Decadal long sub-lethal temperature increases alter the production of non-structural carbohydrates in Zostera...

Globally, seagrass ecosystems are affected by a variety of natural and anthropogenic stressors under a warming climate. Understanding how organisms cope with current stressors, when presented with future warming scenarios, is crucial for predicting ecosystem responses. Here, we use thermal plumes in an east Australian estuary as a proxy for future climate change scenarios to investigate the effects of in situ warming on Zostera muelleri resilience, measured through non-structural carbohydrate (NSC) concentration in the rhizome. NSC's are stored in the rhizome during periods of high photosynthetic efficiency, and are metabolised when photosynthesis is not possible, such as during a storm surge or other stress event. Thermally affected sites in this system experience temperatures 1-6 °C above ambient conditions, with greatest differences in winter months. Despite temperatures frequently exceeding the upper thermal limit (32 °C) for Z. muelleri, we found no significant differences in total NSC concentrations between ambient (80-210 mg g^-1) and thermally affected populations (80-170 mg g^-1), with only a difference in starch concentration being higher at ambient sites than thermally affected sites during October. A notable shift in NSC composition occurred in July-August 2022 after heavy rainfall in the area, with starch decreasing from 120 to 160 mg g^-1 to 20-30 mg g^-1, coinciding with increases in fructose and glucose. Thermally affected sites exhibited dampened seasonality in energy reserves suggesting that these populations have adjusted their energy management strategy in response to extended favorable conditions, effectively reducing the amplitude of seasonal NSC fluctuations. These findings support previous findings that larger changes in carbohydrates are driven by temporal patterns and local weather events. They also suggest that Z. muelleri exhibits resilience to elevated temperatures through maintaining comparable total NSC reserves, but with altered temporal patterns of accumulation that likely reflect adaptation to an effectively extended growing season under warming conditions.