water cores. The warm cores of these rings are hundreds of meters deep, providing a
temporary habitat for subtropical plankton in slope waters as they move west and
south along the North American coast before eroding away over a few months (Wiebe
et al. 1985). They can “rub” along the southern flanks of Georges Bank, an advective
heat wave in a usually subarctic habitat. Eddies and current meanders transfer
expatriate individuals between distinct but adjacent habitats along all oceanic
distributional boundaries, sometimes generating uncertainties about the extent of the
patterns and the tolerance limits of species, unless the physical circumstances of
sampling stations are fully recognized. Coastal plankters are carried seaward from
their home waters by topographically steered upwelling jets and features like the
recurring Haida eddies off British Columbia.
(^) Pattern maintenance in the equatorial zone depends similarly upon current circuits.
Patterns like that of Euphausia distinguenda (Fig. 10.8a) can be sustained by the
circuits of the north equatorial current headed west and north equatorial
countercurrent headed east and the comparable current pair to the south of the equator.
Southern subtropical gyres are closed circuits, much like those of the northern
hemisphere. The Southern Ocean is a suite of concentric circumglobal currents, loops
closed around the sphere of the Earth matching numerous species distributions (e.g.
Fig 10.2d – Clausocalanus laticeps and Fig. 10.2f – Clausocalanus brevipes).
(^) Again, while we have examined mesozooplankton distributions, the basic patterns
demonstrated by them recur in all high-seas pelagic organisms: phytoplankton; squid;
epipelagic and mid-water fish; and cetaceans. On the other hand, nekton that swim
substantially faster than currents can move between distinctive habitats, and many of
them regularly migrate to exploit the seasonal advantages of different regions.
Diversity and Community Structure
(^) The patterns that recur for numerous species do not have equal numbers of
subscribing species. Diversity, particularly the length of the species lists, is much
higher in tropical and subtropical seas than in subpolar and polar ones. For example,
according to Razouls et al. (2005–2011) the North Pacific central gyre is home to
more than 520 species of copepod, while the subarctic region has around 300. The
North Atlantic appears to have more in both regions, but that is likely to be due to the
concentration there of taxonomic effort. Many species on these lists in all regions are
deep-living, many rare. There is no certain answer to why the tropics are more
diverse, but it applies to most terrestrial groups as well – tropical rainforests have
more plant, bird, insect, and other species than mid- and high-latitude rainforests.
Likely a major part of the story is that over long geological times warm habitats have
been more stable, i.e. less subject to the strong temperature and productivity shifts