Ph.D student, The University of the West Indies, St. Augustine Campus
Orinoco plume affected change in NW Tobago planktonic foraminiferal assemblages more than the eddy.
Planktonic foraminifera are shelled microscopic marine organisms and are sensitive to changes in water conditions. Different species of planktonic foraminifera make up an assemblage, and throughout geological time, the composition of the assemblage change due to changing environments or conditions. The assemblage turnover index (ATI) is a quantitative method to measure the change in assemblages. The mean ATI of a section determines the community stability of the area. Wakes are oceanic 'shadow zones' such that ocean currents deflect or bifurcate at islands, the region behind the island is blocked from the active current. Within the wake, a stationary eddy forms and circulating system concentrates particles that get entrained in it. The assemblages of planktonic foraminifera from 3 piston cores off NW Tobago (Core 1, 2 and 3) showed that the oceanic eddy that forms behind the island does not affect the changes in the planktonic foraminiferal assemblages throughout the late Holocene. Rather, nutrients from the Orinoco plume influences the composition of species in the assemblages. Using a suite of statistical analyses, it was also found that Cores 1, 2 and 3 were of similar community stabilities, meaning the frequency and size of the changing assemblage was similar. However, the boundary at which the Orinoco plume front abuts the open ocean water was determined to be between the narrow gap of Core 1 and 2. Also, the planktonic foraminiferal community structure of Core 1 was different (Type -1) to Cores 2 and 3 (Type 0), possibly due to the Cores 2 and 3 being within the Orinoco plume's extent.
Abstract: Mesoscale ocean eddies (closed circular currents typically 100–300 km in diameter) are ubiquitous features of the world's oceans. They form partially isolated environments with distinct physical and chemical conditions capable of supporting and transporting whole plankton communities. The productivity and biodiversity of these communities is ultimately dependent on an eddy's ability to retain planktonic organisms. Our aim was to estimate eddy retention time‐scales across a range of oceanic environments and larval behaviours, with implications for both distributions and future changes in plankton communities.The Pacific Ocean, Indian Ocean, Southern Ocean and Mediterranean Sea.A particle‐tracking model was forced using ocean currents from a number of validated hydrodynamic models covering environments ranging from shelf seas to the open ocean and equatorial to high‐latitude waters. Eddies were seeded with large numbers of particles and their rate of loss from the eddy was used to estimate retention times. The influences of common plankton swimming behaviours were explicitly captured in the model.Eddy retention times of modelled plankton ranged from 5 to 67 days, with a median of 19 days. Retention times were not correlated with latitude or eddy size. However, plankton residing near the surface of eddies rotating cyclonically (anticlockwise in the Northern Hemisphere) had significantly shorter retention times than those residing in the same eddy at depth, and vice versa for eddies rotating anticyclonically.We show that ocean eddies have the potential to retain and support planktonic (and even nektonic) communities over many generations and are likely to enhance larval survival for many invertebrate and fish species. Differences in retention with depth suggest that cyclonic and anticyclonic eddies will support differing plankton communities. If their relative geographical distributions change with global climate, then the relative proportions of diatom‐based and dinoflagellate‐based communities may also change, with potential implications for higher trophic animals.
Pub.: 22 Jun '16, Pinned: 25 Jan '18