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PLOS ONE
<br />Identifying wastewater management tradeoffs in Kona, Hawai'i
<br />potential management interventions and reduction of nutrient levels in Kihei, annual benefits
<br />will likely decline for at least a decade while the reef takes time to recover [62]. Long-term
<br />eutrophication of coasts that may result from the selection of cheaper but less effective waste-
<br />water management strategies sets up situations for likely invasive macroalgal and/or turf algal
<br />blooms [63, 54], loss of coral health [65], and increases in pathogenic microbial communities
<br />[66], changing profoundly the reef communities that draw people to live in coastal regions.
<br />Although we did not explicitly model the potential impact of change in nutrient exposure
<br />on the nearshore habitat, wastewater impact from cesspools on coral reefs has been recognized
<br />as a major driver of groundwater and nearshore water quality degradation [5, 67, 68]. An
<br />increase in land -based nutrient exposure can promote benthic algae growth and hinder corals
<br />[48, 52, 53]. For example, a study in the Pacific region has shown that turf algae have an advan-
<br />tage over corals by becoming a fast colonizer in nutrient enriched conditions [55]. Adverse
<br />wastewater effect on coral reefs have also been identified around the tropics, including the
<br />Caribbean [69], the Red Sea [70], the Indian Ocean [71], the Florida Keys [72], and the Great
<br />Barrier Reef [73]. Therefore, wastewater management can reduce nutrient exposure of coral
<br />reefs and thereby minimize turf and macroalgae growth, providing space for coral recruits.
<br />This can also promote coral recovery post -bleaching events, especially in dry regions or shal-
<br />low back -reef areas with limited water circulation [74], like the Kona coast. Additionally, it is
<br />important to note that whereas this study used mean annual values of nitrogen inputs to the
<br />coastline, future research could usefully explore temporal tidal variation, which is likely to
<br />have an important influence on algal growth rates.
<br />While this work does not explicitly predict macroalgal blooms from elevated nutrients in
<br />SGD alone, it is important to consider how the combined impacts of nutrients and tempera-
<br />ture increases due to wastewater management, climate change (e.g., increased sea surface tem-
<br />peratures), and fishing pressure may lead to decreases in herbivore abundance and accelerated
<br />growth of invasive macroalgae. A potential shift towards turf -algal dominated systems is likely
<br />in the scenarios where nutrient inputs are increased. The combination of decreased herbivore
<br />pressure and increased nutrients could lead to widespread, and ecologically disruptive macro -
<br />algal blooms, as seen on nearby Maui and Oahu, with severe negative impacts to charismatic -
<br />herbivores such as the green sea turtle [5, 52-54, 63, 65, 75]. This point is of special concern in
<br />areas with frequent inter -island and international boat traffic, such as that seen near the
<br />WWTP at Honokohau Harbor, as boat traffic provides likely vectors for invasive algal intro-
<br />duction. Cascading effects of declining nearshore water quality may also include decreased
<br />water clarity from increased phytoplankton abundance, decreases in coral resilience to bleach-
<br />ing, as well as decreased fish and coral quantity and diversity [4, 19]. These ecological changes
<br />are linked to reduced human enjoyment of some of Kona's most prominent activities for resi-
<br />dents and tourism industries, including nearshore snorkeling, scuba diving, shore fishing and
<br />spearfishing. Because the long-term biological consequences of excessive nutrient levels are
<br />generally not immediately reversible, careful consideration of such impacts is critical for
<br />coastal planning and management.
<br />Supporting information
<br />S1 Fig. Histogram of OSDS risk scores across the Keauhou basal aquifer. Risk scores were
<br />calculated by [25].
<br />(DOCX)
<br />S2 Fig. Numerical model grid setup for the Keauhou basal aquifer. The vertical cross section
<br />A -A' consists of 21 layers. The top layer follows the local topography and bathymetry and the
<br />bottom layer follows a flat elevation of 550 m below mean sea level. The bottom elevation of
<br />PLOSONE IIhu�tlps://doaa.oirg/n0.t:171/�OLuirirus.Il,lpoine,0257125 September8,2021 21/26
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