Abstract
Understanding regional‐scale food web structure in the Southern Ocean is critical to informing fisheries management and assessments of climate change impacts on Southern Ocean ecosystems and ecosystem services. Historically, a large component of Southern Ocean ecosystem research has focused on Antarctic krill, which provide a short, highly efficient food chain, linking primary producers to higher trophic levels. Over the last 15 years, the presence of alternative energy pathways has been identified and hypotheses on their relative importance in different regions raised. Using the largest circumpolar dietary database ever compiled, we tested these hypotheses using an empirical circumpolar comparison of food webs across the four major regions/sectors of the Southern Ocean (defined as south of 40°S) within the austral summer period. We used network analyses and generalizations of taxonomic food web structure to confirm that while Antarctic krill are dominant as the mid‐trophic level for the Atlantic and East Pacific food webs (including the Scotia Arc and Western Antarctic Peninsula), mesopelagic fish and other krill species are dominant contributors to predator diets in the Indian and West Pacific regions (East Antarctica and the Ross Sea). We also highlight how tracking data and habitat modeling for mobile top predators in the Southern Ocean show that these species integrate food webs over large regional scales. Our study provides a quantitative assessment, based on field observations, of the degree of regional differentiation in Southern Ocean food webs and the relative importance of alternative energy pathways between regions.
Highlights
Southern Ocean food webs are of major importance to humans and the global system, underpinning diverse values and services including the existence of wildlife populations, high-value fisheries, and carbon sequestration (Grant et al, 2013)
Through analysis of network structure using available dietary metrics at various levels of complexity, we confirm that trophic groups other than Antarctic krill are the major contributors to energy flow pathways in the Indian and West Pacific sectors, consistent with previous studies (McCormack et al, 2019; Nicol & Raymond, 2012; Pinkerton et al, 2010)
In regions surrounding the Antarctic Peninsula (Atlantic and East Pacific sectors), we found that Antarctic krill dominates energy flow pathways through mid-trophic levels (Figure 5a,c and Figure 6b,d)
Summary
Southern Ocean food webs are of major importance to humans and the global system, underpinning diverse values and services including the existence of wildlife populations, high-value fisheries, and carbon sequestration (Grant et al, 2013). Simplicity is commonly achieved by lumping species into functional groups, but this can lead to a situation in which the diet observed in an arena does not represent the population-level diets of, and energy transfer to, the predators (Hill et al, 2009, 2012; Murphy et al, 2012). These issues can affect the utility of ecosystem models, and so, it is important that both the spatial scale and the taxonomic resolution of food web linkages are well-understood to ensure ecological interactions appropriately inform model development. We use network analysis to explore three questions: (a) What can existing dietary observations reveal about food web structures in different sectors of the Southern Ocean? (b) What mid-trophic level organisms provide pathways to transfer energy to higher predators in each sector of the Southern Ocean? (c) Can broad functional groups commonly used to represent Southern Ocean food webs assist in highlighting variations in food web structure between each sector? After addressing these questions, we discuss key results in the context of previous hypotheses regarding the potential structure and function of food webs in each sector and the implications for the future management of the Southern Ocean
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