Award: OCE-1657489

The shelfbreak is the region of costal ocean where the ocean suddenly deepens and where the water on the shelf of the ocean meets the warmer, saltier slope water. In this region, sometimes people have observed from satellites or from the patterns of seabirds and mammals, suggestions of increased biological productivity. But other observations and models suggest this region is not more productive than other parts of the coastal ocean. This proposal set out to conduct three detailed studies of the shelfbreak region in the Mid-Atlantic Bight, specifically in the coastal waters offshore of Woods Hole, MA in the vicinity of the ocean observatory PIONEER array. On these surveys, detailed physical, biological and chemical measurements were made to investigate whether there was enhanced productivity at the shelf. My role in particular was to use gas tracers, O2/Ar and triple oxygen isotopes, both made with an on-board mass spectrometer and on samples taken back to the lab to quantify rates of net community production (NCP) and gross primary production (GPP). NCP is the balance of photosynthesis and respiration and reflects the total amount of carbon dioxide taken up by the biological system. Gross primary production is equal to the total photosynthesis and reflects the energy available at the base of the food chain. We found that there was an ephemeral increase in NCP within 5 km of the surface expression of the shelfbreak front about 1/3 to ½ of the time in the spring. In the summer, we never found an increase in NCP at the shelfbreak front. Interestingly, this increase in NCP was on such a small spatial scale that it was mostly missed by the traditional oceanographic sampling method of sampling at fixed stations, even though the stations were only 7 km apart. Since our samples for GPP were measured at stations, we did not see an increase of GPP at the front. Because of both the ephemeral nature of the enhancement and its very small spatial scale, when rates of productivity were averaged in regions (shelf, frontal, slope, etc.), no enhancement of productivity was seen. The most likely mechanism for this enhancement, at least some of the time, is due to an area just shoreward of the front having a much shallower zone of light-filled water, enabling phytoplankton to grow. A secondary mechanism that may be at work at other times is that enhancements in productivity that are likely sometimes due to upwelling of nutrient rich water at the front. On two of the cruises, we also found fascinating phytoplankton blooms. In early spring, we found a huge bloom of Phaeocystis, and were able to characterize many aspects of it thanks to the plethora of techniques used in this research. In the summer, we found an extraordinary bloom of diatoms below the surface arising from of gulf stream water being brought into the coast in a physical process called a warm core ring. The gulf stream surface water is very low in nutrients so people haven’t thought that warm core rings would cause an increase in biological productivity. However, the deeper gulf stream water has lots of nutrients and when it came onto the shelf, it reached the light-filled zone and fueled a bloom of diatoms. This bloom mechanism is especially interesting because not only was it unexpected and unpredicted, but such warm core ring intrusions are likely to increase as climate changes – will more deep diatom blooms be observed in the future? What impact will they have on coastal productivity? In summary, this project closely examined biological productivity at the shelfbreak front and, through the use of physical, biological, and chemical means, developed an increased understanding of biological productivity in the coastal ocean. Last Modified: 01/05/2022 Submitted by: Rachel Stanley