One of the leading threats to our planets coral reefs is the release of sediment and nutrients from land into coastal areas, known as eutrophication. These nutrients come from both local and distant sources on land, generally from development, deforestation, agriculture, and pollution/waste water discharge. Slow growing corals are unable to utilize these increased nutrient levels, but fast growing macro-algae can. When coastal areas are enriched with nutrients, coral reefs can quickly be lost, and replaced with much less productive and diverse macro-algae ecosystems.
Although this effect is directly observable and has been scientifically recorded again and again, combating the issue is difficult. Nutrient levels and ocean chemistry fluctuate greatly on a global scale, and agricultural and development projects far from the sea can still wreak havoc through transport by rivers. The scope and scale of the problem make identification of nutrient or pollution sources difficult.
On Koh Tao, development and deforestation have directly contributed to the decline of coral reef cover in several areas, including Tanote Bay, Sairee, Mae Haad, Ao Tien Og, and Chalok Ban Kao. Although many great initiatives to solve the problem have been started in the past, most development on the island remains unregulated and poorly planned. Furthermore, a lack of centralized waste water treatment means that each business or resort is responsible to manage their own waste water.
Over the years we have seen increases in the amount of macro-algae on the reef, and in some areas such as Shark Bay we believe that macro-algae has been a significant reason for the lack of coral recovery following the 1998-1999 global bleaching event. But we have never been able to determine with any confidence where the nutrients leading to all that algae growth was coming from.
In 2014, CMBC Master’s student, Laurence Romeo, completed a research project on the island to investigate the abundance of macro-algae at various sites around the island. his paper, titled “Tracing Anthropogenic Nutrient Inputs Using δ15N Levels in Algae Tissue
Koh Tao, Thailand” used what is known as a bioassay technique to determine the source of the nutrients. In this technique, macro-algae samples are collected from the various reef sites at fixed distances from the suspected source of waste water. Next the samples are dried and run through a mass spectrometer. The spectrometer analyses the atoms in the sample, and then provides a ratio on the quantity of different isotopes, in this case Nitrogen 14 and Nitrogen 15.
Nitrogen 14 (N14), is the ‘lighter’ form of the Nitrogen atom, it has 7 neutrons and 7 protons in its nucleus, and thus a molecular weight of 14. Nitrogen 15 (N15), on the other hand has an extra neutron in its atom, and is thus a bit ‘heavier’. In nature, different plants and animals can have a preference for one or the other, and so in certain food chains one will show up in a higher abundance than the other.
As it happens, the human diet is high in N15, and thus so is sewage. Researchers can then look at where algae is growing, and test the amount of N15, and know if the algae growing there is getting its nutrients from deforestation and agriculture, or from raw sewage. As Laurence explains in his paper, looking at the amount of macro-algae coverage can tell a researcher about the amount of nutrient loading in an area, then testing the isotopes of Nitrogen can tell the researcher what the source of the nutrients is. Combined with water testing data, source points for pollution can be identified and addressed to relieve this chronic threat to corals growing near human development.
In Laurence’s project, he tested two sites in Sairee, and one site in Shark Bay. In the Sairee sites, a clear pattern is shown in the ratio of N15, with the highest abundance near the outlet for the ‘Sairee Canal’. The southern site at Sairee also showed levels of N15 high enough to indicate human sewage, but was lower than the site in the North. This data strongly suggests that the treatment of sewage by the businesses along the Sariee canal is inadequate. Not only are the businesses dumping tons of grey water (from laundry, cleaning, cooking, etc.), but also sewage water into the canal. These findings by Laurence also correlate to the findings of a paper on coral diseases around Koh Tao by Lamb et al. (2014), in which elevated levels of disease and bacterial related bleaching were identified in Sairee.
The site in Shark Bay tested the highest for all nutrient tests (NH4, NO3, NO2, PO4), yet had a low abundance of N15. This means that although the site at Shark Bay is more nutrient enriched than Sairee, it must be sources other than sewage. Most likely the high levels or macro-algal growth in Shark Bay are due to deforestation and the loss of topsoil on the adjacent mountains as discussed in our 2014 report. To address the problem here, regulations would need to be made on development and construction in the bay’s watershed.
Through our ecological monitoring data, we have previously identified the problems in Sairee and Shark Bay in terms of increases in macro-algae abundance. Through more in-depth and analytic work such as Laurence’s project, we can further understand the relationships between coral reef degradation and human activities. This information can be used to pin-point sources, and work with local administrators to address these issues.
We welcome such beneficial projects here on our island. Often, independent researchers come here to do studies which can be classified in the category of ‘science for the sake of science.’ Or projects which will further our understanding of some process or the mechanisms involved, but not have any implications for management, policy, or technology. Although these projects also have some intrinsic value, we at the NHRCP feel that in a time in the world where ecosystem managers are scrambling to save species before they go extinct that science should led to action. Laurence’s project takes a developing field of science, the bioassay for N15, and uses it to recommend management changes to address the issues. And furthermore, Laurence does a great job at articulating this problem at the end of his literature review when he states:
“It seems from the literature that there has been a continuous and ongoing debate between scientists from different corners on what may be causing the decline of coral reefs worldwide. The problem here may well be trying to find a ‘silver bullet’ that may not necessarily exist. This search would seem rather dangerous and somewhat fruitless with coral reefs in rapid decline. What is needed, urgently as time is running out, may be less trying to find what the biggest cause of coral decline is and more immediate action in bringing the reefs back to a natural resilient state. On a local level, this means tracing and reducing anthropogenic effects of pollution, sedimentation and overfishing. Reducing these effects locally would be a step in the right direction and would create a more resilient coral reef in a time when global risks, such as bleaching and disease, are on the rise.”
Congratulations to Laurence Romeo for completing this valuable project and completing his Master’s Degree with CMBC.
You can click here to download the full paper:
Romeo, L. (2014) Tracing Anthropogenic Nutrient Inputs Using δ15N Levels in Algae Tissue Koh Tao, Thailand. Master’s Thesis, MAS Marine Biodiversity and Conservation, CMBC, Scripps Institute of Oceanography, UCSD. 29 pp.
Laurence would like to acknowledge for his project:
My Advisors: Dr Jennifer Smith & Heidi Bachelor. My mentors: Maggie Johnson & Emily Kelly. Mike Fox, Bruce Deck, Dr Suchana (Apple) Chavanich, Chad Scott and all at New Heaven Dive School, All my MAS classmates and staff.