Abstract
Euphausiids are a keystone species in coastal food webs due to their high lipid content and seasonally high biomass. Understanding the habitat and environmental drivers that lead to areas of high biomass, or ‘hotspots’, and their seasonal persistence, will support the identification of important foraging regions for mid- and upper- trophic level predators. We quantify the distribution of hotspots of the two dominant species of euphausiid in the north-east Pacific Ocean: Euphausia pacifica and Thysanoessa spinifera, as well as euphausiid larvae (mixed species). The Canadian coast encompasses the northern California Current Ecosystem and the transition zone to the Alaska current, and is a highly productive region for fisheries, marine mammals, and seabirds. We used spatiotemporal modelling to predict the distribution of these three euphausiid groups in relation to geomorphic and environmental variables during the important spring-summer months (April through September) when euphausiid biomass is highest. We quantified the area, intensity, and persistence of biomass hotspots across months according to specific oceanographic ecosections developed for marine spatial planning purposes. Persistent hotspots of both adult species were predicted to occur along the 200 m depth contour of the continental slope; however, differences were predicted on the shallower Dixon shelf, which was a key area for T. spinifera, and within the Juan de Fuca Eddy system where E. pacifica hotspots occurred. The continental slope along the west coast of Vancouver Island was the only persistent hotspot region common between both adult species and euphausiid larvae. Larval distribution was more correlated with T. spinifera than E. pacifica biomass. Hotspots of adults were more persistent across months than hotspots of euphausiid larvae, which were seasonally patchy. The persistence of biomass hotspots of forage species through periods of low overall biomass could maintain trophic connectivity through perturbation events and increase ecosystem resilience to climate change.
Highlights
Euphausia pacifica and Thysanoessa spinifera are the most abundant euphausiid species in the north east Pacific Ocean [1,2,3]
Large variability in the spatial and temporal distribution of E. pacifica and T. spinifera has been linked to seasonal changes in the timing and intensity of the spring transition, which is reflected by ocean temperature and productivity cycles [2, 9,10,11]
Euphausiid research in the California Current ecosystem (CCE) has focused on the physical mechanisms that lead to regions of elevated biomass, or ‘hotspots’, of euphausiids e.g. [12,13,14,15,16,17,18]
Summary
Euphausia pacifica and Thysanoessa spinifera are the most abundant euphausiid species in the north east Pacific Ocean [1,2,3]. Physical processes that lead to hotspot formation tend to be localised and are linked to the timing and local intensity of upwelling and the complexity of seafloor topography [16]. Bathymetrical edges, such as along continental margins and around the edges of canyon systems, are regions where hotspots of euphausiids often occur [7, 15, 16, 25] and are often important foraging regions for their predators [6, 26]. E. pacifica is approximately 100 times more abundant than T. spinifera [10, 28], and they exhibit differences in life history, reproductive timing, organism size and habitat associations [e.g. 3, 10, 12, 29], which control their distributions
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