Mediterranean karst springs: diatom biodiversity hotspots under the pressure of hydrological fluctuation and nutrient enrichment

Mediterranean karst springs are affected by strong climatic seasonality, with long, dry and hot summers and are increasingly threatened by anthropogenic pressures. In Sardinia (western Mediterranean Sea, Italy), they are largely unexplored and their biocoenoses are mostly unknown. The diatom flora from two substrates (cobbles and macrophytes) in eight springs of different areas of the island was investigated in summer 2016 and winter 2017. A total of 162 diatom taxa (58 genera) were found of which 27 (17 genera) only on cobbles and 26 (18 genera) only on macrophytes. The most abundant species from both substrates were Achnanthidium minutissimum, Amphora indistincta, Amphora pediculus, Cocconeis euglypta, Planothidium frequentissimum and P. lanceolatum. Overall, 67 taxa (40 genera) were recorded in single sites and some of these taxa showed high affinity with specific environmental conditions. Hydrological stability (water flow permanence), discharge and nutrients were the main environmental factors influencing diatom assemblages. Our results suggest that diatoms can reflect important local factors related to the vulnerability of these spring ecosystems and underline the importance of their preservation both for biodiversity and water quality maintenance

Diatom assemblages from different substrates of the Casteldoria thermo-mineral spring (Northern Sardinia, Italy)

The thermo-mineral springs of Sardinia have historically been the object of several geochemical studies, whereas the biota and ecology have been totally neglected. The aim of this work is to contribute to the knowledge of the diatom microflora and ecology of the Casteldoria spring (northern Sardinia), in the hottest geothermal district of the Island. A total of 132 diatom taxa (56 genera) were found from three investigated substrates: rock (near the water emergence point), cobbles and sediments (in the spring-fed rivulet). Overall, 62 taxa (47% of the total) were common to other thermal springs of the Mediterranean area. Significant differences in the species composition among all substrates were highlighted by the ANOSIM test (global R = 0.554; p = 0.1%). The assemblages from rock formed a separated cluster from assemblages from cobbles and sediments in the nMDS plot. Rock-dwelling assemblages also had a lower species richness, probably due to the higher water temperature. Total suspended solids, discharge, water temperature and bromine were the most correlated environmental variables with diatom species and seem to influence both species composition and structure of assemblages. Some abundant taxa, such as Pinnularia joculata and Rhopalodia operculata, which fit well on high temperature and mineral content, can be considered representative taxa of the studied spring. This study underlines the importance of investigating different substrates to document diatom biodiversity in a more exhaustive way. It contributes to the knowledge of diatoms and ecology in thermo-mineral springs of Sardinia, providing first information on species composition at small-spatial and seasonal scale

Water quality evaluation in Mediterranean lagoons using the Multimetric Phytoplankton Index (MPI): Study cases from Sardinia

1 - Water quality in four Sardinian lagoons (western Mediterranean Sea) was assessed using the Multimetric Phytoplankton Index (MPI), which is consistent with the EU Water Framework Directive. The index was developed using data on phytoplankton abundances, species structure and chlorophyll a concentrations in Venice Lagoon, Italy. 2 - The aim of this study was to test the MPI on a larger geographical scale and across a range of lagoon types. Therefore, it was applied to assess water quality in the Cabras, S'Ena Arrubia, Santa Giusta and Calich lagoons in Sardinia. These lagoons are all "choked", but exhibit a range of sizes and morphometric features. They are directly affected by human activity within the lagoons themselves, such as fisheries, aquaculture and the construction of dams and canals, and are indirectly affected by anthropogenic activities in their catchments, including intensive agriculture, industrial activity and urban development. 3 - The data used in the present study were collected monthly over a period of 4 years (Calich, Santa Giusta and S'Ena Arrubia) to 7 years (Cabras). Samples were collected at three stations at each of the Cabras, Santa Giusta and Calich lagoons, and at two stations at the S'Ena Arrubia Lagoon, providing a total of 220 samples. 4 - The water quality in three of the four lagoons investigated (Cabras, S'Ena Arrubia and Calich) was classified as bad using the MPI. Among these three, water in Cabras Lagoon exhibited the worst condition. Water quality in Santa Giusta Lagoon was classified as poor using the MPI. 5 - Although we present preliminary results that require further verification, the index appears to be a useful tool for assessing the ecological status of typical Mediterranean lagoons. © 2013 University of Salento - SIBA

Diatom biodiversity in Karst springs of Mediterranean geographic areas with contrasting characteristics: Islands vs Mainland

Karst ecosystems are considered as priority environments for the protection of biodiversity on a global scale. This study provides a first comparative analysis of epilithic diatom flora from karst springs in two Mediterranean geographic areas (Spain and Italy) with contrasting characteristics (islands vs mainland). We investigated twenty-three springs with different anthropogenic impact levels once in the winter season between 2007 and 2017 (N = 23). A total of 176 diatom taxa (56 genera) were found of which 101 (44 genera) were observed in single sites. A general good biotic integrity was revealed by structural indices (species richness, diversity and evenness). However, crenophilous species were generally present and abundant in less impacted springs. Comparing islands and mainland, significant differences were found in species composition and diversity (H') based on multivariate analyses (global R = 0.610; p = 0.001) and t-test (t = 2.304; p = 0.031). Discharge and Cl?? were the most significant variables in determining diatom assemblages. Our results confirm the role of springs as multiple ecotones and refuges for rare species and suggest that the geographic insularity may be an important factor in maintaining diatom biodiversity

Multiannual phytoplankton trends in relation to environmental changes across aquatic domains: A case study from Sardinia (Mediterranean Sea)

<p>We investigated multiannual trends in phytoplankton in relation to several environmental drivers. We analyzed ecological data collected during the past two decades from three aquatic sites: an artificial lake, a coastal lagoon, and a marine coastal area. Hydrographic, nutrient, and phytoplankton data were statistically analyzed to detect trends and interactions. In all ecosystems, the chlorophyll <i>a</i> concentration decreased with increasing abundance of small-sized phytoplankton. Phytoplankton dynamics were related to decreased nutrient concentrations in the lake, to dynamics of phosphorus and decreased salinity in the lagoon, and probably to combined top-down and bottom-up processes in the marine gulf.</p