Copepod Biodiversity
Being planktonic, copepods drift with the ocean currents; therefore, they are excellent ecological indicators of the type of water being transported into the NCC (Northern California Current). Copepod biodiversity (or species richness) can be used to index water types. For example, the presence of subtropical species off Oregon indicates transport of subtropical water into the northern California Current.
Likewise, the presence of coastal, subarctic species indicates transport of coastal, subarctic waters. Thus the presence of certain copepod species offers corroborative evidence that the changes in water temperature and salinity observed during our monitoring cruises were in fact measuring different water types. Figure 14 shows average copepod species richness (i.e., the average number of species from all plankton samples) for each month from 1996 to 2004 at station NH 05.
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| Figure 14. |
Vertical bars are the climatology of monthly averaged copepod species richness, a measure of biodiversity, at station NH 05 off Newport OR. Dashed line with filled triangles is the climatology of monthly averaged copepod biomass (Y–axis on right side of graph). Note the inverse relationship between copepod biodiversity and copepod biomass. |
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 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 14 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 14). 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 15 shows monthly anomalies of copepod species richness during 1996–2007. This time series is derived by taking the average number of species for each month, then subtracting the observed monthly average for that month.
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| Figure 15. |
Upper panel shows time series of the PDO and MEI (bars and dots, respectively) from 1996 to the present. Lower panel shows copepod species richness during the same period. Note the time lag of a few months between long–term persistent shifts in the PDO/MEI and copepod species richness, as seen in mid–1998, mid–2002, and late 2007. |
Also shown in Figure 15 are time series of the Pacific Decadal Oscillation and Multivariate ENSO Index and copepod species richness. Comparisons among these time series show clear relationships between interannual variability in basin–scale physical climate indicators (PDO and MEI) and copepod species richness anomalies at Newport Oregon.
Note that two pronounced changes in copepod species richness lagged the PDO and MEI 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 in July and the MEI in August. 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 MEI in August and April, respectively.
We saw earlier that local sea surface temperatures off Newport showed strong correspondence with the PDO (Figure 5). 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 is 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).
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