Summer primary production of Arctic kelp communities is more affected by
duration than magnitude of simulated marine heatwaves
Abstract
Fjord systems in the Norwegian Arctic are experiencing an increasing
frequency and magnitude of marine heatwaves. These episodic heat stress
events can have varying degrees of acute impacts on primary production
and nutrient uptake of mixed kelp communities, as well as modifying the
biogeochemical cycling in nearshore systems where vast areas of kelp
create structural habitat. To assess the impact of future marine
heatwaves on kelp communities, we conducted a 23 d mesocosm experiment
exposing mixed kelp communities to warming and heatwave scenarios
projected for the year 2100. Three treatments were considered: a
constant warming (+1.8C from the control), a medium magnitude and long
duration heatwave event (+2.8C from the control for 13 d), and two
short-term, more intense, heatwaves (5 d long scenarios with peaks at
+3.9C from the control). The results show that both marine heatwave
treatments reduced net community production whereas the constant warm
temperature treatment displayed no difference from the control. The long
marine heatwave scenario resulted in reduced accumulated net community
production, indicating that prolonged exposure had a greater severity
than two high magnitude, short-term heatwave events. We estimated an
11C temperature threshold at which negative effects to primary
production appeared present. We highlight that marine heatwaves can
induce sublethal effects on kelp communities by depressing net community
production. These results are placed in the context of potential
physiological resilience of kelp communities and implications of reduced
net community production to future Arctic fjord environmental
conditions.