Major Shifts in Pelagic Micronekton and Macrozooplankton Community Structure in an Upwelling Ecosystem Related to an Unprecedented Marine Heatwave

Abstract

The community structure of pelagic zooplankton and micronekton may be a sensitive indicator of changes in environmental conditions within the California Current ecosystem. Substantial oceanographic changes in 2015 and 2016, due to the anomalously warm ocean conditions associated with a large-scale marine heatwave perturbation, resulted in onshore and northward advection of warmer and more stratified surface waters resulting in reduced upwelling. Here we quantify changes in the macrozooplankton and micronekton community composition and structure based on five highly contrasting ocean conditions. Data from fine-mesh pelagic trawl surveys conducted off Oregon and Washington during early summer of 2011 and 2013-2016 were examined for interannual changes in spatial distribution and abundance of fish and invertebrate taxa. Overall species diversity was highest in 2015 and lowest in 2011, but 2016 was similar to the other years, although the evenness was somewhat lower. The community of taxa in both 2015 and 2016 was significantly different from the previously sampled years. Crustacean plankton densities (especially Euphausiidae) were extremely low in both of these years, and the invertebrate composition became dominated mostly by gelatinous zooplankton. Fishes and cephalopods showed mixed trends overall, but some species such as age-0 Pacific hake were found in relatively high abundances mainly along the shelf break in 2015 and 2016. These results suggest dramatically different pelagic communities were present during the recent warm years with a greater contribution from offshore taxa, especially gelatinous taxa, during 2015 and 2016. The substantial reorganization of the pelagic community has the potential to lead to major alterations in trophic functioning in this normally productive ecosystem.

Recent Ecosystem Disturbance in the Northern California Current

“…Increased abundances of species such as the California market squid, age‐0 Pacific Hake, age‐0 rockfish, and pyrosomes were observed off the California coast before similar changes occurred in our more northern survey region (Sakuma et al. ; Brodeur et al., ). Warmwater anomalies first occurred in southern California coastal waters during spring 2014 and were subsequently detected farther north later in that year (Gentemann et al.…”

Fisheries Volume 44 Number 10 October 2019

“…Several studies in the NCC have reported very low abundances of adult euphausiids during the past few years Peterson et al 2017 ;Brodeur et al, 2019 ). In strong contrast, we found an anomalously high biomass of E. pacifi ca furcilia larvae during our study in 2014-2016.…”

“…A question mark indicates that changes in abundance might be due to a change in depth distribution, but we had no data with which to test that possibility. Increased abundances of species such as the California market squid, age-0 Pacifi c Hake, age-0 rockfi sh, and pyrosomes were observed off the California coast before similar changes occurred in our more northern survey region Brodeur et al, 2019 ). Warmwater anomalies fi rst occurred in southern California coastal waters during spring 2014 and were subsequently detected farther north later in that year (Gentemann et al 2017 ).…”

“…Pyrosomes were extremely abundant in our 2017 catches, while other gelatinous species returned to more typical abundance levels (Figure 2 ). In 2014, other surveys encountered low numbers of pyrosomes further south of our study area as well as offshore (Wells et al 2017 ;Brodeur et al 2018Brodeur et al , 2019. By 2015, the surveys captured pyrosomes at least as far north as Willapa Bay, Washington, but well off the continental shelf.…”

“…These oceanographic phenomena resulted in a prolonged marine heat wave throughout the NCC during (Di Lorenzo and Mantua 2016Gentemann et al 2017 ). This heat wave resulted in shifts in the occurrence and abundance of a broad range of taxa, including copepods , ichthyoplankton Daly et al 2017 ), squid , gelatinous invertebrates, krill and shrimp Peterson et al 2017 ;Brodeur et al, 2019 ) , and fi shes (Leising et al 2015 ;Sakuma et al 2016 ). Trophic shifts were also evident in juvenile salmon diets .…”