Understanding the structure and functioning of polar pelagic ecosystems to predict the impacts of change

Abstract

The determinants of the structure, functioning and resilience of pelagic ecosystems across most of the polar regions are not well known. Improved understanding is essential for assessing the value of biodiversity and predicting the effects of change (including in biodiversity) on these ecosystems and the services they maintain. Here we focus on the trophic interactions that underpin ecosystem structure, developing comparative analyses of how polar pelagic food webs vary in relation to the environment. We highlight that there is not a singular, generic Arctic or Antarctic pelagic food web, and, although there are characteristic pathways of energy flow dominated by a small number of species, alternative routes are important for maintaining energy transfer and resilience. These more complex routes cannot, however, provide the same rate of energy flow to highest trophic-level species. Food-web structure may be similar in different regions, but the individual species that dominate mid-trophic levels vary across polar regions. The characteristics (traits) of these species are also different and these differences influence a range of food-web processes. Low functional redundancy at key trophic levels makes these ecosystems particularly sensitive to change. To develop models for projecting responses of polar ecosystems to future environmental change, we propose a conceptual framework that links the life histories of pelagic species and the structure of polar food webs.

Spring fasting behavior in a marine apex predator provides an index of ecosystem productivity

“…Their broad spatial distribution, long lifespans, and position at higher trophic levels mean that changes in their overall reproductive success, F I G U R E 2 Modeled distribution of fasting and non-fasting probabilities based on serum urea and creatinine levels in 1448 samples from subadult and adult polar bears captured in the Chukchi and Beaufort Seas 1983-2016 survival of age classes, and population size may be indicative of regional scale changes in the ecosystems in which they reside. Polar pelagic food webs consist of a small number of species where energy flows to higher trophic levels via fewer pathways than more complex systems (Murphy et al, 2016). This results in Arctic systems being sensitive to changes in abundance of keystone species, such as Arctic cod which link lower and higher trophic levels (Murphy et al, 2016).…”

“…Polar pelagic food webs consist of a small number of species where energy flows to higher trophic levels via fewer pathways than more complex systems (Murphy et al, 2016). This results in Arctic systems being sensitive to changes in abundance of keystone species, such as Arctic cod which link lower and higher trophic levels (Murphy et al, 2016). Spring fasting of the three polar bear subpopulations in this study varied temporally and spatially consistent with observed variation in measures of primary productivity and the health of prey and other species within their Arctic food web.…”

“…As an apex marine predator occupying a relatively simplified ecosystem (Murphy et al, 2016), polar bears (Ursus maritimus) may be a useful indicator of changes in species productivity at lower trophic levels. The ability of a polar bear to hunt depends directly on seal abundance and behavior, and the sea ice conditions that promote successful predation (i.e., encounter and capture).…”

Southern Ocean Phytoplankton in a Changing Climate

“…Copepods and mesopelagic fish, particularly myctophids, are important primary and secondary consumers of the phytoplankton in these waters and form an alternative food web for squid, predatory mesopelagic fish, and penguins (Kozlov, 1995;Cherel et al, 2010;Murphy et al, 2016). Measured rates of microzooplankton grazing (Jones et al, 1998;Griffiths et al, 1999;Safi et al, 2007;Pearce et al, 2011), together with high grazer biomass (Kopczyńska et al, 2001) suggest that grazers consume much of the primary productivity in this region.…”

“…Thus, declines in ice algal abundance will likely have a significant negative effect on critical links in the SO food web, especially krill, and promote different and less energy efficient trophic pathways such as consumption of phytoplankton by salps or via copepods to myctophids. Such changes would reduce the capacity of the SO to support the current abundance of iconic, krill-dependent Antarctic wildlife (Murphy et al, 2007(Murphy et al, , 2016.…”