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Presentation Abstract
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°C. Gametophytes were able to germinate at temperatures up to 20°C, with gametophyte growth peaking around 16°C, but sporophyte production ceased above 16°C. Second, we used a temperature-controlled lab experiment to determine the effects of temperature (12, 16 & 20°C) 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°C, but similar among 12 and 16°C treatments. Respiration rates were higher at 20°C, 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°C, 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.
Session Title
Ocean Productivity & Nutrients
Conference Track
SSE3: The Circle of Life
Conference Name
Salish Sea Ecosystem Conference (2022 : Online)
Document Type
Event
SSEC Identifier
SSE-traditionals-325
Start Date
26-4-2022 9:45 AM
End Date
26-4-2022 11:15 AM
Genre/Form
conference proceedings; presentations (communicative events)
Subjects – Topical (LCSH)
Nereocystis luetkeana--Effect of temperature on--Salish Sea (B.C. and Wash.); Plants--Effect of nitrogen on--Salish Sea (B.C. and Wash.)
Geographic Coverage
Salish Sea (B.C. and Wash.)
Rights
Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.
Type
Text
Language
English
Format
vnd.ms-powerpoint
Included in
Fresh Water Studies Commons, Marine Biology Commons, Natural Resources and Conservation Commons
Effects of heat stress and nitrogen limitation on the physiology of gametophytes and sporophytes of the bull kelp, Nereocystis luetkeana
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°C. Gametophytes were able to germinate at temperatures up to 20°C, with gametophyte growth peaking around 16°C, but sporophyte production ceased above 16°C. Second, we used a temperature-controlled lab experiment to determine the effects of temperature (12, 16 & 20°C) 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°C, but similar among 12 and 16°C treatments. Respiration rates were higher at 20°C, 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°C, 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.
