On the Frontline: Tracking Ocean Acidification in an Alaskan Shellfish Hatchery

<div><p>The invasion of anthropogenic carbon dioxide (CO₂) into the ocean is shifting the marine carbonate system such that saturation states of calcium carbonate (CaCO₃) minerals are decreasing, and this is having a detrimental impact on early life stages of select shellfish species. The global, secular decrease in CaCO₃ saturation states is occurring on top of a backdrop of large natural variability in coastal settings; progressively shifting the envelope of variability and leading to longer and more frequent exposure to adverse conditions. This is a great concern in the State of Alaska, a high-latitude setting vulnerable to rapid changes in the marine carbonate system, where an emerging shellfish industry plans major growth over the coming decades. Currently, the Alutiiq Pride Shellfish Hatchery (APSH) in Seward, Alaska is the only hatchery in the state, and produces many shellfish species with early life stages known to be sensitive to low CaCO₃ saturation states. Here we present the first land-based OA measurements made in an Alaskan shellfish hatchery, and detail the trends in the saturation state of aragonite (Ω<i>_arag</i>), the more soluble form of CaCO₃, over a 10-month period in the APSH seawater supply. These data indicate the largest changes are on the seasonal time scale, with extended periods of sub-optimal Ω<i>_arag</i> levels (Ω<i>_arag</i> < 1.5) in winter and autumn associated with elevated water column respiration and short-lived runoff events, respectively. The data pinpoint a 5-month window of reprieve with favorable Ω<i>_arag</i> conditions above the sub-optimal Ω<i>_arag</i> threshold, which under predicted upper-bound CO₂ emissions trajectories is estimated to close by 2040. To date, many species in production at APSH remain untested in their response to OA, and the data presented here establish the current conditions at APSH as well as provide a framework for hatchery-based measurements in Alaska. The current and expected conditions seen at APSH are essential to consider for this developing Alaskan industry.</p></div

Bar graphs showing histograms of Ω_<i>arag</i> observations at APSH.

<p>The center panel is the data shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0130384#pone.0130384.g004" target="_blank">Fig 4</a> with 2014 atmospheric pCO₂ levels (398 μatm; gray). Top and bottom panels are Ω_<i>arag</i> computed using total CO₂ (TCO₂) adjusted to atmospheric pCO₂ levels of 280 (blue) and 500 (red) μatm, respectively, following the approach of Harris et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0130384#pone.0130384.ref017" target="_blank">17</a>] by assuming sea-air CO₂ disequilibria and the processes that determine TA, temperature and salinity variability are constant in time. The vertical dashed black line in all panels is the sub-optimal 1.5 Ω_<i>arag</i> threshold where some early life stages of marine bivalves become stressed [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0130384#pone.0130384.ref007" target="_blank">7</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0130384#pone.0130384.ref008" target="_blank">8</a>]. An atmospheric pCO₂ of 500 is expected by 2040 if the IPCC AR5 RCP 8.5 emissions trajectory is realized.</p

Map showing the location of Alutiiq Pride Shellfish Hatchery (APSH) in Resurrection Bay, the position of the Gulf of Alaska OA (GAKOA) mooring at the mouth of Resurrection Bay.

<p>Red dots are locations of discrete total alkalinity (TA) and salinity measurements, with the number of measurements made during cruises in May and September from 2008 through 2013 shown in the insert.</p

Temperature (°C)–salinity diagram with contours of seawater potential density anomaly (σ_t) and Ω_<i>arag</i> as colored dots.

<p>Note the two areas of sub-optimal Ω_<i>arag</i> (Ω_<i>arag</i> < 1.5; warm colors) at the lowest salinities (<29) and coldest temperatures (< 7°C).</p

Ten months of data collected from intake water entering the Alutiiq Pride Shellfish Hatchery (APSH).

<p>The top panel is temperature (°C; blue) and salinity (red), and the bottom panel is seawater pCO₂ (μatm) with the red dashed horizontal line as the mean atmospheric concentration (398 ± 6.7 μatm).</p

Relationship between total alkalinity (TA; μmol kg^-1) and salinity in the northern Gulf of Alaska calculated using the data described in Fig 1.

<p>The linear fit (gray line) for these data is: TA = 48.7709*S + 606.23 μmol kg^<b>-1</b> (r^<b>2</b> = 0.94, root mean square error = 17.21 μmol kg^<b>-1</b>). This fit was calculated using the MathWorks MATLAB robust linear regression algorithm with only salinity data < 33.6 (blue dots). Measurements above this salinity range are deep samples collected over the outer continental shelf that have a steeper TA-salinity relationship (black dots). Twenty-three validation TA measurements (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0130384#pone.0130384.s001" target="_blank">S1 Table</a>) were made and are shown here as red dots.</p