Benthic foraminifera in the plankton: how might this impact on palaeoecological interpretations?

The Western Channel Observatory (Smyth et al., 2015) was established by the Natural Environmental Research Council (NERC), with Plymouth Marine Laboratory managing the two autonomous buoys that are located to the south of Plymouth in the English Channel (Stations L4 and E1). These two locations are now monitored continually and there is regular sampling of the water column and the sea floor at Station L4. At this location, despite it being located in water with a depth of 50 m, benthic foraminifera are regularly found in the surface water plankton samples. Some of these benthic foraminifera appear to contain algal symbionts, indicating that they may have been living at the time of capture. If benthic foraminifera can be entrained in the water column, while still living, then this provides a mechanism for dispersal or migration that is much more rapid and efficient than the rate at which protists could migrate within, or on, the sediment surface. Re-colonization by foraminifera, following disturbance, could well be facilitated by this mechanism which has only rarely been reported in the literature (Murray, 1965). It is clearly limited to depths impacted by fair weather (~30 m) or storm wave base (80–100 m).

Observations of vertical plankton tow (20 μm mesh) samples collected at Station L4 during the three winters (2015–2018) have shown that, following significant storms, the numbers of benthic foraminifera in the plankton tows are increased (Hart et al., 2017). Some of the specimens contain sediment, indicating that they have been picked up from the sediment surface and are in the process of being re-deposited. Such assemblage mixing has significant implications for the interpretation of both modern, and ancient, environments. Analysis of sea floor samples in the area has shown that the recorded species are from the area of Station L4 or Hillmars (in 50 m water depth) although some may have been transported from shallower-water settings by increased run-off during the storm events.

Clearly, re-distribution of foraminifera in the environment might make subsequent interpretations of ecology less accurate if the ‘living’ assemblage is not identified (by staining), but in the fossil record such changes could go completely un-detected and lead to inaccuracy in interpretations of palaeoecology.

 

Hart, M.B., Molina, G.S., Smart, C.W. and Widdicombe, C.E. 2017. The Western Channel Observatory: benthic foraminifera in the plankton following storms. Geoscience in South-West England, 14(1), 39–45. [for 2016]

Murray, J.W. 1965. Significance of benthic foraminiferids in plankton samples. Journal of Paleontology, 39, 156–157.

Smyth, T., Atkinson, A., Widdicombe, S., Frost, M., Allen, I., Fishwick, J., Queiros, A., Sims, D. and Barange, M. 2015. The Western Channel Observatory. Progress in Oceanography, 137, 335–341.