Ridge to Reef Management Implications for the Development of an Open-Source Dissolved Inorganic Nitrogen-Loading Model in American Samoa

Abstract

Excessive nutrient discharge to tropical island coastlines drives eutrophication and algal blooms with significant implications for reef ecosystem condition and provision of ecosystem services. Management actions to address nutrient pollution in coastal ecosystems include setting water-quality standards for surface waters discharging to the coast. However, these standards do not account for the effects of groundwater discharge, variability in flow, or dilution, all of which may influence the assessment of true nutrient impacts on nearshore reef habitats. We developed a method to estimate dissolved inorganic nitrogen (DIN) loads to coastal zones by integrating commonly available datasets within a geospatial modeling framework for Tutuila, American Samoa. The DIN-loading model integrated an open-source water budget model, water-sampling results, and publicly available streamflow data to predict watershed-scale DIN loading to the island’s entire coastline. Submarine groundwater discharge (SGD) was found to deliver more terrigenous DIN to the coastal zone than surface water pathways, supporting findings from other tropical islands. On-site wastewater disposal systems were also found to be the primary anthropogenic sources of DIN to coastal waters. Our island-wide DIN-loading model provides a simple and robust metric to define spatially explicit sources and delivery mechanisms of nutrient pollution to nearshore reef habitats. Understanding the sources and primary transport modes of inorganic nitrogen to nearshore reef ecosystems can help coastal resource managers target the most impactful human activities in the most vulnerable locations, thereby increasing the adaptive capacity of unique island ecosystems to environmental variation and disturbances.

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Data Availability

Recent advancements in cloud-computing technologies, particularly in open-source sharing of online projects, significantly increase methodological transparency and reproducibility of models. We provided dynamic open-source access to this project for managers, researchers, regulators, and others by developing the project with GitHub, which manages versioning and retains all necessary raw data files, model code, descriptive information, and output files in a public repository (Shuler and Comeros-Raynal 2019). We also archived the most recent version of the model for long-term storage in an open-access digital artifact repository (https://doi.org/10.5281/zenodo.3462869). Note that sensitive information or datasets not intended to be publicly available, are not posted in raw forms. The model code is licensed under the GNU General Public License v3.0, which is an open-access license designed to explicitly affirm any user’s unlimited permission to run, copy, and use the unmodified code from this repository. By designing the model as an interactive live-code document (Jupyter Notebook), it can be modified to potentially address new management questions that come up as we continue to work closely with stakeholders and managers toward the betterment of American Samoa’s terrestrial and marine environments. Most importantly, we hope that applying this open-source paradigm will provide a blueprint for others to improve upon and translate this method to other locations throughout Oceania.

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Acknowledgements

We would like to express our appreciation for the many individuals and agencies who made this work possible by generously providing their time, expertise, and assistance with data collection and field support. These people and agencies include The American Samoa Department of Marine and Wildlife Coral Reef Advisory Group Staff, specifically Alice Lawrence, Motusaga Vaeoso, and Kim McGuire. The National Marine Sanctuary of American Samoa, especially Dr. Mareike Sudek. The American Samoa Environmental Protection Agency, specifically, Fa’amao Asalele, Jr, Christianera Tuitele, Jewel Tuiasosopo and Josephine Regis, and University of Guam—Dr. Peter Houk. This is contributed paper WRRC-CP- of the Water Resources Research Center, University of Hawai‘i at Manoa.

Funding

Funding for this work was provided by the United States Environmental Protection Agency Region IX Wetland Program Development Grant, through the American Samoa Environmental Protection Agency and NOAA’s Pacific Regional Integrated Sciences and Assessments Program (Pacific RISA) [grant number NA15OAR4310146].

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All authors contributed to the study conception and design. Model development was performed by CS. The first draft of the paper was written by CS and MC-R contributed considerably to the betterment of subsequent drafts, with significant additions to the text. All authors read and approved the final paper.

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Correspondence to Christopher K. Shuler.

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Shuler, C.K., Comeros-Raynal, M. Ridge to Reef Management Implications for the Development of an Open-Source Dissolved Inorganic Nitrogen-Loading Model in American Samoa. Environmental Management (2020). https://doi.org/10.1007/s00267-020-01314-4

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Keywords

  • Nutrient loading
  • Geospatial model
  • Dissolved inorganic nitrogen
  • Water quality
  • Island management