Warming sensitivity limits the proliferation of Undaria pinnatifida (Phaeaophyceae) in the Dutch delta

The non-native kelp Undaria pinnatifida has invaded European coastlines for decades, yet its presence in the Dutch Delta remains non-dominant. The mechanisms underlying this limited proliferation are poorly understood, but thermal sensitivity has been proposed as a potential constraint under ongoing ocean warming. To evaluate this, we conducted a controlled 21-day laboratory experiment to assess the effects of temperature on the survival, growth, photosynthetic efficiency, and nutrient (NO 3 - and PO 4 3- ) uptake of juvenile U. pinnatifida sporophytes. Survival declined significantly at elevated temperatures, with less than 50% of individuals surviving after one week, in contrast to >90% survival at lower temperatures. Growth rates fluctuated around zero across most treatments and became consistently negative at higher temperatures. Nutrient uptake rates remained positive at all temperatures and were significantly enhanced at higher temperatures. Overall, our results indicate that elevated temperatures reduce the survival and growth of U. pinnatifida despite sustained nutrient uptake. This physiological sensitivity suggests that ongoing ocean warming may already be limiting its proliferation in the Dutch Delta and potentially in other regions. These findings offer valuable insight for anticipating future invasion dynamics and refining risk assessments under climate change. • Undaria pinnatifida shows restricted proliferation in the Dutch Delta despite decades of presence. • The Dutch U. pinnatifida population may have undergone cold adaptation, exhibiting reduced growth performance at the elevated temperatures. • Enhanced nutrient uptake alone is insufficient to compensate for thermal sensitivity in U. pinnatifida. • Thermal stress acts as a natural bottleneck for the proliferation of U. pinnatifida.