Substrate Cover of Pa'ea Lagoon Fish Pond - French Polynesia - Summer 2024

Authors: Maria Miller, Alex Lombardi, Diya Sterling
Survey Dates: July 31 - August 1
Location: Pa'ea Lagoon, Tahiti Nui, French Polynesia

Introduction

Wildlands Studies research teams have surveyed the Pā’ea Lagoon Aua i’a over Winter and Spring seasons for the past year to document substrate variety and abundance. Throughout this time, changes in substrate have been observed, and have been used as a basis for future observations. Continuing this research we intend to focus specifically on mineral content to thoroughly represent the mineral cover within the fish pond.

As Tahiti enters the winter season, new data is significant in determining if there is a correlation between dominant substrate groups and their location within the fish pond, as opposed to other seasons and data collections. As our methods and focus vary from past surveys, we will only be comparing their inside substrate data, and not focusing on specific species of algae, but instead categorizing algae as a substrate type.
Our client is most interested in understanding the collection of sand around the North and South walls, and a general mapping of where different substrates are located within the fish pond. In order to develop a hypothesis as to why the collection of sand is allocated, it is necessary to first get a strong understanding of where these collections of sand are located along the walls. For the purpose of this survey, we will be using the term channel to describe the openings in the north and south rock walls that are likely developed from erosion. We hypothesize that these channels in the rocks are resulting in the deposition of sand along the walls. Overall, our study aims to determine mineral substrate cover of the fish pond, with a focus on sediment deposition along the North and South walls.

Methods

The materials used include a transect that reached 20 meters, snorkel gear with fins, neoprene socks, an underwater slate, one 50 centimeter by 50 centimeter quadrant, a camera, and 24 plastic cups labeled by quadrant points. Before the survey commenced, a preliminary survey was conducted to get a general idea of the surface area and to determine the best way to separate transects.
We began our survey by allocating roles between three group members. Two members held the transect, one on each end, and one member swam along the transect surveying the substrate cover within a 50 centimeter by 50 centimeter quadrant. We measured the dimensions of the fishpond and found it to be a 15.25 meter by 15.25 meter area. With this, cross-shore transects were separated by 5 meters from the inside part of both the North and South walls, creating two transects inside of the pond. Three more transect areas were placed at the center of the perimeter wall of the pond. Two of these were cross-shore along the North and South walls. The West wall had a longshore transect to close off our sampling universe within the fishpond.
At each transect, a quadrant was placed every 3 meters five times, starting at 0 meters, with the top left of the quadrant placed on each point on the left side of the transect. For the two transects inside the pond, we only surveyed the left side of the transect. For the transects lining the perimeter, we surveyed both the left and right side of the transects to gain insight on the immediate outside of the pond to fulfill our client’s interest. At each site, sand portions were lightly dusted through to uncover the possibility of different sized sand particles under the first layer.
For the transects inside the pond, once the left side was surveyed at all five points, the swimmer went back to shore to immediately take a picture of the data gathered and switch out the slate and quadrant for 5 plastic cups labeled for each survey point. Once again moving every 3 meters starting at 0 meters, the swimmer scooped the top layer of sand at each of the five points. For the perimeter transects, this was only done on the inner wall at each of the five points. Once we completed the entire survey, two swimmers were designated to get 2 samples of the top layer of sand right outside of the North and South walls of the fishpond where the large piles of white sand were found in order to more deeply analyze the sediment per request of our client.
For data analysis, we used a Google Sheet to create a table for each of the transects and substrate types which included fine white sand, fine black sand, coarse white sand, coarse black sand, rubble, rock, artificial stone, algae, live coral, and dead coral. We differentiated fine and coarse sand by individual particle size, provided by the spring research group. The spring research group concluded that fine sand was particles less than 1 mm, while coarse sand was particles over 1 mm on average. We had overlapping substrate cover, so each area summed up to a percentage over 100%. From this, we were able to make pie charts for each of the transects which used ratios for each of our percentages so that they could be a total of 100%. We then created a second table that took the average coverage of all the substrate types overall transects and found the standard deviations for each group. A pie chart was formed out of this table as well. Once this data was analyzed, a map was formulated showing each of the dominant substrate types in each quadrant area to encapsulate the variation across the fishpond floor for our client.

Results
Figures and Data: https://docs.google.com/spreadsheets/d/1aaYnemKM8PBG3YQlgV00L5wFx-vuUwom1tuDsJfMhdA/edit

Discussion

An overall average of 18.4% of fine black sand and 17.5% white fine sand cover dominated the sampling universe. Fine and coarse grain (white/black) sand was the main type of substrate cover throughout the fish pond. Our client was specifically interested in the piles of fine-grain white sand accumulating on both North and South walls. It was unclear as to how these piles occur, postulating whether it was current direction, biological erosion, or holes in the fish pond wall causing sand to pile up in these two areas. Upon further observations, our team discovered that these substrate piles consisted of a sediment deposition of fine black sand on the outer south wall, with fine white sand on the inner south wall. Fine white sand dominated the outer north wall, and fine black sand dominated the inner north wall. These findings may suggest that waves are being refracted as they travel through the North and South walls, accumulating fine black sand on the South sides of the walls and depositing fine white sand on the opposite side.
Though we had different methods for surveying and conducting calculations, we were still able to compare our data with the Spring data through the dominant substrate found inside the fishpond and directly outside. In the Spring, it was found that coarse sand was the dominant substrate inside of the pond and fine sand was dominant outside of the pond. For our current data conducted this Winter, the dominant substrate found both inside the pond and directly outside the North and South walls was fine white sand. This differs between the seasons. Despite the Spring group not including their survey conditions, we can infer that winds are stronger during the Winter in Tahiti, likely increasing the level of erosion and sifting sand substrate in the channel regions of the fishpond. Colder temperatures may also increase nutrients in the water attracting more bioeroders and increasing algae composition. Halimeda algae also disintegrates fairly quickly and adds to the fine white sand substrate content, as mentioned by our client. We suggest that future research projects should collaborate with another team observing Bathymetry to get a deeper understanding as to why the sediment depostion and wave current sieves out the white and black sand this particular way in the channel. Something to note is the wave direction was traveling from the South, with some wave interference from strong winds possibly impacting these filtrations.

Areas for Improvement/Suggestions to Future Groups:

For this survey, we separated algae substrate from mineral substrate to ensure a further in-depth analysis of each substrate cover as suggested by past groups. This worked very well for us, so we suggest continuing this in future surveys. We also adjusted the methods by using systematic sampling along transects instead of random coordinates to gain a more accurate area cover and standardization.
We did have areas for improvement. Once we analyzed our data, we realized some points in the fishpond weren’t accounted for, so we recommend allocating transects in a way that reaches all corners of the fishpond. For example, the entrance to the fishpond was missed in our data and could be significant when looking at the channel on the sides of the pond. For samples taken, it is important to ensure saving them for future further analysis done by the client. Also, ensure a scale is included for comparison in any photos taken of the sand samples for a better visual of the differing sand sizes and context. During the survey, we faced some opportunities for human error. When we entered the water, the Beaufort scale reached a 2. While in the water, we found the Beaufort scale changed to a 4 and stayed this way until we exited the water. In combination with the Southern winds, this created difficulties as the transect moved with the strong current and the quadrant placed had to be held firmly to keep it from drifting. There is also the likely occurrence of bias due to having only one person surveying the substrate cover and only having the ability to use human vision which is limited. For the future, we recommend having multiple swimmers surveying the quadrants to ensure a lower likeliness of bias. We were unable to do this due to time constraints and limited participants.

Conclusion

Our studies show the collections of sand around the North and South walls were a result of sediment deposition from filtration through the fish pond walls. We designed a general map of substrate cover in the fish pond to visually display the variation of substrate cover, as well as the location of the sand piles in the channel. The North wall accumulated fine black sand on the inside of the wall, with fine white sand on the outside. The South wall had accumulated fine black sand on the outer wall, with fine white sand on the inside. We suggest that this pattern of accumulation may be due to wave refraction as the current hits different parts of the wall, and the influence of the 1m hole in the West wall causing a stronger current within the pond. Further studies need to focus on the North and South wall channels to analyze the sediment deposition and mineral content in order to predict future erosion patterns of the fish pond substrate. This could also raise the question of how this impacts species abundance and behavior in the fishpond.

Posted on August 2, 2024 11:36 PM by langzi