Persistence of trophic hotspots and relation to human impacts within an upwelling marine ecosystem

Abstract

Human impacts (e.g., fishing, pollution, and shipping) on pelagic ecosystems are increasing, causing concerns about stresses on marine food webs. Maintaining predator-prey relationships through protection of pelagic hotspots is crucial for conservation and management of living marine resources. Biotic components of pelagic, plankton-based, ecosystems exhibit high variability in abundance in time and space (i.e., extreme patchiness), requiring investigation of persistence of abundance across trophic levels to resolve trophic hotspots. Using a 26-yr record of indicators for primary production, secondary (zooplankton and larval fish), and tertiary (seabirds) consumers, we show distributions of trophic hotspots in the southern California Current Ecosystem result from interactions between a strong upwelling center and a productive retention zone with enhanced nutrients, which concentrate prey and predators across multiple trophic levels. Trophic hotspots also overlap with human impacts, including fisheries extraction of coastal pelagic and groundfish species, as well as intense commercial shipping traffic. Spatial overlap of trophic hotspots with fisheries and shipping increases vulnerability of the ecosystem to localized depletion of forage fish, ship strikes on marine mammals, and pollution. This study represents a critical step toward resolving pelagic areas of high conservation interest for planktonic ecosystems and may serve as a model for other ocean regions where ecosystem-based management and marine spatial planning of pelagic ecosystems is warranted.

Predicting cetacean abundance and distribution in a changing climate

“…For seven of the eight species considered in this study, the best predictive models developed using 1991-2009 survey data successfully captured shifts in distribution and change in absolute abundance during an anomalously warm year. The average 2014 SST value was substantially higher than in 1991-2009 (Table 3), consistent with multiple studies that revealed dramatic changes in SST and other oceanic properties in 2014 (Bond et al, 2015;Cavole et al, 2016;Leising et al, 2015 (Rykaczewski et al, 2015), and there could be shifts in the locations of such hotspots (Santora et al, 2017) and consequently the foraging grounds for blue and humpback whales.…”

“…These predictions are consistent with abundance estimates derived from the 2014 survey data that indicate a decrease in blue whale abundance in the study area as compared to the 1996-2009 average, and a more than doubling of estimated humpback whale abundance in 2014 (Table 2; Barlow, 2016). Blue and humpback whales were the only species for which Model 3 made the best novel predictions, consistent with observations that these large migrators return to the same largely coastal foraging grounds every season, perhaps due to learned behaviour and/or the persistence of trophic hotspots (Baker et al, 2013;Barlow, Calambokidis, et al, 2011;Calambokidis et al, 2008Calambokidis et al, , 2015Irvine et al, 2014;Santora et al, 2017).…”

“…That said, the inclusion of longitude or latitude in any species habitat model may ultimately limit its forecasting ability under potential climate change scenarios. For example, although it is unclear how trophic hotspots may respond to extreme climate changes, it is likely that upwelling patterns will change (Rykaczewski et al, ), and there could be shifts in the locations of such hotspots (Santora et al, ) and consequently the foraging grounds for blue and humpback whales.…”

Nutrient supply, surface currents, and plankton dynamics predict zooplankton hotspots in coastal upwelling systems

“…Santora et al . [, ] also found significant coupling between physics, primary production, micronekton (including krill), and top predators off central and southern California.…”

“…High prey concentrations increase predator survival [Pitchford and Brindley, 2001], and patchiness is a key characteristic of oceanic ecosystems. The term "hotspot" has been used in the literature to describe areas of high prey and predator abundance that form at scales ranging from meters to hundreds of kilometers [e.g., Hazen et al, 2013;Bertrand et al, 2014;Santora et al, 2017]. Of particular interest are hotspots of large (meso/macro) zooplankton such as copepods and krill that are a primary food source for fish and marine mammals including sardines, anchovies, and whales [Van Der Lingen, 2002;Espinoza et al, 2009;Croll et al, 2005].…”