Ecosystem structuring by pulses of internal wave breaking

Vertical mixing in the ocean is an essential mechanism controlling how marine ecosystems work. All marine life builds on primary production that mostly occurs in the upper ocean, where sunlight is available for growth of phytoplankton. Besides sunlight, primary production also requires nutrients originating from the deep ocean. Strong phytoplankton bloom can occur when nutrients from the deep ocean are mixed upwards to meet sunlight near the surface. This requirement is foremost met during spring time, as winter is generally too dark and nutrients that were brought upward during winter storms are quickly consumed before the summer. However, another mechanism that can mix nutrients vertically are breaking internal waves in the ocean. Internal waves are disturbances at the interface between heavy bottom water and lighter surface water. In the Norwegian Sea such waves are generated when tidal flow is forced over subsea mountain ridges. During wave breaking, strong vertical mixing can occur throughout the entire depth of the ocean providing vast amounts of nutrients to the upper ocean. Primary production can thereby be sustained throughout the entire spring and summer, with tides as a periodic and predictable trigger. Surface signatures of internal waves are frequently observed in satellite images in the Norwegian Sea off Lofoten and Vesterålen, a spot with a remarkable abundance of marine life. This location even hosts rare colonies of cold water coral, a species requiring rather large and steady supply of nutrients. In the EcoPulse project, mathematical models are beeing used to investigate criteria for breaking of internal waves. Along with field observations of ocean currents, tides and nutrient transport, the research group is investigating the role of internal wave breaking for the local ecosystem and cold water coral in particular. Update autumn 2021: - To understand and quantify the exact mechanisms of internal waves that contribute to vertical mixing, idealized numerical experiments have been carried out. The simulations tell us under which criteria internal waves at Hola are expected to break, and which part of the water column is succeptible to enhanced vertical transport, transferring nutrient between benthic and palagic layers. - Case studies on synergy between satellite imagery, wave theory and regional ocean circulation models is beeing conducted, looking at historic wave breaking events at Hola. - Field work to collect data of hydrography, ocean currents, turbulence and nutrient transport near the Hola reef is completed. - Localisation of coral reef sites are analysed in context with ocean circulation patterns on the Norwegian Shelf. Update autumn 2022: - Field work at Hola has been repeated with additional measurements - The numerical experiments in Basilisk are completed Update January 2024: - Field measurements and idealized simulations have been complemented with an analysis of horizontal flow on the Lofoten-Vesterålen continental shelf - The topography plays a central role on controlling ocean currents and transport of particles towards the Hola cold water reef

We now have a better understanding of what drives the observed strong internal waves at Lofoten Vesterålen, at which conditions they occur and how strong they are. In particular, laboratory and numerical experiments have been shown to match up for breaking criteria of such waves. Regarding the ecosystem functioning at Hola, we have developed an understanding of particle transport pathways in the regions that can explain the abundance of cold water coral at the Hola reef.

The biophysical interactions between tidally driven, breaking internal waves and vertical structuring and functioning of plankton and key benthic species, where the internal waves is a ubiquitous phenomenon of the stratified ocean, are addressed in this project. A hot-spot for the internal waves, near the coral reef at Hola, located in the fishing rich Lofoten-Vesterålen area in Norway, is investigated. Depending on the tide, local currents, hydrography and bottom topography, the nonlinear waves drive a strong turbulence in the entire water column. The waves transport matter and living organisms vertically and horizontally. A national team of complementary expertise in biological processes, nonlinear internal waves, remote sensing and ocean circulation models, from Institute of Marine Research, University of Oslo and the Norwegian Meteorological Institute, join forces to: (i) Develop a new mooring for in-situ observations of the strong internal waves and vertical fluxes of nutrients, extending the LoVe Observatory. The existing infrastructure provides reference measurements where the internal waves do not break. (ii) Data from satellite (synthetic aperture radar and visual) are used to develop spatio-temporal statistics of the internal waves and the potentially associated primary production at the ocean surface, at LoVe and other locations of the Norwegian continental shelf. (iii) By operational ocean circulation models analyse the hydrographic structure and response to tidal and atmospheric forcing, particularly the internal tide and its fronts, with comparison to the in-situ measurements and remote sensing data from LoVe. (iv) By dedicated models of the short nonlinear internal waves, obtain the upward flux of bottom nutrients in breaking conditions and turbulence by the shear and overturning instability. (v) Quantify the role of the additional nutrient source for cold water coral as key benthic species, and ecosystem structure in general.