Abstract
In the coming decades, environmental change like warming and acidification will affect life in the ocean. While data on single stressor effects on fish are accumulating rapidly, we still know relatively little about interactive effects of multiple drivers. Of particular concern in this context are the early life stages of fish, for which direct effects of increased CO2 on growth and development have been observed. Whether these effects are further modified by elevated temperature was investigated here for the larvae of Atlantic herring (Clupea harengus), a commercially important fish species. Over a period of 32 days, larval survival, growth in size and weight, and instantaneous growth rate were assessed in a crossed experimental design of two temperatures (10°C and 12°C) with two CO2 levels (400 μatm and 900 μatm CO2) at food levels mimicking natural levels using natural prey. Elevated temperature alone led to increased swimming activity, as well as decreased survival and instantaneous growth rate (Gi). The comparatively high sensitivity to elevated temperature in this study may have been influenced by low food levels offered to the larvae. Larval size, Gi and swimming activity were not affected by CO2, indicating tolerance of this species to projected "end of the century" CO2 levels. A synergistic effect of elevated temperature and CO2 was found for larval weight, where no effect of elevated CO2 concentrations was detected in the 12°C treatment, but a negative CO2 effect was found in the 10°C treatment. Contrasting CO2 effects were found for survival between the two temperatures. Under ambient CO2 conditions survival was increased at 12°C compared to 10°C. In general, CO2 effects were minor and considered negligible compared to the effect of temperature under these mimicked natural food conditions. These findings emphasize the need to include biotic factors such as energy supply via prey availability in future studies on interactive effects of multiple stressors.
Highlights
Ocean warming (OW) and acidification (OA), both caused by rising atmospheric CO2 levels [1,2], will intensify in the future
We developed three hypotheses to test in this study: 1. Elevated CO2 levels negatively affect length, weight, instantaneous growth rate, development and survival, while increasing swimming activity; 2
The temperature effects detected in the first phase of the experiment on instantaneous growth rate and survival may originate from different response times in yolk utilization [62] and the switch from endogenous to exogenous feeding mode [60]
Summary
Ocean warming (OW) and acidification (OA), both caused by rising atmospheric CO2 levels [1,2], will intensify in the future. Sea surface temperature is projected to increase by at least 1.5 ̊C while seawater pH could decrease by 0.4, when pCO2 levels of ~800 μatm are reached by the year 2100 [3,4]. These environmental changes will likely interact in their effects on fish populations and are of great interest to society due to the socio-economic value of fishing and other ecosystem services [5,6]. Few studies have tested the interaction of increased temperature and pCO2 for the early life stages of fish, these two exemplary studies highlight the complexity in the interacting effects of temperature and pCO2
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