The effects of ocean warming and elevated CO2 on the feeding behavior and physiology of two sympatric mesograzers

Atmospheric CO2 concentrations have increased significantly since pre-industrial times, leading to ocean warming and acidification. These environmental changes affect the physiology of marine organisms as they modify metabolic processes. Despite the critical role of temperature and pH in marine biology, studies of their combined effects are limited. This study investigated the interactive effects of ocean warming and acidification on the feeding behavior and physiology of two sympatric amphipods, Hyale niger and Cymadusa filosa. Using an orthogonal experimental design with two temperatures (27 °C and 30 °C) and two pH levels (7.8 and 7.5), we assessed feeding rates, respiration rates, ammonia excretion, and O/N ratios. Results indicated that C. filosa was less tolerant to these stressors than H. niger. While H. niger showed no significant changes between treatments, C. filosa showed reduced feeding rates and altered physiological responses to elevated temperature and decreased pH. Reducing the feeding rate of C. filosa may favor macroalgal biomass and strengthen bottom-up control in phytal communities. In addition, increased ammonia excretion in C. filosa suggests increased protein catabolism to meet energy demands at higher temperatures, despite reduced oxygen consumption. This indicates a compromised metabolism and a reduction in circulating oxygen capacity for C. filosa. The study shows heterogeneous responses to climate change, highlighting the need to assess combined environmental stressors in different species to accurately understand the impacts of climate change.