Spring 2024 Benthic Cover Analysis of Fish Pond
SURVEY DATES: 05/19/2024 - 05/21/2024
SURVEY LOCATION: Paea Lagoon Aua i’a Fish Pond
AUTHORS: Jessie Segnitz, Uma Pant, Nicole Pianalto, and S. Tara Grover
INTRODUCTION:
In this survey, we observed the substrate inside, on top, and outside the Paea Lagoon Aua i’a Fish Pond. The fish pond is a Polynesian traditional practice of small scale fish collection that fell into disuse over time due to the effects of European colonization, commercial fishing, and globalization. We are building upon the previous studies done by Wildlands students in 2022 and 2023 in order to meet the needs of our client, a private individual interested in species and land conservation who owns a marine observatory on Tahiti.
The objective of our study is to determine the sea floor substrate and algae covers on the inside and outside of the fishpond, and top of the rock wall top itself. Further objectives of the research team were to establish measurement definitions for the average particle sizes of both "fine" and "coarse" sand to set a standard for future use, and second, evaluate the presence of patterns on the seafloor of fine and coarse sand areas. There is a current along the shore running south to north through the fishpond and our client is specifically interested in the influence of this current and how the rock wall may act as a sieve or filter, changing the concentration of the sand types on the different sides of the wall. Earlier studies concluded that there was no statistically significant difference in substrate coverages outside and inside the fishpond, however, they did not account for the difference between fine and coarse sand, which is the knowledge gap we addressed with our survey.
We hypothesized that there would be a significant difference in the coverage of the different sand types on the south and north side of the wall. We further hypothesized that there would be a difference in overall substrate coverage inside and outside of the fish pond walls because of the barrier effects of the wall on the current's ability to move and transport substrate types through the area.
METHODS:
FIRST SURVEY — INSIDE FISH POND
We used a 50 x 50 cm quadrant to survey percent coverage of different substrate and algae types. We used a random number generator to generate 10 coordinates within the size of the fish pond which is roughly 15 x 15 meters. Using the bottom right corner of the fish pond (Northern corner) as our (0,0) origin point, we used a transect to measure out the predetermined coordinate points to the South along the shore (x-axis) and out into the water (y-axis) and placed the bottom right corner of the quadrat at each point. Our randomly-generated coordinates were (8, 12), (5, 1), (2, 7), (10, 9), (14, 3), (9, 11), (6, 12), (7, 9), (12, 9), (13, 2).
For this survey as well as Survey #2, we evaluated percent coverage of the following categories: fine sand (with the majority under 1mm length of grain on average), coarse sand (over 1mm length of grain on average), bare rubble (chunks of substrate between 2.5-10 cm, including stone, dead coral, shells), bare rock (over 10 cm with no algae cover), and five types of algae. These types were turf algae (under 1 cm), and the macroalgae genuses: Halimeda, Padina, Turbinaria, and Dictyota. Two researchers both independently estimated the coverage of each type and then double verified with each other.
We made sure to step lightly to avoid disturbing substrates or moving any particles into or out of the quadrats. We also exercised a high level of caution to avoid contact with dangerous benthic species including the stonefish, by wearing neoprene boots and swimming when possible without touching the floor.
SECOND SURVEY —OUTSIDE FISH POND
Starting from the north fish pond edge at the point closest to the shore, we measured out 5 meters parallel to the shore. That would be our starting point of our survey line of 15 meters to the end of the fish pond walls. We did systematic sampling, so every 5 meters starting from 0 meters on the transect we would sample using a 50 x50 cm quadrat, putting the quadrat on the left side of the transect with the bottom right corner at the starting point. We did this for the north, west and south walls of the fish pond. We evaluated percent coverage of the same categories and methods as for Survey #1 (above).
THIRD, FOURTH, AND FIFTH SURVEY — SEDIMENT TRANSECT
In this survey we used line intercept sampling by using a transect adjacent to the interior and exterior wall, which we identified as the area on the seafloor closest to the rock wall that did not include any large rocks that made up the foundation of the wall. We evaluated percent sediment coverage of the following categories: fine sand (under 1mm length of grain on average), coarse sand (over 1mm length of grain on average), rubble (2.5-10 cm), rock (greater than 10 cm), and alive coral, all regardless of any algae cover on top. By looking at what sediment lay directly underneath the transect line we classified what sediment was present and the length of the section it created, for the entire 14.5 meters. We conducted this survey for the north, west and south side walls of the fishpond on both the inside and outer side of walls.
SIXTH SURVEY—- ON TOP OF WALL
For this survey we used systematic sampling using the 50 x50 cm quadrat and a transect laid out from the start of the rock wall for all three walls. We placed the bottom of the quadrant at 0m, 5, and 10 meters for each wall. The quadrat was placed in the very center of the wall, and at all survey points, the wall was thicker in width than 50 cm so the quadrat consisted completely of substrate from the wall itself. We looked down from an aerial view and measured the same substrate and algae types as previously mentioned in the other surveys. We repeated this method for all three walls for a total of 9 survey points.
SEVEN SURVEY — SAND MEASUREMENT
For this survey a team of two took samples of fine and coarse sand from inside the fishpond, scooping only the surface layer of sediment. We collected approximately 20 mm of sand and water for each. The samples were chosen based on visual differentiation, where the fine sand was scooped
from the left wall delta closest to shore inside the fishpond, and coarse sand from the center of the fish pond. The sand was laid out on a paper towel and a randomization method was used to choose 50 grains of sand to measure from each sample. Calipers were used to measure out the various particles in millimeters.
IMPROVEMENTS/ CHANGES FROM PREVIOUS SURVEYS
We used a 50 x 50 centimeter quadrat instead of the previous group's 1x1 m. This allowed us to take more accurate and detailed data on the coverage within our survey areas while still being a large enough surveyed area to be generalizable to the entire pond.
We know that the algae species and concentration can change due to seasonal and climate patterns. We did our own preliminary analysis of what types of macroalgae genuses were present when we got in the water, and created our own list of them to measure instead of using the previous groups. This was the same as the previous groups except for one exclusion, Sargassum, which was not present at this time, and one new inclusion, Dyctyota, which was present.
We did a randomized point intersect method for the inside of the fishpond instead of the previous groups' strategic sampling method because the fishpond is a big enough sample universe to benefit from a random sample to eliminate bias or accidental disproportionate inclusion or exclusion of substrate patterns.
We continued the original method of strategic sampling for the outside fish pond sampling universe, and for the walltop itself, because we felt that the narrow range of these areas would be better represented by consistent sampling. A visual analysis of the general substrate cover of the entire survey area confirmed that we were not overlooking any patterns due to this sampling method.
Surveying the wall top itself was also a new addition to this survey project that was not present in previous years. This allows us to set a baseline time zero (t=0) standard for wall composition which gives insight into the structural integrity of the stones based on how close together they are, and what substrate types are present among the cracks..
Another new addition was the specific line-intercept survey along the outer and inner walls that focused on determining patterns of sand dispersal through the walls.
RESULTS:
Please copy and paste the link below into your browser to view data sheets with graphical analysis
https://docs.google.com/document/d/1296DmEP9zlzIvRDUheGtx5jx7ANnlSfT8JGAyNMvuSI/edit
DISCUSSION:
Survey one data illustrates that the substrate cover inside the fish pond is mostly coarse sand and a scattering of rubble across the area. While the dominant algae inside the fish pond was Halimeda. In the survey two data, it portrays that the sediment composition outside the fishpond was mostly made up of fine sand and some coarse sand, while the algae was mostly turf. The differential sediment composition inside and outside the pond demonstrates that the fish pond creates a different sediment environment, which is reflected by the dominant algae that is growing in the area.
The survey three, four, and five data show sand patterns that are representative of the current flowing through the pond. The currents are coming from the south going north, parallel to the shore. Outside the southern wall there was a larger section of fine sand, while the interior of the fishpond there were three distinct sections of fine sand. This illustrated that the southern wall is filtering the fine sand into the pond at a relatively slow rate, with most of it concentrated on three sections that correlate with thinner and lower wall sections. While on the other hand, the opposite was reflected by the north wall, by the sand filtering out of the pond. The exterior of the wall substrate consisted of more fine sand than the interior of the wall reflecting that there is a large rate the fine sand is leaving the fish pond. With the influx of fine sand coming in at a slower rate and the outflux leaving at a greater rate, the inventory of the fine sand in the fish pond would be lower. This is also reflected in the data from survey one, the sediment coverage inside the fish pond, portraying that there was very little fine sand within the sampling sites that is a portrayal of the entire pond. Then with the west wall the sediment makeup of the interior and exterior was fairly similar with the percent of fine sand being around 30 percent. This similarity illustrates their is not that much if any sand movement between this wall.
For Survey 6 looking at substrate composition on top of the wall itself, we found that the turf (algae less than 1mm long on top of rock/rubble) dominated the bulk of the substrate available in this location, at 83%, as expected. The wall is composed of large rocks, and can resist wave action more so than coarse or fine sand, which can be washed away. Since the stones here are the original foundation of the fishpond from whare our client rebuilt it many years ago, it makes sense that turf covers almost all of the present stone content. The coverage of bare rock with no turf is only the parts of stones that are still above water even at high tide, so there was no ability for turf to settle and grow there. The areas that were not turf or stone are what is visible between the rocks when looking from an aerial view, ad thus they represent the cracks between the rocks which allow a view of algae content below and occasionally all the way down to the sand on the floor.
During Survey 7, we revealed key information about classifying the difference between the size of coarse and fine sand. We found that the coarse sand had a median of more than 1mm in particle size, and the fine sand had a median size of less than 1mm in particle size. We used this understanding to clarify our data for both types of sand during the other survey collections, where we used visual markers to identify each type. This classification can also be used for future studies as a standardization of the monitoring project.
It's important to note there was an ocean swell a couple weeks ago that washed away different substrate from the location that may have previously been present in the area. For example, certain algae species and fine sand may have been washed away, influencing the current composition of the inside and outside of fishpond wall barriers. This may contribute to significant differences between this survey and the previous ones, although other factors such as seasonal differences and simply the regular wave and wind action of many months will also produce these differences.
Overall, there are noticeable trends in the differences with sand cover on the inside versus the outside of the fishpond.
The higher concentration of fine sand outside could be due to current and wave action pushing coarser, heavier sand particles inside the fishpond that has nowhere to go, whereas Fine sand can be lifted away by wave action. The fact that turf on rock substrate covered most of the fish wall shows how the stones have been present long enough to accumulate turf cover.
Another explanation for the lack of other types of substrate and algae coverage may be due to the nature of intertidal zone harsh conditions, which only support life for the most hardy species that can survive high and low temperatures, water and salinity levels, and have increased mechanical wave action that make it difficult to establish a presence on the rocks and moves sediment rather quickly, not allowing for settlement.
We also noted an interesting observation from this year's data versus last year's projects. Halimeda is the most common algae found in and around the pond, which is different from the rest of the coral reef areas that the research team has seen across Tahiti and Moorea.
PROPOSALS FOR FUTURE METHODS:
We propose that future projects continue to differentiate between fine and coarse sand, so that a standard can be maintained for data collection over time in consideration of factors such as current speed and wave action.
A proposal for future groups is to analyze the structure of the wall to determine what qualities exactly lead specific sections to allow more fine sand through the stones.
The line-intercept survey of sand cover along the sides of the walls should be repeated to track change over time.
We also recommend separating the data collection between algae biodiversity (different genuses) and mineral substrates (coral, sand types, rock, rubble) as separate surveys, where the mineral substrate survey does not regard algae coverage.
The algae biodiversity survey should also include a difference between turf on rock vs. turf on rubble, to make sure that it is clear to differentiate the preferential turf substrate.
We also recommend continuing data collection on specific algae types over time, or making sure to note if there is a decrease in a specific population during that year to maintain standard measurements, just to keep a clear comparison of the different populations in the fish pond over time.
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