DOI: 10.5061/dryad.9kd51c5dt
title: Data from: Biogeography of ocean acidification: differential field performance of transplanted mussels to upwelling-driven variation in carbonate chemistry

description: Ocean acidification (OA) represents a serious challenge to marine
 ecosystems. Laboratory studies addressing OA indicate broadly negative
 effects for marine organisms, particularly those relying on calcification
 processes. Growing evidence also suggests OA combined with other
 environmental stressors may be even more deleterious. Scaling these
 laboratory studies to ecological performance in the field, where
 environmental heterogeneity may mediate responses, is a critical next step
 toward understanding OA impacts on natural communities. We leveraged an
 upwelling-driven pH mosaic along the California Current System to
 deconstruct the relative influences of pH, ocean temperature, and food
 availability on seasonal growth, condition and shell thickness of the
 ecologically dominant intertidal mussel Mytilus californianus . In 2011
 and 2012, ecological performance of adult mussels from local and commonly
 sourced populations was measured at 8 rocky intertidal sites between
 central Oregon and southern California. Sites coincided with a large-scale
 network of intertidal pH sensors, allowing comparisons among pH and other
 environmental stressors. Adult California mussel growth and size varied
 latitudinally among sites and inter-annually, and mean shell thickness
 index and shell weight growth were reduced with low pH. Surprisingly,
 shell length growth and the ratio of tissue to shell weight were enhanced,
 not diminished as expected, by low pH. In contrast, and as expected, shell
 weight growth and shell thickness were both diminished by low pH,
 consistent with the idea that OA exposure can compromise shell-dependent
 defenses against predators or wave forces. We also found that adult mussel
 shell weight growth and relative tissue mass were negatively associated
 with increased pH variability. Including local pH conditions with
 previously documented influences of ocean temperature, food availability,
 aerial exposure, and origin site enhanced the explanatory power of models
 describing observed performance differences. Responses of local mussel
 populations differed from those of a common source population suggesting
 mussel performance partially depended on genetic or persistent phenotypic
 differences. In light of prior research showing deleterious effects of low
 pH on larval mussels, our results suggest a life history transition
 leading to greater resilience in at least some performance metrics to
 ocean acidification by adult California mussels. Our data also demonstrate
 “hot” (more extreme) and “cold” (less extreme) spots in both mussel
 responses and environmental conditions, a pattern that may enable
 mitigation approaches in response to future changes in climate.
description: <b>Mussel
 transplants</b> Using a standard protocol,
 performance of California mussels (30-45 mm total length) was quantified
 during 2011 and 2012 upwelling seasons [61,62,64]. We first collected
 mussels for pre-study measurements and individual tagging. Under permits
 from the Oregon Department of Fisheries and Wildlife and the California
 Department of Fish and Game (Oregon Department of Fish and Wildlife, 2010
 permit #15122 and California Department of Fish and Wildlife
 S-183160003-18316-001), mussels were haphazardly collected from the
 vertically middle portion of <i>M. californianus</i> beds. In
 2011 but not 2012, to assess genetic or persistent phenotypic influences
 on mussel performance, we translocated intermingled local-source (i.e.,
 those from each site) and common-source (mussels from a single site, Bob
 Creek, Oregon, USA). To distinguish them from local-source mussels,
 common-source mussels were also marked with a bead of epoxy.
 In the lab, translocation
 mussels were marked with a 1-2 mm triangular notch filed on the posterior
 shell edge (growing lip) to establish an indicator of initial length.
 Pre-outplant shell weight was estimated using a buoyant-weight method
 similar to [65]. Briefly, the process involved collecting separate mussel
 samples for model calibration at Bob Creek, Bodega Marine Reserve
 (northern California), Sandhill Bluff in central California and Lompoc
 Landing (southern California). The buoyant weight of each “calibration”
 mussel was measured by placing the live mussel on a platform submerged in
 water. The shells of each mussel were pinched closed during transfer
 through air, to prevent the confounding effect of air intake on buoyancy.
 Thus, submerged weight was an estimate of the negatively-buoyant shell
 weight. Soft tissue was then dissected from the shell and, after drying,
 shell weight was directly measured. The site-specific relationship between
 buoyant weight and dry shell was modeled using linear regression. The
 slope and intercept of each model was then used to estimate pre-study
 shell weight for translocated mussels. The Bob Creek regression model was
 used for Fogarty Creek and Strawberry Hill mussels, the Bodega Marine
 Reserve model for Van Damme and Bodega Marine Reserve, the Sandhill Bluff
 model for Terrace Point and Hopkins Marine Station, and the Lompoc Landing
 model for Lompoc Landing and Alegria.  <b>Mussel
 translocation</b> After pre-outplant processing,
 mussels were translocated back to the field for the April through October
 upwelling season. In 2011, mussels were sorted into 5 replicate groups of
 50 per site, with each group consisting of 25 local- and 25 common-source
 individuals. For the 2012 season, mussels were sorted into 5 replicate
 groups of 30 per site.  Mussel translocation used
 established methods [59]. Briefly, at each site, mussels were placed
 ventral side down in cleared plots 2-5 m apart within existing mussel
 beds.  Because bed heights varied among sites along the coast, tidal
 height of transplants varied (Table 1). We accounted for these differences
 by using tidal height as a covariate in data analyses. Mussels were held
 in place with plastic mesh (1-cm x 1-cm mesh) that was fastened using
 stainless steel lag screws inserted into pre-drilled holes with wall
 anchors. Two to four weeks later, the mesh was loosened to encourage more
 byssal thread production, and then 2-4 weeks later loosened further into a
 “dome” to allow space for growth while protecting the mussels from
 predation.  <b>Sample processing and
 growth measurements</b> Within 12 hours of collection,
 all mussels were placed in seawater tables, then within two days of
 collection, frozen at -20<sup>o</sup>C. During processing,
 mussels were thawed, measured (length, width, and depth to the nearest
 0.01 mm. Epibionts and byssal threads were removed from the shell
 exterior, and mussels were then dissected into two constituent parts –
 shell and soft tissue. These were dried separately at 80ºC for ≥ 5 days
 then weighed to the nearest mg.    Shell-length growth was measured
 as mm new shell accumulated between the pre-study notch and the growing
 edge of the shell. Growth was standardized by dividing by initial length.
 Shell-weight growth was measured as the difference in pre- and post-study
 shell weight (g), standardized to the individual’s estimated pre-outplant
 shell weight and the study-season duration at each site. Shell-weight growth of each
 mussel was calculated as the difference between the measured dry shell
 weight at the end of the season and the pre-season shell weight as
 determined by the previously described buoyant weighting
 method.  The
 condition index (unitless) of each mussel was measured as the dry tissue
 mass per total (tissue + shell) dry mass. Higher condition index mussels
 have proportionately more soft tissue mass and may reflect energy
 allocation favoring tissue development [66,67]. Higher condition index may
 also reflect higher resource quality for mussel predators. Mean shell thickness index
 (mg/mm<sup>2</sup>) was estimated by calculating the dry shell
 mass per shell surface area, with surface area
 (<i>A</i> ) calculated by the ellipsoid model <i>A=l×</i><i>h</i><sup><i>2</i></sup><i>+</i><i>w</i><sup><i>2</i></sup><sup><i>1/2</i></sup><i>×π÷2</i> , where <i>l</i> , <i>h</i> , and <i>w</i>  are mussel length, height and width, respectively [68]. All shell dimensions were measured to the nearest 0.1 mm, and shell weight was measured to the nearest 0.01g. The resulting index assumes a constant crystalline density of the shell structure. Major predators of <i>M. californianus</i> include <i>Nucella</i> whelks consume mussels through holes drilled their shells. Therefore, mean shell thickness index may correspond to drilling susceptibility [69]. <b>Temperature</b> Temperature data were obtained using mussel biomimetics, which mimic the thermal properties of living mussels [70,71].  Each logger consisted of a thermistor-based temperature recorder (Tidbit logger, Onset Computer Corp., Bourne, MA) embedded in an epoxy mold shaped like an adult mussel. Using Z-spar epoxy, one to two loggers were deployed per site near replicate mussel plots, then covered with a plastic mesh cage to mimic conditions experienced by the transplanted mussels. Loggers recorded temperatures at 10-minute increments. Air and water temperature data were separated [72] and used to calculate mean temperatures by site and upwelling year. <b>Phytoplankton abundance</b> Phytoplankton are the primary food of <i>M. californianus </i>[73]<i>. </i>Food availability was quantified using chlorophyll-<i>a</i> concentrations ([Chl-<i>a</i>]) as a proxy for phytoplankton abundance. Chl-<i>a</i> was measured by periodically collecting water samples in opaque bottles during low tide at each site [74-76]. Replicate (n=3) bottle samples were collected at low tide from the shore at ~0.5m below the water surface. In the field, fifty ml of water was passed through 25-mm pre-combusted 0.7-µm Whatman GF/F glass-fiber filters. Filters were placed on ice and taken to the lab where Chl-<i>a</i> concentrations were quantified using a fluorometer. Because discrete sampling was not consistently conducted at all study sites in both study years, for analysis we averaged all bottle samples across all sample years creating site-specific long term mean summaries of Chl-<i>a</i> data. Prior research has shown spatial variability but temporal consistency in the levels of Chl-<i>a</i> among subsets of the sites used in this study [74,77]. <b>pH measurements</b> pH data were collected at 10-minute intervals using autonomous sensors deployed at each site within 20 meters from the mussel plots. Sensors were attached to the rock using methods similar to the mussel translocations except that they were held down with stainless steel mesh. Care was taken to ensure that the sensing electrode remained wet even at low tide. Details on these custom-designed sensors can be found elsewhere [49,78], but briefly each was based on an ion-sensitive Honeywell Durafet® with an integrated data logger and power supply [79]. Sensors were calibrated either directly against certified reference materials or indirectly using spectrophotometric pH samples that were calibrated using certified reference materials. pH is reported on the total hydrogen ion concentration scale [80]. Calibrations occurred pre- and post-deployment for all sensors. To spot-check sensor performance, sensor data were periodically (2-4 weeks) compared to discrete water samples collected at all sites except at the two southern California sites in 2012 (Lompoc Landing and Alegria ). To investigate how different aspects of the pH environment might influence mussel performance at each site and in each study year, we compiled summary statistics for the pH mean, standard deviation, and percentages of exposure below two thresholds: pH 7.8 and pH 7.7. These thresholds were chosen for their alignment with model predictions of average global pH conditions by the year 2100 [4,5,6]. Using tide tables, sensor data collected when tides were below the sensor were excluded from analysis. 
description: Robomussel temperature
 data 
 Field
 
 Descriptors 
 datetime Calendar date and time of sensor
 measurement 
 yearday Julian day and time of sensor
 measurement 
 temp_c Measured temperature in degrees
 Celsius 
 tide_m Tidal height (to nearest 0.01 meters)
 at time of measurement 
 air20/water0 Boolean notation indicating if mussel
 sensor was exposed to air (“20”) or under water (“0”) at time of
 measurement 
 cutoff_m Tidal height (to nearest 0.01 meters)
 at which sensor was located in the field 
   Site information; NA=no
 data 
 Field
 
 Descriptors 
 Site Location where mussels were
 transplanted during study duration; FC=Fogarty Creek, SH=Strawberry Hill,
 VD=Van Damme State Park, BM=Bodega Marine Reserve, TP=Terrace Point,
 HM=Hopkins Marine Station, LL=Lompoc Landing, AL=Alegria
 Lat
 Latitude of study site
 Long
 Longitude of study site
 
 outplant.date Calendar date that mussels were
 transplanted to the field at study outset 
 
 recover.date Calendar date that mussels were
 recovered from the field at the end of the study season
 
 days.exposed The duration in days for which mussels
 were exposed to field conditions 
 Year
 Year of study – 2011 or 2012
 
 chla.mean Long-term mean of chlorophyll-a
 concentration, measured by bottle sampling 
 
 chla.sd Long-term variability of chlorophyll-a
 concentration, measured by bottle sampling 
 chla.n
 Total number of bottle samples
 collected to establish mean and sd 
 
 tidal.height.mean Average tidal height (to nearest 0.001
 meters) of transplant replicates 
   Regression mussels; NA=no
 data 
 Field
 
 Descriptors 
 ID Unique numerical identifier
 site
 Site of origin; BC=Bob Creek, BY=Bodega
 Marine Reserve, LP=Lompoc Landing, SB=Sandhill Bluff 
 
 buoyant_wt Weight (to nearest 0.01 grams) of live
 mussel when suspended in seawater 
 length
 Total length of mussel shell (to
 nearest 0.1mm) measured from umbo to posterior shell margin
 width
 Maximum width of mussel shell (to
 nearest 0.1mm) measured from dorsal to ventral margins
 girth
 Maximum breadth of mussel shell (to
 nearest 0.1mm) measured across the breadth of both shell halves when
 closed 
 tin_wt Weight (to nearest 0.01 grams) of tin
 dish in which mussel soft tissue was dried 
 
 shell_wt Weight (to nearest 0.01 grams) of dry
 mussel shell 
 tissue_dry_wt Weight (to nearest 0.01 grams) of dry
 soft tissue; Includes tin dish weight since dish was inseparable from
 desiccated tissue. 
  
 Transplant mussels; NA=no
 data 
 Field
 
 Descriptors 
 ID Unique numerical identifier
 Site
 Location where mussels were
 transplanted during study duration; FC=Fogarty Creek, SH=Strawberry Hill,
 VD=Van Damme State Park, BM=Bodega Marine Reserve, TP=Terrace Point,
 HM=Hopkins Marine Station, LLPU=Lompoc Landing, AL=Alegria
 year
 Year of study – 2011 or 2012
 
 replicate Experimental transplant
 replicate 
 source Identifies if mussel was sourced from
 the local (“L”) transplant site (see “site” field) or common-source (“C”)
 site of Bob Creek, OR 
 init_length Length of mussel shell (to nearest
 0.1mm) measured from umbo to the notch filed on the posterior margin
 before transplanting mussel to the field 
 
 post_length Total length of mussel shell (to
 nearest 0.1mm) measured from umbo to posterior shell margin after mussel
 was recovered from the field 
 growth
 Shell length (to nearest 0.1mm) between
 pre-study notch and post-study posterior margin of the shell
 
 post_height Maximum width of mussel shell (to
 nearest 0.1mm) measured parallel to line from umbo to the opposite shell
 margin after mussel was recovered from the field 
 
 post_girth Maximum breadth of mussel shell (to
 nearest 0.1mm) measured across the breadth of both shell halves when
 closed after mussel was recovered from the field 
 tin_wt
 Weight (to nearest 0.01 grams) of tin
 dish in which mussel soft tissue was dried 
 
 shell_wt Weight (to nearest 0.01 grams) of dry
 mussel shell after mussel was recovered from the field
 
 tissue_dry_wt Weight (to nearest 0.01 grams) of dry
 soft tissue after mussel was recovered from the field; Includes tin dish
 weight since dish was inseparable from desiccated tissue.