Impacts of ocean climate variability on biodiversity of pelagic forage species in an upwelling ecosystem

Abstract

Monitoring essential marine biodiversity variables is an effective means for assessing impacts of climate change and human-related stressors such as pollution, overfishing and habitat destruction. Yet little is known about the natural variability of biodiversity in pelagic upwelling marine ecosystems, which are often subject to substantial interannual and decadal variability of ocean climate conditions. Using data from a pelagic midwater trawl survey, we quantified diversity indices of epipelagic forage species collected over 26 yr (1990 to 2015) to determine the natural variability and environmental determinants of biodiversity within the California Current upwelling ecosystem. Biodiversity time series indicate there are 2 alternate forage species assemblages that relate to differences in cool/strong and warm/weak upwelling years that vary in 3 to 5 yr cycles. Cooler years are associated with increased biodiversity of juvenile groundfish, whereas warm years coincide with increased biodiversity of coastal and mesopelagic fishes and species originating from southern and subtropical waters. During 2015, a year of anomalous warm surface ocean conditions, we observed unprecedented high levels of biodiversity and attribute it to high abundance of juvenile groundfish combined with an unusually high transport of subtropical and offshore species into neritic waters. Using a combination of remote sensing and in situ hydrographic data, we compare 2015 to previous anomalous ocean climate conditions and discuss how biodiversity of forage species may impact trophodynamics of upwelling ecosystems and predator−prey interactions. Attributing changes in marine biodiversity to productivity cycles and anomalous climate events, and detecting long-term biodiversity trends, provides a critical index toward understanding climate forcing on upwelling ecosystems.

Habitat compression and ecosystem shifts as potential links between marine heatwave and record whale entanglements

“…The impact of the MHW is observable in the anomaly of epipelagic species richness ( Fig. 2b), which increased to record levels in 2015 and is attributed in part to a strong presence of sub-tropical and warmwater affinity species 29 . Increased epipelagic species richness in the CCLME during the recent MHW is now considered to reflect an anomalous ecosystem state, characterized by unusual abundance patterns and species assemblages, but with greater productivity than in previously documented warm years in which primary and secondary productivity were extremely reduced 51,52 .…”

“…During 2014-2016, a MHW occurred in the North Pacific that resulted in an unprecedented multi-year warming event 5 . The impacts of the MHW were wide ranging 9,10 , but notably caused a sustained bloom of toxic Pseudo-nitzschia diatoms that led to the persistence of domoic acid (a neurotoxin impacting marine wildlife; e.g., shellfish poisoning 10,[26][27][28] , record changes in biodiversity of pelagic species 29 , and an unprecedented delay in the opening of the commercial Dungeness crab (Metacarcinus magister) fishery in California (a fixed-gear trap fishery with vertical lines 6 ). The crab fishery, which in recent decades has been among the largest by both volume and value in California [30][31][32] , normally opens in November and continues through mid-July, with catches peaking shortly after the initial opening and tapering to low levels throughout spring and early summer.…”

“…We hypothesize that onshore compression of the coastal upwelling ecosystem was at the root of the unusually high concentrations of whales occurring within the primary area of the crab fishery. Our hypothesis is summarized as the following sequence: (a) the MHW contributed to upwelling habitat compression, coinciding with the prevalence of domoic acid 10,28 , increases in epipelagic biodiversity due to the combined high abundance of warm-and cool-water species, and altered forage species availability (krill and anchovy abundance) 29,42 ; (b) humpback whales exhibited prey-switching behavior and distribution shift in response to upwelling habitat compressionrelated changes in forage availability 43,44 , resulting in (c) an amplified spatial overlap of whales and crab fishing gear during 2014-2016. This overlap was intensified in spring 2016 when the opening of the crab fishery was delayed in response to domoic acid contamination in crabs such that it coincided with the migratory peak arrival of whales in the CCLME.…”

Satellite sensor requirements for monitoring essential biodiversity variables of coastal ecosystems

“…, Wood and Kellermann , Santora et al. ). Top‐down pressures due to the harvesting of top predators and other higher trophic levels also often have impacts that can cascade down the food web (Pace et al.…”

“…We have increasing evidence that biomass production increases with species richness in a wide range of marine and terrestrial ecosystems and not simply in response to abiotic effects (Duffy et al 2017). Moreover, changes in the community composition of lower trophic levels can have major impacts on higher trophic levels, determining the success or loss of animal populations such as fish, waterfowl, and marine mammals (Platt et al 2003, Ji et al 2010, Wood and Kellermann 2015, Santora et al 2017. Top-down pressures due to the harvesting of top predators and other higher trophic levels also often have impacts that can cascade down the food web (Pace et al 1999).…”