Boca Do Rio A Study of Portuguese Foz

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1 summer 2008 I don t know about you, but I like fish. Not in a way where I want to hold them, pet them, hug them or idolize them, but on my plate, fresh off the grill. I love sushi: spicy tuna rolls, and eel. And I love salmon, baked, poached, or grilled with a nice twist of lemon and dusting of black pepper and salt. There is a problem with this love though; fish stocks are in a downward spiral and we all face the sobering fact that if we can t mitigate this population freefall, we might all be eating chicken and only chicken for the rest of our days. Horrifying. Some of these threatened fish species, in particular salmon and steelhead, have very complex lifecycles before they grace our dinnerware. Adult fish migrate to freshwater streams and rivers from the vast deep oceans to lay their eggs in coarse gravel river beds, after which they die, never to see their offspring. Sometime later, the baby fish emerge from their underwater nests and spend the first few months swimming in cold inland streams, eating and growing. During the spring and summer, these young fish migrate toward the oceans where they spend most of the rest of their lives swimming, eating, and in large numbers, becoming food for whales, seals, and yes, at University of California, Berkeley Boca Do Rio A Study of Portuguese Foz us humans. After 3 to 5 years in the sea, a lucky few make it back to the rivers of their birth to spawn and complete the cycle. We know that the population of fish who make it back to the rivers to spawn has been perpetually falling, and scientists, for years, have been trying to figure out what factors have been driving this decline. Up until this point, studies have focused on dynamics in the ocean, on the features of freshwater streams, and on the influences of landuse in watersheds. Recently, though, research has emerged that indicates there is a strong correlation between the number of fish who survive long enough in the ocean to return to spawn and the amount of time they spend not in the ocean, and not in freshwater streams of their birth, but at the intersection of these two habitats: in small coastal lagoons. A new study out of the University of California at Santa Cruz shows that juvenile salmon and steelhead reared in coastal lagoons that are disconnected from the ocean during the dry summer months by a large sand bar are nearly twice as big as fish that swim directly into the ocean. This gives them a higher chance of survival in their marine habitat and a greater chance of returning to their streams to spawn. 1

2 methods This project focused on determining what characteristics were shared by coastal lagoons that separate from the ocean and what characteristics were shared by rivers that maintained a surface connection with the ocean. By knowing what characteristics have strong correlations with the open or closed status of the lagoon, I can begin to examine the processes that drive the physical formation of coastal lagoons which will then have implications on land, water resource, and coastal management strategies for salmon and steelhead preservation. In the summer of 2008, I spent five weeks along the beautiful Portuguese coast inventorying foz, the Portuguese word for estuary or river mouth. I visited over 30 foz from Vila Praia de Âncora near the Spanish border in the north to Lagos in the Algarve and characterized the physical attributes of each foz. I measured the salt content in each river and lagoon to get an idea of the mixing that happens between the freshwater in the rivers and the salt water that comes in with the tides; I also collected sand samples from each beach from which I later determined the dominant grain sizes in the geotechinical engineering lab at the University of Porto. I was most amazed by the snowy white sand at Praia D el Rey whose grains were so fine that as I walked barefoot across the beach, the sand to squeeked and sang every time I took a step. I also evaluated the amount of flow in the river, the topography of the beach, the shape of the river, the shape of the river mouth, and the presence of bluffs or rocks along the coastline. In addition, I used aerial photographs to look at the shape of the coastline and the angle of the beach with respect to north. results The most striking correlation I found in this study was between the mean diameter of the sand grains on the beach and the tendancy for that beach s foz to be closed. I found that every beach with an average grain size smaller than 0.6 millimeters has a surface water connection to the ocean while every beach with an average grain size greater than 0.7 millimeters had a sand bar separating the lagoon from the sea. From a physics point of view, this makes sense; larger grains require more energy to move. The energy to move the grains that are deposited on the beach and in the river s outlet channel by ocean waves is proportional to the amount of flow in the channel. Thus, as flows decrease and the energy provided by the The river winds through the beach at Sao Pedro de Muel at University of California, Berkeley summer

3 A sorted sand sample (above). Lagoa de Melides, a closed lagoon (left), and Rio Onda, an open river (right). Notice the sand bar shutting off the Melides Lagoon from the ocean and the continuous channel of the Rio Onda through the beach to the ocean (photos from Google Earth). river wanes, the rivers that flow through beaches with large sand grains are expected to become trapped by the sand deposited by the ocean waves and to form closed lagoons. In Mediterranean climates, river flows typically decreases through the summer months and after revisiting the sites a few weeks later, I found that many of the rivers with low flows that were open in the first survey, were closed. This leads to the conclusion that there is a natural order and progression of lagoon closure along the Portuguese coast. I found another very strong correlation between the slope of the beach and the tendancy for the river to form a closed lagoon. As the beach steepness increases, so does the likelihood that the lagoon will be closed. This is intuitive, for in order to backwater up behind a sandbar, there needs to be some significant vertical relief to pile the river water behind. Another interesting finding is that the azimuth of the beach is correlated with if a river forms a closed lagoon. Beaches that have large positive azimuths (or that tend to face in a more northerly direction) are more likely to remain open than those rivers that emerge onto beaches that face more toward the south. This leads me to believe that the north to south ocean current present in the Atlantic ocean off the coast of Portugal plays a role in keeping foz that face into it open while foz that face in the direction of the current or have their backs to it, so to say, do not experience the same energy provided by the ocean. This energy plays a role in removing sand from the river mouth and transporting it along the shore, so it is possible that we see more closed lagoons facing in a southerly direction because the ocean current along these sheltered beaches does not have the energy to transport the quantity of sand whose deposition naturally increases during the spring tidal cycles regardless of orientation. The variables that correlate most significantly with if a foz is open or closed are sand size, beach steepness, the flow in the river, and the angle of the beach with respect to north. Since flow is significant, I can also at University of California, Berkeley summer

4 Mouth of Rio Onda on June 22, 2008 Mouth of Rio Onda on July 17, In the latter photo there is no longer a surface connection to the river or the Lagoon. say that watershed size is significant as these two are generally proportional to one another within a specific geographic region. Variables that have weak correlations with if a closed lagoon forms are the degree of mixing of fresh and saltwater in the estuary, the shape of the coastline (if the coast is concave, convex, or straight), the width of the beach, and the location on the beach where the river enters the ocean. Variables that have little to no correlation with if a foz is open or closed include the beach length, the shape of the river as it runs across the beach and enters the ocean, the length of the river on the beach, and the presence of rocks and bluffs at the river mouth. conclusion As you could well imagine, I got some strange looks doing my research. I went to the beach looking like a normal person, swimsuit, sunglasses, hat and beach bag, but then my behavior took a strange turn when I pulled out electronics with probes, filled bottles with water from the ocean and plastic baggies with sand. On one particularly windy day at Praia de Apulia, two young men came over, sat down, and asked me with a puzzled look, what I was doing. I explained the study and we talked for an hour or so about the data, and of course, football, while getting sandblasted on the beach. During the course of this hour, I notice something interesting: sand dunes formed in the dry at University of California, Berkeley summer

5 river channel. This piqued my interest and brought up a whole new set of questions about the role of wind in building and reinforcing the sand bars that form closed lagoons. This is a topic that I would like to spend more time on; originally I had thought that the sand bars are the result of a competition between the ocean waves and the river s current, but after witnessing this, I feel that wind may have a strong part to play as well. Thank goodness those two Portuguese men were curious enough to come ask me what exactly it was that I was doing on the beach with my probes and bottles. In addition to really enjoying living in Portugal for five weeks, having a great time running around the city hitting complete strangers on the head with a squeaky plastic hammer during the festivities of São João in Porto, and eating more grilled sardines than any American probably should, the time I spent in Portugal was extremely valuable for my research and helps to inform my dissertation work here at the University of California at Berkeley. I feel that now, after this experience, I have a good grasp on the suite of characteristics that are common to small coastal lagoons and can from this point forward pose more sophisticated questions about the processes that drive the formation of these biologically important features. acknowledgements I would like to thank the Pinto-Fialon Endowment for the grant to perform this research and the Portuguese Studies Program for supporting my research along the beautiful Portuguese coast. at University of California, Berkeley summer