Title of dataset Water quality data, Wailupe, HIAbstract Coastal groundwater dependent ecosystems take advantage of low salinity, nutrient rich submarine groundwater discharge (SGD). Across the Pacific islands marine macroalgae have been challenged by and adapted to the stress of lowered salinity with a trade-off of nutrient subsidies delivered by SGD. Human alterations of groundwater resources and climate change-driven shifts brought modifications to the magnitude and composition of SGD. This paper discusses how native macroalgae have adapted to SGD nutrient and salinity gradients, but that invasive algae are outcompeting the native ones near SGD with nutrient pollution, due to their higher salinity tolerance. It is important to re-evaluate land and water use practices by modifying groundwater sustainable yields and improving wastewater infrastructure to keep SGD reductions minimal and nitrogen inputs in optimal ranges. This task may be particularly challenging amidst global sea level rise and reductions in groundwater recharge, which threaten coastal groundwater systems and ecosystems dependent on them.Keywords submarine groundwater discharge, salinity water level, nitrate, ammoniumDataset lead author Henrietta DulaiPosition of data author Professor, principal investigatorAddress of data author 1680 East-West Rd POST 707 Honolulu, HI 96822Email address of data author hdulaiov@hawaii.eduPrimary contact person for dataset Henrietta DulaiPosition of primary contact person Professor, principal investigatorAddress of primary contact person 1680 East-West Rd POST 707 Honolulu, HI 96822Email address of primary contact person hdulaiov@hawaii.eduOrganization associated with the data University of Hawaiʻi at MānoaUsage Rights publicly available and free to useGeographic region Wailupe, O’ahu, Hawai’I, USAGeographic coverage 21.2759N, 21.2751N, 157.7624W, 157.7606WTemporal coverage - Begin date Sep 10, 2015Temporal coverage - End date Oct 7, 2015General study design A known coastal spring area at Wailupe, HI was monitored for 28 days.Methods description water salinity measurements were collected at 1-hour intervals with YSI multiparameter sonde (6920 V2-2) deployed 0.3 m below the surface and a 5 m lateral distance from a major spring. The instrument was attached to a float. Water depth measurements at 1-hour intervals were performed using a CTD Diver (Schlumberger Inc. CTD Diver) fixed at the ocean bottom.Discrete sampling was done at low, mid, and high tide for dissolved nutrients. Samples were filtered onsite through a 0.45 μM filter and kept on ice until returning to the lab.Laboratory, field, or other analytical methods Dissolved nutrients (total dissolved nitrogen, total dissolved phosphorus, nirate + nitrite, ammonium, and phosphate) were analyzed with a SEAL AutoAnalyzer 3 HR in the S-Lab at the University of Hawaiʻi at Mānoa. Quality control YSI and CTD salinity were calibrated before each field excursion against a known standard in the lab.For nutrient samples, all bottles were pre-cleaned to appropriate standards. Over 10% of nutrient samples were analyzed in duplicate to assess laboratory analysis accuracy.Additional information