Nutrient enrichment can increase the thermal performance of Galápagos seaweeds

Thermal sensitivity in ectothermic organisms is often contingent upon environmental factors. Nutrient availability in particular is believed to influence the physiological responses of primary producers to global warming and is thus relevant to consider when forecasting the structure and functioning of future marine ecosystems. This study measured the effect of nutrient enrichment on the thermal sensitivity of 4 genera of Galápagos seaweeds (Ulva, Caulerpa, Padina, and Ochtodes), estimated as the thermal optimum (T_opt), performance maximum (P_max), activation energy, and deactivation energy. These parameters were quantified by modeling thermal performance curves for net photosynthesis under ambient and nutrient-enriched conditions. Our findings revealed variation in T_opt among genera, ranging from 27.6° to 36.0°C. Nutrient additions enhanced T_opt by ~2°C for 2 (Padina and Caulerpa) of the 4 taxa and also significantly increased P_max in Padina, suggesting the ability for warming-induced ocean stratification and associated effects (i.e. decreasing nutrient availability) to reduce the capacities of these populations to maintain and support new growth. No significant differences in T_opt or P_max were observed for either Ulva or Ochtodes with enrichment. Ambient net photosynthesis and respiration rates were also compared across genera; P_max rates for net photosynthesis were consistently higher than those for respiration (i.e. until just beyond T_opt); however, photosynthetic T_opt values were lower. Thus, this study suggests that further warming could reduce overall net primary productivity, with potentially far-reaching implications for marine food webs.