Comparative Thermal Performance of Kelp and Herbivores Across a Latitudinal Gradient in Ocean Temperature

The interaction between herbivorous sea urchins and their macroalgal food source plays a fundamental role in regulating community structure on temperate reefs across the globe. Water temperature strongly influences the strength of this interaction, through its effects on the rate of metabolic processes. In southeastern Australia, warming temperature has also facilitated the range extension of some urchin species, which are now threatening macroalgal habitats across $\sim$ 2000 km of coastline. Despite the importance of temperature in this system, the impact of warming on the relative performance of macroalgae and urchins across their range is unknown. We measured the temperature dependence of respiration rates in two barren-forming sea urchin herbivores (Centrostephanus rodgersii and Heliocidaris erythrogramma), and of photosynthetic rates of their macroalgal food source (Ecklonia radiata). This was done at 6-9 temperatures ranging from 5°C-35°C at each of six sites in Australia. These sites span these species' latitudinal distributions, covering a latitudinal range of 12° and a temperature range of 8°C. We found clear differences across latitude in the performance of all three species. At warmer latitudes, E. radiata showed decreased maximum photosynthetic rates while respiration rates for C. rodgersii demonstrated increased sensitivity to acute temperature change. For H. erythrogramma, cool-edge populations showed a plateau in respiration rates at the highest temperature tested, while rates in central and warm-edge populations continued to increase. Overall these patterns indicate that temperatures at the warm range edge may be approaching upper thermal tolerance limits for all three species, although C. rodgersii may have a slightly higher thermal threshold compared to E. radiata , and may persist longer in a warming environment. This study provides novel insights into the comparative thermal performance of kelp and urchins across their entire latitudinal distribution and highlights the need to consider different populations of species to understand the impacts of warming on marine ecosystems.