Mediterranean Transitional Waters Research Articles

Dense Water Formation Variability in the Aegean Sea from 1947 to 2023

The formation of dense water in the Aegean Sea is important as it affects the deep circulation and the hydrography of the Eastern Mediterranean Sea. In this study, the variability of dense water formation is investigated in relation to forcing mechanisms from 1947 to 2023 in the subbasins of the Aegean Sea, utilising in situ observations from various sources, which have been analysed in combination with satellite altimetry and reanalyses products. The analysis reveals that the Aegean Sea has been in a state of increased dense water formation since 2017 due to the combination of increased surface buoyancy loss and reduced Black Sea water inflow. Extremely high salinity has been recorded in the intermediate layers of the Aegean Sea since 2019. The anticyclonic circulation of the North Ionian gyre during 2017 and 2018 probably also contributed to the rapid transport of highly saline waters in the intermediate and, through dense water formation, the deep layers of the Aegean Sea in 2019. Until 2022, the dense waters formed during the peak of the Eastern Mediterranean Transient still occupied the bottom layers of some deep subbasins of the North and South Aegean; however, the 29.4 kg m−3 isopycnal in the North Aegean and the 29.3 kg m−3 isopycnal in the Southeastern Aegean have gradually deepened by 800 m, permitting the waters forming in the last ten years in the Aegean Sea to settle at ever greater depths. Temperature controls the density variability of the Cretan intermediate water up to the decadal time scale. Increased data availability since 2010 was sufficient to clarify that intrusions of dense water from the North–Central Aegean Sea contributed to the erosion of the Eastern Mediterranean transitional waters in the South Aegean Sea after 2017, as well as to raising the intermediate water masses of the South Aegean to shallower depths. The erosion of the transitional Mediterranean waters in the South Aegean Sea between 1947 and 1955 and 1973 and 1980 coincided with increased dense water formation in the North–Central Aegean Sea. During the peak of the Eastern Mediterranean Transient, the North Ionian circulation, the Black Sea water inflow, the Atlantic Multidecadal Oscillation, and the surface buoyancy fluxes favoured dense water formation in the Aegean Sea.

Assessing benthic macroinvertebrate communities’ spatial heterogeneity in Mediterranean transitional waters through eDNA metabarcoding

Transitional waters are important habitats both for biodiversity and ecological functions, providing valuable natural resources and relevant ecosystem services. However, they are highly susceptible to climate changes and anthropogenic pressures responsible for biodiversity losses and require specific biomonitoring programs. Benthic macroinvertebrates are suitable as ecological indicators of transitional waters, being affected by biological, chemical, and physical conditions of the ecosystems about their life cycles and space-use behaviour. The advent of high-throughput sequencing technologies has allowed biodiversity investigations, at the molecular level, in multiple ecosystems and for different ecological guilds. Benthic macroinvertebrate communities’ composition has been investigated, at the molecular level, mainly through DNA extracted from sediments in marine and riverine ecosystems. In this work, benthic macroinvertebrate communities are explored through eDNA metabarcoding from water samples in a Mediterranean transitional water ecosystem. This research highlighted the validity of eDNA metabarcoding as an efficient tool for the assessment of benthic macroinvertebrate community structure in transitional waters, unveiling the spatial heterogeneity of benthic macroinvertebrate communities correlated to the measured environmental gradients. The results suggest that peculiar features of transitional water ecosystems, such as shallow waters and limited currents, facilitate the assessment of benthic macroinvertebrate communities through environmental DNA analysis from surface water samples, opening for more rapid and accurate monitoring programs for these valuable ecosystems.

Environmental DNA detects biodiversity and ecological features of phytoplankton communities in Mediterranean transitional waters

Climate changes and anthropogenic pressures are causing a biodiversity decline in terms of species number and genetic diversity, reducing the adaptability and evolvability of natural communities. Transitional water ecosystems are more sensitive to habitat reduction and degradation and, thus, are more exposed to biodiversity declines requiring biodiversity monitoring programs for their conservation. Environmental DNA (eDNA) metabarcoding represents a high-throughput tool for biodiversity assessment that is facilitating data collection for biodiversity monitoring. In this study, we applied, for the first time, eDNA metabarcoding in a Mediterranean coastal lagoon to assess the ecological features of eukaryotic phytoplankton communities. We sampled water in seven different lagoon sites and amplified the extracted DNA with primers targeting the variable region 4 (V4) of the 18S rRNA gene marker. The results demonstrated the validity of eDNA studies to provide insights into lagoon phytoplankton composition, establish the structure and spatial variation of phytoplankton communities, and evaluate its correlation to abiotic factors. Finally, the genetic distances analysis suggests that the different spatial distribution of OTUs, at least for the Tetraselmis genus, reflects the genetic background.

Non-indigenous aquatic fauna in transitional waters from the Spanish Mediterranean coast: A comprehensive assessment

Understanding drivers of spatial variation in non-indigenous species (NIS) is a key goal in invasion biology, but comprehensive assessments providing high-resolution data are extremely scarce. Anthropogenic modifications to transitional waters facilitate the invasion of NIS where they cause both ecological and economic important damage. By screening validated data sources, we conducted a comprehensive assessment of non-indigenous aquatic fauna in Spanish Mediterranean transitional waters (30 sites), as well as assessed introduction pathways, native regions, NIS assemblage patterns and temporal introduction rate. One hundred and twenty-nine NIS were inventoried, with 72 % established and more than half listed before 1980. Two intentional (release, escape) and two unintentional (contaminant, stowaway) introduction pathways were dominant. Recorded NIS originated mostly from North America and Asia. A clear nested pattern in NIS assemblages was observed across sites, suggesting secondary spread from the most invaded waters placed in the northern regions. Our updated inventory should be pivotal for designing prevention protocols and informing specific management plans on non-indigenous fauna in transitional waters.

Joint use of biological traits, diversity and biotic indices to assess the ecological quality status of a Mediterranean transitional system

In the present study, we tested the effectiveness of biological trait analysis (BTA) as an ecological indicator in support of the assessment of the Ecological Quality Status (EQS) of a Mediterranean transitional system by commonly used macrobenthic assemblage diversity (e.g. H’ and Margalef) and biotic (BENTIX, M−AMBI and M−bAMBI) indices. Ten biological traits describing morphological, behavioral and life history characteristics were considered for the analysis of the functional structure of macrobenthos. As a test study, a historical dataset obtained from the Cabras lagoon (Sardinia, Italy) comprising a riverine site, the lagoonal main body, a confined pond, a seaward creek and a marine channel connected to the Gulf of Oristano was analyzed. Our results showed clear spatial changes in macrobenthic density, number of species and diversity indices along the riverine-lagoon-sea gradient, which were well reflected in the EQS values based on M−AMBI and to a lesser extent on M−bAMBI. The abundance-based functional diversity was significantly related to the structure and taxonomic diversity of the benthic community. Similarly, the Community-level Weighted Mean test computed with abundance data, rather than with biomass data, supported major evidence for differences in the ecological conditions of the different study sites. In particular, all riverine, lagoonal and confined stations had poor or bad EQS, indicating unstable and warning conditions in this Mediterranean transitional water body. Overall, the BTA approach appeared to be a very promising method in reporting the expected environmental and biotic variability in the Cabras lagoon system and in supporting the EQS assessment by M−AMBI. We conclude that an in-depth knowledge of the benthic communities in terms of both taxonomic and functional aspects is an indispensable basic tool for understanding the variability of the brackish-water systems and for properly assessing their environmental quality.

Testing the Influence of Incomplete DNA Barcode Libraries on Ecological Status Assessment of Mediterranean Transitional Waters.

Simple SummaryThe biodiversity and ecological status assessment of transitional water ecosystems by benthic macroinvertebrates investigation could use DNA barcode tools for more rapid and efficient outputs. The principal limits of this application are the incompleteness of DNA barcode databases, the identification of optimal primers set, and the gap in the species sequences. The influence of the incompleteness of DNA barcode libraries on species diversity indices, ecological indicators, and ecological status assessment in transitional waters of the southeast Mediterranean were analysed, underlying the importance to implement DNA barcode libraries and to put an effort toward specific species at a local level.The ecological assessment of European aquatic ecosystems is regulated under the framework directives on strategy for water and marine environments. Benthic macroinvertebrates are the most used biological quality element for ecological assessment of rivers, coastal-marines, and transitional waters. The morphological identification of benthic macroinvertebrates is the current tool for their assessment. Recently, DNA-based tools have been proposed as effective alternatives. The main current limits of DNA-based applications include the incompleteness of species recorded in the DNA barcode reference libraries and the primers bias. Here, we analysed the influence of the incompleteness of DNA barcode databases on species diversity indices, ecological indicators, and ecological assessment in transitional waters of the southeast Mediterranean, taking into account the availability of commonly sequenced and deposited genomic regions for listed species. The ecological quality status assigned through the potential application of both approaches to the analysed transitional water ecosystems was different in 27% of sites. We also analysed the inter-specific genetic distances to evaluate the potential application of the DNA metabarcoding method. Overall, this work highlights the importance to expand the barcode databases and to analyse, at the regional level, the gaps in the DNA barcodes.

Successions of phytobenthos species in a Mediterranean transitional water system: the importance of long term observations

The availability of quantitative long term datasets on the phytobenthic assemblages of the Mar Piccolo of Taranto (southern Italy, Mediterranean Sea), a lagoon like semi-enclosed coastal basin included in the Italian LTER network, enabled careful analysis of changes occurring in the structure of the community over about thirty years. The total number of taxa differed over the years. Thirteen non-indigenous species in total were found, their number varied over the years, reaching its highest value in 2017. The dominant taxa differed over the years. The number of species in each taxonomic division also varied. In addition to the centuries-old exploitation of its biotic resources, mainly molluscs, the basin has been subject for a long time to a range of anthropogenic driving forces linked to urbanisation, shipbuilding, agriculture and military activities, which have caused chemical and biological pollution, eutrophication and habitat destruction. It may therefore be assumed that these changes were closely related to human activities. Indeed, it was observed that the reduction of only one of these forces, i.e. urban sewage discharge, triggered the recovery of phytobenthos. Therefore, it may be assumed that if the anthropogenic pressure on the Mar Piccolo was eased, it could once again become the paradisiacal place it was held to be in ancient times.

Assessing the effectiveness of surrogates for species over time: Evidence from decadal monitoring of a Mediterranean transitional water ecosystem

The use of higher taxa or alternative approach to species surrogacy, such as the BestAgg procedure, could represent cost-effective solutions to avoid expensive species-level identifications in monitoring activities, especially on the long term. However, whether a set of surrogates would be effective in subsequent reiteration of the same assessment remains largely unsolved. We used a long-term dataset on macro-benthic assemblages to test the hypothesis that family-level and BestAgg surrogates which are effective for a limited period of monitoring could be successfully applied to quantify community patterns also in subsequent monitoring programmes. The effectiveness of surrogates in detecting temporal variations in assemblage structure as at species level remained basically unaffected over a decade. Recognizing once and for all if species surrogacy may have a practical value for monitoring will strongly depend on future assessments of the potential of surrogates to reflect community changes and to retain this prerogative over time.

Population Dynamics and Reproduction of Mediterranean Green Crab Carcinus aestuarii in Parila Lagoon (Neretva Estuary, Adriatic Sea, Croatia) as Fishery Management Tools

AbstractPopulation structure, age, growth, mortality, and reproduction patterns of the Mediterranean green crab Carcinus aestuarii were determined for the native population in Parila Lagoon (Neretva Estuary, Middle Adriatic, Croatia). The population size structure showed two distinct cohorts: (1) specimens with a carapace width of 20–34 mm and dominated by females and (2) large‐sized specimens with a carapace width > 34 mm with males significantly dominating and no females found above 46 mm. Males appeared to grow faster than females in the first and second year of the life cycle. Most of the natural mortality (70.4%) occurred during the first year of life. This indicates high predation pressure from fish and other crab species on small‐sized (less than 25 mm) C. aestuarii cohorts. The peak of ovigerous female occurrence occurred in January 2015. A very small percentage of ovigerous females appeared in June 2015. The 50% ovigerous size for the population was estimated at a carapace width of 29.65 mm and weight of 10.39 g. The average fecundity was 61,017 eggs/female, with an average fecundity per gram of female wet weight of 4,804 eggs. The biological characteristics and population dynamics of C. aestuarii show that life history traits of this species (such as the smallest and average ovigerous female size, the mortality rate during the first year of life or the establishment of a second year, and a late‐spring reproduction period) should be used to monitor potential changes in ecosystem properties of Mediterranean transitional waters and to manage potential fisheries.

Fate of two invasive or potentially invasive alien seaweeds in a central Mediterranean transitional water system: failure and success

AbstractOver the past 15 years, populations of two invasive or potentially invasive alien seaweeds have been studied by means of both quantitative and qualitative methods in the Mar Piccolo basin in Taranto (Southern Italy, Mediterranean Sea). The dynamics of the cold-temperate brown seaweed

Stable isotope-based statistical tools as ecological indicator of pollution sources in Mediterranean transitional water ecosystems

N-stable isotope analysis of macroalgae has become a popular method for the monitoring of nitrogen pollution in aquatic ecosystems. Basing on changes in their δ15N, macroalgae have been successfully used as biological traps to intercept nitrogen inputs. As different nitrogen sources differ in their isotopic signature, this technique provides useful information on the origin of pollutants and their extension in the water body. However, isotopic fractionation potentially resulting from microbial nitrogen processing, and indirect isotopic variations due to effects of physicochemical conditions on algal nutrient uptake and metabolism, may affect anthropogenic N isotopic values during transportation and assimilation. This in turn can affect the observed isotopic signature in the algal tissue, inducing isotopic variations not related to the origin of assimilated nitrogen, representing a “background noise” in isotope-based water pollution studies.In this study, we focused on three neighbouring coastal lakes (Caprolace, Fogliano and Sabaudia lakes) located south of Rome (Italy). Lakes were characterized by differences in terms of anthropogenic pressure (i.e. urbanization, cultivated crops, livestock grazing) and potential “background noise” levels (i.e. nutrient concentration, pH, microbial concentration). Our aim was to assess nitrogen isotopic variations in fragments of Ulva lactuca specimens after 48h of submersion to identify and locate the origins of nitrogen pollutants affecting each lake. δ15N were obtained for replicated specimens of U. lactuca spatially distributed to cover the entire surface of each lake, previously collected from a benchmark, unpolluted site. In order to reduce the environmental background noise on isotopic observations, a Bayesian hierarchical model relating isotopic variation to environmental covariates and random spatial effects was used to describe and understand the distribution of isotopic signals in each lake.Our procedure (i) allowed to remove background noise and confounding effects from the observed isotopic signals; (ii) allowed to detect “hidden” pollution sources that would not be detected when not accounting for the confounding effect of environmental background noise; (iii) produced maps of the three lakes providing a clear representation of the isotopic signal variation even where background noise was high. Maps were useful to locate nitrogen pollution sources, identify the origin of the dissolved nitrogen and quantify the extent of pollutants, showing localized organic pollution impacting Sabaudia and Fogliano, but not Caprolace. This method provided a clear characterization of both intra- and inter-lake anthropogenic pressure gradients, representing a powerful approach to the ecological indication and nitrogen pollution management in complex systems, as transitional waterbodies are.

Towards the implementation of the Water Framework Directive in Mediterranean transitional waters: the use of macroinvertebrates as biological quality elements

During the last decade the Water Framework Directive (WFD) has driven scientific community endeavours towards the development of assessment tools to determine the Ecological Quality Status (EQS) for all surface waters, including transitional waters (TWs). Macroinvertebrates being used as Biological Quality Elements encouraged the development of distinct multimetric and multivariate indices, initially based on taxonomic approaches. Those indices were mostly developed for the marine environment and applied extensively on TWs. The main discrepancies in the ecological quality status assessment arise on TWs, partially due to the difficulty in discriminating the effects of natural stress from anthropogenic impact. As a response, indices following functional approaches are being developed and applied in assessing the EQS in these environments. Next, the validation and intercalibration of the metrics as well as the settlement of reference conditions are additional sources of variability inherent to any assessment...

Towards the implementation of the Water Framework Directive in Mediterranean transitional waters: the use of macroinvertebrates as biological quality elements

During the last decade the Water Framework Directive (WFD) has driven scientific community endeavours towards the development of assessment tools to determine the Ecological Quality Status (EQS) for all surface waters, including transitional waters (TWs). Macroinvertebrates being used as Biological Quality Elements encouraged the development of distinct multimetric and multivariate indices, initially based on taxonomic approaches. Those indices were mostly developed for the marine environment and applied extensively on TWs. The main discrepancies in the ecological quality status assessment arise on TWs, partially due to the difficulty in discriminating the effects of natural stress from anthropogenic impact. As a response, indices following functional approaches are being developed and applied in assessing the EQS in these environments. Next, the validation and intercalibration of the metrics as well as the settlement of reference conditions are additional sources of variability inherent to any assessment. This paper aims at briefly presenting the different steps needed for the implementation of WFD on Mediterranean TWs. It highlights existing difficulties and possible research lines to be explored in order to reduce sources of variability and better assess the status of such water bodies.

Influence of sampling effort on ecological descriptors and indicators in perturbed and unperturbed conditions: A study case using benthic macroinvertebrates in Mediterranean transitional waters

The effectiveness and accuracy of biomonitoring programs, based on benthic macroinvertebrates, is strictly related to the sampling design and effort, whereas the feasibility depends on the economic sustainability of sample collection and processing methodologies. In the last decade, how to improve the Rapid Bioassessment Protocols (RBPs) maintaining the accuracy of the results has been a topic recurrently debated among researchers. It is well known that the sample unit size (i.e., surface of the sampled area, SUS) and the sieve mesh size (SMS), selected to collect and to retain benthic macroinvertebrates from soft-bottom samples, may affect the evaluation of the aquatic ecosystem ecological status; however, studies analyzing the combined influence of SUS and SMS on assessment tools are lacking, in particular for transitional water ecosystems. Even if the Water Framework Directive (WFD) suggests rapid and cost-effectiveness sampling effort and procedures, the identification of optimal SUS and SMS is a basic step to improve the RBPs and to meet WFD suggestions. Therefore, this research analyses the effects of four soft-bottom sample unit sizes (0.0225m2, 0.0450m2, 0.0675m2, 0.0900m2), and three sieve mesh sizes (4mm2, 1mm2, 0.25mm2) on the selection of benthic macroinvertebrates and, thus, on assessment tools, in a Mediterranean lagoon. A sampling survey was performed in September 2009 at a perturbed and an unperturbed study site in the Lesina lagoon (SE Italian coastline); three replicates were taken for each SUS and SMS using an Ekman–Birge grab (15cm×15cm). The samples were sieved on a column of three sieves, with decreasing mesh size. Benthic macroinvertebrates were sorted, identified, measured, weighted and included in twelve datasets (4 SUS×3 SMS). Sampling effort (SE) was calculated for each SUS and SMS combination as: SE=[SUSm2×(1/SMSmm2)]×100. Four simple community descriptors (numerical density, taxonomic richness, biomass density, individual body-size) and four ecological indicators (AMBI, BENTIX, BITS, M-AMBI) were compared for each combination of SUS and SMS in both study sites. Simple community descriptors and ecological indicators varied significantly between perturbed and unperturbed study site. The results showed that SMS had significant effects on simple community descriptors and ecological indicators, except for BITS index. Conversely, no significant differences were observed for different SUS analyzing simple community descriptors and ecological indicators, except for taxonomic richness and M-AMBI index. The response of the ecological indicators was only slightly affected by the SMS, whereas SUS choice did not influence the ecological status assessment. Anyway, using the larger SMS (4mm2), all ecological indicators showed either the same ecological quality status as the 1mm2 and 0.25mm2 SMS or, in some cases, one class higher, except for the AMBI index.

Phenotypic plasticity in habitat use and growth of the European eel (Anguilla anguilla) in transitional waters in the Mediterranean area

AbstractLagoons and estuaries are transitional waters (TW), saline in character but substantially influenced by freshwater (FW) flows and the most productive habitat after upwelling areas. The European eel, Anguilla anguilla, is a typical inhabitant of these habitats and a target of important lagoon fisheries since ancient times. Notwithstanding this, in the Mediterranean region, where the numerous coastal lagoons are the most eligible habitat for this species, eel habitat use and growth, which display a high inter‐individual variability, are poorly studied. To gain knowledge about the migratory behaviour and the relative growth history of this species in TW in the Mediterranean area, the otolith Sr/Ca ratios of 56 individuals were analysed. The study sampling sites were two typical coastal lagoon environments, Caprolace lagoon and Lesina lagoon, located, respectively on the Tyrrhenian and the Adriatic coast of Italy, and the Tiber River (TR) estuary, in Central Italy. Otolith Sr/Ca profiles revealed that in all the sites, the resident contingent is substantial, while the proportion of nomad eels, or habitat inter‐shifter, was different among sites. Mean annual grow rate of resident eels is higher in productive environments (TR and lagoon of Lesina), while in Caprolace lagoon, an oligotrophic lagoon, resident eels grow slower. The observed patterns of habitat use and growth seem linked to local ecological conditions: facultative movements of eels with a nomadic behaviour seem affected by food availability rather than by the salinity gradient. This consideration supports the hypothesis that the facultative catadromous migration of European eel in Mediterranean TW, and the trophic shifts that this species exhibits, depend primarily on the productivity of the environment rather than on its salinity gradient.

Cell biovolume and surface area in phytoplankton of Mediterranean transitional water ecosystems: methodological aspects

1 - A series of experimental researches highlighted the role of morphometric parameters of phytoplankton guilds as descriptors of the ecological status of transitional water ecosystems (TWs). 2 - However, at present, standardized or common methodologies for their use do not exist. In this work, we develop a procedure for the determination of biovolume and surface area in phytoplankton guilds of Mediterranean TWs. 3 - Phytoplankton biovolume and surface area are included among the most studied morphometric descriptors. They can be estimated by associating the algae with similar geometric forms and determining the volume of these by measuring the linear dimensions required for its calculation under the light microscope. 4 - Here, a set of geometric models is suggested for calculating the cell biovolumes and surface area of 235 phytoplankton genera, deriving from the analysis of 869 phytoplankton species, found in transitional water ecosystems of the Mediterranean Ecoregion. The equations were designed to minimize the effort of microscopic measurements. 5 - The similarities and differences between the geometric models here proposed and previously published are discussed.

Estimation of benthic macroinvertebrates taxonomic diversity: testing the role of sampling effort in a Mediterranean transitional water ecosystem

1 - The accurate evaluation of benthic macroinvertebrate taxonomic diversity in transitional water ecosystems is strictly related to sampling effort and, usually, biomonitoring protocols define the sampling effort needed to the elaboration of a specific ecological indicator. The time-lag between the sampling event and the final assessment of ecological status, and to overall costs for sampling, personnel and sample treatment suggest a reduction of sampling effort. 2 - How to simplify methods and to reduce efforts without compromising the ecological validity of taxonomic diversity indicators is a topic recurrently debated in the procedures for sampling protocol implementation. Regarding this topic, the identification of optimal sample unit size (SUS) and sieve mesh size (SMS) is still lacking, mainly for benthic macroinvertebrates of Mediterranean transitional water ecosystems. 3 - The present study analyzes the effect of the increasing the sampling effort in terms of sample unit size (SUS; 0.0225 m2, 0.0450 m2, 0.0675 m2, 0.0900 m2) and sieve mesh size (SMS; 0.5 mm, 1 mm, 2 mm) on the estimation of taxonomic diversity in a Mediterranean lagoon. Benthic macroinvertebrates were collected in September 2009 at two locations, considering a perturbed and a relatively unperturbed study site of Lesina lagoon (South-East Italy). Samples were sieved on a column of three decreasing mesh sizes of sieves. Taxonomic richness (S), Shannon–Weaver index (H’), Simpson index (λ) and Taxonomic distinctness (TD) were calculated for each study site, SUS and SMS combination, and replicate. The difference between perturbed and relatively unperturbed site was tested according to the variation of sampling effort using three-way ANOVA tests. 4 - As expected, the accuracy of the results increased with increasing of SUS and SMS, the difference between perturbed and relatively unperturbed study site were always highlighted by each taxonomic diversity index, independently by used SUS and SMS. The variation of taxonomic diversity indicators seems to depend mainly by used sieve mesh size suggesting the reduction of sampling effort through the reduction of sample unit size. 5 - Finally, this contribution could be useful in harmonizing sampling methodologies for the cost-effectiveness taxonomic diversity estimation and biomonitoring programs.

Functional diversity in three Mediterranean transitional water ecosystems

Biological trait analysis (BTA) is a method that describes ecological functioning of species assemblages incorporating information on species' distributions and their biological characteristics. In the present study, soft bottom communities of three Mediterranean coastal lagoons with different degree of salinity range were analyzed for seven biological traits (mobility, habitat, feeding type, habitat modification, body form, body size and feeding apparatus) in order to investigate the differences in communities' structure and functional diversity across a scale of natural stress. In more variable environments semi-mobile and epibenthic organisms prevailed, while predators were found under more stable conditions. Multivariate analyses using biological traits gave similar results with those obtained by the traditional analyses using species abundances; however, the distinction among lagoons was less evident, indicating that in transitional waters species can be different but their biological traits similar. This supports the idea of species redundancy in transitional water ecosystems.

Applicability and intercalibration of macrophyte quality indices to characterise the ecological status of Mediterranean transitional waters: the case of the Venice lagoon

AbstractIn the framework of the WFD 2000/60/EC intercalibration process the updated versions of the EEI and R‐MaQI, proposed by Italy and Greece for the transitional waters, have been applied to the macrophytes of the Venice lagoon to test their comparability and relationships with the pressure gradients. Submerged macrophytes were collected during spring 2007 at 60 sites spread within the lagoon. At each site, five random samples were collected and the total coverage of benthic macrophytes in the field was estimated based on a number of tests of the bottom. To assess seagrass epiphytes, five shoots were collected for each replicate. Physico‐chemical data were collected in the water column at 14 sites selected to reflect the main hydro‐geomorphological and trophic gradients of the lagoon. The analyses performed indicated that the two metrics appeared to be weakly intercalibrated and only the 30% of the sampling sites displayed the same quality class. The main differences fell into the Moderate and Low classes and the two indices provided discordant results in the intermediate and confined areas of the lagoon. In contrast, the two indices showed good affinity in the marine areas of inlets, which are characterised by seagrass meadows and late‐successional macroalgae. Similar results were evidenced also in a redundancy analysis by the different relationships between quality classes and the physico‐chemical gradients. The main reason for this seems to be the heterogeneity of species–environment relationships inside the groups of species on which the indices are based. Critical aspects of methodological differences and applicability of the macrophyte indices proposed by Italy and Greece for the transitional waters of the Mediterranean eco‐region are discussed.

Long-term phytoplankton dynamics in a Mediterranean eutrophic lagoon (Cabras Lagoon, Italy)

The temporal variations in phytoplankton abundance and diversity were analysed from 1999 to 2009 in Cabras Lagoon, a shallow eutrophic Mediterranean lagoon (west coast of Sardinia). It is one of the aquatic ecosystems listed in the “Marine Ecosystems of Sardinia” by the Italian Network of Long-Term Ecological Research (LTER Italy). The objectives of this work were to assess the general features of phytoplankton succession and its dynamics in relation to environmental factors along the decade 1999–2009, and to identify the main forces driving phytoplankton variability in terms of the general trends in Mediterranean transitional waters. Our results highlight several important aspects: a shift in the annual peak density from the summer–autumn season, observed in 1999–2002, to the winter–spring season in subsequent years; simultaneous reductions in chlorophyll a concentrations and cell size from 1999 to 2008; and the modification of the structure of the phytoplankton assemblages, from a more differentiated class composition in 1999–2002 to the subsequent dominance of the Cyanophyceae. Moreover, analysis of similarity on both abiotic and phytoplanktonic data shows significant differences among the hydrological years considered. Pearson's correlations indicate temperature, salinity and nutrients as the environmental parameters that most influenced the phytoplankton composition and dynamics.