Temperature and nutrient source drive population-specific patterns of dissolved inorganic nitrogen uptake in a common kelp

Ocean warming and marine heatwaves are among the greatest threats to kelp forests globally, with warm temperatures affecting kelp performance and survival. Resolving the thermal response in nutrient uptake by kelp is an important step toward modelling functional physiological changes in these taxa under future temperatures. We tested the effect of temperature on the dissolved inorganic nitrogen (DIN) uptake of 2 geographically isolated populations of golden kelp Ecklonia radiata that experience substantial differences in ocean conditions (warm-adapted Western Australia, WA; and cool-adapted Tasmania, TAS). DIN uptake from 2 sources (nitrate, NO 3 - ; and ammonium, NH 4 + ) was measured at 3 concentrations (2, 5 or 20 µM) across 8 temperature incubations (4-30°C) over 2 h. Based on these treatments (population × DIN concentration × DIN source), we modelled thermal performance curves (TPCs) to quantify changes in thermal optima (T opt ) and maximum uptake rates (R max ). Most treatments followed TPCs, except NO 3 - uptake by the TAS population, which showed no temperature dependence. As predicted, T opt values for NH 4 + uptake by the warmer WA population (24.3-30.0°C) were consistently higher than the cooler TAS population (17.7-23.1°C) across all concentrations. Except for 1 treatment (TAS 20 µM), R max values for NH 4 + uptake were also higher in the WA population. Our results indicate that DIN uptake by kelp occurs across a broad range of conditions and that the impacts of temperature depend strongly on the availability of DIN and population-level differences.