Invasive seaweeds in rising temperatures: impacts and risk assessments

Europe is a hot-spot for aquatic introductions with around 600 alien species recorded at present. Introduction of Invasive Alien Species (IAS) is regarded a serious threat to European biodiversity and ecosystems. Introduced seaweeds represent one of the largest groups of marine aliens in Europe, and constitute between 20 and 29 % of all alien marine species. Seaweeds are major primary producers in coastal areas, and are extremely important for coastal ecosystems by supporting high biodiversity through structuring complex habitats for associated species. Large-scale substitution of dominant native seaweeds with alien species will consequently alter coastal productivity and food web structure, and therefore impact ecosystem services. Little is known about effects of alien seaweeds in Europe, or how temperature variation influences the relationships between alien and native seaweeds. The aim of the project is to assess present and future impacts of invasive alien seaweeds on the North-Atlantic coastal biodiversity, by addressing six different main objectives. The project aims specifically at predicting the effects of alien seaweeds under climate variability and rising sea surface temperatures in the North-Atlantic. The project consortium includes seven different European partners, where two are Norwegian institutes. The main objective of the Norwegian partners is to investigate the ecological processes responsible for substituting native seaweeds with invasive ones. Project work has consisted of field studies and laboratory experiments on the invasive seaweeds Codium fragile subsp. fragile and Sargassum muticum. Four stations with transect clearings were initiated in the field during 2013 and 2014, at sites where both native habitat forming algae (Laminaria spp., Saccharina latissima, Fucus serratus) and the invasive C. fragile subsp. fragile were present. Regrowth of native and alien seaweeds in the cleared transects has been monitored during visits to the stations. Additional transects have been monitored as controls. In addition the abundance of C. fragile subsp. fragile and other introduced seaweeds have been monitored annually at a total of 50 stations during the project, in order to record large scale changes related to environmental factors. A field experiment on the competitive relationships between the native habitat forming seaweeds F. serratus, S. latissima (sugar kelp) and the invasive S. muticum was carried out during 2014, and was repeated during the summer of 2015 in contrasting temperatures. Growth and survival of the three species alone or in combinations in constructed patches was investigated after three months in each experiment. While summer temperatures during 2014 were extraordinary high, the summer of 2015 had lower and more normal sea temperatures. High deterioration and mortality was seen of both S. latissima and S. muticum during 2014, while S. latissima showed normal growth and production during the summer 2015. Laboratory experiments on growth of C. fragile subsp. fragile in different temperatures have continued during 2015, and a 3 degree increase related to both normal winter and summer temperature have shown a positive effect on growth. In addition surveys have been carried out to assess the effect on local biodiversity of habitat structuring introduced seaweeds their associated fauna is being investigated. The fauna associated to C. fragile subsp. fragile have been recorded from three different sites and compared with the one on the native F. serratus. In addition, the fauna associated to S. muticum have been identified and analyzed.

Europe is a hot-spot for aquatic introductions with around 600 alien species recorded at present. The assessment of present and future impacts of invasive alien seaweeds on the North-Atlantic coastal biodiversity is a challenge. The project aims specifica lly at predicting the effects of alien seaweeds under climate variability and rising sea surface temperatures in the North-Atlantic. This will be done by addressing temperature effects on expansion rate and range shifts, abundance and ecological functioni ng of invasive or potentially invasive alien seaweeds. The project consortium represents seven different partners from five countries, covering the North-Atlantic European coast from Norway to Portugal. By using a combination of modelling, field studies, ecological experiments and molecular work, we will study the impact of alien seaweeds on the native biodiversity under variable climatic conditions. We will focus on case studies of selected seaweeds currently regarded as invasive or potentially invasive. The scientific objectives are organized in six Work Packages representing six objectives: Finding the new pathways for alien seaweeds to European coasts, performing species distribution by developing spatial modelling, examining processes related to spac e monopolization by invasive seaweeds, examining impact of invasive seaweeds on aquatic macrophyte associated species composition, studying acclimation and adaptation processes of invasive seaweeds and studying biochemical adaptation under different tempe ratures of invasive seaweeds and their impact on native marine organisms. The first two objectives are related to introduction vectors and spreading of alien seaweeds under changing climate conditions, the next ones concerns effects and impacts of alien s eaweeds under changing climate conditions, and the two last objectives are related to adaptation and acclimation of alien seaweeds and their impact on native communities.