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Dryad

Latitudinal investigation of Olympia oyster elevational distribution

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Jan 24, 2025 version files 52.28 MB

Abstract

The goal of our investigation was to characterize the upper elevational limit of Olympia oysters (Ostrea lurida) across the full latitudinal range of the species, and to determine how this limit varies as a function of air temperature. At each of 26 sites from British Columbia to Baja California, we located the highest live Olympia oysters to identify the upper distributional limit at that site. Where Pacific oysters (Magallana gigas) were present, we also identified their upper elevational limit. In addition, we identified the upper limit of acorn barnacles at the site as a biological indicator of the upper limit of sessile invertebrate communities in the intertidal zone, analogous to the treeline on mountains. We examined how the upper elevational limits of the two oyster species and barnacles varied across latitudes and as a function of air temperature, obtained from a nearby weather station. Given that other studies have shown negative effects of extreme heat events on intertidal organisms, we hypothesized that the elevational distribution of these species would be compressed at sites experiencing more frequent high temperatures. Thus, we expected the upper limit of these species to be lower at hotter, lower latitude sites. However, timing of the tides affects exposure to heat events and sites where the lowest tides in summer occur near midday are exposed to more heat stress than sites where they fall at night or early morning. Consequently, we expected that temperatures during low tide exposure would be a better predictor of the upper limits than temperatures across all time periods. To further explore whether high temperatures might cause mortality near the species’ upper limit, we also compared the elevations of the highest dead vs. live Olympia oysters and examined size distributions in relation to elevation for this species. For example, if a recent thermally induced die-off had occurred, dead oysters should occur higher than live oysters, and small, newly settled oysters should occur above larger, older ones. We also expected that the highest oysters at a site would be in shaded rather than sun-exposed microhabitats. Ultimately, the goals of this investigation across an expansive latitudinal range were to improve understanding of the ecology and climate sensitivity of Olympia oysters to inform future restoration planning for this important foundation species, and to serve as a model for evaluating climate resilience of other foundation species that occur along steep environmental gradients.