Dataset: Results of a study examining how foundation species loss alters multiple ecosystem functions based on community surveys and biogeochemical sampling of tidepools in the Otter Rock Marine Reserve (ORMR) and Marine Garden, Oregon USA from June to August 2019

We selected 32 tide pools within the Oregon Department of Fish and Wildlife’s (ODFW) Otter Rock Marine Reserve (ORMR) and Marine Garden, Oregon USA (44°45'09.1"N 124°03'58.0"W) during the months of June to August 2019. One mussel species (M. califorinanus) and two surfgrass species (P. scouleri and P. torreyi) were present at the site. Of the 32 tide pools, 16 were dominated by the California mussel (M. califorinanus) and 16 were dominated by surfgrass (Phyllospadix spp.; 10 pools contained Phyllospadix scouleri and six pools had both Phyllospadix scouleri and Phyllospadix torreyi). Percent cover of foundation species ranged from 45.3 to 98.9% in the mussel pools and 49.5 to 100% in the surfgrass tide pools. Some tide pools had both mussels and surfgrasses; however, the presence of a second foundation species did not exceed 7.2% of the remaining tide pool cover. Tide pools were located in the mid to low intertidal zone ranging from 0.71 meters to 1.77 meters above mean lower-low water. At this tide height, tide pools were isolated for 4-6 hours during summer low tides.

A Before-After-Control-Impact (BACI) design was used to account for changes in ocean chemistry, timing of low tide, and variability within tide pools throughout the experimental period. The BACI design consisted of two 29-day time periods (before and after removal) with control and removal tide pools (Stewart-Oaten et al. 1986) where foundation species were removed from removal tide pools (n = 8 per foundation species) between the before and after removal periods. The before-removal period occurred June to mid-July 2019 and the after-removal period occurred mid-July to August 2019. Tide pools were selected for control or removal groups using a random number generator while accounting for the surface area to percent cover of foundation species. Removal of all foundation species from removal tide pools occurred in mid-July 2019 and tide pools had a one-week recovery period before any surveys or samples were taken. Rhizomes were also removed in surfgrass removal pools due to their ability to alter nutrient cycling (Terrados & Williams 1997). During each time period, we characterized physical parameters (pool volume and tide height), community composition, light, temperature, and biogeochemical fluxes (e.g., dissolved oxygen, pH, nutrients) in each tide pool.

Tide pool physical parameters included tidal height (location within intertidal) and tide pool volume (size of pool). Tide heights for each pool were surveyed with a laser level and stadia rod (DeWalt, Towson, MD, USA). Tide pool volume (V) was determined using a dye method (Pfister 1995) and measured with a SmartSpec3000 spectrophotometer (Bio-Rad Lab, Hercules, CA, USA). Water volume in the tide pools changed by 0 - 20% between the two timepoints due to removal of foundation species. To account for the slight effect of changing biomass on volume, volume was re-measured post-removal and the average between both time periods were used in statistical analyses.  

We conducted two rounds of community composition surveys for sessile percent cover and mobile organism counts: three-weeks before removal and one-month post-removal. We temporarily removed seawater from the tide pool and placed a flexible mesh quadrat (Nielsen 2001) with demarcations in 10 x 10 cm squares over the bottom of each pool to survey the entire community (pools ranged from 58 to 754 squares). We measured percent cover for sessile organisms and counted mobile organisms, identifying down to the lowest possible taxonomic unit in the field (usually genus level). We normalized the sum of non-foundation species sessile cover to 100% cover per tide pool, including the opposite foundation species if present (e.g., mussel cover within surfgrass pools was also normalized to 100% cover). Each foundation species cover in their respective tide pools remained the raw percent cover and did not exceed 100%. Both sessile and mobile organisms were grouped into larger functional groups based on their ecological role for data visualizations. All tide pool characterization and community composition methods were completed at least 24 hours before any water sampling event to allow the pools to be flushed at least twice by the ocean before measurements.

Temperature (°C) and light intensity (lumens m-2) were recorded continuously every 15 minutes for 29 days during each time period using HOBO® Pendant loggers bolted facing up on the flattest part of the tide pool on a level platform in the interstitial spaces of the foundation species (Onset® HOBO® Pendent Light Intensity Data Logger MX2202, Bourne, MA, USA). Light intensity (lumens m-2) was converted to photon flux density (PFD; µmol photons m-2 s-1) following Long et al. (2012) field experiment values. We measured the change in maximum temperature between the before and after removal period (i.e., the average of the daily hottest temperature from 7/18/2019 – 8/16/2019 minus the average daily hottest temperature from 6/16/2019 – 7/15/2019) and the percent change in maximum light (i.e., the average maximum light from 7/18/2019 – 8/16/2019 minus the average maximum light from 6/16/2019 – 7/15/2019 divided by the average maximum light from 6/16/2019 – 7/15/2019 multiplied by 100). For the causal model, maximum temperature was extracted from the logger data for the specific dates and times of water collection for comparison with biogeochemistry and ecosystem metabolism measurements. Hourly ocean temperatures over the experimental period were extracted from a nearshore ODFW Marine Reserve mooring sensor at 1 meter depth within ORMR to descriptively compare tide pool temperatures to the local ocean.

To determine biogeochemistry fluxes and ecosystem metabolism (NEC and NEP) before and after removal of foundation species, we collected daytime and nighttime water samples during low tide. We used a block design for water sampling with two daytime and two nighttime sampling events due to the timing of low tide and time restraints of sampling, where n = 16 pools (n=8 per foundation species) were measured on separate day and night sampling events. Each sampling event had an equal number of pools per foundation species and treatment group (removal or control: n = 8 pools). In situ temperature, DO, salinity, pH, and discrete samples for dissolved inorganic nutrients (NH4+, NO2− + NO3−, PO43 −) were collected hourly over a four-hour period in each pool and the adjacent ocean following methods by Silbiger & Sorte (2018). Temperature, DO, and salinity were measured with a calibrated multi-parameter pro meter directly in each pool (YSI Pro 2030, Lot #18B100763, Yellowsprings, OH, USA). For pH, nutrients, and TA, we collected 400 mL discrete water samples into a sealed Erlenmeyer flask using a vacuum hand pump (Mityvac, St. Louis, MO, USA) from the deepest part of the pool. Discrete samples (~250mL) for total alkalinity (TA) were taken four times over the low tide period. To compare tide pool conditions to the open ocean, ocean measurements were taken from the surface adjacent to the site.

pHT was measured within one hour of water collection from the sealed Erlenmeyer flask using an Orion Star Multiparameter Meter with a ROSS Ultra glass electrode (Thermo Scientific, USA, accuracy = ±0.2 mV, resolution = ±0.1, drift < 0.005 pH units per day) and a traceable digital thermometer (5-077-8, accuracy = 0.05 °C, resolution = 0.001 °C; Control Company, Friendswood, TX, USA) following Dickson SOP 6 (Dickson et al. 2007). The glass electrode measured millivolts (mV) and was calibrated within 48 h of each sampling event using a multipoint calibration to a tris standard solution from the Dickson Lab at Scripps Institution of Oceanography following Dickson SOP 6a (Dickson et al. 2007). TA seawater samples were placed in 250 mL Nalgene bottles with 100 µl of 50% saturated HgCl2 to preserve the water within five hours of collection. Seawater samples for nutrient analysis were filtered through GF/F filters (0.7µm) with a syringe into designated 50 ml centrifuge tubes and frozen within five hours of collection. All sampling and storage containers were soaked in 10% HCl for 24 hours, rinsed with MilliQ water, and rinsed three times with sample water before sampling events.

In situ pH was calculated using the seacarb package in R (Gattuso et al. 2018) by correcting for the in situ temperature in each tide pool from a multi-parameter pro meter. Total alkalinity seawater samples were processed using open-cell potentiometric titrations on a Mettler-Toledo T5 auto-titrator (Columbus, OH, USA) following Dickson SOP 3b (Dickson et al. 2007). A certified reference material (CRM) from the Dickson Lab at the Scripps Institution of Oceanography was used at the beginning of each sample group run. The accuracy of the CRM never exceeded ± 0.79% (precision = 5 µmol kg-1) from the standard value and TA samples were corrected for deviations from the standard value. Dissolved inorganic nutrients (NH4+, NO2− + NO3−, PO43 −) were analyzed at Moss Landing Marine Laboratory using a Lachat Quickchem 8000 Flow Injection Analyzer (± 1.21% NH4+, ± 0.26% NO2− + NO3−, ± 3.57% PO43 − instrument precision; Hach, Loveland, CO, USA). After processing nutrients, one mussel control pool (Pool ID 30) was removed from all resource flux analyses due to abnormally high values of ammonium compared to other pools (4290 ± 188.1 µmol g-1), likely due to contamination, on one sampling day leaving n = 7 control pools.

We used the total alkalinity anomaly technique (Chisholm & Gattuso 1991) to calculate net ecosystem calcification ( mmol CaCO3 m−2 hr−1) where total alkalinity values were divided by 1000 to convert from μmol kg−1 to mmol kg−1. ΔTA/2 is the salinity-normalized and nutrient-corrected TA (mmol kg−1) between each time point (n = 2−3 values per pool) divided by 2, where one mole of CaCO3 is formed per 2 moles of TA times ρ is the density of seawater (1023 kg m−3) times V is the volume of water in the pool at each time point (m3) divided by SA is the bottom surface area of the tide pool (m2) and t is the time between sampling points (h). and net ecosystem production rates (mmol C m-2 hr-1) were calculated from differences in dissolved inorganic carbon (DIC), calculated from pHT and TA (error propagation = 7.09 ± 0.06 mmol kg -1) with the seacarb package in R (Gattuso et al. 2018), using Gattuso et al. 1999’s equation: NEP = ΔDIC is the difference in salinity-normalized DIC (mmol kg−1) between each time point (around n = 3 values per pool) times density of seawater times the volume of the tide pool divided by the surface area of the tide pool and the time between sampling. NEC is subtracted to account for changes in DIC by the precipitation or dissolution of CaCO3, and FCO2 (mmol m−2 h−1) is the air−sea flux of CO2, which was subtracted to account for the flux in CO2 from the air−sea exchange. FCO2 is k is the gas transfer velocity (m h−1) calculated from wind speed (Ho et al. 2006) using the closest weather station, around 10 miles south of Otter Rock (NOAA Station NWPO3: 44° 36’ 46.8” N, 124° 04’ 01.2” W) multiplied by s (the solubility of CO2 in seawater calculated from in situ temperature and salinity (Weiss 1974) times CO2 (μatm) in water is calculated from pHT and TA values minus CO2 in the air was 410 μatm based on concurrent measurements at the Mauna Loa Observatory (Tans & Keeling 2019).

Known Issues:

• Tide pool 30 was removed because of high nutrient values in the after period. All time period 5 data were removed from analysis due to issues with sampling with the incoming tide.
• Missing 8 samples from nutrient analysis out of 612 samples: N_17_5_BC was lost before taking nutrients, D_24_1_BC, N_24_1_BC, N2_Ocean_5_BC, D_1_2_AI, D_16_2_AI, D_4_3_AI, N_20_3_AI were missing from lab analyses. We removed time points that nutrients were missing. N_26_1_BC has very low phosphate that yields it to be an outlier in later analysis could be a typo or the tide pool was just exposed from the ocean during sampling so time point was removed.
• Tide pool 18 was removed from mobile community composition analysis.