Physiology of juvenile hydroids - High food availability mitigates stress responses of Hydractinia echinata to increasing seawater temperatures

Ocean warming due to climate change is predicted to profoundly affect marine ecosystems. These effects are expected to be intensified for shelf seas like the North Sea, where sea surface temperatures are predicted to increase 3 °C by 2100. Increasing seawater temperature is suggested to alter physiological performance of benthic marine invertebrates, while predicted changes in ocean dynamics and primary production might limit nutrient availability. At present, the combined effect of temperature and nutrition stress on marine biota is largely understudied. We investigated physiological responses in highly polymorphic hydroids Hydractinia echinata from two locations in the North Sea in response to two temperatures (18 °C = ambient, 21 °C = increased) cross factored with two food regimes (high, low). After 50 days of experimental exposure, morphological (biomass), physiological (standard metabolic rate) and biochemical performance (contents of protein and protein carbonyl) was determined in juvenile hydroids. Reduced growth and low total protein contents combined with high standard metabolic rates and high protein carbonyl contents in H. echinata exposed to 21 °C/low food compared to 18 °C/high food point towards an energy deficiency in the former animals. Meanwhile, high food availability seems to mitigate negative effects of elevated temperature, as energy budgets were sufficient to maintain growth and to keep oxidative damage accumulation low at 21 °C/high food. Our results suggest that high nutrition will increase resilience in juvenile hydroids to seawater temperatures predicted for 2100 in the North Sea. This study illustrates that habitat energy availability is a major driver of species distribution ranges and should be considered in when predicting responses of marine invertebrates to future environmental stressors.