This study was conducted in Lake Varano (southern Italy) in order to determine the environmental parameters that influence nematode taxonomic and functional diversity and how they influence them, and to classify for the first time the ecological quality status of a transitional environment. The general composition and level of taxonomic diversity were comparable with those reported for transitional environments worldwide. Principal component analysis revealed that the main environmental factors controlling the assemblages in terms of both composition and biological traits were grain size, organic matter, pollution load index and, only secondarily, salinity.
Este estudio tiene como objetivo analizar cuál y en qué medida los parámetros ambientales influyen en la diversidad taxonómica y funcional de los nematodos y clasificar por primera vez el Estado de la Calidad Ecológica (EQS) de un ambiente de transición como la laguna de Varano (Italia meridional). La composición y el nivel de divesidad taxonómica son comparables con los reportados en todo el mundo para TEs. El análisis de componentes principales reveló que los factores que controlan la composición y las características biológicas de las comunidades son el tamaño del gránulo, la materia orgánica (MO), el índice de aportes contaminados (PLI) y sólo secundariamente la salinidad.
Transitional environments (TEs) are unpredictable aquatic systems characterized by large environmental fluctuations of physicochemical and geochemical parameters which may influence benthic assemblages in terms of their distribution and structure (
Free-living nematodes are the most diverse and numerically dominant metazoans in aquatic ecosystems (
Currently, knowledge of the meiofauna in southern Italian TEs is limited to the Lesina Lagoon (
This investigation documents the taxonomic and functional diversities of the nematode assemblages in the poorly known TE of Lake Varano (southern Adriatic Sea) and evaluates the ecological quality status (EQS) of this TE, thereby opening up new potential perspectives for the conservation and management of coastal areas. In particular, the following questions are addressed: (a) Are there changes in taxonomic and functional nematode diversity in response to the environmental parameters? (b) Are there significant differences in the nematode assemblages that may be imputable to anthropogenic disturbance?
Located on the north side of the Gargano National Park (Foggia, Italy), Lake Varano is a coastal lagoon comprising an area of ca. 65 km2 with an average water depth of ca. 4 m (
Samples from 21 stations were collected using a modified model of the Van Veen grab in late March 2012. This modified model permits the insertion of a plexiglas corer from the central part of the grab, enabling portions of almost completely undisturbed sediments to be captured. A set of sub-samples of the sediments was taken from each station for the purposes of sedimentological, geochemical and nematode analyses (two and three replicates for geochemical and nematode analyses, respectively). All of the analyses were carried out on the upper surface sediment layer (0-2 cm). During the sampling process, physico-chemical parameters of the bottom water, such as temperature, pH, salinity, oxidation reduction potential (ORP) and dissolved oxygen (DO, expressed as mg/l) were measured using a conductivity, temperature and depth (CTD) probe. In particular, these environmental parameters selected as independent variables are well known to affect the structure, abundance and diversity of the nematode assemblage (
Grain size analyses were conducted at Lille 1 University (France) following the principle of diffraction described in
The quantity and quality of the OM were estimated by analysing the total carbon (TC), the total organic carbon (TOC), the total sulfur (TS), the total hydrogen (TH) and the total nitrogen (TN) content in the sediment (for details, see
Concentrations of 37 trace elements were obtained at Activation Laboratories Ltd. (Ontario, Canada,
Samples utilized for the nematode analysis were treated with a 7% MgCl2 aqueous solution for narcotizing fauna, fixed in a 4% formaldehyde solution in buffered sea-water and stained with Rose Bengal (0.2 g L–1). In laboratory, the samples were rinsed with a gentle jet of fresh water through a 0.5-mm sieve to separate the macrofauna (
The trophic and life strategies were among the functional traits considered. Following
The biodiversity pattern of the assemblage was described by means of the Shannon (H’, using log-base 2), Pielou, Margalef and Simpson indices.
All of the statistical analyses were performed on the relative abundances of the nematode species. Following
The data on the environmental parameters are summarized in Supplementary material Table S1. The pH was slightly alkaline, ranging from 7.6 (V1, V15) to 8.0 (V29, V39). The salinity varied from 23.0‰ (V50) to 31.9‰ (V13), with a clear increase in the outer part of the lake and, in particular next to the Capoiale Channel. The DO values varied between 5.7 (V15) and 12.8 (V35), with the highest values found in the central part of the lake and the lowest near to the two channels. The ORP values ranged from 164.0 (V39 and V41) to 286.0 (V1), with the highest values in the outer part of the lagoon. The sediments were primarily composed of silt (47% on average) and sand (51.9% on average), with a low amount of clay (1.1% on average). The outer part of the lake, particularly in front of the two channels and the baymouth bar, was dominated by sand, whereas silt was predominant in the southeastern part of the lake. The CaCO3 varied between 9.5% (V4) and 80.8% (V37), and was particularly abundant on the western side of the lake. The TN ranged between 0.1 (V4, V7, V11, V41, V48, V49 and V50) and 0.4 (V27 and V47), whereas TS was between 0.9% (V7) and 1.8% (V1). The TC values varied from 1.3 (V4) to 10.9 (V37 and V42), while TH ranged from 0.1 (V4) to 1.3 (V39 and V47). The TOC varied from 0.2% (V4) to 4.2% (V27), with higher values being found in the central and southwestern parts of the lake. A clear spatial distribution of most of the trace elements was visible, with the highest concentrations being found in both the central and southeastern parts of the lake. The PLI values ranged from 1.0 (V4) to 11.4 (V41), and revealed a general enrichment of the concentrations of trace elements in both the central and southeastern parts of the lake.
Fifty-five nematode species, belonging to 37 genera and 17 families, were recorded at Lake Varano (Supplementary material Table S2). The richest families were Chromadoridae (10 species), Linhomoeidae (9) and Xyalidae (8). Almost half of the selected specimens belonged to either the Desmodoridae (35%) or Linhomoeidae (16%) families, followed by Leptolaimidae (12%), Microlaimidae (9%) and Xyalidae (9%). Three species had very high abundances and a wide distribution:
The richness of the species ranged from 4 (V29) to 27 (V7). The H’ ranged from 0.9 (V29) to 4.2 (V7), the Margalef index from 0.7 (V29) to 5.6 (V7), the Pielou evenness from 0.4 (V29) to 0.9 (V7), and the Simpson evenness from 0.33 (V29) to 0.94 (V7 and V50) (Supplementary material Table S3). These indices revealed that the nematode assemblage in the central part of the lake was less diversified.
The trophic structure of the assemblage was mainly characterized by 1A and 1B (60.1% and 21.8%, respectively, on average), followed by 2A (16.2%) and 2B (2%) (Supplementary material Table S3). The group 1A was widely distributed, whereas the group 1B was particularly associated with freshwater inputs and the Varano Channel. The group 2B and, in particular, 2A were more abundant in the outer part of the lake.
The MI ranged from 2.1 (V4) to 2.9 (V27), with higher values mainly in the central part of the lake. All of the classes of the colonizers-persisters (c-p) were revealed, with the only exception being class c-p5 (Supplementary material Table S3). On average, the c-p3 species were the dominant component of the nematode assemblage (58.5%) followed by c-p2 (38.4%), c-p1 (2.8 %) and c-p4 (0.4%).
The results of the R-mode CA enabled two main (A and B) clusters and two different subclusters (A1 and A2) to be recognized (
Subcluster B2 included all the other stations, and can be further subdivided into two subclusters, B2a and B2b. The subcluster B2a grouped a few stations located in front of the Varano Channel, in the southern margin and in the central part of the lake. It was dominated by
The eigenvalues of the PCA revealed that the first two factors explained ca. 49.3% of the total variance, whereas the factor variable plans showed how the species (those used for the CA and representing the main variables) and secondary variables (abiotic) were related to the different factors (
Strong relationships among the fine sediment fractions of both silt and clay were evidenced with
The description of the distribution patterns of species is one of the fundamental starting blocks in the ecology of biological assemblages. However, there is very little information available on nematode taxonomic composition in Italian TEs and in the entire southern part of the Adriatic Sea (
The nematode assemblage of the study area appeared to be mainly comprised of representative species of the fine sediments (
Studies carried out in both marine and coastal lagoon ecosystems have revealed that the quantity and quality of the OM and salinity are among the most important factors influencing meiofaunal and nematode assemblages (
As shown in the PCA, the sediment grain size appeared to be one of the most important environmental variables affecting the nematode species distribution of Lake Varano. This parameter may, in fact, exert significant influence on the structure of the nematode assemblage, which appears to be very sensitive, even to slight sedimentological variations (
As suggested by the CA and the PCA, the species closely associated with the silt were mainly
As highlighted by the statistical analyses, the enhancement of ORP and the sand percentage values led to a higher abundance of
In the second factor of the PCA, TN, TOC, TH, TS and salinity were the most important environmental variables. Salinity was not strongly correlated with the nematode assemblages. As stated above, this could be due to the small variations in salinity detected in the area, and also confirms the macrofaunal spatial distribution observed by
Pielou and Simpson indices seemed to be more affected by the second factor plan of the PCA, increasing with the reduction in the OM supply and salinity. This could be related to the discharge from the San Antonino and Francesco Canals, as documented by
The nematode assemblage in Lake Varano appeared to be mainly represented by colonizer species. This group has been reported to increase in abundance under stress conditions (
Although the MI may generally give good results in ecological assessments (
In order to classify the EQS of Lake Varano in accordance with the Water Framework Directive (WFD, Directive 2000/60/EC), we used the thresholds for nematode descriptors proposed by
We warmly thank the two anonymous referees for their constructive and critical reviews of the manuscript and Dr. Anabella Harriague-Covazzi for the translation of the abstract into Spanish.