Advection and trophic subsidies

TMF STARTING GRANT PROJECT - START SUMMER 2025

The world’s open ocean appears barren, but is it its low productivity, transported by currents, what fuels the rich ecosystems along shelves and coasts?

The open ocean, often regarded as a marine desert due to its low per-area productivity, harbours between 1–16 billion tonnes of small mesopelagic fishes, roughly the size of a finger. These fish, inhabiting the twilight or mesopelagic zone, are estimated to comprise 50–95% of the global fish biomass, likely making some of them the most abundant vertebrates on Earth.

Numerous predators, including birds, fishes, squids, seals, and even baleen whales rely on vision—and thus light—to locate their prey. In the open ocean, the rapid attenuation of light with depth provides essential cover from visual predators. Many mesopelagic fish, along with pelagic crustaceans, squids, and gelatinous zooplankton, remain hidden in the twilight zone at depths of several hundred meters during the day. At dusk, they migrate to surface waters to feed under the cover of darkness, returning to the depths at dawn.

A mixed catch from a midwater trawl haul, consisting of small mesopelagic fishes, pelagic shrimps, krill, and gelatinous zooplankton. © Tom Langbehn

Adult-sized Glacier lantern fish (Benthosema glaciale). © Tom Langbehn

Adult-sized Mueller's pearlside (Maurolicus muelleri). © Tom Langbehn

Fish and pelagic crustaceans, such as this Pasiphaea sp., from the mesopelagic zone are often similar in size. © Tom Langbehn

Mesopelagic ecosystems have primarily been studied for their vertical connections between surface waters and the deep sea, focusing on sinking particles and vertical migration. However, there is a bias, with little attention given to how this vast biomass moves horizontally or integrates into the food chains of nearby, shallower ecosystems.

Small and scattered across vast ocean volumes, mesopelagic organisms often drift passively with currents, much like plankton. Advection effectively transforms ocean currents into food conveyor belts, delivering vast amounts of prey to distant feeding hotspots where thriving ecosystems emerge, thus ecologically linking systems distant in space and time.

© Tom Langbehn — **To what degree does advected mesopelagic biomass fuel high-latitude food chains?** Currently, there are two primary explanations for the poleward limits of otherwise ubiquitous mesopelagic communities. Observations show a rapid drop in acoustic backscatter, accompanied by shifts in pelagic fish community structure, at the transition to cold polar water masses. This suggests that temperature is a key factor restricting their poleward distribution. Alternatively, mechanistic models propose that extreme seasonal light conditions at high latitudes create a bottleneck for mesopelagic diel vertical migrators, as continuous daylight during the midnight sun makes surface foraging unsafe. The first explanation highlights the presence of mesopelagic fish in warmer waters north of the polar circle. However, this perspective overlooks the origin of these water masses, which reflect the conditions of source populations from which advected expatriates originate. Polar waters flowing south are sourced from regions with extreme seasonal light environments, likely acting as sinks for certain species. In contrast, Atlantic water masses flowing north originate from latitudes with a distinct day-night cycle, likely serving as sources for mesopelagic communities.

We hypothesize that advected prey from the open ocean subsidize local production in shelf seas at a much larger scale than currently acknowledged and that this represents an overlooked yet significant explanation.

Being widely dispersed and hard to locate, the organisms inhabiting the twilight zone are difficult for predators to feed on. However, advection into shallow waters forces them into the light and brings them within reach of bottom-dwelling predators lining the upstream flanks of banks, seamounts, islands, shelf breaks, or underwater canyons, forming a “wall of mouths,” much like mussels filtering food from passing currents (see number one on figure below)

Unlike upwelled nutrients, which require time to stimulate phytoplankton growth and for energy to propagate through the food web, advected prey immediately supports local predator populations.

Trawl catch from a slope community, showcasing a highly diverse assemblage of species.

The large eye of the Greater Argentine (<i>Argentina silus</i>) suggests that light and vision play a significant role in the species' ecology, even at depths of several hundred meters.

Advection brings mesopelagic scattering layers into the sight and reach of demersal predators, where the seafloor prevents mesopelagic organisms from escaping into the dark at greater depths.

Advected nutrients fuel phytoplankton blooms, advected prey fuel predator hotspots. Diverse and thriving predator communtites emerge where prey is brought in by the currents, © Tom Langbehn.

This may explain why regions such as the marginal ice zone, seamounts, and shallow banks attract high densities of predators. These are areas where prey that normally remain hidden in the dark are first exposed to light: (1) when prey is washed into shallow areas where the seafloor prevents escape into deeper and darker waters (Langbehn et al., 2023); (2) when prey is flushed from the darkness beneath sea ice into sunlit open ocean (Langbehn et al., 2023; Langbehn & Varpe, 2017); and (3) when prey is carried by currents into high-latitude areas during summer, when day-night cycles fade into the midnight sun with continuous daylight (Langbehn et al., 2022; Ljungström et al. 2021).

**Rich feeding grounds are found where prey that usually hide in darkness become exposed to light, and thus an easy catch for visual predators**. Terrestrial rules do not apply in advective systems. The fact that prey is advected while predators remain stationary is opposite to terrestrial ecosystems with mostly active predators, forcing us to think differently to understand much of marine life, © Tom Langbehn.

Objectives:

Our goal is to identify the mechanisms that determine when and where prey from the open ocean become accessible to predators, to map these foraging hotspots throughout the North Atlantic, and to predict how advected biomass structures production, life-history strategies, and species distributions in the receiving ecosystems.

Approach and research questions ❯

The project is organized into three interconnected work packages (WPs), each addressing a central research question with a distinct methodological approach: Each WP is divided into specific tasks with clearly defined outcomes and milestones. While the WPs will start sequentially, they will progress in parallel, creating an iterative feedback loop of modelling-observation-modelling where data and insights from each WP inform and refine the others. This structure ensures a rigorous, systematic investigation, adhering to the principles of strong inference (sensu Platt 1964).

Fieldwork ❯

As part of the project we have planned a multi-week research cruise on board the RV G.O. Sars to gather biological, oceanographic, and acoustic data, focusing on benthopelagic interactions around the Faroe Plateau. This region's unique topography and its position between major ocean currents make it an ideal location for studying the dynamics between advected prey and stationary predators. We will perform cross-shelf transects on both the windward and leeward sides of the plateau, capturing a range of hydrographic conditions. This approach will help us understand how advection influences marine communities and quantify the energetic subsidies provided by advected prey. Additionally, we will analyze fisheries data and a time series of ADCP ocean current data, comprising over 200 years of observations aggregated from multiple devices, collected by our Faroese project partners.

The project will benefit from trawl data and CTD casts collected since 2019 during our annual BIO325 research cruise, part of the master's course in fisheries and marine biology. In 2026, we are planning a research cruise to study interactions between advected prey and stationary demersal predators across the Faroe Shelf and the Iceland-Faroe Ridge, © Tom Langbehn.

Project team ❯

Project leader: Tom J. Langbehn
Researcher
I am a quantitative ecologist, or "ocean-going modeller," with a broad interest in global change ecology, evolution, and sustainable fisheries. My primary focus lies in the pelagic ecology of high-latitude oceans and the ocean twilight zone, aiming to understand the mechanisms that link individual behaviours, interactions, and life histories of zooplankton and fishes to large-scale patterns in the environment.

Collaborator: Marius Årthun
Researcher at the Geophysical Institute, University of Bergen
Marius Årthun is a physical oceanographer and the leader of the Polar Climate group at the Bjerknes Centre for Climate Research. His research focuses on basin-scale ocean circulation and air-sea interactions in the northern North Atlantic and the Nordic Seas.

Collaborator: Katja Enberg
Professor
My main interest is sustainable fisheries, including the effects of fisheries on genetic and phenotypic traits as well as dynamics of the harvested populations and ecosystems. I am keen on finding sustainable solutions for using our natural resources for feeding the growing human population.

Collaborator: Aino Hosia
Associate Professor at the University Museum of Bergen
Aino Hosia studies the diversity, systematics, and ecology of ctenophores and medusozoan cnidarians, focusing on integrative taxonomy and DNA barcoding. Her research spans faunistics, spatial-temporal distributions, and ecological roles of gelatinous zooplankton, utilizing physical sampling, optical methods, eDNA, and experiments.

Collaborator: Eydna í Homrum
Research scientist and head of the Resources Department at the Faroe Marine Research Institute
Eydna í Homrum's expertise lies in the ecology, assessment, and management of pelagic fish stocks in the North Atlantic. Eydna is currently leading a project on 'Key Processes Governing Pelagic Productivity in Sub-Arctic North Atlantic Fjord Ecosystems,' with advection as one of the key drivers.

Collaborator: Hjálmar Hátún,
Oceanographer with the Faroe Marine Research Institute and Associate Professor at the University of the Faroe Islands
Hjálmar Hátún is an expert on the influence of the subpolar gyre on marine climate and ecosystems. He participates in one to two scientific cruises annually and has compiled over 200 years of ocean current measurements from Faroese waters, aggregated from multiple ADCP devices.

Collaborator: Dag L. Aksnes
Professor Emeritus
Two of my current research questions are about mesopelagic fishes and their potential role in carbon sequestration and about effects of increased terrestrial greening on marine ecosystems. I am also engaged in science advise on how to achieve more sustainable food and feed production.

Collaborator: Stein Kaartvedt
Guest Researcher (Professor Emeritus at University of Oslo)
My interest is pelagic ecology, and how environment, predators, and prey together affect abundance, distribution, and behaviour of zooplankton and fishes.

Scientific Advisory Board: Ken H. Andersen
Professor in theoretical marine ecology at AQUA, DTU
Ken H. Andersen is an expert in combining theoretical physics with ecology to model ocean life across all scales, from bacteria to whales. His mission is to simulate global ecosystems to understand the past and present and to inform policy and management for the future.

Scientific Advisory Board: Sally Thorpe
Ecological Modeller and leader of the Pelagic Ecosystems Team at the British Antarctic Survey in Cambridge, UK.
Sally Thorpe, with a PhD in physical oceanography and extensive field experience, specializes in developing coupled physical-biological models. She has published research on how krill behavior, such as diel vertical migration, interacts with ocean currents to shape their spatial distribution.

Science support: Frank Midtøy
Senior Engineer at the Department of Biological Sciences, University of Bergen
Frank Midtøy has more than 1200 days of experience on research vessels and over 25 years’ experience conducting lab experiments with fish.

Science support: Heikki Savolainen
Senior Engineer at the Department of Biological Sciences, University of Bergen
Heikki Savolainen has extensive field experience and is a skilled taxonomist across multiple groups, including fish and birds.

MSc student 2024-2025: Yola Keenlyside
I am interested in learning how models can aid our understanding of real-life biology. In my Master’s thesis I will explore the ecological niche of the deep-water redfish Sebastes mentella. Supervisors: Tom J. Langbehn and Christian Jørgensen.

Vacancies ❯

As part of the project, we will be recruiting one PhD candidate (biological oceanography) and one postdoctoral researcher (acoustics), both for 36 months. Position calls will be advertised in due course across various channels. Please stay tuned for updates.
Additionally, we are seeking several master's students > with an interest in quantitative ecology and/or oceanography. If you are interested, please contact Tom Langbehn for more information.

Project News ❯

Tom Langbehn Gave Early Career Keynote at ICES Annual Science Conference
Tom Langbehn gave the Early Career Keynote at the 2024 ICES Annual Science Conference in Gateshead, UK, at the end of September. The recorded talk has now been made available online by ICES. [3 October 2024]

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This project is funded as a Starting Grant Project by the Trond Mohn forskningsstiftelse and the University of Bergen, lasting for four years from 2025-2028.