This dataset is part of a larger experiment to investigate how pH influences trophic links between intertidal sea stars (Leptasterias hexactis), snails (Tegula funebralis), and macroalgae (Mazzaella flaccida). These data were gathered during an experiment at the Bodega Marine Laboratory. Wet weight (hand blotted; grams) of algae was determined before and after the experiment using an analytical balance (Mettler AE 160).

Organisms were placed for 7 days in mesocosms containing seawater at either ambient (~7.9) or low pH (~7.0). The pH was modified using equimolar additions of sodium bicarbonate (NaHCO3) and hydrochloric acid (HCl). The water in each container was changed daily. The mesocosm array consisted of 40, 13-liter (L) circular plastic containers with a mesh barrier down the center to separate predator, prey, and/or basal resource but allowing for passage of waterborne cue. Mesocosms were filled halfway with seawater, allowing 10 centimeters (cm) of refuge space for snails above the waterline. Mesocosms were held within a seawater table under constant flow to maintain consistent temperatures.

Each mesocosm was assigned to one of four trophic treatments and one of two pH levels, resulting in five replicates per treatment and pH (4 trophic × 2 pH × 5 replicates = 40 mesocosms). The first trophic treatment was a “no-predator” configuration, composed of four snails and four 3-cm-diameter circular pieces of Mazzaella macroalgae cut out of blades (four pieces = 0.33 g ± 0.03 in total, with each piece standardized to have similar initial mass), both placed on one side of the central barrier of the mesocosm. The second trophic treatment was a “cue only” treatment in which one sea star was housed on one side of the barrier with four snails and macroalgae on the other side. The third was a “complete interaction” treatment in which one sea star, four snails, and macroalgae were all located on the same side of the barrier. The final trophic treatment was a “no prey/no grazing” configuration, for which one sea star was placed on one side of the barrier with the macroalgae on the other. The final trophic treatment was a “no prey/no grazing” configuration, for which one sea star was placed on one side of the barrier with the macroalgae on the other. It was used primarily as a control for predator behavior in the absence of prey, and also provided a control for any changes in macroalgal mass that might occur without grazing, due, for example, to growth or senescence.

Consequences of pH for the strength of net top-down indirect effects were estimated from relative reductions in mass of macroalgae in the no-predator treatment, the cue-only treatment, and the complete interaction treatment across pH levels. Wet weight (blotted dry) of disks excised from Mazzaella blades was measured before and after the 7 days to assess macroalgal mass loss. First-order estimates of macroalgal consumption were calculated as the difference in macroalgal mass loss between each container of a trophic treatment and the group mean from the no-grazing treatment for each pH level (algae eaten).

See Jellison, B.M. & Gaylord, B. Oecologia (2019).