Being planktonic, copepods drift with the ocean currents; therefore, they are good indicators of the type of water being transported into the Northern California Current. Copepod biodiversity (or species richness) is a simple measure of the number of copepod species in a plankton sample and can be used to index the types of water masses present in the coastal zone off Oregon and Washington.
For example, the presence of subtropical species off Oregon indicates transport of subtropical water into the northern California Current from the south. Likewise, the presence of coastal, subarctic species indicates transport of coastal, subarctic waters from the north.
Thus the presence of certain copepod species offers corroborative evidence that the changes in water temperature and salinity observed during our monitoring cruises are in fact measuring different water types. Figure CB-01 shows average copepod species richness (i.e., the average number of species from all plankton samples) for each month from 1996 to present at station NH-5.
Generally, species diversity is lower during the summer months and higher during winter months. This pattern is the result of seasonally varying circulation patterns of coastal currents. During summer, source waters to the Oregon coast flow from the north, out of the coastal subarctic Pacific. This is a region of low species diversity.
Conversely, during winter, the source waters originate offshore and from the south, bringing warm, low–salinity water into the coastal waters of the northern California Current. With it comes a more species–rich planktonic fauna with subtropical neritic and warm–water offshore affinities. Variations in species richness from the average values shown in Figure CB-01 index the relative contribution of subarctic vs. subtropical water to the northern California Current.
The annual cycle of copepod biodiversity and copepod biomass are related in an inverse manner (Figure CB-01). During the winter months, when biodiversity is high, the biomass of copepods is low; during summer, when biodiversity is low, biomass of copepods is high. We also find that during summers when biodiversity is high that copepod biomass is low (not shown).
Figure CB-02 shows monthly anomalies of copepod species richness during 1996-present. This time series is derived by taking the average number of species for each month, then subtracting the observed monthly average for that month.
Note that three pronounced changes in copepod species richness lagged the PDO and ONI by about 6 months. The first of these was in 1998, when a change to a negative anomaly of species richness in December was preceded by sign changes of the PDO and ONI in July. The second pronounced change was seen in 2002, with the shift to a positive anomaly of copepod species richness in November, which followed changes in the PDO and ONI in August and April, respectively.
The third and most pronounced change occurred followed the switch to a positive PDO in the beginning of 2014. Species richness from the autumn of 2014 through 2016 has been the highest we have observed since 1996. These species are subtropical and tropical in origin and were delivered to the continental shelf in the warm water mass called “The Blob”. The exact origin of these species is a topic of ongoing study, but we do know that these are not coastal species that are often delivered to Oregon via northward flowing coastal currents as occurs during El Niño events (Peterson et al. 2017).
Additional persistent signal changes occurred in summer 2007, 2010 and 2014, although species richness showed only a moderate response to these events. Note that the El Niño event of 2009-2010 (shown by moderately positive PDO and ONI values) resulted in high species richness during February-August 2010 and a switch back to low species richness in early 2011.
We saw earlier that local sea surface temperatures off Newport showed strong correspondence with the PDO (Figure TA-01). The interpretation of simultaneous change in sea surface temperature and copepod species richness is that when the PDO is in a cool phase, cold water from the subarctic Pacific dominates the northern California Current. Moreover, there can be a time lag of about 6 months between a changes in the PDO sign and changes in water temperature and copepod species composition. For further detail on the relationships between copepod species richness and oceanographic conditions, see Hooff and Peterson (2006).
We have found that this simple measure of species richness is correlated with salmon survival (Figure CB-03). This suggests that the copepod community, when these salmon first enter the ocean two and one years prior for Chinook and Coho respectively, is a reasonably good indicator of adult salmon survival.
The relationship with salmon survival and copepod species richness is somewhat biased and complicated by the trend towards increasing species richness with time. Figure CB-04 shows that species richness has increased at a rate of 4.4 species over the past 40 years. Although this increase in biodiversity may be due to climate change, it is probably too soon to draw this conclusion (see Peterson 2009).