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.
![Plots of coho survival versus length of the upwelling season (top) and day of the year when the copepod community transitioned (bottom).](EcInFigs/ecinfigtwenty.jpg) |
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.
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Table 2. |
Historical dates of the Biological Spring Transition, as measured by the timing of change in the zooplankton from a winter to a summer community. |
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Upwelling season |
Length of season |
|
Year |
|
Start date |
End date |
(in days) |
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|
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1970 |
|
~20 Mar |
20 Oct |
|
214 |
|
|
1971 |
|
20 Mar |
6 Nov |
|
231 |
|
|
1983 |
|
20 Jun |
13 Sep |
|
85 |
|
|
1996 |
|
3 Jul |
31 Oct |
|
120 |
|
|
1997 |
|
15 May |
27 Aug |
|
104 |
|
|
1998 |
|
never |
never |
|
0 |
|
|
1999 |
|
14 May |
4 Nov |
|
174 |
|
|
2000 |
|
6 Apr |
4 Nov |
|
200 |
|
|
2001 |
|
11 Apr |
7 Nov |
|
210 |
|
|
2002 |
|
30 Apr |
1 Nov |
|
185 |
|
|
2003 |
|
5 June |
3 Oct |
|
120 |
|
|
2004 |
|
25 May |
14 Oct |
|
142 |
|
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2005 |
|
18 Aug |
21 Oct |
|
64 |
|
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2006 |
|
30 May |
31 Oct |
|
154 |
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2007 |
|
22 Mar |
-- |
|
-- |
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2008 |
|
3 Mar |
-- |
|
-- |
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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.
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