M. Roberto García-Huidobro

Nutrient availability and temperature have a strong effect on the performance of organisms and ultimately determine the productivity of marine ecosystems. To evaluate how nutrient supply and temperature variability influence trophic dynamics, we simulated in the laboratory the main environmental conditions of upwelling and downwelling sites, contrasting high and low nutrient availability under current and end-of-the-century projected temperatures. We used a green alga ( Ulva sp.) and a prominent herbivore, the sea urchin Loxechinus albus , to test 2 hypotheses: (1) the positive effect of nutrient availability on algal biochemical composition, observed in cold upwelling zones, is attenuated or reversed under future warming scenarios due to the increased energy demand imposed by warmer temperatures; and (2) reduced nutrient availability in algal tissues reduces the physiological performance and thermal tolerance of herbivores compared to those consuming diets of higher nutritional content. Our results show that nutrient availability had a significant influence on the nutritional properties of the algae and indirectly on the food consumption and absorption efficiency rates of the sea urchins in addition to 2 thermal performance parameters. Temperature, and its interaction with nutrients, also had a significant effect on the herbivore’s growth rate and critical thermal maximum. Considering various climate model predictions that suggest warmer waters in or near upwelling systems, our results suggest negative effects for algal nutritional quality, and the feeding ecology of sea urchins and other consumers relying on these and possibly other macroalgae.

Predicting vulnerability to global warming remains an elusive goal in thermal biology. In marine fishes, ongoing changes in distribution contrast with their apparent capacity to tolerate temperatures from 5°C up to 25°C higher than current conditions. Employing a data set of 786 upper critical temperatures across 213 species and recent theoretical developments, we provide conclusive evidence that these so‐called thermal safety margins overestimate the resilience to warming and that most species inhabit thermal conditions approaching their physiological tolerance limit. This result holds across latitudes and based on historical records, several populations have encountered stressful temperatures in the recent past. While warming tolerance remains similar across geographic regions, behavioural responses are constrained at low latitudes as distribution shifts required to encounter cooler waters are disproportionally higher in the tropics. Overall, our results illustrate how thermotolerance measures can be extrapolated to the field and used to quantify vulnerability to warming.

Artificial Light at Night (ALAN) is expanding worldwide, and the study of its influence remains limited mainly to documenting impacts, overlooking the variation in key characteristics of the artificial light such as its intensity. The potential dose–response of fitness-related traits to different light intensities has not been assessed in sandy beach organisms. Hence, this study explored dose-responses to ALAN by exposing the intertidal sandy beach isopod Tylos spinulosus to a range of light intensities at night: 0 (control), 20, 40, 60, 80 and 100 lx. We quantified the response of this species at the molecular (RNA:DNA ratios), physiological (absorption efficiency) and organismal (growth rate) levels. Linear and non-linear regressions were used to explore the relationship between light intensity and the isopod response. The regressions showed that increasing light intensity caused an overall ~ threefold decline in RNA:DNA ratios and a ~ threefold increase in absorption efficiency, with strong dose-dependent effects. For both response variables, non-linear regressions also identified likely thresholds at 80 lx (RNA:DNA) and 40 lx (absorption efficiency). By contrast, isopod growth rates were unrelated (unaltered) by the increase in light intensity at night. We suggest that ALAN is detrimental for the condition of the isopods, likely by reducing the activity and feeding of these nocturnal organisms, and that the isopods compensate this by absorbing nutrients more efficiently in order to maintain growth levels.