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2022 Feb 12;122:e8607. doi: 10.1002/ece3.8607.
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Low-pH seawater alters indirect interactions in rocky-shore tidepools.
Ocean acidification is expected to degrade marine ecosystems, yet most studies focus on organismal-level impacts rather than ecological perturbations. Field studies are especially sparse, particularly ones examining shifts in direct and indirect consumer interactions. Here we address such connections within tidepool communities of rocky shores, focusing on a three-level food web involving the keystone sea star predator, Pisaster ochraceus, a common herbivorous snail, Tegula funebralis, and a macroalgal basal resource, Macrocystis pyrifera. We demonstrate that during nighttime low tides, experimentally manipulated declines in seawater pH suppress the anti-predator behavior of snails, bolstering their grazing, and diminishing the top-down influence of predators on basal resources. This attenuation of top-down control is absent in pools maintained experimentally at higher pH. These findings suggest that as ocean acidification proceeds, shifts of behaviorally mediated links in food webs could change how cascading effects of predators manifest within marine communities.
FIGURE 1. Trajectories through time of pH (total scale) in replicate experimental tidepools for each of three trials in November and December 2017. Lines represent time series of pH for individual pools across the three pH treatments. Raised pH (blue), natural pH (black), and reduced pH (red). Points represent the average pH over the nighttime tidal period for each pool after chemical additions
FIGURE 2. Experimental reduction of tidepool pH impairs the refuge‐seeking behavior of black turban snails (T. funebralis), causing them to act as if predators are absent. This trend appears as a decline under reduced pH in the maximum proportional increase of snails out of water in the presence of the sea star Pisaster ochraceus (convergence of the with‐predator [blue] and without‐predator [gray] trend lines). Panels indicate data from three nighttime trials and pH is calculated as the nightly average pH for each pool. Lines are based on a linear mixed‐effects model of log‐transformed data and shading represents 95% confidence intervals. Note that a y‐axis value of 1 represents a doubling of snails using refuge
FIGURE 3. Fewer snails exited the water under low pH which led to increased capture rates by sea stars. Lines represent the number of snails caught by sea stars in pools with predators, based on a generalized linear mixed‐effects model with a log link function (Poisson distribution). Shading represents 95% confidence intervals
FIGURE 4. Cascading effects of predators on algal consumption are eliminated under low pH. This outcome is apparent from the convergence of the with‐predator (blue) and without‐predator (gray) trend lines at reduced average pool pH levels. Fitted lines are based on a linear mixed effects model of log‐transformed data, and shading represents 95% confidence intervals