DOI: 10.5061/dryad.ngf1vhj4v
title: Latitudinal investigation of Olympia oyster elevational distribution

description: 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.
description: <strong>Geographic scope</strong>
 We conducted surveys at 26 sites located in 18 estuaries across
 almost the entire biogeographic range of Olympia oysters, more than 21
 degrees latitude, from 30.5° N to 51.6° N. Sampling occurred May-November
 2023, mostly in summer. This effort was a collaborative project with more
 than 100 people involved in the field. We chose sites that had appropriate
 substrate for oysters distributed across a range of elevations, at least
 from Mean Lower Low Water (MLLW) to Mean Higher High Water (MHHW); we thus
 could be confident that the absence of oysters from high elevations was
 not due to lack of suitable substrata.
 <strong>Characterizing intertidal
 distributions</strong>  
 <em>Olympia oyster elevational
 patterns</em>   Our
 primary focus at all 26 sites was to identify the upper elevational limit
 of Olympia oysters –  to identify their landward boundary in the
 intertidal zone. At all sites, we placed markers on the highest 10-20 live
 Olympia oysters that could be found. We only included oysters that were in
 fairly exposed locations; that is, we excluded oysters that inhabited deep
 crevices, under cobble, or in seeps or pools that retain water at low
 tide. These oysters were at least 1 m apart from each other within 100 m
 of shoreline. In these relatively rapid field surveys for oysters, we
 would not have detected newly settled juveniles &lt;0.05 cm in
 size.   We measured the elevation of the highest
 Olympia oysters flagged at each site. At most sites we used a networked
 RTK GPS. At the most northern two sites and one Oregon site we used laser
 leveling from the waterline and estimated relative tidal elevations using
 the predicted waterline elevation at that time. At the three most southern
 sites, we used differential GPS. We surveyed a landmark (known/stable
 ground control point) at the start and end of our fieldwork at the site,
 and in most cases these measurements differed by &lt;3 cm.
 To compare the upper limits of oysters across estuaries with
 different tidal ranges and different vertical datums (CGVD2013 converted
 to Chart Datum in Canada, NAVD88 in US and WGS84 in Mexico), it was
 critical to use a consistent universal currency. For each site, we
 obtained an estimate of MLLW and MHHW in the same vertical datum as the
 elevation measurements. In the US the estimates were provided using NOAA’s
 vDatum (https://vdatum.noaa.gov/). The only exception within the US was at San Dieguito Lagoon where the nearest NOAA water level station was not representative of local water level conditions; in this case, MLLW and MHHW were calculated using water level data collected with HOBO water level loggers deployed by the San Onofre Nuclear Generating Station Mitigation Monitoring Program during 2021. In Canada, MLLW and MHHW were calculated from the closest government tide station (https://www.tides.gc.ca/en/stations). In Mexico, MLLW and MHHW were calculated using the astronomical tidal models from CICESE (https://predmar.cicese.mx/calendarios/ ), and the local tidal datums were validated against benchmarks defined by the Mexican Navy (Secretaría de Marina).  We used the following formula to standardize all elevation measurements to a relative scale that takes into account tidal range: <em>Relative elevation of measurement </em> <em>= (field elevation measurement - elevation of MLLW ) / </em> <em>(elevation of MHHW - elevation of MLLW)</em> Critically, all four elements in this formula were in the same vertical datum for a site, e.g. Chart Datum in Canada but NAVD88 in USA. On this scale, MLLW has a relative elevation of 0, MHHW of 1. Because we were interested in the upper tolerance limit at each site, we used the highest 5 elevation values for each site that were within 0.1 unit on the relative scale of the highest measurement.  We preferred 5 measurements to a single one, to avoid anomalous outliers. However, at sites with very low oyster densities, sometimes there were fewer than 5 oysters near the upper limit, so in these cases some of the top 5 measures spanned a large elevational range. To obtain a more accurate estimate of the true upper limit, we only used those within 0.1 elevational unit. At those sites where the tide was low enough after conducting the above surveys, we also characterized the elevational zone with the densest Olympia oysters, which was always lower than the upper limit. At 12 sites we placed quadrats (15 x 15 cm) in 10 of the densest oyster locations at the site and counted the oysters. We took the elevation of these 10 quadrats as described above. <strong>Pacific oyster and barnacle upper limits compared to Olympia oysters</strong> At a selection of sites where time and tide permitted, we measured the elevation of the 10 highest live Pacific oysters using the same methods described above. At most (22 of 26) sites, we measured the elevation of the 10 highest barnacles, which represented the highest sessile invertebrates present at the site. At sites with both oyster species present, individuals &lt;7 cm in size were sacrificed following measurement to view diagnostic features for definitive identification. Individuals &gt;7 cm were assumed to be Pacific oysters. <strong>Relationships among air temperature, latitude, and Olympia oyster upper limits</strong> We used the nearest reliable weather stations to obtain hourly air temperature for the five years prior to and including our field surveys (2019-2023) for each site. In a few cases, there were gaps in the datasets. Where these gaps lasted a week or more (South Sequim Bay and Brickyard Park, San Francisco Bay), we substituted data from the next closest weather station. We were unable to fill a 3-month gap (Jan-Mar 2023) at one site (RV Park, Estero Punta Banda). To examine air temperatures during low tide exposure, we also filtered the above dataset of all air temperatures to only include hours when tide level was below MLLW. We used the nearest tide stations for water level estimates to obtain predicted hourly tide levels relative to MLLW for each site for the same five-year period.  We were interested in the role of extreme temperatures in shaping the intertidal distribution of oysters, and in exploring which methods of calculating extreme temperatures would be most ecologically relevant. We therefore examined multiple metrics for characterizing temperatures that were extreme across the range of the Olympia oyster. We recognize that these “extreme” temperatures were not extreme on a global scale, but rather only in relative terms of what organisms at these sites experience. To characterize exposure to these air temperature extremes, we calculated the number of temperature values &lt;5°C and &gt;30°C both before and after filtering data for water levels below MLLW. Due to occasional missing data, we divided that by the number of hourly readings present to get the proportions of readings below 5°C and above 30°C. Totals and proportions were for the entire 5-year period of 2019-2023. We used 5°C and 30°C as thresholds for extreme temperatures based on inspection of temperatures across the range; we had initially considered using 0°C and 40°C, but determined they occur so rarely at most sites as not to be useful for correlative analyses. As an alternative approach, we also examined the 10th and 90th percentile of temperatures for each site both before and after filtering data for water levels below MLLW. <strong>Within-site variation in upper elevational limits</strong> We collected and analyzed ancillary data to determine whether within-site variation in Olympia oyster elevations showed patterns that would be consistent with mortality associated with extreme high temperature events. <strong>Sun exposure</strong> If high temperatures set the maximum elevation of an intertidal species, upper distributional limits should be related to exposure to sunshine. For the highest Olympia oysters at each site, we assigned each oyster to one of three levels of exposure, 1) shady: oyster would be mostly shaded at midday, for instance under overhang; 2) exposed: oyster would be mostly in full sun at midday, for instance on top of rock; and 3) partial: somewhere in between shaded and exposed. At some sites, there was no variation (e.g., all of the highest oysters were exposed); at other sites, low tides during our field survey occurred at night and exposure could not be assessed. <strong>Evidence of recent die-off</strong> If recent die-offs caused by low tide exposure to heat occurred, one would expect to find dead oysters above live oysters at a site. In addition to searching for the highest live Olympia oysters, field teams also searched for the highest dead Olympia oysters when time permitted. We focused on recently dead individuals that were gaping (oysters still cemented to the substrate by the bottom valve, with the top valve still attached). As with live oysters, we identified individuals at least 1 m apart within 100 m of shoreline, and measured elevations as described above. If the local populations experienced periodic die-offs of the highest oysters due to heat waves, the highest oysters at a site should be smaller than lower ones. At all except one site, we measured shell sizes (longest dimension) for the highest Olympia oysters while collecting their elevations. <strong>Substrate size</strong> Large substrates heat up more slowly than small ones during low tide exposure, so if high temperatures were setting the upper limit, the highest oysters might be limited to the largest substrates at the site. We measured the size (maximum linear dimension) of the substrate to which the highest live Olympia oysters were attached. Unattached single oysters were assigned an arbitrary substrate size of 1 cm and oysters on bedrock were assigned a substrate size of 1000 cm.
description: # Latitudinal investigation of Olympia oyster elevational distribution
 [https://doi.org/10.5061/dryad.ngf1vhj4v](https://doi.org/10.5061/dryad.ngf1vhj4v) ## Description of the data and file structure These four datasets were used for the analyses presented in our article. ### Files and variables #### File: Wasson\_Elevation\_Regions.csv **Description:** This is the most critical data file for this paper, with the elevation data used for most analyses. ##### Variables * Region: regional grouping used for visualizations * State: state or province * Estuary: estuary  * Site: site * Date: date * Measurement: type of elevation measurement -- "highest live lurida" (highest live Olympia oyster), "highest live gigas" (highest live Pacific oyster), "highest live barnacle" (highest live barnacle), "highest recently dead lurida" (highest dead Olympia oyster with both shells still present), "highest long dead lurida" (highest dead Olympia oyster with only bottom shell present), "densest live lurida" (densest Olympia oyster zone where quadrats were taken), "lowest live lurida" (lowest live Olympia oyster), "highest live lurida microhabitat" (highest live Olympia oyster in a microhabitat); "highest live gigas microhabitat" (highest live Pacific oyster in a microhabitat) * ID: shorthand labels for above used for field markers * Oyster Size (mm): longest of oyster dimension in mm * Substrate size (cm): longest dimension of substrate in cm * Substrate type: descriptor of substrate * Shadiness Index: index where 1 is shaded and 3 is exposed, 2 is partial * Latitude: latitude in whatever currency was provided by contributor, mostly decimal degrees * Longitude: : latitude in whatever currency was provided by contributor, mostly decimal degrees * Microhabitat: description of microhabitat if noted * Elevation: in local currency, NAVD88 for US, chart datum for Canada * MHHW: elevation of Mean Higher High Water in same currency as elevation measurement * MLLW: elevation of Mean Lower Low water in same currency as elevation measurement * Relative_Elevation: elevation converted to relative scale where 0=MLLW, 1=MHHW #### File: Wasson\_Summary\_Temperatures.csv **Description:**  This file has a single value per site of extreme temperatures calculated in different ways, which is what was used for the model selection linking Olympia oyster upper elevation limits to potential thermal predictors. The temperatures were calculated from five years of weather and water level data for each site. ##### Variables * Site: site * Estuary: estuary * State_Province: state or province * Country: country * Region: regional grouping used for visualizations * Latitude: latitude in decimal degrees * Elev_lurida: average elevation of the highest live Olympia oysters at the site, on the relative scale where 0=MLLW and 1=MHHW * Elev_gigas: average elevation of the highest live Pacific oysters at the site, on the same scale as above * Elev_barnacle: average elevation of the highest live barnacles at the site, on the same scale as above * Proportion_of_all_temps_that_were_below_5: proportion of all temperatures 2018-2023 that were below 5 C * Proportion_of_all_temps_that_were_above_30: proportion of all temperatures 2018-2023 that were above 30 C * Top_10%_of_all_temps: top 10% of all temperatures 2018-2023 * Bottom_10%_of_all_temps: bottom 10% of all temperatures 2018-2023 * Proportion_of_temps_at_low_tide_that_were_below_5: proportion of temperatures during low tide 2018-2023 that were below 5 C * Proportion_of_temps_at_low_tide_that_were_above_30: proportion of temperatures during low tide 2018-2023 that were above 30 C * Top_10%_of_temps_at_low_tide: top 10% of temperatures during low tide 2018-2023 * Bottom_10%_of_temps_at_low_tide: bottom 10% of temperatures during low tide 2018-2023 #### File: Wasson\_Elevation\_Paired.csv **Description:** This file was used for paired Wilcoxon tests, where each site was used as replicate to examine differences in elevation between the highest live Olympia oysters and a) highest dead Olympia oysters, b) highest live Pacific oysters, c) highest live barnacles, and d) densest live Olympia oysters. ##### Variables * Site name: Name of the site * Estuary: Estuary that the site was in * State/Province: State or province * Region: A regional grouping variable we used for visualizations * Highest_Live_Olympia_Oysters: Average elevation (on relative scale where 0=MLLW, 1=MHHW) of highest live Olympia oysters at the site * Highest_Dead_Olympia_Oysters: Average elevation (on same scale as above) of highest dead Olympia oysters at the site * Dead_Analysis: "yes" if this site had data on both dead and live and should be included in paired test * Highest_Live_Pacific_Oysters: Average elevation (on same scale as above) of highest live Pacific oysters at the site * Pacific_Analysis: "yes" if this site had data on both Olympia and Pacific oysters and should be included in paired test * Barnacle_Upper_Limit: Average elevation (on same scale as above) of highest barnacles at site * Barnacle_Analysis: "yes" if this site had data on barnacles and Olympia oysters and should be included in paired test * Densest_Olympia_Oysters: Average elevation (on same scale as above) of 10 quadrats with densest Olympia oysters at the site * Dense_Analysis: "yes" if this site had data on densest and live Olympia oysters and should be included in paired test #### File: Wasson\_Elevation\_Correlations.csv **Description:** The file contains data on the highest live Olympia oysters at each site, and was used for correlative analyses between elevation and potentially associated variables (oyster size, substrate size, exposure). ##### Variables * State: state or province * Estuary: estuary * Site: site * Date: date * Oyster_Size: size across longest dimension in mm * Substrate_Size: size across longest dimension in cm * Size_Include: "yes" if size varied within a site, in which case the data were included in a correlation of elevation vs. substrate size * Substrate type: descriptor of substrate type * Latitude: latitude mostly in decimal degrees, though some contributors used different currencies * Longitude: latitude mostly in decimal degrees, though some contributors used different currencies * Elevation: elevation in local vertical datum * MHHW: Mean Higher High Water in the same vertical datum * MLLW: Mean Lower Low Water in the same vertical datum * Relative_Elevation: Relative elevation calculated from the above, on scale where 0=MLLW, 1=MHHW * Exposure: see methods text; either exposed, shaded, or partially shaded * Exposure_Include: "yes" if there was sufficient variation within a site to compare elevations of oysters with different exposures #### File: Wasson-Tide-All-Sites.csv **Description:** The file contains hourly temperature and water level data for five years (2019-2023) for 26 sites.  It is thus a longer file than can easily be opened in Excel.  In this file, blank cells are ones for which we were not able to obtain data (weather station not reporting values). ##### Variables * Estuary: estuary * DateTime_LST: date and time in local standard time * Temp_C: temperature in Celsius * Tide: water level in meters relative to Mean Lower Low Water (=0)