Latitudinal investigation of Olympia oyster elevational distribution

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.