Good afternoon, everyone, and thank you for tuning into my talk. My name is Catherine Hudson and I'm a PhD candidate in the School of Marine Science and Policy at the Lewis Campus. Today I will be presenting on a chapter of my dissertation titled a subsurface Eddie facilitates retention of dial vertical my graders in a biological hotspot. This study focuses on organisms such as 0 plankton that perform DO vertical migrations or deviant behavior. When performing this behavior, zooplankton and spend their days away from surface waters where their food is abundant. Avoid visual predation by efficiency birds. At night they migrate up to the surface waters defeated when visual predation risks are low. This behavior is considered the world's largest migration. Studies from around the world have shown that this behavior is cued by sun angle, as illustrated in the simplified simulations. In the figure on the right. The gray line here illustrates our sine angle. And as you can see when the sun angle dips below 0 here are my greater start to swim up in the water column, may remain in the surface water until the sun rises again in the morning, in which they migrate back down to their depth. While DVM is believed to be primarily used to avoid predation, it may also help retain particles in certain systems. These systems traditionally consist of a surface mixed layer and a subsurface layer below. The surface mixed layer is the upper part of the ocean that is well-mixed and can be distinct from the sub-surface layer below it. This surface layer moves much faster than the subsurface layer in some scenarios due to a variety of forcing mechanisms like wind, solar radiation, as shown here in this figure from Woods Hole oceanographic institute on the right. We define the boundary between these two layers as the mixed layer depth or MLT. And for Dial vertical migration behaviors to facilitate retention in this two layer system, Maya graders must move out of the rapidly moving surface layer here and into the slower moving subsurface layer below. Retention can only occur however, if the subsurface layer can help reduce the net movement of migraineurs by either moving in the opposite direction as the surface layer or recirculating or stain in one area. We will test the effects of dial vertical migration on retention in Palmer deep canyon, which is outlined in green here. It is a subsurface submarine canyon along the coast of the western Antarctic Peninsula here in gray. And my previous work in this region has shown that a subsurface recirculate Eddie persists over the canyon during the Austral summer. And this is really important because there are surrounding area, serves as a biological hotspot, supporting several penguin colonies and transiting whale populations during the summertime. And so the retention of critical food resources, such as applied to that perform these behaviors, may actually be what drives the hotspot to begin with. So the questions we will ask here, the first being, does this vertical migration behavior increase the retention of particles near palmar deep canyon? We hypothesize that the presence of this behavior will increase the residence times of particles over the canyon when we compare them to near surface particles are. Second question is, how does mixed layer? Which we're using as the proxy for the boundary between the surface and subsurface layers and or day length which affects the periodicity of these behaviors, affect residence times. And we believe that both shallow mix layers and long days will increase residence times by increasing the amount of time these particles are spending adapt. To answer these questions. We use the Regional Ocean modelling system or rooms. We used a simulation that ran from 2008 into 2009. We seeded particles on this approximately four kilometer grid here, shown by the pink dots, every two days from November to March, and calculated a residence time over the over poverty Canyon using e folding time, which is basically we look at how long it takes a concentration of particles seated over the canyon to drop to one over natural e, which is about 37 percent. We tested migrations down to 50, 150, and 300 meters, and also calculated residence times for non migrating particles at those depths. So to start off with the residence times of the non-migrant particles, this bar graph shows the median plus or minus the 95% confidence interval of our residence times and days on the y-axis for the four depths, we release particles that, and this horizontal dotted line represents approximately 60 days, which is the length of Biola, peak biological activity in the system. As we can see, residence times increase with depth. We performed a Kruskal-Wallis test on these groups. And the different letters over the bars represent statistically different groups. Obviously a residence times increase with depth, with residence times upwards of nearly 200 days at 150 meters. Comparison to residence times of approximately six days at 10 meters. Panel B of this figure on the right is the same figure except now just notice that the y-axis is very different here for our migrating particles. So there was no statistically significant difference in resonance times between our particles that migrated down to 50 meters versus our particles, at least at the surface. But we did see significant differences with depth. In Crete. Deeper migrations, increased residence times upwards of 35 days. And now looking at the effect of our mixed layer depth, again, we're using that as a proxy for the boundary between the surface and subsurface layers here, day length on residence times. To do this, we performed multiple regressions. And on these figures here again, our residence time is on the vertical axis, days. The left horizontal axis is our mixed layer depth in meters. And I'll write horizontal axis is the day length and hours. On the plane you see here is the plane predicted by the models. For particles migrating down to 50 meters only mixed layer depth had a significant effect on resonance times. With shallower mix layers resulted in slightly higher residence times. When particles migrated down to a 150 meters, only day length was significant here. Again, with longer days producing higher resonance times. And when you migrated down to 300 meters, both mixed layer depth and Daley had a significant effect on our residence times with shallower mix layers and longer days resulting in residence times upwards of 50 days. So in summary, we did show that diagonal vertical migration behavior into the subsurface. Eddie over Palmer deep did increase residence times relevant in comparison to near surface resonance times here. When he migrated down 50 meters, there wasn't a significant difference here. But when you migrated down to 150 and 300 meters, there were significant differences. Illustrating the benefits of vertical migration in the system. We also found that shallow mix layers on longer days increased your residence times in the canyon system. And we believe this is due to the amount of time spent at depth. So what does this retention mean for the hotspot? Migraineurs such as this curl here, spend their days in this recirculating eddies shown here by the lines. And they're attained in the system for approximately 30 upwards of 30 days depending on the depth of their migration. This retention, we believe, provides a constant feed source for nearby foragers such as these penguins. The foragers can travel out to the EDI, as I've illustrated here. Or they can rely on surface currents and the surface mixed layer to move them closer to their colonies. When zooplankton migrate into the surface layer at night. And we've shown that mixed layer depth and day length are significant drivers of increased retention. Shallow mix layers, ink decrease the distance needed to travel into that subsurface retentive layer, and therefore help increase the time in the retentive layer itself. Longer days also mean more time is spent in the subsurface layers. And it's important to note here that these processes aren't unique to the Antarctic ecosystem. We've used Palmer deep canyon as an example here. These factors may play a significant role in retaining critical food sources and other biological hotspots worldwide. The coupling of subsurface flows, dial vertical migration behavior and the fact here's that influenced the time in these different flow regimes may be a driving feature of other biological hotspots and other ecosystems worldwide. And with that, I'd like to say a quick thank you to the graduate student college for awarding me the dissertation fellowship that allowed me to conduct this analysis over the past year. And also that the graduate student government for giving me the opportunity to present my research to you today. I would like to give a big shout out to our collaborators on this project with which we've titled swarm. Especially to Mike did, um, and, and, and Dr. John clink at Old Dominion University for their work on the ROM simulations. I look forward to your questions. Thank you.

Subsurface Eddy Facilitates Retention of Diel Vertical Migrators in a Biological Hotspot, Katherine Hudson

From Priyanka Mondal April 15, 2021

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Diel vertical migration (DVM) is a common behavior in zooplankton populations world-wide. Every day, zooplankton leave the productive surface ocean and migrate to much deeper and darker waters to avoid visual predators and return to the surface at night to feed. While this behavior is primarily for predator avoidance, it may also help retain migrating zooplankton in biologically productive regions. Compared to fast surface currents, deep ocean currents are sluggish, and can recirculate. We test the hypothesis that DVM into these recirculating currents increases local retention of migrating zooplankton in a biological hotspot along the Western Antarctic Peninsula. A subsurface, recirculating eddy has recently been described in Palmer Deep Canyon, a submarine canyon adjacent to a biological hotspot. Previous simulations have shown that residence times of particles increase with depth within this feature. Here, we use in-situ observations to illustrate the presence of vertical migration behavior in local zooplankton populations. We then use model simulations to demonstrate that vertically migrating particles have residence times on the order of 30 days, which is significantly greater than residence times of non-migrating particles. The presence of this seasonal, retentive feature, and the resulting retention of critical zooplankton populations may serve as an important resource for local predator populations. The interaction of DVM with this subsurface feature may be important to the establishment of the biological hotspot within Palmer Deep Canyon. Similar interactions between DVM behavior and subsurface circulation features, modulated by mixed layer depth and day length, may also increase residence times of local zooplankton populations elsewhere.

Department Name: School of Marine Science and Policy
Appears In: GSG presents 10th Annual Graduate Students’ Forum

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