Double trouble at high latitudes: ocean acidification and warming challenge embryogenesis in Atlantic cod Gadus morhua and polar cod Boreogadus saida

Chair: Elvira Poloczanska

Flemming T Dahlke (1)*, Jasmine Nahrgang (2), Velmurugu Puvanendran (3), Atle Mortensen (3), Hans-Otto Pörtner (1), Daniela Storch (1)

1 Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, 27570, Germany.
2 The Arctic University of Norway, Tromsø, NO-9037, Norway.
3 Nofima AS, Tromsø, NO-9291, Norway.

Background
Changes in PCO2 and temperature will impact fish populations in future oceans, specifically those living close to their lower or upper thermal limit, such as Atlantic cod Gadus morhua (lower) and polar cod Boreogadus saida (upper) in the subarctic Barents Sea. Embryogenesis may represent a developmental bottleneck in this respect since organ systems supporting homeostasis are not jet fully developed. Embryos are also limited in their ability to compensate for increased metabolic costs possibly associated with acid-base regulation and thermal acclimation. This may lead to trade-offs in resource allocation to vital maintenance functions and developmental processes.

Methods
Embryos of both Barents Sea source populations were exposed to factorial combinations of two PCO2 levels (400 & 1100 µatm) and five temperatures (0, 3, 6, 9, 12 °C for G. morhua and 0, 1.5, 3, 4.5, 6 °C for B. saida). In addition to hatching success, we measured respiration rates (MO2) of embryos as a proxy for metabolic activity, while larval morphometrics at hatch were analysed to assess embryonic growth and resource allocation.

Findings
In both species, ocean acidification (OA) exacerbated negative effects of warming on hatching success. MO2 increased with temperature and in response to OA but declined upon extreme warming, especially in combination with OA. This pattern indicated increased energy demand for acclimation to OA and suggested a mechanistic link between oxygen uptake and heat tolerance. The increase in MO2 in response to OA was paralleled by reduced larval size, while the amount of consumed resources (yolk) remained unaffected.

Conclusions
OA has the potential to aggravate effects of thermal stress on embryogenesis in both species tested. Furthermore, our results showed that acclimation to OA required reallocation of limited resources; supporting the idea that energy supply to life-sustaining functions takes priority over other developmental processes such as embryonic growth.