Assessment of Terrigenous Nutrient Loading to Coastal Ecosystems along a Human Land-Use Gradient, Tutuila, American Samoa

Abstract

Anthropogenic nutrient loading is well recognized as a stressor to coastal ecosystem health. However, resource managers are often focused on addressing point source or surface water discharge, whereas the impact of submarine groundwater discharge (SGD) as a nutrient vector is often unappreciated. This study examines connections between land use and nutrient loading through comparison of four watersheds and embayments spanning a gradient of human use impact on Tutuila, a high tropical oceanic island in American Samoa. In each study location, coastal radon-222 measurements, dissolved nutrient concentrations, and nitrogen isotope values (δ15N) in water and in situ macroalgal tissue were used to explore SGD and baseflow derived nutrient impacts, and to determine probable nutrient sources. In addition to sampling in situ macroalgae, pre-treated macroalgal specimens were deployed throughout each embayment to uptake ambient nutrients and provide a standardized assessment of differences between locations. Results show SGD-derived nutrient flux was more significant than baseflow nutrient flux in all watersheds, and δ15N values in water and algae suggested wastewater or manure are likely sources of elevated nutrient levels. While nutrient loading correlated well with expected anthropogenic impact, other factors such as differences in hydrogeology, distribution of development, and wastewater infrastructure also likely play a role in the visibility of impacts in each watershed.

Submarine Groundwater Discharge and Stream Baseflow Sustain Pesticide and Nutrient Fluxes in Faga'alu Bay, American Samoa

“…The MODFLOW-derived groundwater fluxes were compared to the measured values collected during a previous study in 2014-2015 (Shuler et al, 2019), which consisted of baseflow separation and indirect measurement of SGD entering the bay calculated by a 222 Rn mass balance (Shuler et al, 2019). Groundwater flow as a sum of SGD and stream baseflow was 5,500-14,400 m 3 /d (Shuler et al, 2019) by comparison to 6,550 ± 980 m 3 /d in this 2016 study. Although both measurements occurred during American Samoa's dry season under baseflow conditions, some variability in the elevation of the water table due to seasonal conditions likely exists.…”

“…A fringing coral reef exists at the shoreline and extends 50-400 m offshore in Faga'alu Bay. Water quality of the bay is currently of major concern due to turbid, nutrient-enriched waters (Messina and Biggs, 2016;McCormick, 2017;Vargas-Angel and Schumacher, 2018;Shuler et al, 2019).…”

“…As 90% of the municipal water on the densely populated island is sourced from groundwater, the concern over polluted drinking water resources has become a serious issue (Shuler et al, 2017). Additional concern is that pollution from groundwater propagates to streams and coastline as well (Shuler et al, 2019). Groundwater discharge to streams and the coastal ocean is often hard to quantify and is the subject of this study, which focuses on the Faga'alu watershed on the island of Tutuila, American Samoa (Figure 1A).…”

“…Nutrient fluxes have been the focus of many SGD studies because they are widely known to leach into the water table and to be carried by groundwater flow to the coastline (Richardson et al, 2015;Amato et al, 2016;Shuler et al, 2019). Pesticides also commonly leach into and may be dispersed in aquifers (i.e., Schuette, 1998), although their mobility is species dependent and is usually lower than those of nutrients (Davidson, 1995).…”

“…The insecticide, however, was sprayed heavily across the island in the mid-twentieth century and is still persistent thanks to its long residence time in the environment (Travis et al, 1946;NPIC Oregon State University, 1999). Also quantified in this study were nutrient fluxes, in order to provide comparison to other island studies (e.g., Glenn et al, 2013;Shuler et al, 2019).…”