Luka Wright1, Markus Molis2, Juan Vicente de Miguel2, Anea Rakvåg2, Thomas Wernberg1,3, Karen Filbee-Dexter1,3
1University of Western Australia, Australia - 2UiT Norges Arktiske Universitet, Norway - 3Institute of Marine Research, Norway
Sea urchin barrens form alternative states to kelp forests and occur in every ocean. Yet the ecological function of urchin barrens in the kelp carbon cycle has received much less attention than overgrazing. On one hand, urchins undoubtedly diminish the total organic carbon pool by consuming fresh kelp from fringing forests as well as detritus exported to the barrens. On the other hand, they may act as detritus shredders, enabling longer suspension times and long-distance kelp carbon transport, thereby potentially increasing carbon export and sequestration. Here, we aim to better understand the role of urchins, using fringing Laminaria hyperborea forests located next to Strongylocentrotus droebachiensis barrens as a model system. Ex situ feeding experiments were conducted in spring and autumn to estimate kelp absorption and defecation rates. To test equivalence of urchin faeces and kelp, we then measured carbon, nitrogen and phenol content, microbial enzyme activity and feeding preference by amphipods. We found that urchins do not perform the role of detritus shredders. They absorb a substantial proportion of consumed kelp, with only 53% (39% in autumn, 67% in spring) of biomass transferred as faeces. When re-expressed in terms of carbon, the transferred fraction becomes 32% since the carbon content of faeces is 15% lower than that of fresh kelp in absolute terms. Beyond this immediate effect, urchin faeces also contained more nitrogen and practically no phenols, which we found made them more palatable to microbes and secondary grazers. Our data suggest that the carbon sequestration potential arguably gained by long-distance export of urchin faeces is outweighed by the carbon lost to absorption, biochemical transformation and consequent increased secondary remineralisation. Since most urchin barrens capture and transform all detritus exported from fringing forests, this implies that the role of urchins in the kelp carbon cycle exacerbates the negative effects of urchin barrens.
Biography
Luka Seamus Wright is a chemical and microbial ecologist primarily interested in marine plant systems. Having graduated from the University of Plymouth’s Marine Biology and Coastal Ecology programme with a first class Honours, Luka went on to pursue research at Queen’s University’s Portaferry Marine Laboratory and commenced his PhD at the University of Western Australia’s Oceans Institute in early 2022. Luka has published 12 papers (58% first author, 33% second author, 17% single author).