Dataset: Dissolved nutrient data from samples collected from 1998 through 2025 from the surface waters of Tatoosh Island, Makah Reservation, Washington, USA

All censusing was done from shore or with small boats. Sampling stations were established on the shoreline of Tatoosh Island and adjacent offshore sites to the east of the island beginning in August 1998. Seven of the sites (Finger, Glacier Control, Hedophyllum Cove, Ladd's Finger, Simon's Landing, Strawberry Point, Toad Point) corresponded to wave-exposed shores on the periphery of the island used in a prior nutrient manipulation experiment (Wootton et al. 1996), and an eighth (Northwest Point) was another wave-exposed shore selected to expand peripheral coverage of the island. Two additional relatively wave-sheltered sites were also added to the sampling, both located inside extensive kelp beds: Main Beach and Strawberry Draw. Water at all these sites was collected 2-3 hours after a strong spring tide on a day when the investigators were departing from the island to facilitate rapid sample freezing. Water samples were collected in 2001, 2015, 2016, and 2022 at an additional shoreline site (Hydrolab Pool) subject to intensive sampling of ocean conditions. In 2000, four offshore sampling sites were established at varying distances from the island (Inside Kelp, ~30 meters (m); Outside Kelp, ~100 m; Gut, ~0.6 kilometers (km); Mushroom Rocks, ~ 1.8 km from island, ~0.5 km from the mainland). Water at the offshore sites was collected by boat approximately 4-5 hours after the tide. Water was extracted from the ocean at the surface using 50-milliliter (mL) syringes, which were first rinsed in the water at the site before a sample was drawn. The sample was then pushed through a syringe filter loaded with a disk of Whatman GF/F glass fiber filter paper, and collected in 40 mL acid-washed Nalgene bottles, leaving enough headspace to allow water expansion when frozen.

Water samples were kept in coolers following collection and immediately frozen at -20 degrees Celsius upon return to the mainland field base. Frozen water samples were express shipped to the University of Washington Ocean Chemistry Laboratory, where they were analyzed following the methods outlined in (UNESCO 1994) using a Seal Analytical AA3 nutrient autoanalyzer. In 2025, the analysis of samples from all dates/sites was carried out by the Moss Landing Marine Laboratory Environmental Analytical Laboratory, by colorimetric analysis using a Lachat QuickChem 8000 Flow Injection Analyzer and 50 mL Falcon Tubes as sample containers. We sent duplicate samples from 29 May 2025 and 23 August 2025 to the University of Washington Marine Chemistry Laboratory for comparative purposes.