Full methods are found in La Valle et al. 2020.

Site descriptions and SGD characterizations: The study sites are located along the southern shoreline of O'ahu, Hawai'i, in Maunalua Bay (21.2743°N, 157.7492°W). The two study sites, Black Point (21.2586°N, 157.7899°W) and Wailupe (21.2756°N, 157.7624°W), are on the western side of the bay and are macroalgae-dominated, with 50% and 15% fleshy macroalgal benthic cover at Wailupe and Black Point, respectively, from the shoreline to the reef crest. There is a close to conservative relationship between SGD-derived inorganic nutrients (nitrate, phosphate, and silicate) and salinity at Black Point and Wailupe (linear regressions statistics described in Table 1 of La Valle et al. 2019). SGD fluxes and nutrient delivery were described in detail in Holleman (2011), Nelson et al. (2015), Richardson et al. (2017a-b), Lubarsky et al. (2018) and summarized in La Valle et al. (2019).

Salinity time series (see instruments section) were used for EOF and community composition data analyses were related to the EOF loading scores.

Sampling and analytical procedures:
Benthic algal surveys were done at the grid intersection points (n = 97 for Wailupe and n = 115 for Black Point). The survey consisted of 25 cm by 25 cm quadrats, where species-specific percent algal cover, species-specific invertebrate percent cover and counts, and substrate type were measured. All taxa were identified to lowest taxonomic level.

Empirical orthogonal functions (EOFs) were applied to the spatially indexed salinity time series at both sites. The continuous time series were inputted in the EOFs. EOFs were used to reduce dimensionality of the spatially explicit time series and to reveal the spatial structure of the time series data. EOFs are the spatiotemporal manifestation of principal components analysis (PCA) (Wikle et al. 2018). The output for EOFs includes a spatial map of loadings and an associated normalized principal-component time series for the salinity dataset obtained using a singular value decomposition of a space-wide matrix. The singular value decomposition was done using the function svd in the base library in RStudio (version 1.0.44, R Core Team 2016).

Instruments:
Salinity time series were not included in this dataset because they are redundant for analyses (as EOF1 and EOF2 are included in the datasets) but are available from Florybeth La Valle (flavalle@ucsd.edu).

Twenty-three autonomous salinity sensors (Odyssey Temperature and Conductivity loggers, 3 to 60 mS cm-1) were deployed in a sparse grid across each site. The sensors were attached a few centimeters off of the benthos at Black Point for 30 days (29 May - 29 June 2015) and for 27 days at Wailupe (4 April - 21 May 2015) and took readings every 10 minutes. Two water samples were taken every week, synchronously with salinity sensor measurements, for the duration of the deployments at both sites and were analyzed using a Portasal Salinometer 8410A (accuracy 0.001) for quality control purposes and to check for sensor measurement drift.