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Morphological responses and interactions of clonal marine macrophytes with their nutrient environment

Final Activity Report Summary - CLONMACMORPH (Morphological responses and interactions of clonal marine macrophytes with their nutrient environment)

The CLONMACMORPH project formed the first year of a three-year project, lasting from 2005 to 2008, which was co-funded by the Portuguese Science and Technology Foundation (FCT). The objectives of this project were to study the interaction between aquatic and sediment nutrient cycling and clonal aquatic macrophytes, both intertidal and subtidal, as well as the consequences of the clonal plant strategy for these interactions. In addition, the strategies of the clonal plants would be compared with the life-history strategies of green ulvoid algae and the driving forces for their growth and biomass dynamics would be studied.

The plant architecture of clonal growth was exhibited by many higher plant species. It basically implied that a plant, i.e. genet, consisted of units, called ramets, which were repeated during growth. Ramets were interconnected through horizontal rhizomes and were further composed of a set of leaves, i.e. the shoot, and a root system. This clonal architecture had clear consequences with respect to life-history traits, competition, etc. Terrestrial plants could use their clonal structure to most efficiently locate and explore soil nutrient micropatches by alternatively investing in new ramet production or in rhizome expansion. In the marine realm, the importance of exploration of sediment nutrient resources was not clear. Seagrass species all shared a common architecture of clonal growth and responses in architecture to nutrient availability were recorded. Many types of seaweed also possessed this clonal integration; however, they were usually ignored in this context. There were very strong indications that rooting macroalgae, such as caulerpa spp. also exhibited these morphological responses; nevertheless, this was never experimentally confirmed.

Due to lack of knowledge on the meaning of clonal growth in seaweeds, the mayor part of the first project year, i.e. the CLONMACMORPH year, was dedicated to the study of the clonally growing green seaweed caulerpa prolifera. The effect of nitrogen (N) load and irradiance on morphology, growth and photosynthetic performance was studied in specimens from the Gulf of Cadiz, in south Spain. Clear effects of N load on morphology were observed. At low N loads, this alga produced significantly more and longer stolons, which were the analogue of rhizomes in higher plants, whereas plants grown at high loads produced more assimilators, as an analogous to shoots. When algae were subjected to combinations of high and low N loads, HN and LN respectively, and irradiance (HL and LL) levels, the highest growth rates were observed in the HNLL treatment, whereas the reverse combination rendered the lowest growth rate. High irradiance and high N load both led to increased biomass allocation to assimilators; at low N, the bulk of the biomass, which was greater than 75 % in the HLLN treatment, was allocated to the stolons.

Despite its prolific nature, c. prolifera should be characterised as a slow-growing but highly nitrophilic alga, which had the capacity to forage for nutrients by allocating biomass to the stolons. It could hence be concluded that this seaweed followed a clonal growth strategy that was used for the optimal acquisition of nutrients and light. This was the first time that this was demonstrated for seaweed, and could thus be considered an important scientific achievement. The results of this study were published in the Marine Ecology Progress Series journal, in issue 298, pages from 101 to 114, in 2005. An attempt to further test the advantages of the clonal growth strategy in comparison with seagrasses was made in the field. Sediment nutrient concentrations were experimentally enriched in an area where the seaweed occurred together with the seagrasses cymodocea nodosa and zostera noltii. However, erosion caused by heavy storms destroyed the field site before the experiment could be terminated. The remaining of the first year was mainly dedicated to research on green opportunistic algae co-occurring with the seagrasses.