Organisms inhabiting the intertidal zonehave been used to study natural ecophysiological responses and adaptations to thermal stress because these organisms are routinely exposed to high-temperature conditions for hours at a time. While intertidal organisms may be inherently better at withstanding temperature stress due to regular exposure and acclimation, they could be more vulnerable to temperature stress, already living near the edge of their thermal limits. Strong gradients in thermal stress across the intertidal zone present an opportunity to test whether thermal tolerance is a plastic or canalized trait in intertidal organisms.  Here, we studied the intertidal pool-dwelling calcified alga, Ellisolandia elongata, under near-future temperature regimes, and the dependence of its thermal acclimatization response on environmental history. Two timescales of environmental history were tested during this experiment. The intertidal pool of origin was representative of long-term environmental history over the alga’s life (including settlement and development), while the pool it was transplanted into accounted for recent environmental history (acclimation over many months). Unexpectedly, neither long-term nor short-term environmental history, nor ambient conditions, affected photosynthetic rates in E. elongata. Individuals were plastic in their photosynthetic response to laboratory temperature treatments (mean 13.2°C, 15.7°C, and 17.7°C). Further, replicate ramets from the same individual were not always consistent in their photosynthetic performance from one experimental time point to another or between treatments, and exhibited no clear trend in variability over experimental time. High variability in climate change responses between individuals may indicate the potential for resilience to future conditions, and thus may play a compensatory role at the population or species level over time.

Twenty-seven turf samples of E. elongatawere collected from nine tidal pools at Cape Cornwall, Penzance,England (50°07’44.8” N, 5°42’16.4” W) using hammer and chisel on October 28, 2015. Collected specimens included the basal thallus still attached to rocky substrate, with healthy, mature fronds emerging from across the basal crust (approximately 4-cm x 4-cm basal crust area).  Thus, collected specimens were all estimated to be at least one year old.  Pools were chosen to be representative of a gradient in thermal stress, with smaller pools located high in the tidal range (upshore) representative of the highest thermal stress, and large, low-shore pools experiencing the lowest thermal stress. Three tidal pools were chosen within each category of low, medium, and high thermal stress, and three turf samples of E. elongatawere collected from each pool for transplantation.

After sample collection, tidal pools were partially drained to allow installation of HOBO temperature and irradiance loggers (Onset Corp.) below the water line of each pool at low tide. Collected samples were kept in outdoor buckets overnight and reciprocally transplanted using marine epoxy (Z-SPAR, A-788 Splash Zone) on October 29, 2015 after emptied pools had refilled naturally over the tidal cycle. Transplants from each pool were dispersed between low, medium, and high stress pools, including samples transplanted back into their original ‘home’ pool of collection (i.e., out of three samples collected from a low-stress pool, one was returned to its original pool, one was transplanted to a medium-stress pool, and one was placed in a high-stress pool).  

At the end of the field portion of the study (221 days), photosynthetic rates of E. elongatanative to each pool (not manipulated in experimental transplants) were measured in ambient summer sunlight unobstructed by clouds in the morning (08:00 GMT, mean irradiance 1,186 ± 781 Lux, mean pool temperature 12.8 ± 0.1°C) and in the afternoon (15:00 GMT, mean irradiance 103,402 ± 30,896 Lux, mean pool temperature 21.0 ± 0.5°C) on 6 June 2016. 

Evolution of O₂ gas in seawater was measured over 12 minutes using a four-channel FireStingO₂ oxygen meter fitted with air-tight 4-mL vials containing fibre-optic sensors (PyroScience). During each incubation, one vial was incubated with seawater from the tidal pool without an algal sample as a seawater blank. Each of the remaining three vials contained one frond that was plucked at its base from a non-transplanted individual within the tidal pool and was filled with ambient pool water.  Algal fronds used in each incubation were collected and air dried at the laboratory for one week prior to weighing, allowing O₂evolution to be normalized to dried sample mass.  

Lab -

Experimental tanks were held at control (mean 13.2°C), medium (mean 15.7°C) and high (mean 17.7°C) temperature treatments using electric heaters. Water temperature was measured twice daily and adjusted manually if temperature deviated by >0.2°C.  Approximately 10% of the water mass was exchanged each week with freshly collected seawater from the L4 Station of the Western Channel Observatory (50°15.0’N, 4°13.0’ W).

Once per week, evolution of O₂ gas in seawater was measured over 12 minutes using a four-channel FireStingO₂oxygen meter fitted with air-tight 4-mL vials containing fibre-optic sensors (PyroScience) for each sample in the laboratory at photosynthetically active radiation (PAR) averaging 27.2±0.5 photosynthetic photon flux density. For each tank, one seawater blank was measured using a vial incubated with seawater from the tank without an algal sample and used as a correction for all measurements from that tank.  To measure algal photosynthesis, one frond was plucked at its base from each transplant and placed in a vial filled with ambient treatment water.  Each incubated algal frond was rinsed with distilled water and air dried for one week prior to weighing and used to normalize O₂evolution to dried sample mass. 

Metadata is included in the last tab of each spreadsheet file.  Temperature files are zipped by field and lab, and labelled by either tidepool or experimental tank, as relevant.