Distribution of macroalgae in the area of Calvi (Corsica)

Abstract

Macroalgae play an essential role in benthic ecosystems, which makes it very important to monitor their cover rates and study their community structures. A good knowledge on algae distributions of an area can tell us a lot about the dynamics of the ecosystem, its resilience and the pollution in the area. The Bay of Calvi is an obvious choice as a study site because not only does it harbor STARESO, a research station that has years of data on macroalgae distributions and other physical parameters of the region, it also acts as a reference for the study of Mediterranean coastal communities because it harbors all types of coastal habitats of the Mediterranean Sea while at the same time being relatively un-polluted. But due to its evergrowing local tourism and to on-going global climate changes, monitoring and conservation actions should be taken in the near future to prevent the degradation of this pristine area.Marine biodiversity studies are usually led by autonomous divers and often don't generate sufficient data to provide enough material for robust conservation plans. This project was meant to evaluate the potential of a complimentary data acquisition method: annotating videos filmed with an underwater remotely operated vehicle (ROV).A secondary objective was to study the distribution of some important macroalgae in the Bay of Calvi. The focus was put on Cystoseira brachycarpa, an algae species often used as an indicator of unpolluted water. In the Master thesis that produced the dataset of this project, the objective was to study spatio-temporal variations of this algae's distribution in the Bay of Calvi. Nevertheless, the abundance of several other macroalgae, which could be identified on the ROV images, were also noted. By studying our recent macroalgae distributions on the videos, we were able to compare older data and discuss spatio-temporal changes in the cover of several important macroalgae. Furthermore, in order to assess the status of the colonization of the invasive algae Caulerpa cylindracea, the ROV was sent to some deeper parts of the Bay of Calvi. The project, while providing some interesting insights on using a ROV as a tool to study marine biodiversity, generated a dataset of the distribution of 19 macroalgae on both standardized and new transects in the Bay of Calvi (Katz et al. 2021). The species of macroalgae are the following : Acetabularia acetabulum, Amphiroa rigida, Caulerpa prolifera, Caulerpa cylindracea, Codium bursa, Colpomenia sinuosa, Corralinales (order), Cystoseira brachycarpa, Cystoseira crinita, Cystoseira spinosa, Cystoseira zosteroides, Dictyota (genus), Flabellia petiolata, Halopteris (genus), Halopteris scoparia, Jania (genus), Osmundaria volubilis, Padina pavonica, Peyssonnelia squamaria.By comparing this dataset to older data from the same area, we noted that the cover rate of Cystoseira brachycarpa significantly increased since 1991 (after having disappeared north of the Punta de l'Oscellucia between 1979 and 1991). Furthermore, our spatial study showed a lot of heterogeneity in the vertical and horizontal distributions of Cystoseira brachycarpa. This can be explained by variations of the benthic habitat caracteristics such as hydrodynamics and thermal regime, nutrient or pollutant concentration, human activities and substrate type, along with competition with other macroalgae species.The videos also showed that the invasive algae Caulerpa cylindracea has spread significantly between 2016 and 2019, and that more focused studies should be held in the bay to assess the actual reach and possible impacts of this invasion.Finally, our ROV video transects have also underlined the significant presence of particular benthic macroalgae communities over habitat zones described as "soft-bottom" on benthic habitat maps. Although the biomass per unit area of these community is probably lower than for most coastal rocky bottoms, this widely spread habitat type holds a contribution to primary production to be considered in coastal ecosystem models.

Keywords

• macroalgae
• ROV
• Calvi
• <i>Cystoseira</i>
• <i>Caul