To explore the relationship between water type, copepod species richness, and the PDO, we developed two indices based on the affinities of copepods for different water types. The dominant copepod species occurring off Oregon at NH 05 were classed into two groups: those with cold–water and those with warm–water affinities. The cold–water (boreal or northern) group included the copepods Pseudocalanus mimus, Acartia longiremis, and Calanus marshallae. The warm–water group included the subtropical or southern species Mesocalanus tenuicornis, Paracalanus parvus, Ctenocalanus vanus, Clausocalanus pergens, Clausocalanus arcuicornis and Clausocalanus parapergens, Calocalanus styliremis, and Corycaeus anglicus.
The cold–water group usually dominates the Washington/Oregon coastal zooplankton community in summer, whereas the warm–water group usually dominates during winter (Peterson and Miller 1977; Peterson and Keister 2003). This pattern is altered during summers with El Niño events and/or when the PDO is in a positive (warm) phase. At such times the cold–water group has negative biomass anomalies and the warm group positive anomalies. Figure NSC-01 shows a time series of the PDO, along with biomass anomalies of northern and southern copepod species averaged over the months of May–September. Changes in biomass among years can range over more than one order of magnitude. When the PDO is negative, the biomass of northern copepods is high (positive) and biomass of southern copepods is low (negative), and vice versa.
Figure NSC-02 shows the same data, but as a scatter plot, with copepod anomalies plotted against the PDO. Relationship of northern copepod anomalies and the PDO during the summer upwelling season (May - Sept). Data are from 1969 - 1973, 1983, and 1996 - present. Units of copepod biomass are Log10 mg carbon m-3. Strongly negative PDO values lead to high biomass of cold-water copepods and vice versa.
When winds are strong from the north (leading to cool water conditions and a PDO with a negative sign), cold–water copepod species dominate the ecosystem. During summers characterized by weak northerly or easterly winds, (e.g., 1997–1998 and 2004–2005), the PDO is positive, warm–water conditions dominate, and offshore animals move onshore into the coastal zone.
Perhaps the most significant aspect of the northern copepod index is that two of the cold–water species, Calanus marshallae and Pseudocalanus mimus, are lipid–rich. Therefore, an index of northern copepod biomass may also index the amount of lipid (wax–esters and fatty acids) transferred up the food chain. These fatty compounds appear to be essential for many pelagic fishes if they are to grow and survive through the winter successfully. Beamish and Mahnken (2001) provide an example of this for coho salmon.
Conversely, the years dominated by warm water, or southern copepod species, can be significant because these species are smaller and have low lipid reserves. This could result in lower fat content in the bodies of small pelagic fish that feed on "fat–free" warm–water copepod species as opposed to cold–water species. Therefore, salmon feeding on pelagic fish, which in turn have fed on warm–water copepod species, may experience a relatively lower probability of surviving the winter.