We've been studying the ecology in seagrass meadows. You probably already understand that seagrasses are plants in the sea. It happens, however, that people confuse seagrass with seaweed which is what algae are called sometimes. Seagrasses are not related to algae; instead, seagrasses are flowering plants (the group of plants that usually grow on land) that rarely have flowers but uses clonal reproduction via their rhizomes (roots). This way of growing makes the seagrass form "meadows" that has the appearance of grasslands on land, but the seagrass grows in the water in shallow coastal areas. In Sweden we only have one species of seagrass, but in tropical coastal areas there are several species that sometimes grow mixed together.
In the tropics the seagrass meadows are closely associated to mangrove forests and coral reefs. Among other effects, the reefs and the mangroves protect the coast from the strong wave action of the open ocean and the seagrass with their epiphytes (mostly algae and bryozoans), that grow on the seagrass, takes up nutrients which makes the water clear. Both the mangroves, corals and seagrass are extremely important habitats for invertebrates (seastars, sea cucumbers, shells etc.) and fish. Since the fish from one ecosystem often migrates to one of the others (eg. to search for food or to reproduce) it is very important that all three parts of the system are not disturbed by for example overfishing. From a human perspective, it is also very important that the people living in tropical coastal areas can continue fishing for invertebrates and fish, since it’s their primary source of protein and they make a living out of it. A lot of science has been conducted that focuses on the ecology of mangroves and corals, but not so much on seagrass.
In our project we aim to discover how different variables in seagrass meadows affect each other and see what effect protection against fishing has on the seagrass meadows around Zanzibar. To do so, we needed to collect data about the seagrass and animals by visual observation and also take physical samples of seagrass and sediment on as many places as possible around Zanzibar. The data we collected on each sampling point had to be collected using the exact same method for all points, and the original goal was to complete sampling on 10 points in 10 different sites, resulting in a total of 100 sampling points. In the end we managed to complete 60 sampling points in the sites Chwaka Bay, Mbweni, Chumbe Island, Prison Island, Nungwi East and Fumba.
As I mentioned in an earlier blog post, we had some problems with the project in the beginning. The biggest problem we had was being able to do the visual observations on the same day we were taking physical samples. We had to do it on the same day because we couldn’t leave our markers for the sampling points (a weight attached to a float with a flag on plus transect lines on the bottom) in the ocean over night or longer. It turned out visual observations, i.e. seeing fish in the water at least a couple of meters away, could only be made during neap tide, because that’s when the tidal current is weak and the water is clear. The physical sampling of seagrass, however, could only be made during spring tide which happens two weeks after a neap tide. That’s when the tide makes the water low enough for a person to stand on the bottom and use muscle strength to cut through the strong rhizomes of the seagrass using a machete. The seagrass species we’re focusing on in the project, called Thalassodendron, is a species that often grows much deeper than a man can stand and has the thickest rhizomes.
After trying for one and a half month to make everything work at the same time during spring tides we had to abandon taking samples of the rhizomes. We also had to give away the isopropanol we had bought for preserving those samples (worth 700 Euro). Floriaan was the one who was supposed to write a thesis about the animals that live among the rhizomes and in the sediment. When we abandoned that, we started focusing on the epiphytes that grow on the seagrass (algae and bryozoans) instead, and this will be the focus of my thesis. Floriaan got my old part of the project, which is studying invertebrates that live on the bottom; seastars, sea cucumbers, shells and such. This group of animals can be called epifauna, “on-the-bottom-animals”. Elisa will still write her thesis about fish.
Now I’m going to describe what one day of fieldwork was like:
We started in the morning by carrying bags containing our field equipment from our apartment in Stonetown to a hired taxi. The equipment consisted mostly of plastic bottles filled with wet sand, attached to rope, which were used to keep the markers in place on the bottom. With the taxi we picked up our hired local assistant and travelled to a site, for example Chwaka Bay. At the site we met up with a local boatman (they usually didn’t speak much English) and we all helped out with carrying our equipment to the boat, which was on deeper water. We also paid the boatman some money for gas so we could use the engine. We then asked him (using our assistant’s translation and by showing pictures) where there’s a Thalassodendron seagrass meadow in the area. The boatman always knew the area very well and during the trip there we all put our sunscreen and snorkeling gear on. We then jumped into the water and located the exact location of the Thalassodendron. Then we got up in the boat again, moved the boat to the center of the Thalassodendron meadow, and Floriaan noted the GPS coordinates of the central point. Then, using random coordinates that he had prepared in advance, he inputted waypoints in the GPS so that we could place out our sampling point markers in the real seagrass area. He then guided the boatman to steer the boat to the waypoints and me and Elisa helped with putting the markers in the right places.
Floriaan to the boatman: “Can we go… that way?”
Floriaan to Michaela and Elisa: “Ready on marker no. 1… Chug!”
I and Elisa chugged first the weight and then the bottle with the flag that’s attached with rope to the weight, into the water. “OK!”
Floriaan to Michaela and Elisa: “Ready on marker no. 2…” and so on. We put out six markers one day, and worked on those points, and then the next day we put out the remaining four.
When the markers were all set out, we jumped into the water again to set out transect lines starting at the marker point, all in the same direction. Each of these consisted of a 25 m long brightly colored rope with six bottles filled with sand attached to it, to mark every 5 meters. When they were set out we got up in the boat for a 10-minute break and to leave the transects undisturbed so that the fish would continue acting as normal. We drank water, ate bananas and chocolate chip cookies, and refilled our sunscreen. We also made a plan of which order we wanted to do each transect.
Then Elisa got into the water again and swam to the first. She slowly swam back and forth along the transect counting and identifying each fish that passed within the transect, and noted them on her underwater slate. This was of course hard to do when for example a school of tiny fish swam by. When she was ready she gave the “OK” sign to me and Floriaan and then continued on the next transect.
Floriaan swam along the transect counting and identifying sea cucumbers, sea urchins and seastars. This was hard to do in the Thalassodendron because the seagrass was long and covered all the epifauna, which meant he had to dive down to the bottom and comb through the seagrass with his hands to uncover the animals. He noted them on his underwater slate.
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| One transect without seagrass. |
I stopped every 5 meters, by every bottle, along the transect line and estimated a percentage cover of plant species (seagrass and algae) and sessile fauna (sponges, corals and anemones) in a 50 x 50 cm quadrant, and noted this on my underwater slate.
When the visual observations of all transects were done, we took a lunch break; more cookies, a roll of Pringles and bananas, mangos or watermelon, drank more water and refilled the sunscreen again. On the basis of my cover estimates, I decided which of the six quadrants had been most representative of each transect. Floriaan then carried a very heavy 1 x 1 m iron frame to the representative quadrant I had chosen and placed it there to mark our physical sampling spot. He then dived down and combed through the seagrass in the 1 x 1 m quadrant, looking for shells to identify and count. He wore gloves for protection against the spines of the urchins and the shells that could be potentially lethal. He picked some of the harmless shells for later identification. Floriaan then placed a 25 x 25 cm iron frame inside the big quadrant and dived down several times picking all seagrass shoots within it. He then dived down again with a plastic tube which he used to take a sediment sample with. It was very hard physical work that Floriaan had to do because he’s obviously the strongest one of us and also most skilled at snorkeling down deep. Elisa and I assisted by handing him tools, putting the samples in plastic bags and swimming back and forth to the boat to label and drop off the samples in a cooler box. When the sampling of all transects was completed we swam back to the boat for another short break. After the break we swam out to the points again and rolled up the transect lines and the markers carefully so the ropes and bottles wouldn’t get entangled, and then carried them one by one to the boat.
Afterwards, we had a very nice trip with the boat back to our taxi, where our driver had been napping during the day, and then we drove to Stonetown. From the taxi we carried our weird bags and iron frames along the touristiest street in town back home to our apartment. There we put our samples in the freezer, rinsed our snorkeling gear, showered, inputted our data in our computers, prepared coordinates and labels for the next day’s fieldwork and went for drinks while watching a beautiful sunset and had dinner at one of our favorite restaurants.
Since we didn’t have much time for labwork in the end, and the labs in Sweden are better equipped, we did as little labwork as possible in Zanzibar. We took subsamples of the different seagrass species, and preserved them in tiny bottles to take to Stockholm. The rest of the seagrass shoots we scraped to separate the different seagrass species from the epiphytes that grew on them, and then we weighed the seagrass and epiphyes to get the wet weight. We also put them in a drying cabinet and then weighed them again to get the biomass data. We did some labwork at home in the apartment too, where we picked out tiny leaves and twigs from the sediment so we got only sediment and then we homogenized it and preserved in tiny bottles to take to Stockholm.
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| The lab in Zanzibar. |
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| An epiphyte from a seagrass leaf. |
Thanks for reading this blog. Zanzibar was an incredible experience and a dream come true, I loved it, it made me feel much more alive than in my safe Swedish home and I wish to go back to that tropical paradise many more times. If you get the opportunity to come to Zanzibar, don’t just stay by the pool at your resort or luxury hotel! Buy the book “The Seashores of Eastern Africa”, a snorkel and mask, maybe also fins, and take a boat to any coral reef where you’ll see beautiful things.
Now the promised video:
Cheers!
Michaela
