Effects of heat stress and nitrogen limitation on the physiology of gametophytes and sporophytes of the bull kelp, Nereocystis…

Bull kelp (Nereocystis luetkeana) are declining at some locations within the Salish Sea, particularly in areas that experience elevated temperatures, low nitrogen (N) concentrations, and low current velocities. High temperatures and low N concentrations can be stressful for marine primary producers, yet these stressors often occur simultaneously in temperate marine ecosystems, making it difficult to unravel their effects. First, we determined the reproductive thermal tolerance limits of bull kelp by growing gametophytes at 10, 12, 14, 16, 18, 20 and 22 degreesC. Gametophytes were able to germinate at temperatures up to 20 degreesC, with gametophyte growth peaking around 16 degreesC, but sporophyte production ceased above 16 degreesC. Second, we used a temperature-controlled lab experiment to determine the effects of temperature (12, 16 & 20 degreesC) and N concentrations (2 & 30 µM NO3) on the physiology of bull kelp adult sporophyte blades. After 7 days, we found significant effects of temperature on blade growth and metabolic rates, and significant effects of N concentrations on nutrient uptake rates. Blade elongation rates were lowest at 20 degreesC, but similar among 12 and 16 degreesC treatments. Respiration rates were higher at 20 degreesC, suggesting elevated metabolic demand, but photosynthesis rates did not increase to meet this demand. Nutrient uptake measurements performed in N-rich seawater revealed that N-depleted kelp assimilated NO3 at a significantly faster rate compared to N-enriched kelp. After 7 days at 20 degreesC, N-depleted blades began to degrade, preventing us from assaying this treatment, while N-enriched kelp remained intact, suggesting that low N availability may be more stressful near their upper thermal limits. Taken together, these results suggest that bull kelp populations in warm and nutrient-poor areas, including South Puget Sound during the summer, may be at risk due to the impacts of these stressors.