Dataset: Sitka Sound Kelp Wet Weights

Experiment

To test whether and at what distance the presence of Pycnopodia can reduce prey densities or suppress grazing on kelp, we performed an underwater caging experiment at three urchin barren sites approximately six km east of Sitka in February 2023: Ellsworth Cut (57.036, -135.280), Harris Island (57.033, -135.277), and Whale Park (57.033, -135.255). Each experimental array consisted of a central cage with four 4 m long radial transect lines (Figure 1). We constructed cages (30 x 30 x 15 cm; l x w x h) using a PVC frame covered in ~1cm Vexar mesh fastened with zip ties that could be opened underwater to add a Pycnopodia. We attached a 4m long lead line to each corner of the cage, forming a plus pattern (Figure 1). We simultaneously deployed four of these arrays at each site in two blocks. In each block, one cage served as an experimental treatment (with a Pycnopodia) and the second cage served as a control (an empty cage). We placed all arrays in areas with high sea urchin density, hard rocky substrate, and low rugosity, and the cages within a block were ~20-30 m apart.

The experiment began when we sealed a Pycnopodia (9.5-19.5 cm radius, lab acclimated for >2 weeks) into the experimental treatment cage and the control cage was sealed with a dive weight. To each lead line of all cages, we attached a yellow nylon “kelp line” with 1 m distances demarcated by tape (Figure 1). Kelp lines consisted of pre-weighed, individually labeled Macrocystis pyrifera blades at regular intervals (metre marks: 0, 0.25, 0.5, 0.75, 1, 1.5, 2, 2.75, 3.75) woven into the nylon line. M. pyrifera blades were harvested from surface canopies over 1 mile away from closest experimental urchin barren sites and held in recirculating seawater for no more than a week before deployment. After the 24 hours, we removed the arrays and kelp lines and re-weighed each individual kelp blade in the lab to calculate any change in wet mass. 

Analyses

We assessed the main and interactive effects of Pycnopodia treatment and continuous distance from the cage on the percent of kelp biomass lost over 24 hours using a mixed effects hierarchical linear model, fit using the lmer() function in the lme4 package in R. We included site, block (nested within site), and array (nested within site, and block, and treatment), and transect (nested within site and array) as random factors. 

We then calculated the net effect of the sea star at 24 hours on kelp abundance as the difference in the average percent of kelp remaining, respectively, in the sea star treatment minus its paired control treatment (i.e., the differences between treatments in each block). In other words, for each block and at a given distance from the cage at 24 hours we calculated: [avg. percent remaining in sea star treatments at each distance - avg. percent remaining in paired control treatments at each distance]. We tested the main and interactive effects of continuous distance from the cage and kelp on the net sea star effect using a linear model (lm()) in base R. We originally performed a model that included site, block and array as random factors matching the model construction as above, but were forced to simplify the model to avoid singularity errors. We then ran follow-up, individual linear fits (lm()) for each species separately to obtain the equation for each line (i.e., net effect = intercept + (slope * distance from cage)). Finally, we calculated the radius of the ‘halo of influence’ of the sea star by setting the net effect to zero and solving for the distance to the cage (i.e., the distance at which the sea star effect was no longer detectable). We calculated the area of influence using pie*(radius2), with the radius being defined as the distance from the cage statistic solved from the previous equation. This gives us an approximate measure of how far from an inactive sea star we can expect grazing to be suppressed for at least 24 hours, even when kelp is present in an urchin barren.