankton, empty shells of planktonic foraminifera act as ballast to sinking particles and thus play an important role in ocean carbon cycling. To evaluate the significance of the ballasting, the scientific programme of the expedition incorporates investigations of particle flux and dust deposition in the same region, mutually linked through shared access to moorings which will provide long records of particle flux, including foraminifera and airborne dust, at unprecedented resolution.
The principal aim of the cruise is to provide data and samples that are needed to characterise the biology and ecology of planktonic foraminifera, and their coupling with biogenic and mineral particle flux. To this end, the research will combine three approaches:
- 1) Sampling of the water column by filtration and by plankton tows, combined with physical water properties profiles and water sampling will be used to determine horizontal and vertical species distribution, diversity and physiology of planktonic foraminifera. The sampling will be carried out using a vertically resolving plankton sampler (multiple closing plankton net) along transects between the mooring stations and in a replicated 26 hour continuous sampling scheme at mooring stations M3 and M1. Plankton samples will be processed during the cruise and foraminifera will be fixed for genetic analysis, for later TEM study of digestive content and symbiont content and the photosynthetic activity of the symbionts will be measured on board.
- 2) Short-term particle flux and particle composition in the water column will be studied by deployment of drifting particle traps, marine snow catchers and in situ cameras in the CB working area. Short term dust flux will be studied by sampling from air along the entire cruise.
- 3) Long-term particle flux will be studied by analysis of samples from sediment traps and dust collecting buoys. We will recover sediment trap and dust buoy moorings at position M3, and recover and redeploy at positions M1, CB and CBi.