Effect of Mesoscale Eddy Dynamics on Bioproductivity of the Marine Ecosystems

Abstract

Different types of mesoscale eddy dynamics are considered in the paper from the viewpoint of their effect on the plankton (mainly phytoplankton) amount and its taxonomic structure. The eddy structures of all types, including cyclonic, anticyclonic, water-body anticyclonic and frontal ones, as well as the dipole structures, actively affect plankton. Theoretical schemes of the influencing mechanisms, which are illustrated by the examples of such an impact on the plankton in the Black Sea, are examined. The analyzed responses of the marine plankton ecosystems to the eddy dynamics and the scientific literature review unambiguously testify the important role of these processes in formation of biological productivity in the seas and oceans. A cyclonic eddy forms the isopycnals rise (a dome-like bend) in its core both in the thermocline and in the pycno-halocline that elevates nitrocline; it promotes bioproductivity increase. In the center of the anticyclonic eddy, the thermocline and pycno-halocline deepen (deflection) which negatively affects bioproductivity. At the same time, the rise of the isopycnals occurs at the eddy periphery that, on the contrary, contributes to increase in primary production. In contrast to a regular anticyclone, a water-body (or lens-like) eddy induces the water rise in a layer above the depth of the maximum orbital velocity of the eddy, in other words, in its upper part it often acts like a cyclone. Thus, in any eddy there are the areas where the thermocline rises to the surface and, therefore, the prerequisites for the bioproductivity increase are formed. Strong winds not only enhance the effect of the eddies on biota, but can completely change the nature of this impact. When exposed to wind, the intensity of the biogenic elements transport to the photic layer in the cyclones can decrease, whereas in the lens-like anticyclones it can increase. The important point is that the long-living eddies change the influencing mechanisms depending on the stage of their evolution. At last, the eddy structures often promote changing in the dominant phytoplankton species that can significantly alter the flow of organic matter to the bottom and affect the global carbon cycle.