11. Krill (Euphausia pacifica) Development in the Laboratory is Impaired at Currently Observed pCO2 Levels

Anna K. McLaskey (1)*, Julie E. Keister (1), Paul McElhany (2), M. Brady Olson (3), D. Shallin Busch (4), Mike Maher (2), Amanda K. Winans (1)

1 University of Washington, School of Oceanography, Seattle, WA, 98105, USA
2 NOAA Northwest Fisheries Science Center, Seattle, WA, 98112, USA
3 Western Washington University, Shannon Point Marine Center, Anacortes, WA, 98221, USA
4 NOAA Northwest Fisheries Science Center and Ocean Acidification Program, Seattle, WA, 98112, USA

Background
Euphausiids are a critically important component of marine ecosystems around the world, but ocean acidification (OA) work on euphausiids is limited. Research to date has shown that hatching of Euphausia superba eggs and survival of Nyctiphanes couchii sub-adults are reduced at elevated pCO2 levels. Euphausia pacifica is a dominant and trophically important zooplankton species throughout the North Pacific and the California Current Ecosystem, where naturally low pH conditions occur. The areal and temporal extents of these low pH conditions are expected to increase under OA.

Methods
We sampled the vertical distribution of E. pacifica eggs and larvae with simultaneous carbonate chemistry measurements to characterize their exposure in Puget Sound, WA, a large estuary connected to the California Current. We also collected adult females and spawned them under a wide range of pCO2 conditions in the laboratory, tracking hatching success, larval development and survival. Our laboratory experiments were designed to cover the broad range of pH conditions that E. pacifica currently experiences in Puget Sound (pH 8.0, 7.7, 7.4) and may experience in the future (pH 7.3, 7.2).

Findings
In Puget Sound, WA, E. pacifica larval stages, particularly the nauplius 2 and metanauplius stages, are found throughout the water column (20-180m), where they are exposed to pH 7.8-7.5. In the laboratory, E. pacifica hatching is robust to a range of pH levels, but larval development and survival are reduced at lower pH; survival from three days post hatch to the calyptopis 2 stage was reduced by an average of 20% at pH 7.69 compared to pH 7.96.

Conclusions
This study indicates that E. pacifica may be living near the limits of its pH tolerance and continued OA could push these organisms past their threshold, with negative consequences for their populations and higher trophic levels.