Effect of rising sea-surface temperatures on kelp photosynthesis and respiration in the Arctic
University of Bremen
Kelps (large brown seaweed, Laminariales, Phaeophyceae) dominate many rocky shore coastlines in temperate and polar regions, forming submarine forests that are among the most productive ecosystems on our planet. Due to globally rising sea-surface temperatures, models predict a kelp biomass increase in the future Arctic. However, rising temperatures also accelerate glacial run-off and the introduction of sediments to the fjords. The aim of this study was to gain a mechanistic understanding of the effect of rising temperatures on the photosynthetic and respiration rates of kelps and their light requirements to maintain a positive net photo synthesis. Further, we assessed whether these changes are met by the prevailing underwater light climate in Arctic Kongsfjorden, Svalbard to draw conclusions about the kelp forests future depth distribution.
Set-Up and Study Site
To determine kelp light requirements for a positive net photosynthesis, we measured oxygen concentration changes as response to different light intensities within closed incubation chambers, containing kelp meristematic discs. We used contactless sensor spots OXSP5 and a four-channel Firesting-O2 with an integrated temperature sensor for the measurements. The system required several restarts at low temperatures (3 °C). We avoided this by isolating the Firesting connector box against cold surfaces (e.g. with styrofoam). Further, it was crucial for the measurements to ensure a homogenic water body within the incubation chamber by using magnetic stirrers.
-
A) Overview of the experimental set-up in a climate chamber with defined temperature and light conditions. -
B) Incubation chambers, containing meristematic discs of kelps. -
C) The magnetic stirrer inside the incubation chambers ensured a homogenic oxygen concentration and illumination of the discs.
Results
The optodes were very responsive to oxygen concentration changes, and the method represented an easy way to measure kelp photo-respirometry. At stable light intensities, we found the oxygen concentrations to change linearly over time, indicating that the incubation chambers were neither carbon-dioxide or nutrient limited, nor oxygen super-saturated during our measurements. The change in oxygen concentration of all four channels (used as biological replicates) were within the same range. Responding to increasing temperatures, we detected significant differences in kelp respiration rates and light require-ments in order to maintain a positive net photosynthesis.
-
FIG. 2 Dark respiration rate of two kelp species, Alaria esculenta (purple) and Saccharina latissima (orange), in response to temperature changes. Treatment mean (dots) ± SD and average replicate response (cross) (n=4). Different capital letters indicate significant (p<0.05) differences between the temperature treatments and species.
Conclusion from data and application
We related this dataset to the prevailing underwater light climate in Kongsfjorden. Thereby, we showed distinct spatial differences in the underwater light climate and potential kelp habitat. We found that the kelp depth distribution is likely to be reduced with increasing water temperature, due to changes in their photosynthetic and respiration rates. This study is within the scope of the EU project FACE-IT (grant agreement No 869154).
For further information please contact Sarina Niedzwiedz (sarina@uni-bremen.de).
