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Copepods under lockdown – a swarm of copepods blocked in by a Transport Barrier in Northern Norway

Updated: Feb 25

The Stressors team set out to investigate the location and migration of copepods in the waters off Lofoten-Veterålen in northern Norway during the onset of spring. Using a combination of sampling techniques including particle tracking, satellite imagery, and hydrographic data, we found results that tell the story of copepods under a spring lockdown.


The copepod Calanus finmarchicus has spent the winter in diapause – a sleep-like state - at 1000 m deep in the waters off Lofoten-Vesterålen. It’s now February, and the copepods have migrated to the surface waters where their food source, phytoplankton, blooms. Here, the copepods will feed, grow, and mature before the oceanic population migrates off the shelf to reproduce. The population grows as more copepods join and grow. Soon the surface is swarming with copepods – red from synthesizing the carotenoid astaxanthin – that can be observed from space using ocean color satellite image technology. Full of phytoplankton and with plump eggs, it is for the copepods to leave.


Except they can’t. The copepods are under lockdown on the Lofoten-Vesterålen shelf, blocked in by a ‘transport barrier.’


The Transport Barrier


Transport barriers are fronts made by the meeting of different bodies of water. For some species, such as plankton, a transport barrier prevents migration from one location to another. Plankton is not strong enough to swim through it.


The main water masses in the Norwegian are the warm and saline Atlantic Water, the Norwegian Atlantic Slope Current, and the lower saline Norwegian Coastal Current. Atlantic Water is carried up the coast by Norwegian Atlantic Slope Current. Distinctions between these waters can be noted by comparing salinity profiles recorded with salinity-temperature-density profiles.


Ocean color images on April 5 (a), April 10 (b), and April 17 (c), 2019. The FSLE (d−1) fields of the same days are shown in (d)–(f), respectively. Dashed red line in (a)–(c) represents the potential transport barrier shown in Figure 2d. The region inside the green circle represents the potential cross-slope transport of C. finmarchicus on April 17. JGR Oceans, Volume: 126, Issue: 8, First published: 02 August 2021, DOI: (10.1029/2021JC017408)

When water masses meet, the interaction of different physical features can lead to fronts. For example, the front that forms at the meeting of the Norwegian Atlantic Slope Current and the Norwegian Coastal Current.


Yet, the formation of the LoVe transport barrier is more complex than the meeting of salty waters. The LoVe region has a complex bottom topography – the shapes and structures of the seabed – which affects how, when, and where currents flow.


Exodus

Over time, eddies form in the NwASC and split off into the Lofoten Basin. The eddies chip away at the transport barrier, eventually causing it to break and dissolve away. With the transport barrier removed, the oceanic Calanus can migrate out to the open ocean – away from the mouths of their potential predators.


By analyzing satellite images covering ten years, researchers have observed that the LoVe transport barrier can last 30 to 70 days. Such a timeline has ecological consequences for the copepods – and commercially important fisheries such as cod.


The copepod lockdown has consequences up the food chain. An environment rich in copepods provides a large amount of food for hungry little fish. The more the young cod get to eat, the more adult cod is available for the fishery. However, subsequent copepod populations are dependent on the number of copepods that survive to reproduce the previous year. If too many copepods become meals, there is a risk of a lower population in coming years. In turn, there could be fewer copepods for the fish in year two, such as cod.



C. finmarchicus distributions on the Lofoten shelf in early spring from 2010 to 2019. JGR Oceans, Volume: 126, Issue: 8, First published: 02 August 2021, DOI: (10.1029/2021JC017408)

C. finmarchicus distributions from Lofoten shelf to the open ocean after early spring from 2010 to 2019. JGR Oceans, Volume: 126, Issue: 8, First published: 02 August 2021, DOI: (10.1029/2021JC017408)


We know about the barrier – now what?


The length of time Calanus is stuck on the shelf has ecological and economic implications. The Stressor research contributes to fisheries management for commercially important finfish, such as herring and cod, and the Calanus fishery.


The ability to predict and monitor the transport barrier and the geographical distribution of Calanus will enable best practices for scientists and fisheries managers to manage ocean resources. Additionally - and perhaps more crucially in our changing climate – scientists may predict how a change in oceanographic processes – such as currents that affect the fronts, eddies, and meanders that form and break down barriers – alters the distribution of species.


This study is a prime example of how new technologies can answer long-standing questions in marine ecology – questions that have been barriers to our understanding of the ocean and processes within. Without incorporating new technologies, research efforts will come up against a wall of their own.


Methods

Ocean Color Remote Sensing

Ocean Color Remote Sensing was used to identify patches of copepods at the surface of the water. Identifying copepods from space is a research method the Sea Patches team is developing. Check out our previous publication for more details and follow our webpage to learn more we improve our methods with the refinement of new technologies.

Satellite Altimetry and Lagrangian Coherent Structures Analyses

We identified surface currents and eddies through satellite altimetry. Satellite altimetry allows researchers to measure the height of ocean’s surface with high accuracy. Scientists measure the length of time it takes a radar pulse sent from the satellite to the sea surface and back to the satellite. Additional data is also analyzed.

Particle Tracking

A form of mathematical modeling used by oceanographers, particle tracking involves adding virtual organisms (or other suspended matter) into a model of the marine environment that displays the movement and patterns of water masses over a period of time and space. By following the migration of the added particles backwards in time, researchers can visualize the where and when of these virtual organisms over a specified studied period.

Hydrographic Data Set

A Seabird CTD profiler collected data on conductivity-temperature-depth. Salinity and temperature data were collected from the surface to 400 meters deep. Since water masses are characterized by salinity and temperature values, the data collected with the CTD allows researchers to identify which water masses were present in the study area and when.



See the full scientific article here:

Dong, H., Zhou, M., Hu, Z., Zhang, Z., Zhong, Y., Basedow, S. L., & Smith, W. O. (2021) Transport barriers and the retention of Calanus finmarchicus on the Lofoten shelf in early spring. Journal of Geophysical Research: Oceans, 126, e2021JC017408.

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