Aspects of the ecology of the potentially invasive alien macrophyte Caulerpa taxifolia var. distichophylla (Sonder) Verlaque, Huisman & Procacini in Maltese waters

Abstract

The aim of this study was to obtain information on the ecology of the alien macrophyte Caulerpa taxifolia var. distichophylla in the Maltese Islands, following its introduction in 2013. Three aspects were investigated: the spread of the macrophyte from the initial colonised area, the hypothesis that the alga shows seasonal changes in frond density and frond length and the effect of site characteristics on the frond density, morphology and distribution. The spread of the macrophyte was monitored at 38 different sites around the Maltese islands between June 2016 and September 2016 and between June 2017 and September 2017 either by making observations using SCUBA or through an open call for citizen scientists to report sightings, which were then confirmed. When surveyed by SCUBA diving, frond density was estimated and samples were taken for morphological studies in the lab. Seasonal changes were assessed within a study area measuring 2m X 2m at two different sites which were visited monthly between June 2016 and June 2018 to estimate frond density from 12 replicate quadrat counts. Samples for biomass estimation were collected from a different sites by separating out the different algal species and estimating the biomass of each to determine changes in macrophyte community composition. Patchiness in the distribution of the alga and the morphological variation within a patch were studied at five different sites (Bahar ic-Caghaq, Comino, San Tumas, Sliema and Qawra) from June to September 2016. At each site, density counts, depth and if the alga was located in the centre or periphery, were recorded. Replicate samples of the macrophyte were collected for morphological analysis. The results of these studies indicated that the alga is well established in the Maltese Islands and had spread to at least 22 sites by 2016 and an additional four sites in 2017. Of these sites, 46% were located along the eastern coast of Malta and Gozo where the degree of anthropogenic impact is higher. The least affected coast, with 8% of records was the west coast of the Malta and Gozo probably due to the deep bathymetry and reduced anthropogenic impacts. The results also indicate that the frond density of the macrophyte decreases during the winter months and attains is highest values during autumn months giving a pronounced intra-annual variability. There was also an inter-annual variability in the population as during the second year of the study the macrophyte there was no compete regression of the fronds at both study sites as had been observed during the first year. nMDS results indicated that there were no changes in species abundance relative to the algal occurrence. Of the five sites in which studies of the algal patchy distribution were conducted, the highest frond density was at Bahar ic-Caghaq whilst the longest fronds were recorded from Comino. Linear regression analysis indicated a weak relationship between depth and both frond density and frond length. However, PERMANOVA indicated that the factor ‘site’, affects both frond density and frond length whilst the type of substratum and associated flora affects only frond density. Of the microhabitat types a change in population structure was only shown for the ‘mosaic’ microhabitat, where a statistically different mean frond density value was obtained between the periphery and centre of a ‘mosaic’ patch, whilst for the microhabitats ‘Posidonia oceanica’ and ‘rock with sediment cover’ no statistically significant differences were found. Caulerpa taxifolia var. distichophylla has established itself successfully in Malta, colonising areas with high anthropogenic factors that have facilitated its dispersal. Whilst no changes in community biomass were observed, the alga is taking up space that would be otherwise be made available to native species. The species can be considered as ‘invasive’ given the exponential growth it has attained.