Large–scale Ocean and Atmospheric Indicators

Local and Regional Physical Indicators

Local Biological Indicators

Indicators Under Development

Introduction to Pacific Northwest Oceanography

Ocean Sampling Methods

Biological Spring Transition

We suggested earlier that the spring transition could be defined in several ways, one of which was the date that cold water first appeared in mid–shelf waters.  In Figure 11, we saw coho survival correlated with the date when cold water first appeared at our baseline station, NH 05.

Figure 20 shows a similar relationship, but using the date when a northern (cold–water) copepod community first appeared at station NH 05.  This date can also be used to index the biological spring transition. 

Figure 20.	Upper plot:  Regression of coho survival vs. the length of the "biological" upwelling season, measured as the number of days that the summer community structure persisted.  Lower plot:  Regression of coho survival vs. Julian date when copepod community structure transitioned to a summer community.  The earlier this transition takes place, the higher coho survival has been.

We believe this date may be a more useful indicator of the transition in ocean conditions because it also indicates the first appearance of the kind of food chain that coho and Chinook salmon seem to prefer; that is, one dominated by large, lipid–rich copepods, euphausiids, and juvenile forage fish.

Thus we suggest that potential feeding conditions for juvenile salmon are more accurately indexed using both the northern copepod biomass and the biological spring transition date (as compared to an upwelling index, which is presumed to serve as an index of feeding conditions).  We say this in light of the following two instances wherein the upwelling index alone fails to correctly indicate feeding conditions.

First, during El Niño years, or years with extended periods of weak El Niño–like conditions, upwelling can still be strong (as in 1998), but can produce a warm, low–salinity, low–nutrient water type (rather than the expected cold, salty, and nutrient–rich water).  Upwelling of this water type results in poor plankton production.

A second example of upwelling as a misleading indicator occurred during 2005, when mean total upwelling levels from May to September were "average."  However, the zooplankton community did not transition to a cold–water community until August (Table 2).  Therefore, in spite of early upwelling, conditions for salmon feeding, growth, and survival were unfavorable throughout spring and most of summer 2005.

The end of the upwelling season marks the return to a winter community for zooplankton, the timing by which the fall transition is measured. 

These changes in community type occur because of coastal currents, which reverse in spring to flow from the north with the onset of upwelling.  Another reversal occurs in the fall, when the northward-flowing Davidson Current appears on the shelf due to winter downwelling. 

The arrival of the "northern" species in spring signals that the ecosystem is primed to begin a productive upwelling season.  Also listed is the length of the upwelling season, in days, as reckoned by the zooplankton.  Note that the transition in 2007 occurred on one of the earliest dates of the past 12 years.