Fig. 10.10 The distribution pattern of the small copepod Oithona similis (open circles)
is continuous from the subarctic Pacific, along the California Current (our
observations), across the equator and south to the subantarctic and Antarctic waters.
(After Nishida 1985.)
Pattern Maintenance
(^) The resemblance of the Pacific patterns to the distribution of water masses (Fig.
10.11), defined on the basis of temperature vs. salinity (TS) diagrams, is striking. A
TS diagram is simply temperature plotted vs. salinity for a vertical profile of these
variables in the water column. Over wide ocean expanses the TS curves fall into
narrow bands, or envelopes, on the plots, bands identifiable with regional water types
or water masses. These are in a sense the oceanic equivalent of specific habitats. At
least their co-extension with species ranges implies that they are. This is confirmed by
comparing the TS plots for stations at which given species are captured with TS
envelopes for given water masses. There is a general correspondence, usually with the
array of plots for positive sites somewhat larger than the standard envelope (Fig.
10.11). This is because individuals are frequently expatriated from their usual habitat
by mixing processes, and they can survive quite some time outside their usual range.