Necessary and Sufficient Conditions for Phytoplankton Changes in the Vicinity of the Grand Banks
Abstract:A tongue of low salinity cold water extends from the Grand Banks between a southeastward bending portion of the Gulf Stream and a northwestward flowing portion of a neighboring current. In association with this hydrographic structure, there was a marked regional change in the distribution of plankton species in April and May, 1972. The change was characterized by two types of species, those that were produced in the cold tongue of water and those found in the warm oceanic currents to either side. The hydrographic structure was a necessary condition for the production of the two species types. But there was also another necessary condition, for the species of each group had overlapping distributions, which suggests that the species of a group interfered with each other to only a minimal degree. Furthermore, in the cold tongue region the most abundant species rarely occurs in the ocean, while five other species present there do occur commonly in the ocean near the Gulf Stream. Thus, absence of severe interference by the common oceanic species was a most necessary condition for the change from rarity to abundance by the dominant non-oceanic species; otherwise the common oceanic species would have blocked its growth.
Minimal interference, however, was not sufficient for the growth of the cold tongue species. Lack of severe interference between species is not a cause of growth. Nutrients (and light) cause growth and did so, it is inferred, to a greater degree in the cold tongue, where greater cell concentrations occurred and where deep water, as indicated by low temperature and high salinity, came close to the surface and could have been a source of nutrient for the plankton population above.
Conditions sufficient for the abundance of five common species and one rare species in the cold tongue do not guarantee a similar plankton in the future. The species occurred in a quasi-chance manner.
Document Type: Research Article
Publication date: January 1, 1975
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