Spatial, seasonal, and climatic variability in mesozooplankton size spectra along a coastal-to-open ocean transect in the subarctic Northeast Pacific

Canyon Downwelling and Water Mass Influences on Winter Zooplankton Distributions in the Coastal Northeast Pacific

“…Northern Neocalanus species undergo an ontogenetic migration during which they enter diapause at depth over winter and return to the upper ocean in late-winter/early-spring (Conover, 1988;Tsuda et al, 1999); thus, it is plausible that the observed increase in mid-depth zooplankton S v in the latter part of this record results from the onset of this seasonal migration. We note that the percent of total biomass of copepod groups in the net tow data was larger than that of euphausiids (Kwong et al, 2022), suggesting that copepoda may have been in sufficiently high concentrations to dominate the zooplankton S v signal. However, no adult Neocalanus were observed in the net tow data and the younger stages of Neocalanus are unlikely to be large enough to be efficiently detected by the 300 kHz echosounder.…”

“…Finally, net tow data collected in the region (see Supporting Information and Kwong et al. (2022)) suggest an increased abundance of northern zooplankton species, specifically young Neocalanus (northern copepods), on February 18 relative to February 7, which raises questions about the idea that the observed mid‐depth zooplankton variability is driven by an increasing influence of southern‐sourced PEW over time.…”

“…Both northern and southern taxonomic groups of copepods were present (Figure S1 in Supporting Information S1), though at net-abundances that are unlikely to dominate the acoustic signal. Euphausia pacifica were also present (Figure S1 in Supporting Information S1), but their fraction of total net-biomass was low compared to the preceding decade, whereas the copepod fraction was comparable to previous years (Kwong et al, 2022). Note that given their size relative to the mouth opening of the bongo nets, we would expect more inter-net variability for euphuasiids.…”

“…We assumed the resulting ensemble‐averaged S v was a direct proxy for zooplankton biomass and refer to it as “zooplankton S v .” While remaining cognizant that a change in zooplankton species composition, for example, the introduction of new species with very different sound‐scattering properties, could also influence the value of S v (Sato et al., 2015; Stanton et al., 1994), we took advantage of the dominance of crustacean species in the zooplankton of the WCVI (Kwong et al., 2022; Mackas, 1992) and the remarkable similarity of the ratio of echo energy to biomass across many species (e.g., Table 2 of Stanton et al., 1996) to justify this assumption. Other limitations in the interpretation of acoustic data as a proxy for zooplankton biomass include the unknown and perhaps changing orientation of the animals in the water column and neglecting the possible presence of small fishes.…”

Vertically Resolved Pelagic Particle Biomass and Size Structure Across a Continental Shelf Under the Influence of a Western Boundary Current

“…This temporal instability in some regions suggests that repeated surveys under different oceanographic conditions may be necessary to fully understand the drivers of zooplankton on continental shelfs. The long‐term cross‐shelf survey in the northeast Pacific may provide a good template for seasonal surveys (Kwong et al., 2022), and demonstrated stability in the cross‐shelf zooplankton patterns. The current study had no temporal replication and while it was shown that the conditions which were sampled are regularly occurring features in terms of both upwelling (Schaeffer & Roughan, 2015), EAC influence and chlorophyll production (Everett et al., 2014), additional studies would be useful to confirm the regular occurrence of the patterns in particulate biomass and size structure.…”