A comparative analysis of coastal and shelf-slope copepod communities in the northern California Current system: Synchronized response to large-scale forcing?

The synchrony between coastal and shelf-slope copepod communities was investigated in the northern California Current (NCC) system, a strong upwelling zone, using time series of zooplankton sampled from a nearshore station (9 km offshore, water depth 62 m) and a shelf-slope station (46 km offshore, water depth 297 m). Long-term trends and seasonal changes were constructed for the dissimilarity index (Euclidean distance) between the two stations and for the biomass of three different copepod assemblages at the two stations: cold neritic, southern, and warm neritic copepods. The dissimilarity between the community structures of the two stations showed little variation in the long-term trend, but exhibited a clear seasonal pattern. All three copepod assemblages showed similar long-term trends in relation to the large-scale forcing as indexed by the Pacific Decadal Oscillation at both stations, but variations in the long-term trend at the nearshore station were much higher than the offshore station. Most copepod groups exhibited regular seasonal patterns at both stations except southern copepods at the nearshore station. All three copepod assemblages exhibited more pronounced seasonal fluctuations at the nearshore station compared with the slope station, and this difference is likely driven by higher productivity nearshore fueled by nutrient-enriched upwelled water. Copepods in the inshore and offshore waters in the NCC ecosystem showed synchronized response to the large-scale variability in physical forcing and copepods in the coastal waters were more responsive to local perturbations than were those in the slope waters. Large-scale climate variability has a clear effect on zooplankton communities and likely has substantial ecosystem consequences (Roemmich and McGowan 1995; Beaugrand et al. 2002; Richardson 2008). In the northern California Current (NCC) system, it is well-established that the zooplankton community structure is related to largescale ocean and climate variability, such as El Niño (Keister and Peterson 2003) and the Pacific Decadal Oscillation (PDO; Hooff and Peterson 2006; Peterson 2009). When the PDO is persistently positive, there are more warm neritic and southern copepods and relatively fewer cold neritic copepods in shelf waters off Oregon; conversely, when the PDO is negative, there are more cold neritic copepods and few-to-none warm neritic and southern copepods. Both Bi et al. (2011b) and Keister et al. (2011) have shown that these PDO-related differences in copepod community composition are due to variations in transport of source waters that feed the NCC. These changes in zooplankton communities can have profound cascading effects on higher trophic levels (McGowan et al. 1998). Understanding biophysical interactions at different alongshore and cross-shelf regions offers a unique opportunity to investigate how ecosystems respond to large-scale and local climate variability. Zooplankton in the entire California Current system from southern California to British Columbia exhibit a cohesive response to large climate events such as El Niño, although the zone most strongly affected extends from northern California to southern British Columbia (Mackas et al. 2006). Meanwhile, in the nearshore upwelling region, zonal variations in physical forcing and ecosystem structure are clear because of wind-driven coastal upwelling during the spring and summer months (Peterson et al. 1979; Smith et al. 2001; Huyer et al. 2007). The continental shelf waters in the NCC off Washington and Oregon are enriched during periods of upwelling as surface waters are moved offshore through Ekman transport and replaced with deeper waters high in nutrients. In comparison, offshore waters tend to have much lower concentrations of nutrients and less production. While Ekman pumping helps fertilize offshore surface waters (Rykaczewski and Checkley 2008) and a portion of the nutrients and phytoplankton found nearshore can be transported offshore (Keister et al. 2009; Yokomizo et al. 2010), a cross-shelf zonal gradient in nutrients and primary production is generally maintained. Cross-shelf zonal variations in copepod production (Peterson et al. 2002a) and community structure (Keister and Peterson 2003) are also observed, with typically higher secondary production nearshore relative to offshore. Variations in copepod species composition are also closely related to upwelling. Cold neritic copepod species dominate coastal waters during the summer upwelling season, while warm neritic and southern copepod species become relatively more abundant in coastal waters during the downwelling season (Hooff and Peterson 2006). Cross-shelf zonation is also important for species such as euphausiids, whose young life-history stages aggregate nearshore and benefit from higher phytoplankton standing stock relative to offshore regions (Gómez-Gutiérrez et al. 2005, 2007). Many other coastal invertebrate and fish species rely on cross-shelf transport and zonation to not only disperse their larvae and * Corresponding author: [email protected] Limnol. Oceanogr., 57(5), 2012, 1467–1478 E 2012, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2012.57.5.1467