Generally unseen and infrequently measured, submarine groundwater discharge (SGD) can transport potentially large loads of nutrients and other land-based contaminants to coastal ecosystems. To examine this linkage we employed algal bioassays, benthic community analysis, and geochemical methods to examine water quality and community parameters of nearshore reefs adjacent to a variety of potential, land-based nutrient sources on Maui. Three common reef algae, Acanthophora spicifera, Hypnea musciformis, and Ulva spp. were collected and/or deployed at six locations with SGD. Algal tissue nitrogen (N) parameters (δ¹⁵N, N %, and C:N) were compared with nutrient and δ¹⁵N-nitrate values of coastal groundwater and nearshore surface water at all locations. Benthic community composition was estimated for ten 10-m transects per location. Reefs adjacent to sugarcane farms had the greatest abundance of macroalgae, low species diversity, and the highest concentrations of N in algal tissues, coastal groundwater, and marine surface waters compared to locations with low anthropogenic impact. Based on δ¹⁵N values of algal tissues, we estimate ca. 0.31 km² of Kahului Bay is impacted by effluent injected underground at the Kahului Wastewater Reclamation Facility (WRF); this region is barren of corals and almost entirely dominated by colonial zoanthids. Significant correlations among parameters of algal tissue N with adjacent surface and coastal groundwater N indicate that these bioassays provided a useful measure of nutrient source and loading. A conceptual model that uses Ulva spp. tissue δ¹⁵N and N % to identify potential N source(s) and relative N loading is proposed for Hawaiʻi. These results indicate that SGD can be a significant transport pathway for land-based nutrients with important biogeochemical and ecological implications in tropical, oceanic islands.

Study regionMaui, Hawaii, United States.Study focusWe investigated connections between land uses and submarine groundwater discharge (SGD) nutrient fluxes to coastal waters of Maui, Hawai'i. Nutrient contributions from agricultural lands, wastewater injection, and septic-cesspool systems were examined by combining a numerical groundwater model with δOH2O18, δNNO3−15, and δONO3−18 modeling to identify groundwater pathways, recharge elevations, and nitrate sources. Fresh and total SGD rates and nutrient fluxes were quantified using 222Rn mass balance modeling.New hydrological insights for the regionLow nitrate+nitrite (N+N) SGD fluxes (24mols/d) were measured where groundwater flowed beneath primarily undeveloped land on transit to the coast. By contrast, of all land use types, sugarcane and pineapple fields contributed the largest amount of N to coastal waters via SGD (3800mols/d). Despite their much smaller freshwater flux, these SGD sources provide substantially larger N fluxes than the State's largest rivers (avg. 700mols/d). Septic systems, cesspools, and near coast wastewater injection wells also contribute N+N to groundwater and coastal waters, although in much smaller quantities. This study demonstrates that numerical groundwater modeling combined with geochemical modeling can be used to determine sources and flux of nutrients in SGD and provides a unique, original, and practical framework for studying the effect of land use and its impact on nutrient delivery to coastal waters