Codium fragile subsp. fragile and Sargassum muticum: Non-native seaweeds in a changing environmen
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Non-native species have received increasing scientific and public attention over the last three decades, and have been identified as a threat to biodiversity. However, coastal environments may also be affected by several other anthropogenic stressors, including climate change, eutrophication, overfishing and coastal development. How such stressors interact is difficult to predict and, especially in the marine environment, understudied. In particular, it is uncertain how climate change may facilitate non-native seaweeds. This is important as seaweeds have a key role in coastal temperate ecosystems.
The aim of this thesis was therefore to examine factors affecting the success of two common and widespread non-native seaweeds, and whether environmental change will benefit them. The green seaweed Codium fragile subsp. fragile (Suringar) Hariot and brown seaweed Sargassum muticum (Yendo) Fensholt were chosen as both are canopy-forming species and can be abundant in the infra- and sublittoral in southwestern Norway. They both have their northern limits along the Norwegian coast, which along with their relatively high temperature optima suggests that warming may facilitate them in Norway. However, their success may also be limited by abiotic or biotic conditions which could continue to limit them despite environmental changes occurring.
For C. fragile subsp. fragile, I investigated how abundance and/or local distribution was related to wave-exposure (within a semi-exposed to sheltered range), hard substratum type, disturbance of existing canopy seaweeds, and a native fucoid (Fucus serratus Linnaeus). In terms of environmental change, I examined how increases in minimum and maximum temperatures would affect the growth and abundance of the alga. These factors were assessed by field surveys, with the exception of disturbance and temperature where effects were also/instead assessed experimentally. For S. muticum, I experimentally investigated how its cover was influenced by disturbance of existing canopy seaweeds, and how growth and survival of laterals was affected by competition in canopies with two native seaweeds (F.serratus and the kelp Saccharina latissima C.E. Lane, C. Mayes, Druehl & G.W. Saunders). For environmental changes, I studied whether nutrient enrichment or increased summer temperatures could affect growth, survival or competitive relationships.
Non-native and native subspecies of C. fragile have frequently been misidentified, thus to ensure that my studies were carried out on the non-native subspecies I genetically sequenced populations along the coast and in herbaria. This revealed that subsp. atlanticum is present in Norway, but most C. fragile in southwestern Norway is the non-native subspecies, subsp. fragile. The morphological traits investigated could be variable and overlap between the subsp. atlanticum and subsp. fragile, indicating that these are not reliable for identification here.
With regard to abiotic and biotic factors affecting success, C. fragile subsp. fragile (hereafter C. fragile) was most successful at stony sites (cobble/boulder substratum). This substratum type was associated with higher abundances, higher persistence over time, and deeper lower depth limits than on bedrock, where C. fragile populations tended to be transient and limited to the infralittoral. Exposure also played a role; it did not affect abundance, but C. fragile distribution tended to expand closer to the surface at more sheltered sites. At stony sites C. fragile could persist as the dominant canopy for at least 5 years.
The vertical distributions of C. fragile and Sargassum muticum are mostly infra- and sublittoral, overlapping with those of F. serratus and some native kelp species (commonly Saccharina latissima and Laminaria digitata (Hudson) J.V. Lamouroux). Distribution patterns suggested that C. fragile may limit F. serratus, but only in the infralittoral zone at favourable sites. Disturbance of canopy seaweeds did not lead to an increase in C. fragile or Sargassum muticum cover, but this may be due to the short length of the disturbance, the heterogeneous nature of the infralittoral fringe habitat, and propagule pressure in the case of Sargassum muticum. In mature mixed canopies, Saccharina latissima and Fucus serratus did not limit the growth of Sargassum muticum laterals, but these native species did have a negative effect on survival of the laterals.
Observations suggest that some local factor(s) may limit the growth and condition of Sargassum muticum in summer, and this likely affected the competitive relationships documented here. Data were consistent with cover of filamentous brown epiphytes (Ectocarpales) having a negative impact, but also suggested that S. muticum is nitrogen limited. Nutrient enrichment was associated with improved condition of S. muticum thalli at one site, but not at another where filamentous epiphytes were abundant. More work is needed on this topic.