On the myths of indicator species: issues and further consideration in the use of static concepts for ecological applications

Good-Moderate boundary setting for the environmental status assessment of the macrozoobenthos communities with the Benthic Quality Index (BQI) in the south-western Baltic Sea

The status assessment of the macrofauna community under the European Marine Strategy Framework Directive (MSFD) requires threshold values that mark the transition from good to moderate conditions (G-M boundaries). Using the example of the Benthic Quality Index (BQI) in the south-western Baltic Sea, we demonstrate the possibilities and restrictions of i) defining G-M boundaries using reference areas, historical data and a statistical method and ii) the subsequent evaluation of the resulting G-M boundaries using disturbance data. The historical data from the period 1911 to 1929 proved to be unsuitable for defining G-M boundaries due to their data quality. The G-M boundaries calculated using the statistical method delineated high disturbance values more reliably than those based on reference areas. We conclude that disturbance data are very useful to evaluate G-M boundaries for their suitability, but data on all state conditions are needed.

Sediment grain size and benthic community structure in the eastern English Channel: Species-dependent responses and environmental influence

This study addresses the scarcity of evidence on the relationship between benthic communities and coarse-grained sediments in the eastern English Channel. The region's geological history contributes to its predominantly coarse sediment composition. The study employs ternary plots to visualize benthic species' preferences and tolerance for sediment types, revealing their effectiveness. Redundancy Analyses (RDA) and species-level quantile regressions explore the influence of grain size on benthic species distribution. The results indicate a moderate impact of grain size, influenced by hydrodynamics. Estuaries, particularly the Seine Estuary, significantly shape benthic species distribution. Quantile regressions underscore the varied responses of benthic communities along the grain size gradient. The study underscores the importance of considering coarse sediments, offering insights into the complex relationship between benthic communities and sediment characteristics.

Assessing ecological health in areas with limited data by using biological traits

A multitude of biotic indices that represent environmental status have been developed over the past decades making status comparisons difficult. However, transferring an existing index to a new region can be problematic due to differing stressors, ecosystem components and lack of knowledge on regional species sensitivities. Here we assess whether calculating species sensitivities to specific stressors based on biological traits offers a solution. We use biological traits of macrofaunal species to assess sensitivity to suspended sediment concentrations and calculated the Benthic Quality Index (BQI) at 47 sites across a suspended sediment gradient. This trait-based modification of the BQI was well correlated (0.82) to suspended sediment. Problems previously highlighted, relating to trait plasticity and differential weightings of indifferent and beneficial species, were investigated but did not strongly affect results. A trait-based approach has the additional benefit that the data could be easily converted to evaluate ecosystem function.

Distribution Patterns of Benthic Foraminifera in Fish Farming Areas (Corsica, France): Implications for the Implementation of Biotic Indices in Biomonitoring Studies

Corsican marine aquaculture is one of the highest contributors of fish production in France, which may result in environmental perturbations caused by organic matter (OM) accumulation under fish farms and impacting natural communities. This study aimed to (1) characterise the environmental conditions at two different fish farms, (2) monitor the response of benthic foraminiferal species to this activity, and (3) assess the accuracy of existing foraminiferal biotic indices. In 2017, sea floor sediment was sampled in transects from two Corsican fish farms for living foraminiferal and sedimentary analyses. Four indices were calculated and compared: exp(H′bc), Foram-AMBI, Foram Stress Index and TSI-Med. A significant increase in total organic carbon (TOC) has been shown, mainly below the fish cages. Communities were characterized by a shift from high density, opportunistic and tolerant species under the cages to lower densities and more sensitive species further away. According to their distribution patterns along the TOC gradient, we propose to update the ecological group classification of seven species to improve Foram-AMBI’s accuracy and sensitivity: Triloculina oblonga and Quinqueloculina lamarckiana to Ecological Group (EG) I; Rosalina bradyi to EGIII; and Bolivina dilatata, Bulimina aculeata and Quinqueloculina stalkeri to EGIV. We recommend prioritising the use of TSI-Med and Foram-AMBI with the updated list to assess ecological quality in coastal waters of the Mediterranean Sea.

A roadmap towards quantitative cumulative impact assessments: Every step of the way

Currently most Cumulative Impacts Assessments (CIAs) are risk-based approaches that assess the potential impact of human activities and their pressures on the ecosystem thereby compromising the achievement of policy objectives. While some of these CIAs apply actual data (usually spatial distributions) they often have to rely on categorical scores based on expert judgement if they actually assess impact which is often expressed as a relative measure that is difficult to interpret in absolute terms. Here we present a first step-wise approach to conduct a fully quantitative CIA based on the selection and subsequent application of the best information available. This approach systematically disentangles risk into its exposure and effect components that can be quantified using known ecological information, e.g. spatial distribution of pressures or species, pressure-state relationships and population dynamics models with appropriate parametrisation, resulting in well-defined assessment endpoints that are meaningful and can be easily communicated to the recipients of advice. This approach requires that underlying assumptions and methodological considerations are made explicit and translated into a measure of confidence. This transparency helps to identify the possible data-handling or methodological decisions and shows the resulting improvement through its confidence assessment of the applied information and hence the resulting accuracy of the CIA. To illustrate this approach, we applied it in a North Sea CIA focussing on two sectors, i.e. fisheries and offshore windfarms, and how they impact the ecosystem and its components, i.e. seabirds, seabed habitats and marine mammals through various pressures. The results provide a "proof of concept" for this generic approach as well as rigorous definitions of several of the concepts often used as part of risk-based approaches, e.g. exposure, sensitivity, vulnerability, and how these can be estimated using actual data. As such this widens the scope for increasingly more quantitative CIAs using the best information available.

Benthic Foraminiferal Indices and Environmental Quality Assessment of Transitional Waters: A Review of Current Challenges and Future Research Perspectives

Transitional waters straddle the interface between marine and terrestrial biomes and, among others, include fjords, bays, lagoons, and estuaries. These coastal systems are essential for transport and manufacturing industries and suffer extensive anthropogenic exploitation of their ecosystem services for aquaculture and recreational activities. These activities can have negative effects on the local biota, necessitating investigation and regulation. As a result of this, EcoQS (ecological quality status) assessment has garnered great attention as an essential aspect of governmental bodies’ legislative decision-making process. Assessing EcoQS in transitional water ecosystems is problematic because these systems experience high natural variability and organic enrichment and often lack information about their pre-human impact, baseline, or “pristine” reference conditions, knowledge of which is essential to many commonly used assessment methods. Here, foraminifera can be used as environmental sentinels, providing ecological data such as diversity and sensitivity, which can be used as the basis for EcoQS assessment indices. Fossil shells of foraminifera can also provide a temporal aspect to ecosystem assessment, making it possible to obtain reference conditions from the study site itself. These foraminifera-based indices have been shown to correlate not only with various environmental stressors but also with the most common macrofaunal-based indices currently employed by bodies such as the Water Framework Directive (WFD). In this review, we firstly discuss the development of various foraminifera-based indices and address the challenge of how best to implement these synergistically to understand and regulate human environmental impact, particularly in transitional waters, which have historically suffered disproportionate levels of human impact or are difficult to assess with standard EcoQS methods. Further, we present some case studies to exemplify key issues and discuss potential solutions for those. Such key issues include, for example, the disparate performance of multiple indices applied to the same site and a proper assignment of EcoQS class boundaries (threshold values) for each index. Disparate aptitudes of indices to specific geomorphologic and hydrological regimes can be leveraged via the development of a site characteristics catalogue, which would enable the identification of the most appropriate index to apply, and the integration of multiple indices resulting in more representative EcoQS assessment in heterogenous transitional environments. In addition, the difficulty in assigning threshold values to systems without analogous unimpacted reference sites (a common issue among many transitional waters) can be overcome by recording EcoQS as an ecological quality ratio (EQR). Lastly, we evaluate the current status and future potential of an emerging field, genetic biomonitoring, focusing on how these new techniques can be used to increase the accuracy of EcoQS assessment in transitional systems by supplementing more established morphology-based methods.

The response of benthic foraminifera to heavy metals and grain sizes: A case study from Hainan Island, China

Benthic foraminifera, heavy metals, and sediment grain sizes were studied in three bays of Hainan Island, and canonical correspondence analysis (CCA) and nonparametric regression were used to reveal the relationship between foraminifera and their environment. According to our survey, the three bays were moderately contaminated by Mo and As and uncontaminated to moderately contaminated by Pb, Zn, Cr, Sb, and Hg. The spatial pattern of heavy metals was comparable to sediment transport trends, indicating that their distribution was determined by sediment transport. Both living and dead foraminiferal assemblages were analyzed, and their compositions were similar, although the latter had a higher density and diversity. Based on the CCA method, species were divided into three groups, each of which responded differently to heavy metals and grain sizes. The response curves of individual species to heavy metals and grain sizes were obtained by using the Loess (locally weighted regression) method.

Coverage and quality of DNA barcode references for Central and Northern European Odonata

Background

Dragonflies and damselflies (Odonata) are important components in biomonitoring due to their amphibiotic lifecycle and specific habitat requirements. They are charismatic and popular insects, but can be challenging to identify despite large size and often distinct coloration, especially the immature stages. DNA-based assessment tools rely on validated DNA barcode reference libraries evaluated in a supraregional context to minimize taxonomic incongruence and identification mismatches.

Methods

This study reports on findings from the analysis of the most comprehensive DNA barcode dataset for Central European Odonata to date, with 103 out of 145 recorded European species included and publicly deposited in the Barcode of Life Data System (BOLD). The complete dataset includes 697 specimens (548 adults, 108 larvae) from 274 localities in 16 countries with a geographic emphasis on Central Europe. We used BOLD to generate sequence divergence metrics and to examine the taxonomic composition of the DNA barcode clusters within the dataset and in comparison with all data on BOLD.

Results

Over 88% of the species included can be readily identified using their DNA barcodes and the reference dataset provided. Considering the complete European dataset, unambiguous identification is hampered in 12 species due to weak mitochondrial differentiation and partial haplotype sharing. However, considering the known species distributions only two groups of five species possibly co-occur, leading to an unambiguous identification of more than 95% of the analysed Odonata via DNA barcoding in real applications. The cases of small interspecific genetic distances and the observed deep intraspecific variation in Cordulia aenea (Linnaeus, 1758) are discussed in detail and the corresponding taxa in the public reference database are highlighted. They should be considered in future applications of DNA barcoding and metabarcoding and represent interesting evolutionary biological questions, which call for in depth analyses of the involved taxa throughout their distribution ranges.

Indicative value of benthic foraminifera for biomonitoring: Assignment to ecological groups of sensitivity to total organic carbon of species from European intertidal areas and transitional waters

This work contributes to the ongoing work aiming at confirming benthic foraminifera as a biological quality element. In this study, benthic foraminifera from intertidal and transitional waters from the English Channel/European Atlantic coast and the Mediterranean Sea were assigned to five ecological groups using the weighted-averaging optimum with respect to TOC of each species. It was however not possible to assign typical salt marsh species due to the presence of labile and refractory organic matter that hampers TOC characterization. Tests of this study species' lists with Foram-AMBI on two independent datasets showed a significant correlation between Foram-AMBI and TOC, confirming the strong relation between foraminifera and TOC. For one of the validation datasets, associated macrofaunal data were available and a significant correlation was found between the foraminiferal Foram-AMBI and the macrofaunal AMBI. The here proposed lists should be further tested with sensitivity-based indices in different European regional settings.

Long-term spatio-temporal changes of the muddy fine sand benthic community of the Bay of Seine (eastern English Channel)

In the English Channel, the eastern Bay of Seine is exposed to numerous anthropogenic disturbances, in particular major changes in sediment dynamics, which are expected to greatly impact benthic communities. To assess the long-term effects of these stressors on the muddy fine sand benthic community, an original long-term monitoring program has been implemented since 1988. It is based on the sampling of a network of 60 stations during seven surveys over 28 years from 1988 to 2016. We investigate changes of species density, species composition and species diversity at different scales (α-diversity, β-diversity and γ-diversity). Contrary to results obtained in many coastal areas, our results showed a long-term persistence of the community in terms of species composition and structure although a general shift towards muddy sediment has resulted in increased colonisation by species associated with muddy habitats and a decrease in spatial beta diversity.

Assessing large-scale macrobenthic community shifts in the Aegean Sea using novel beta diversity modelling methods. Ramifications on environmental assessment

The effects of large-scale environmental gradients on the spatial patterns of macrobenthic communities used in marine health assessment were evaluated using beta diversity methods. In this work, beta diversity partitioning and relevant ecological modelling methods revealed clear patterns between the northern and the southern parts of the Aegean Sea (Greece). A community turnover point was observed in the Evoikos Gulf marking the transition between the northern and the southern communities. The increased beta diversity in this work was due to species replacement driven mainly by the latitudinal gradients of bottom temperature and salinity whereas species richness did not present substantial differences between the Aegean macrobenthic communities. These findings are attributed to the ability of beta diversity methods to detect the environmental filtering that occurs in these marine provinces through the assessment of biotic interactions in respect to geographic distance. We propose a new standpoint of using beta diversity measures for benthic environmental assessment on a large spatial scale and in marine areas characterised by environmental gradients.

Comparing structural and functional approaches to distinguish benthic communities from eutrophic and non-eutrophic estuaries

The performance of three approaches for macrobenthic classification in distinguishing communities subjected to different environmental conditions was compared: i) the traditional species-based approach; ii) a tolerance-based approach according to ecological groups of the AMBI index; and iii) a character-based approach using biological traits. We used a hierarchical sampling design including three progressively smaller spatial scales, considering that the environmental degradation influence benthic communities at the larger spatial scale. Despite small-scale spatial variations, all approaches performed similarly, distinguishing eutrophic from non-eutrophic benthic communities as they varied significantly at the larger scale, corresponding to the scale at which human induced changes act. Our results suggest that all three methodological approaches could be used as operational criteria to assess estuarine environmental quality, and despite more meaningful, there is still a degree of subjectivity in functional approaches, associated to the lack of more detailed information on macrofaunal life-cycles and levels of tolerance to disturbance.

Modelling the effects of climate change on the distribution of benthic indicator species in the Eastern Mediterranean Sea

The potential effects of climate change on the distribution of benthic species commonly used in marine ecological quality assessment were investigated using a spatial modelling approach. In this work, the relevance of the ecological groups that macrofaunal molluscs are assigned according to their sensitivity or tolerance to environmental disturbance was examined under the scope of the RCP 8.5 severe emissions scenario. The effects of climate change were more profound on species that are indicative of a specific suite of climatic conditions regarding temperature and salinity. Significant loss of habitat suitability was observed for the tolerant species Corbula gibba and Abra prismatica whereas the sensitive species Moerella donacina was least affected. In contrast, an overall expansion of the distributional potential was observed for the sensitive species Flexopecten hyalinus as newly suitable habitats are formed. As hypothesised, the current ecological grouping that depicts the sensitivity of a benthic species to an environmental stressor is irrelevant when assessing the effects of climate change. We propose a new standpoint of using benthic species as biotic tools based on their ecological niche requirements.

20th century increase in body size of a hypoxia-tolerant bivalve documented by sediment cores from the northern Adriatic Sea (Gulf of Trieste)

An increase in the frequency of hypoxia, mucilages, and sediment pollution occurred in the 20th century in the Adriatic Sea. To assess the effects of these impacts on bivalves, we evaluate temporal changes in size structure of the opportunistic bivalve Corbula gibba in four sediment cores that cover the past ~500 years in the northern, eutrophic part and ~10,000 years in the southern, mesotrophic part of the Gulf of Trieste. Assemblages exhibit a stable size structure during the highstand phase but shift to bimodal distributions and show a significant increase in the 95th percentile size during the 20th century. This increase in size by 2-3 mm is larger than the northward size increase associated with the transition from mesotrophic to eutrophic habitats. It coincides with increasing concentrations of total organic carbon and nitrogen, and can be related to enhanced food supply and by the tolerance of C. gibba to hypoxia.

Molluscan indicator species and their potential use in ecological status assessment using species distribution modeling

Marine habitat assessment using indicator species through Species Distribution Modeling (SDM) was investigated. The bivalves: Corbula gibba and Flexopecten hyalinus were the indicator species characterizing disturbed and undisturbed areas respectively in terms of chlorophyll a concentration in Greece. The habitat suitability maps of these species reflected the overall ecological status of the area. The C. gibba model successfully predicted the occurrence of this species in areas with increased physical disturbance driven by chlorophyll a concentration, whereas the habitat map for F. hyalinus showed an increased probability of occurrence in chlorophyll-poor areas, affected mainly by salinity. We advocate the use of C. gibba as a proxy for eutrophication and the incorporation of this species in monitoring studies through SDM methods. For the Mediterranean Sea we suggest the use of F. hyalinus in SDM as an indicator of environmental stability and a possible forecasting tool for salinity fluctuations.

Can benthic foraminifera serve as proxies for changes in benthic macrofaunal community structure? Implications for the definition of reference conditions

Benthic macrofauna is one of the most widely used biological groups to assess the ecological status of marine systems. Lately, attention has been paid to similar use of benthic foraminifera. In this study, distribution patterns of benthic foraminiferal and macrofaunal species were investigated simultaneously in 11 fjords in southeastern Norway in order to assess correlations and responses to environmental conditions. Selected fjords allowed to investigate contrasted environmental conditions from low total organic carbon (TOC) content (sediment TOC < 2.7%) in normoxia (bottom-water [O₂] > 2 mL O₂.L^-1) up to high TOC content (> 3.4%) in severe hypoxia (< 0.5 mL O₂.L^-1). Environmental parameters comprised bottom-water dissolved oxygen, grain size, total organic carbon, total nitrogen (TN), pigments and depth below threshold (DBT). Foraminiferal and macrofaunal community data were significantly correlated (Procrustes analysis m² = 0.66, p = 0.001). Hence, benthic foraminiferal distribution patterns mirror those of benthic macrofauna. However, as opposed to the foraminifera, macrofauna was not recorded at the most oxygen-depleted stations and, hence, was more sensitive to severe oxygen depletion. With regard to assigning species to ecological groups for ecological quality status assessment, the results suggest that species, e.g. Spiroplectammina biformis (foraminifera), Scalibregma inflatum (macrofauna), may exhibit different ecological requirements depending on their habitat. Considering the observed congruent patterns of benthic foraminifera and macrofauna, palaeo-communities of benthic foraminifera could be used as indicators of reference conditions for benthic macrofaunal community structure. This would however need further developments of algorithms to perform such a translation.

The importance of applying Standardised Integrative Taxonomy when describing marine benthic organisms and collecting ecological data

The decline of morphologically based taxonomy is mainly linked to increasing species redundancy, which probably contributed to a worldwide disinterest in taxonomy, and to a reduction of funding for systematic biology and for expertise training. The present trend in the study of biodiversity is integrated taxonomy, which merges morphological and molecular approaches. At the same time, in many cases new molecular techniques have eclipsed the morphological approach. The application of Standardised Integrative Taxonomy, i.e. a rigorous, common method of description based on the integration between ecological and morphological characteristics, may increase the precision, accessibility, exploitability and longevity of the collected data, and favour the renaissance of taxonomy by new investments in biodiversity exploration.

Unraveling the performance of the benthic index AMBI in a subtropical bay: The effects of data transformations and exclusion of low-reliability sites

We investigated the relationship between the AMBI index and different contamination proxies in an urbanized bay in South America (SE Brazil), and the effect of (a) abundance data transformation and (b) exclusion of low-reliability sites (high SD; low N) on the index' performance. Poor ecological quality and opportunistic species were related to an increase in contaminants concentrations and mud content. Good ecological status and sensitive species (EG I) were mainly related to increased hydrodynamics. Data transformation caused minimal changes to the overall relationships, but exclusion of low-reliability sites improved the relationship between ecological groups and contamination proxies. Our results show that AMBI is robust in detecting effects of different contaminants in the area and reinforce the importance of the index as a tool for coastal management, but local joint efforts are needed to improve and adjust local species classification in ecological groups to improve the index' performance.

Species contributions to ecosystem process and function can be population dependent and modified by biotic and abiotic setting

There is unequivocal evidence that altered biodiversity, through changes in the expression and distribution of functional traits, can have large impacts on ecosystem properties. However, trait-based summaries of how organisms affect ecosystem properties often assume that traits show constancy within and between populations and that species contributions to ecosystem functioning are not overly affected by the presence of other species or variations in abiotic conditions. Here, we evaluate the validity of these assumptions using an experiment in which three geographically distinct populations of intertidal sediment-dwelling invertebrates are reciprocally substituted. We find that the mediation of macronutrient generation by these species can vary between different populations and show that changes in biotic and/or abiotic conditions can further modify functionally important aspects of the behaviour of individuals within a population. Our results demonstrate the importance of knowing how, when, and why traits are expressed and suggest that these dimensions of species functionality are not sufficiently well-constrained to facilitate the accurate projection of the functional consequences of change. Information regarding the ecological role of key species and assumptions about the form of species–environment interactions needs urgent refinement.

The importance of naming cryptic species and the conservation of endemic subterranean amphipods

Molecular taxonomy often uncovers cryptic species, reminding us that taxonomic incompleteness is even more severe than previous thought. The importance of cryptic species for conservation is poorly understood. Although some cryptic species may be seriously threatened or otherwise important, they are rarely included in conservation programs as most of them remain undescribed. We analysed the importance of cryptic species in conservation by scrutinizing the South European cryptic complex of the subterranean amphipod Niphargus stygius sensu lato. Using uni- and multilocus delineation methods we show that it consists of 15 parapatric and sympatric species, which we describe using molecular diagnoses. The new species are not mere “taxonomic inflation” as they originate from several distinct branches within the genus and coexist with no evidence of lineage sharing. They are as evolutionarily distinct as average nominal species of the same genus. Ignoring these cryptic species will underestimate the number of subterranean endemics in Slovenia by 12 and in Croatia by four species, although alpha diversity of single caves remains unchanged. The new taxonomy renders national Red Lists largely obsolete, as they list mostly large-ranged species but omit critically endangered single-site endemics. Formal naming of cryptic species is critical for them to be included in conservation policies and faunal listings.

Variation in benthic long-term data of transitional waters: Is interpretation more than speculation?

Biological long-term data series in marine habitats are often used to identify anthropogenic impacts on the environment or climate induced regime shifts. However, particularly in transitional waters, environmental properties like water mass dynamics, salinity variability and the occurrence of oxygen minima not necessarily caused by either human activities or climate change can attenuate or mask apparent signals. At first glance it very often seems impossible to interpret the strong fluctuations of e.g. abundances or species richness, since abiotic variables like salinity and oxygen content vary simultaneously as well as in apparently erratic ways. The long-term development of major macrozoobenthic parameters (abundance, biomass, species numbers) and derivative macrozoobenthic indices (Shannon diversity, Margalef, Pilou's evenness and Hurlbert) has been successfully interpreted and related to the long-term fluctuations of salinity and oxygen, incorporation of the North Atlantic Oscillation index (NAO index), relying on the statistical analysis of modelled and measured data during 35 years of observation at three stations in the south-western Baltic Sea. Our results suggest that even at a restricted spatial scale the benthic system does not appear to be tightly controlled by any single environmental driver and highlight the complexity of spatially varying temporal response.

Testing the saprobity hypothesis in a Mediterranean lagoon: effects of confinement and organic enrichment on benthic communities

The macrobenthic community was compared at four sites characterized by varying degrees of freshwater input, organic enrichment and confinement in the Cabras lagoon (Sardinia, Italy). Three sites, riverine (C1), confined (C2) and seaward (C3), were studied on two dates of summer 2010 and on two dates of winter 2011. A fourth site (C12), representative of the central sector of the Cabras lagoon, was included in this study using the extensive historical datasets at our disposal from previously published work. We aimed to test the hypothesis that (1) the benthos is distributed according to the recently proposed concept of habitat saprobity for coastal lagoons that unifies the Pearson-Rosenberg (sensu organic enrichment) and Guélorget-Perthuisot (sensu confinement) models, and (2) indicator species of different saprobic levels can be identified among dominant species occurring along the saprobity gradient. Salinity was also considered as an additional agent of selection in brackish environments. Irrespective of significant seasonal changes within each site, our results highlighted major environmental and biotic differences between sites. At the northward riverine site (C1), most affected by freshwater input and with limited organic matter (OM) enrichment, Corophium orientale was the single dominant species. The most confined site (C2) was characterized by the highest levels of sedimentary OM and benthic Chlorophyll-a and by mesohaline conditions; the site was inhabited mainly by the halolimnobic Hediste diversicolor and Hydrobia spp. Site C12, characterized by a high OM load and high residence time, was dominated by the opportunistic detritivorous Alitta succinea and Polydora ciliata. At the southernmost seaward site (C3) the considerable seawater renewal, resulting in high salinity (only in summer) and limited OM load, favored a much more diverse macrobenthic assemblage, essentially composed of both marine species, such as Corophium insidiosum, Gammarus aequicauda, and brackish-water species, such as Lekanesphaera hookeri and Idotea chelipes. We conclude that the biotic and abiotic characteristics of the Cabras lagoon can be represented by a succession of spatial zones along two main gradients determined by salinity and saprobity. The salinity gradient proved to be the main structural feature in the oligohaline pole, while in the range of variable salinity, saprobity appeared to be the main selection factor. To illustrate our findings, we provide a graphical representation summarizing the changes in environmental parameters and indicator species along the salinity and saprobity gradients.