Thanks to the high density of backscatterers at depth the ADCP reached to greater than 400 m. It’s fun to speculate about what determines these distribution patterns.
Each pixel in these backscatter sections is 8 m x 5 minutes (~2.3 km at 15 Kt vessel speed) so there are close to 1500 profiles in the figures. Origin on the x-axis corresponds to the Reykjanes Ridge crest. The white areas to the right correspond to Faroe Bank and Bill Bailey’s Bank. Observations:
- Backscatter density is greatest in the Irminger Sea, but drops of close to Greenland perhaps because the water there has an increasingly large Arctic component? Questions like that could perhaps be addressed with XCTDs across the East Greenland Current in future studies.
- Backscatter density is conspicuously lower in the Iceland Basin (300-900 km). These are North Atlantic Current waters. Between the ridge and roughly 300 km the waters will have circulated anti-clockwise around the Iceland Basin. Does this suggest that the backscatter population grows with age of the North Atlantic Current Water (i.e., as it loops around the subpolar gyre). What enables this?
- Do variations in depth of backscatter during daylight hours reflect zooplankton response to cloudiness? If we wanted to study these questions further it would help to equip the vessel with a sensor to measure solar irradiance.
- Where did the high density (bright red) of backscatterers at -250 km and ~300 m depth come from (near sunrise). There is a hint of a similar feature at sunset the evening before (at 0 to -50 km).
- But right in the middle of the night at -150 km note the conspicuous drop in backscatter, both near the surface and at ~100-150 m depth. Might this be due to trapping in a coherent eddy? We’ve seen this elsewhere.
- What are the processes that prevent the homogenization of the backscatterers by the ambient eddy field? Perhaps temporarily coherent columns of water that trap the zooplankton and prevent their dispersal as they feed off phytoplankton at night? A paper by Shoosmith et al. identified a large number of both cyclonic and anticyclonic features in these waters (ref. below).
- There is a region of high backscatter at 250-300 m depth and 800 km. What caused this, perhaps a spin-off from the Prime eddy (blog January 25, 2024)?
These guesses hint a range of biological and physical processes. Perhaps I’m ignorant of what is already well-known? If we had a coherent eddy detector (blog February 20, 2024), perhaps we might find a correlation between high/low density patterns and coherent structures? Trapping in coherent features might help promote the growth and concentration of backscatterers, but we also know of examples where concentrations drop in coherent features.
The previous blog showed the enormous difference in backscatter intensity between summer and winter. The lower levels in winter may be because the zooplankton disperse and rest at greater depths during this time. What remains would presumably be the larger myctophids, but they are larger and perhaps less effective backscatterers? Comments and clarifications on this are most welcome. All these data have been processed, archived, and are available for study.
Shoosmith, D. R., P. L. Richardson, A. S. Bower and H.T. Rossby, 2005. Discrete eddies in the northern North Atlantic as observed by looping RAFOS floats. Deep-Sea Research II,52,627-550.