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 PST-02, we saw salmon survival correlated with the date when cold water first appeared at our baseline station, NH 05. Figure BST-01 shows a similar relationship, but using the date when a northern (cold–water) copepod community first appeared at station NH 05. We define this as the date of the biological spring transition.
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 seems most favorable for coho and Chinook salmon; 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 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 failed 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 BST-01). 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 of 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.
Arrival of the "northern" species in spring signals that the ecosystem is primed to begin a productive upwelling season. Also listed is length of the upwelling season in days, as reckoned by the zooplankton. Note that over the years of 2007-2009 and again in 2011 and 2013, the transition date came very early, in March, whereas in 2015 and 2016 for the first time in our time series, the biological spring transition never occurred. Although in 2017, the biological spring transition did happen again, it was very late, which indicates poor ocean conditions for outmigrating juvenile salmonids.
Both the date of "biological spring transition" and "length of the biological upwelling season" also correlate well with counts of adult spring Chinook salmon (Figure BST-02) and adult fall Chinook salmon (Figure BST-03) at Bonneville Dam 2 years later.