Biological processes play important roles in determining how global changes manifest at local scales. Primary producers can absorb increased CO₂ via daytime photosynthesis, modifying pH in aquatic ecosystems. Yet producers and consumers also increase CO₂ via respiration. It is unclear whether biological modification of pH differs across the year, and, if so, what biotic and abiotic drivers underlie temporal differences. We addressed these questions using intensive study of tide pool ecosystems in Alaska, USA, including quarterly surveys of 34 pools over one year and monthly surveys of 5 pools from spring to fall in a second year. Here, we report values for physical conditions and changes in pH and dissolved oxygen during day and night as part of our effort to link physical and biological processes - and particularly the importance of time of year in determining the relationship between community composition and pH conditions - in coastal ecosystems.

Our study was conducted in 34 tide pools on a rocky shore at John Brown’s Beach on Japonski Island near Sitka, Alaska, USA (57.06°N, 135.37°W). Substrate type (hard rock; Sitka graywacke) was consistent across pools. During Year 1 of our study, we sampled all 34 pools quarterly: summer (21 – 29 June 2018), fall (03 – 11 September 2018), winter (16 – 22 January 2019), and spring (24 March – 01 April 2019). In Year 2, sampling was conducted monthly in a subset of 5 pools, beginning with the spring (24 March – 01 April 2019) sampling followed by 28 April – 02 May, 11 – 14 June, 08 – 10 July, 06 – 07 August, and 19 – 21 September 2019.

Tide pool characteristics were quantified during our summer 2018 sampling. For each pool, we determined (mean ± 1 SE) shore height (2.50 ± 0.06 m above mean lower-low water), volume (11.1 ± 1.5 L), and bottom surface area (0.23 ± 0.02 m²). The subset of 5 pools studied in Year 2 was determined randomly and did not differ significantly from the full group of 34 pools in shore height (2.69 ± 0.12 m), volume (9.5 ± 4.7 L), or surface area (0.27 ± 0.01 m²).

During each quarter (Year 1) or month (Year 2), we conducted measurements over one daytime and one nighttime low tide event to characterize change in pH and dissolved oxygen (DO). To complete our sampling during the relatively short daytime (in winter) and nighttime (in summer), we typically sampled half of the pools on each of 2 nights and 2 days. Daytime measurements were taken when irradiance levels saturated photosynthetic rates of the most widespread seaweed species in the pools, Neorhodomela oregona (> 47 µmol photons m^-2 s^-1; M. Bracken, unpublished data). Nighttime measurements were collected when most pools were isolated by the receding tide after dark (0 µmol photons m^-2 s^-1). The order of daytime and nighttime sampling differed by month based on natural variation in tide timing.

Each sampling event took ~3.5 hr (2.4 – 5.3 hr) and started when pools were isolated by the receding tide. Pools not flushed by the previous high tide were flushed manually with buckets of seawater. We took initial readings of pH (HI9829 Meter with 7609829 glass pH electrode, Hanna Instruments, Woonsocket, RI, USA), as well as temperature, DO, and salinity (ProDSS optical DO meter, YSI, Inc., Yellow Springs, OH, USA), at a marked location in the pool center. After intervals of ~1.5 hr, subsequent measurements (for a total of 3 measurements) were taken.

 We calculated rate of change per hour for pH and DO between our first and last (third) measurements reducing the data to single values per pool. Measured pH data (mV) were corrected by pool temperatures and converted to total scale using calibrations with Tris standards (Dickson Marine Physical Lab, Scripps Institution of Oceanography, La Jolla, California, USA).

The data are presented in a comma-delimited .csv text file.