Oregon (USA) corresponding to the seasons. During summer, the very nearshore zone
usually has a copepod fauna dominated by an array of northern species:
Pseudocalanus mimus, Calanus marshallae, Acartia hudsonica, Centropages
abdominalis, Acartia longiremis. Those species are present all year round. However,
in winter they are mostly replaced by an array of copepods with southern affinities:
Paracalanus parvus, Ctenocalanus vanus, Clausocalanus spp., Corycaeus anglicus,
and others. These are forms that have their main ranges of distribution to the south
and in some cases offshore in central waters. They are carried into the region during
the late fall and winter months. The seasonal switching is in agreement with the
direction of the wind, current meter observations and other faunal changes. Summer
winds from the north bring in species of northern affinity, and their populations
increase during the cooling and strong production induced by nearshore upwelling. In
the fall, the winds reverse and inshore flow switches to northward and onshore, so
southern forms are carried into the region and are present through the winter, usually
until April. They do not prosper, specifically reproduction is low or zero and
abundance remains low.
(^) During the strong El Niños of 1983 and 1997, southern plankton were carried into
the region in winter, just as in normal years, but were not carried away again in
spring. During the abnormally warm sea conditions of those summers, southern forms
remained along the shore and were able to spawn and grow. They remained dominant
or subdominant components of the fauna right through the entire summer of 1983 and
1997 (Keister & Peterson 2003). For example, the dominant copepod on many dates
in the summer of 1983 was Acartia tonsa. This is a circumglobal species (or “species
group”) of nearshore waters in warm, temperate zones. Along this coast they are a
dominant summer form south of Point Conception. It is not usual for them to get as
far north as Oregon in most winters. In 1982–1983 they did, and they were able to
increase and stay all summer.
(^) In general, the El Niño results (in agreement with others) suggest that because they
“go with the flow” planktonic life forms have an advective flexibility. They are
carried to newly suitable habitats immediately as those develop, primarily because
habitats are usually made “newly suitable” by advection. Once arrived, new forms
replace the usual inhabitants immediately. However, those usual inhabitants are
themselves displaced farther downstream, and somewhere along the line they may be
doing fine. There is no radical reduction in the global total of the population and no
danger of extinction. When conditions reverse, stocks are carried back to their waters
of origin and re-establish quickly. An important aspect of this flexibility, of course, is
that most plankton are fast growing with relatively short generation times. Thus, they
are resilient to large changes in “ocean climate”. El Niño is no exception. We present
the Oregon example because it is closely familiar to us. Similar applications of the
indicator species concept have been or can be made for virtually every coastal zone.