 Here is Alejandra Ortiz from Colby College, and Alejandra will be talking about atolls and ecogeomorphology. Over to you. Great. Thank you so much. I'm really happy to be here, and I apologize for the really long title. I think it might be the longest title. I'm going to say it's due to COVID, and really it can be summed up with this title, understanding atoll survival, or what's going to happen to atolls in the coming century. And so this work is something that I've been doing for the past couple of years and is really dependent on a large body of students I've been able to work with, including Sean Dailey, Jonathan Woodruff, Carter Rucker, and Faith Johnson, who are all students that I worked with at North Carolina State University, and Melody Thomas, John Connors, and Manny Salas, and Bentley Meyer are more recent students I've been able to work with at Colby College. And I have been really liking some of the memes that have come up on the academic Twitter, so I couldn't resist doing my own version of it for this talk. And one of the things when we look at atolls in the geomorphic community is that in the last 10 to 20 years, there's been a real kind of explosion of interest looking at these landscapes from the point of view of how the processes and feedbacks are driving landscape change. And you kind of tend to get to big camps, which is either that these islands are doomed because of climate change, or they seem to be surviving, and they're going to be okay. And so that's kind of motivating work for a lot of the work that we've been doing. And to answer this question of atoll resiliency or survival, first we have to understand where are the people living. And on those islands, which I call Motu and I'll explain a bit more about what's driving the evolution of those islands. And to understand the evolution, primary control, we need to think about what drives kind of the size and shapes that we're seeing on these Motus. And is it potentially external controls like the offshore wave climate or big hurricanes that they're passing through? Or is it more driven by internal processes like coral reefs or production of sediment in the lagoon? And to answer this, we've started with creating a global atoll database and then looking at quantifying some of these relationships between offshore wave climate and atoll and Motu morphology. So of course that begs the question, how am I defining an atoll? And you'll hear me say sometimes atoll or atoll in a given audience about half the people like one pronunciation, half like the other. So choose your favorite. But I take a very broad definition. I'm saying that there has to be some coral reef platform, so some kind of old carbonate platform that's encircling an inner lagoon. And on this coral reef platform, we may have land. And those subarial land masses are what I'm going to call Motus. Motus is one term. It's the Polynesian term. They are also called sandy caves or islets. And they tend to be really low elevation, usually less than five meters with maybe an average elevation of one to two meters above sea level. And the key part, of course, is that the Motus are where people live. So here is an example of a small vegetated Motu that you can see in the background in Majuro, which is in the Marshall Islands in the Pacific Ocean. And when we kind of think about the shape, we have kind of this offshore ocean that then shallows up on this old carbonate reef platform, which we'll call the Reef Flat. And then on that Reef Flat, you can have these Motus. And there one way of thinking about these Motus is that there's kind of a pile of sand and coral rubble and maybe some vegetation holding it all together. But it's pretty small relative to the size of the entire atel. So when we talk about where our atels found, Sean Daly helped me create a global atel database using this very broad definition. And not unexpectedly, you find these in all of the world's oceans between the tropics of cancer and Capricorn. The Pacific Ocean has the largest number of atels followed by the Indian Ocean with only a few in the Atlantic Ocean in comparison. But because we have this like huge number of boards of five, almost 600 atels globally, it becomes difficult to visit them and do fieldwork for a number of different reasons. A lot of them tend to be in the middle of nowhere. So getting to them is difficult. And that's really kind of started to motivate some of the approach that we've been doing in my lab, which is this kind of three-pronged stool where we're using a combination of remote sensing and computer modeling with some targeted fieldwork to help us think about this question of whether these landscapes are going to be surviving in the coming century. Oh, skip one. So just a brief background in general, when we think about these atels, I've said, you know, there's kind of this reef platform, which you can see here in the upper left in orange and circling this inner lagoon. And then on that reef platform, we might have some of these motos, these islands, which are shown in grain. If we take a cross section through that, we can see that that reef platform, that carbonate base is actually quite large. And that's the product of tens of thousands of years of growth of old corals and happening on very slow timescales compared to the timescales that are driving the motu formation. So these motus are forming much more quickly on timescales of decades to centuries, typically. And that's kind of the piece I'm interested in, because the motus, these reef islets are where people are actually living. And so when we think about kind of the survival of these landscapes, we need to understand what's going to happen to those islands. So here I'm showing you two satellite images of atels in French Polynesia in the Southern Pacific. And we have Rangaroe, which is one of the largest atels on the upper left. And then another one on the far right, Higuero. And what we see is that on a given atel, A, they very rarely are actually circular, but B, and more importantly, we have huge variation in the size and shapes of the motus on just even a given atel. So there's kind of this consistent pattern in French Polynesia, where on the northeastern sides, we see these long connected motus. And you can see kind of the vegetated part here with a pretty narrow reef flat in front of the motu. But when we look on the southern side, we might have significantly fewer motus, and they tend to be much smaller. So if we look here, we can see that these motus are maybe 500 meters in width. There's large variation in how wide the reef flat is. And on some atels, you might not have any motus on that southwestern side. And so the question of course is, well, why is that happening? And there's some work that I did with Andrew Ashton from my PhD, using X-Beach in a simple 1D model that predicts that the offshore wave climate should drive a critical width of the reef flat in front of the motu. So what I mean by that is if we zoom in here to that northeastern side of Rangarola, we've got this long connected motu in green. You can see the lagoon in kind of the lighter blue to the south of it. And then what's been highlighted in these orange boxes is this reef flat. So the carbonate reef flat is usually quite shallow, maybe a meter or two in depth, and doesn't necessarily have a lot of active coral growth. Most of the active coral growth is happening on the ocean side of that reef flat. But what we found was at least with the model, it kind of seems to suggest that there's a reason that the motu doesn't keep growing all the way up to the edge of the reef flat. And so the question of course becomes, do we see that? So to understand this question, I've been working on atomorphometrics and trying to figure out, do I have this behavior kind of seeing globally of consistent offshore or consistent reef flat width in front of the motu versus when we don't have motus? And is that driven by an offshore wave climate? And so to answer this, I'm kind of using two big pieces, the remote sensing and some more modeling work. And this has really been driven by my graduate student, Faith Johnson, and then my two undergrads, Manny Salas and Bentley Meyer. And the methods that we've been using for this are first, we're going to use Landsat imagery and create satellite images composites that we can then automatically classify land cover on them. Then we're going to calculate the metrics of that land cover. So if I've separated an image into motu versus reef flat, let me measure how wide is that reef flat? What's the area of my motu? And then compare that to wave data. And we're going to be using WaveWatch 3 hindcasted wave data as a way to kind of get our global coverage. So when I talk about Landsats and temporal composites, what you can see here on the left is a single Landsat image taken of Bikini Atoll, which is in the Marshall Islands, and is most well known for the fact that the U.S. dropped some atomic bombs and you can see some of the remnants of that in the upper left hand corner of the reef flat. And one of the big issues we have is the presence and prevalence of clouds. And so one way that we can get rid of clouds is by using a temporal composite. So we take every Landsat image over a set time period, in our case, three years, and then we remove the clouds and average the remaining pixels to create this composite of Bikini Atoll shown here in Part B. And it's a very nice method to help us remove some of the outline issues that might affect our images. And so here I'm walking you through Katui Atoll, which is in French Polynesia. And we start with our composite Landsat image in the upper left. We then are using different methods to do automatic land cover classification. Currently, it's using unsupervised canines classification to separate it into water in blue, reef flat in red, and then motu's islands in yellow. From that, we can then start to do some of these morphometrics. And one of the cool things is, even though Landsat has pretty coarse resolution, it's only about 30 meters, when we zoom in on some of those finer features that are visible in a Google Earth high-resolution image like this deep channel that's cutting through our reef flat. Our Landsat imagery and our automated analysis is actually capturing that behavior, and we are seeing those features preserved. So that's actually been really good. You can see here kind of the outline in light blue is the reef flat, and the outline in green is the motu. And obviously we can see when we're looking at a high-resolution image, we might have more variation within those motu's, but ours at Landsat scale kind of resolves it as a single motu. And so what Bentley has been doing, and I'm not going to steal too much of his thunder because he's presenting tomorrow's poster, is taking some of these morphometric measurements that Faith has done and comparing it to the offshore wave climates that we have pulled from WaveWatch3. And we've done it on a relatively compressed piece. So what we did was looking at a couple of key metrics, like the reef flat width in front of motu's, and we've binned them based on the orientation. So we picked the center point of the atoll and then bin it directionally into northeast, south, and west. And we also bin our wave data into those same categories as a way to kind of simplify our comparison. So what we find when we're looking at 43 atolls in French Polynesia, and this isn't all of them, and we look specifically at how long our motu's are. So the motu length versus the reef flat motu width, what we see plotted are all these points. So these are from 43 different atolls, and they're colored based on the direction. So all points in the blues are on the north and eastern sides of our atolls, and the greens and yellows are on the south and western side. And we see a really interesting behavior for small motu's. Motu's that are relatively small in area and length, less than about five kilometers. We have huge variation in how wide the reef flat is in front of those motu's. It ranges from 100 meters to almost a kilometer. But once we get these bigger kind of connected motu's, like on this north side of Rangaroa, we now have almost a near constant reef flat width in front of the motu's. And so what I've now plotted here in the second graph is that reef flat width of the motu's for only these larger motu's. So just kind of looking at this piece of the data and plotting it against our wave heights that we've extracted from WaveWatch3. And in this case, I'm plotting it versus three years worth of wave data that's been in, we've calculated the H3 Sigma, which is the kind of storm, the big infrequent storm that might be hitting these islands. And what we see is there seems to possibly be a trend that we have increasing reef flat widths in front of our motu's with increasing wave heights. Now, these data were generated yesterday. It's very preliminary. We have a huge data set. We're still going through it. So just wanted to kind of highlight it. One of the other pieces we've been getting into is trying to use different methods to help us classify our landscapes at higher details. So right now with K-Means, we've been doing just three classes, land, water, and reef flat. And Manny's been working at using deep neural networks to classify our landscapes into more classes, including vegetation versus sand versus infrastructure. And that's been some work that we're hoping we can then use to look at more places. One of the other things we've actually done is look at some of the impacts at kind of much smaller scales. So what can happen on certain atolls is that to generate building materials, people will actually excavate out holes of the reef flat. And so these excavation pits we can see here off of Majura, which is atoll in the Marshall Islands in the Pacific. And the question became, well, if we create this big hole in our reef flat, how does that change the wave energy across that reef flat? And do we see kind of enhanced erosion of vulnerability of those motus behind those reef pits? And what we were finding was when we have no excavation pit, we've got this black line and wave height decreases as you get closer to the motu, which is what we expect. But then as you create a different pit of different depths, we actually increase the wave height at the motus. And so what we can then look at is kind of, once you've got excavation pits of about two meters or deeper, you do have a bit of enhanced wave energy at the motu location. So this might be an area of future worry for people with rising sea levels of kind of higher vulnerability of these landscapes. So I generally touched upon some of the different pieces we've been working on. And the big piece that has now been postponed to summers is to try and get that third leg of the stool, the fieldwork. And so I'm going to be going hopefully next summer to Glover's Reef and Belize, which is one of the few atolls in the Atlantic Ocean. So it's much easier for me to get to. And we're going to be deploying a lot of instrumentation to help us kind of get a better handle on some of the hydrodynamics and sediment transport processes that are operating at these landscapes. Other aspects is right now we've got 88 atolls that we've done morphometrics on and wave data analysis. We have about 560 in our database. So we need to apply that more broadly. And we want to start tracking how those morphometrics have changed through time. Because we're using Landsat, we could in theory go back to the 1980s. The one downside is it doesn't tend to have great coverage back to the 1980s for a lot of these landscapes. We can usually go back to about 2000. And so even so we could still track 20 years worth of landscape change on these locations globally. And so I think that's going to be really exciting to help us again piece out what might be driving the changes in these islands. So acknowledgements, my students, Andrew Ashton who helped start this, and then a number of different funding pieces. In particular, most recently from Colby with the AI Davis Institute and the Low Latitudes Fund. Here are my references and I'll take any questions. Thank you Alejandra. That was super interesting. So we have some time for questions for Alejandra. Brad has a question. Go ahead Brad. So thank you. That was a very interesting talk. And I was wondering if you can just remind us from the earlier work, what is it that determines that critical reflat width before you get a Mo2? Or that stops the Mo2 from growing out to the edge? And remind us, can you explain the difference between those North and East sides versus the Southwest sides in terms of wave climate? So those are great questions. So first in terms of the potential driver of this critical reflat width, what we found was when I used X-Beach in this kind of simple 1D thing, which I'm showing up here in the upper right and in the schematic, we had offshore ocean, a placement of a Mo2, and then behind the Mo2 a Lagoon. There was a change in the direction of bed shear stress. And so what would happen is that as the Mo2 got closer to the ocean side, it went from having onshore directed bed shear stress, which would theoretically be driving sediment onto the Mo2 and helping it prograde to the edge, the ocean side, to a negative directed bed shear stress where bed shear stress would be directed off towards the ocean, which would then be a way that it would stop accumulation of sediment at the ocean side of that Mo2. Now, the key part with this is because we were doing it as a 1D model, this really was only a good proxy for some of these larger, long connected Mo2s where the dominant direction is probably on and off the reflap. If we look at some of these smaller Mo2s, we would expect that flow would be going around them as much or more than maybe just going on and off the reflap. So we don't actually necessarily expect that to be the primary driver of a critical reflap in front of these smaller Mo2s, but we were thinking that it might be driving a consistent trend in these larger Mo2s. And then when we look at the data, sorry for going through this kind of fast, the interesting thing that popped out is of these 43 atolls, I have a consistent trend. Now this is French Polynesia, and I've added in about another 40 atolls that are more globally spaced, and we still see similar patterns of a near constant reflap with in front of these big Mo2s. It gets to about 120 meters to 150 meters as the mean reflap with. And in terms of really what's driving that, I've got this graph, but there is a lot more data, and honestly, we were just starting to look at this yesterday. So I think the cool part is trying to investigate that more and tease out some of the different pieces.