Habitat partitioning and first microplastic detection in the Argentarola marine cave (Tyrrhenian Sea, Italy)

Due to their location, at the boundary between the marine and terrestrial domain, marine caves are very sensitive to environmental changes, but, at the same time, they are extremely vulnerable due to the enclosed setting. Despite their hostile conditions, they are reservoirs of biodiversity, and there is a need to improve their knowledge for conservation and protection purposes. This research studied sediment characteristics (grain size and organic geochemistry) and benthic foraminiferal assemblages of the Argentarola cave (Tuscany, Italy) to define cave ecozones and characterize their environmental variability. Because microplastic pollution is considered a potential threat for the cave, its presence was researched in sediment samples; moreover, the tests of agglutinated foraminifera were analyzed through Fourier Transform Infrared Spectroscopy (FTIR) to verify the possible inclusion of these contaminants. Result indicated the presence in the whole cave of fine-grained sediment with a low organic matter content of clear marine origin, pointing to oligotrophic conditions. Microplastic items at low concentrations were found in sediments, mainly constituted by very small (0.1–0.5 mm) polyethylene and polypropylene fragments and fibers. Foraminifera were abundant and diversified with dominant Spirillina vivipara and Patellina corrugata, like in other small Mediterranean marine caves. Despite the scarce variability of environmental parameters, foraminifera developed two distinct assemblages in relation to sediment texture and organic matter content. In the relatively more organically enriched one, two agglutinated deep-water species, Lagenammina difflugiformis and Glomospira charoides, never recorded before in marine caves, were common. The first one included polyethylene items among the particles of the agglutinated test. Then, this species may be considered an early indicator of microplastic pollution. However, this pollution did not influence the Ecological Quality Status, which was found to be high to good when applying the Foram-Ambi index. This study constitutes a new approach to studying marine caves, providing new tools for the environmental assessment and microplastic monitoring of this protected habitat.