Food‐web structure and ecosystem function in the Laurentian Great Lakes—Toward a conceptual model

Abstract

The relationship between food‐web structure (i.e., trophic connections, including diet, trophic position, and habitat use, and the strength of these connections) and ecosystem functions (i.e., biological, geochemical, and physical processes in an ecosystem, including decomposition, production, nutrient cycling, and nutrient and energy flows among community members) determines how an ecosystem responds to perturbations, and thus is key to understanding the adaptive capacity of a system (i.e., ability to respond to perturbation without loss of essential functions). Given nearly ubiquitous changing environmental conditions and anthropogenic impacts on global lake ecosystems, understanding the adaptive capacity of food webs supporting important resources, such as commercial, recreational, and subsistence fisheries, is vital to ecological and economic stability. Herein, we describe a conceptual framework that can be used to explore food‐web structure and associated ecosystem functions in large lakes. We define three previously recognised broad habitat compartments that constitute large lake food webs (nearshore, pelagic, and profundal). We then consider, at three levels, how energy and nutrients flow: (a) into each basal resource compartment; (b) within each compartment; and (c) among multiple compartments (coupling). Flexible shifts in food‐web structures (e.g., via consumers altering their diet or habitat) that sustain these flows in the face of perturbations provide evidence for adaptive capacity. We demonstrate the conceptual framework through a synthesis of food‐web structure and ecosystem function in the Laurentian Great Lakes, with emphasis on the upper trophic levels (i.e., fishes). Our synthesis showed evidence of notable adaptive capacity. For example, fishes increased benthic coupling in response to invasion by mussels and round gobies. However, we also found evidence of loss of adaptive capacity through species extirpations (e.g., widespread collapse in the abundance and diversity of ciscoes, Coregonus spp., except in Lake Superior). In large freshwater lakes, fishery managers have traditionally taken a top‐down approach, focusing on stocking and harvest policy. By contrast, water quality managers have focused on nutrient effects on chemical composition and lower trophic levels of the ecosystem. The synthesised conceptual model provides resource managers a tool to more systematically interpret how lower food‐web dynamics influence harvestable fish populations, and vice versa, and to act accordingly such that sustainable resource practices can be achieved. We identify key gaps in knowledge that impede a fuller understanding of the adaptive capacities of large lakes. In general, we found a greater uncertainty in our understanding of processes influencing energy and nutrient flow within and among habitats than flows into the system.

Movement Ecology of Potamodromous Top Predator in a Large Lake: Synchrony and Coexistence of Distinct Migratory Patterns 

“…Our results document and provide key information on the migration patterns of contingents, aggregation points and routes, which are all useful considerations in modelling to understand and predict community dynamics, nutrient cycling and overall ecosystem function. For example, lake trout are considered a key link between the benthic and pelagic near-and offshore areas in the cycling of energy (Ives et al, 2019;Ryder & Kerr, 1990). Thus, understanding their migratory patterns (i.e.…”

Pop‐off data storage tags reveal niche partitioning between native and non‐native predators in a novel ecosystem

“…Lake Ontario is a novel ecosystem (Hobbs et al, ) due to a long list of species extirpations and introductions as well as physical modification of habitat (Christie, ; Ives et al, ; Mills et al, ). The ecosystem is also somewhat unique because, in addition to stocking native salmonids like lake trout as part of a restoration effort, Chinook salmon and other non‐native salmonids continue to be stocked into the system owing to their popularity among anglers despite the possibility that their abundance could challenge the restoration of naturally reproducing populations of native species (OMNRF, ; Scott, Judge, Ramster, Noakes, & Beamish, ; Mumby et al, ).…”

“…), which is ca . 19,000 km 2 in surface area, has a maximum depth of 244 m, and is mesotrophic in some nearshore areas but oligotrophic offshore (Ives et al, ). The methods we used to catch fish and to tag them with data loggers (pDSTs) have been described in detail elsewhere (Raby et al, ).…”

“…Nowhere is this reality more evident than in the Laurentian Great Lakes of North America, the world's largest freshwater ecosystem. As a result of over‐harvest of native fishes, destruction of spawning habitat, pollution, and introduction of >180 non‐native aquatic species, the Great Lakes currently constitute a novel species assemblage that nonetheless continues to provide substantial ecosystem services (Bogue, ; Ives et al, ). In Lake Ontario, there is a vibrant recreational fishery for Chinook salmon ( Oncorhynchus tshawytscha ), a non‐native Pacific salmonid introduced in the 1970s that continues to be stocked today because of its popularity among anglers and its capacity to suppress the abundance of a non‐native forage fish, the alewife ( Alosa pseudoharengus , Dettmers, Goddard, & Smith, ).…”

Modeling potential impacts of three benthic invasive species on the Lake Erie food web

“…Lake Erie has abundant benthivorous and omnivorous fish (e.g., yellow perch, freshwater drum, lake whitefish Coregonus clupeaformis, catfish Ictaluridae spp.) that have adapted to increases in benthic-oriented energy pathways in response to the invasion by dreissenid mussels and round gobies (Ives et al 2018). These benthivores likely would pose strong food competition with, and predation on ruffe and killer shrimp.…”