Dataset: Species cover in 15 permanent survey plots in the lower intertidal zone of Tatoosh Island, Makah Reservation, Washington, USA 2012-2022

Permanent quadrats (50 x 50 cm) were established in the lower intertidal zone (below the lower limit of beds of the mussel Mytilus californianus) by embedding two stainless steel screws of two types (1 hex-head, 1 Phillips-head) into Rawl plug wall anchors located at diagonal corners of each plot. The two types of screws allowed the quadrat to be positioned and oriented in the same location in subsequent sampling years to document dynamics without confounding spatial variation. Quadrats were constructed out of 1/4" schedule 80 PVC pipe connected with 90 degree joints at the corners. Each pipe had a series of 10 equidistant holes drilled through them, which were strung with heavy duty 40 lb test monofilament line, creating a grid of sampling squares., and each line was designated with either a letter (top to bottom) or number (left to right) to allow spatial information to be collected. Plots were established in 2012 and subsequently surveyed from 2014-2026 (except in 2020), with surveys still ongoing. In 2012, plots were visually surveyed by identifying the taxon of individual first encountered below each point where monofilament lines crossed, and then by identifying the taxon of individual adhering to the rock located under each sample point, with the aid of a pen/pencil to carefully move the overstory away. Beginning in 2014, plots were digitally photographed annually for later analysis on computers in the lab. Two photographs, with the quadrat filling the frame as much as possible, were taken: 1) the plot without any disturbance and 2) the plot with all overstory kelp blades moved to the side as much as possible without destructive manipulation. At present, photographs from 2014 through 2022 have been analyzed for the percentage of area covered by mussels, aided by the 121 squares defined by the 10 monofilament lines plus the edges of the quadrat. Additionally, photographs from 2014 and 2019 have been analyzed for percent cover of all identifiable species to the lowest taxonomic level possible from non-destructive photographs. Data are likely to be updated as subsequent analysis of the photographs from other years is carried out. Tide height of plots was determined using a rotary laser level, referenced to permanent markers of known tide height. Latitude and longitude were determined by GPS.