Abstract
Dissolved silicate (DSi) is one of the essential elements of biogeochemical cycles in the coastal zones. It plays a vital role in the preservation of endemic diverse organism's structure like plankton groups. DSi estimation and distribution within coastal aquifer and its fluxes into the ocean has been getting great attention among researchers, managers, and policymakers especially who focus on the coastal environmental health. However, estimating DSi is very challenging due to the fact that the nutrient fluxes magnitude can vary spatially and temporally. In the Hawaiian coastlines, the main sources of DSi are the weathering products of basaltic rock and volcanic ashes, which mainly gets transported to the coast by groundwater flow. In this study, an integrated hydrological modeling approach is considered as a robust way to estimate DSi fluxes both at temporal and spatial scales. The integrated model consists of the SWAT, MODFLOW, and SEAWAT models. Hereto, we also estimated DSi fluxes under different scenarios, such as wetland restoration (LU), climate change (CL), and sea level rise (SLR). While the CL scenarios were run using the Representative Concentration Pathways (RCP) 4.5 and 8.5 scenarios for mid and late 21st century, the SLR of 0.4 and 1.1 meter was assumed. The findings indicated that the average DSi flux under California grassland cover was about 48 moles per day that increased by 15% during the wet season and decreased by 16% during the dry season. The DSi fluxes were highly dependent on fresh submarine groundwater discharge (FSGD). The climate change had a more negative impact on DSi fluxes by 5% when compared to the 1.1 meter of SLR scenario. On the other hand, wetland restoration did not have a significant effect on DSi fluxes. The decrease in DSi fluxes under SLR and climate change had a positive effect on the accumulative storing of DSi within coastal wetland. Overall, the integrated hydrological modeling approach has drawn a comprehensive picture of DSi fluxes and silicate behavior under various conditions within the Heeia coastal zone.
Highlights
The Heeia coastal zone in Hawaii is a typical example of groundwater dependent ecosystems due to the presence of boundary interface among the Island of Oahu’s largest fishpond, the largest federally designated Wetland, and the Kaneohe Bay’s greatest sheltered water body of coral reefs [1, 2]
This study aims to assess the Dissolved silicate (DSi) fluxes as species transport across the Heeia coastal aquifer-ocean interface under different scenarios of dynamic variable land use and climate scenarios
The results illustrated that the average DSi flux was about 48 mole per day (Table 2) and increased by 15% during the wet season but decreased by 16% during the dry season due to the temporal variation of fresh submarine groundwater discharge (FSGD) (Figure 7)
Summary
The Heeia coastal zone in Hawaii is a typical example of groundwater dependent ecosystems due to the presence of boundary interface among the Island of Oahu’s largest fishpond, the largest federally designated Wetland, and the Kaneohe Bay’s greatest sheltered water body of coral reefs [1, 2]. The freshwater flows play vital roles in preserving the native adjacent coastal ecosystems as it provides essential nutrients like DSi to the terrestrial and marine organisms. The high concentration of DSi and low salinity provide a clear signal of freshwater discharge across the coastline, especially in the Hawaiian Islands, where groundwater is the main source of DSi as the result of weathering basaltic bedrocks and volcanic ash [4, 5]. The macronutrient DSi plays a vital role in sustaining coastal and oceanic ecosystems due to the marine organisms like phytoplankton’s dependence on the availability of DSi [6].
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