Spatial distribution of subtidal Nematoda communities along the salinity gradient in southern European estuaries

This study investigated the spatial distribution of subtidal nematode communities along the salinity gradients of two Portuguese estuaries exposed to different degrees of anthropogenic stress: the Mira and the Mondego.The nematode communities were mainly composed of Sabatieria, Metachromadora, Daptonema, Anoplostoma, Sphaerolaimus and Terschellingia species, closely resembling the communities of Northern European estuaries. In both estuaries, nematode density and community composition followed the salinity gradient, naturally establishing three distinct estuarine sections: (i) freshwater and oligohaline – characterised by the presence of freshwater nematodes, low nematode density and diversity; (ii) mesohaline – dominated by Terschellingia, Sabatieria and Daptonema, with low total density and diversity; and (iii) polyhaline and euhaline – where nematodes reached the highest density and diversity, and Paracomesoma, Synonchiella, and Odontophora were dominant.Despite the similarities in community composition and total nematode density, the proportion of different nematode feeding types were remarkably different in the two estuaries. In Mira, selective deposit feeders were dominant in the oligohaline section, while non-selective deposit feeders were dominant in the other sections. On the contrary, in the Mondego estuary, epigrowth-feeders and omnivores/predators were dominant in the freshwater sections and in the euhaline sector of the southern arm.Differences observed along each estuarine gradient were much stronger than overall differences between the two estuaries. In the Mondego estuary, the influence of anthropogenic stressors seemed not to be relevant in determining the nematodes' spatial distribution patterns, therefore suggesting that mesoscale variability responded essentially to natural stressors, characteristic of estuarine gradients. Nevertheless, the proportion of the different feeding types was different between the two estuaries, indicating that the response of nematode feeding guilds is able to reflect anthropogenic-induced stress and can be useful in assessing biological quality in transitional waters ecosystems.     

Estuarine intertidal meiofauna and nematode communities as indicator of ecosystem's recovery following mitigation measures

The Mondego estuary (Portugal) has been under environmental pressure since the early 1990s due to different anthropogenic stresses. The system has been studied following benthic communities’ features from an impacted situation until the recovery phase, focusing mostly on macrobenthos. Following the application of mitigation measures in the estuary, this study is focused on the variability of the intertidal meiobenthic and nematode communities in a system that has recovered after different anthropogenic stresses. While at the spatial level (among areas along the eutrophication gradient) no significant differences were observed regarding the structure and function of the nematode communities, at the seasonal level some differences stood out. These results broadly suggest that the system has recovered from the early situations of pressures being, to the best of our knowledge, the first attempt to investigate the variability of intertidal meiobenthic and nematode communities in the scope of a system's recovery along an estuarine gradient of eutrophication. Even if performed in a short timeline, this study provides a good baseline analysis of conditions, being important for future comparisons.     

Trends in water quality and subtidal benthic communities in a temperate estuary: Is the response to restoration efforts hidden by climate variability and the Estuarine Quality Paradox?

The analysis of temporal patterns in water quality and benthic assemblages in estuaries constitutes an important methodological issue for discriminating the effects of natural and anthropogenic pressures. Temporal trends in water quality and in the subtidal benthic community over a 5-year interval in the Mondego estuary (Portugal) were investigated with the aim of assessing changes in environmental quality as a response to restoration efforts and climate variability. Particularly, we addressed the following questions: (a) Would trends in water quality and benthos behave consistently over the whole study period for the different zones of the monitoring network and indicate improvement or degradation in ecological condition? (b) Could we distinguish the effects of climate variability and restoration efforts in water quality and benthos from trend analysis results? (c) Could the response of the benthic communities and water quality be useful to guide the planning of future management actions in this system?Clear cause–effect relationships regarding the ecological response to restoration efforts and climate variability were indeed challenging to identify and interpret. In fact, the response of water quality and benthic communities to restoration efforts seemed to have been masked by the effects of climatic variability. Furthermore, the present study illustrated clearly the high environmental variability inherent to estuarine systems and the difficulty of clearly distinguishing natural from anthropogenic stressors, in agreement with the “Estuarine Quality Paradox”. Implications for ecological quality assessment and management of the Mondego estuary and other poikilohaline systems are discussed, namely with regard to the “one-out, all-out” principle required by the European Water Framework Directive (WFD).     

Role of meiofauna in estuarine soft-bottom habitats*

Meiofauna are ubiquitous in estuaries worldwide averaging 10⁶ m^?2. Abundance and species composition are controlled primarily by three physical factors: sediment particle size, temperature and salinity. While meiofauna are integral parts of estuarine food webs, the evidence that they are biologically controlled is equivocal. Top down (predation) control clearly does not regulate meiofaunal assemblages. Meiofauna reproduce so rapidly and are so abundant that predators cannot significantly reduce population size. Food quantity (bottom up control) also does not appear to limit meiofaunal abundance; there is little data on the effect of food quality. In estuarine sediments meiofauna: (i) facilitate biomineralization of organic matter and enhance nutrient regeneration; (ii) serve as food for a variety of higher trophic levels; and (iii) exhibit high sensitivity to anthropogenic inputs, making them excellent sentinels of estuarine pollution. Generally mineralization of organic matter is enhanced and bacterial production stimulated in the presence of meiofauna. Tannins from mangrove detritus in northern Queensland appear to inhibit meiofaunal abundance and therefore the role of meiofauna in breakdown of the leaves. Meiofauna, particularly copepods, are known foods for a variety of predators especially juvenile fish and the meiofaunal copepods are high in the essential fatty acids required by fish. The copepod’s fatty acid composition is like that of the microphytobenthos they eat; bacterial eaters (nematodes?) do not have the essential fatty acids necessary for fish. Most contaminants in estuaries reside in sediments, and meiofauna are intimately associated with sediments over their entire life-cycle, thus they are increasingly being used as pollution sentinels. Australian estuarine meiofauna research has been concentrated in Queensland, the Hunter River estuarine system in New South Wales, and Victoria’s coastal lagoons. Studies in northern Queensland have primarily concentrated on the role of nematodes in mineralization of organic matter, whereas those from southern Queensland have concentrated on the role of meiofauna as food for fish and as bacterial grazers. The New South Wales studies have concentrated on the Hunter River estuary and on the structure and function of marine nematode communities. In Victoria, several fish have been shown to eat meiofauna. The Australian world of meiofaunal research has hardly been touched; there are innumerable opportunities for meiofaunal studies. In contaminated estuarine sediments reduced trophic coupling between meiofauna and juvenile fish is a basic ecological question of habitat suitability, but also a question with relevance to management of estuarine resources. Because meiofauna have short lifecycles, the effects of a contaminant on the entire life-history can be assessed within a relatively short time. The use of modern molecular biology techniques to assess genetic diversity of meiofauna in contaminated vs uncontaminated sediments is a promising avenue for future research. Much of the important meiofaunal functions take place in very muddy substrata; thus, it is imperative to retain mudflats in estuaries.     

A biological trait approach to assess the functional composition of subtidal benthic communities in an estuarine ecosystem

Within transitional/estuarine environments ‘ecosystem functioning’ has been mostly investigated with “traditional” taxonomic analysis, based on the taxonomic composition of benthic invertebrate communities. However, ‘ecosystem functioning’ depends also greatly on the functional characteristics (biological traits) of organisms.It was a priori suggested that the biological traits of the subtidal benthic invertebrate communities within an estuarine environment would respond to the high variability of environmental pressures (natural and human induced) within this type of ecosystem.For this study, traditional taxonomic analysis (species richness, species density and Shannon–Wiener diversity) as well as biological trait analysis were used together for the first time to investigate the response of the subtidal benthic invertebrate communities to the environmental pressures within the Mondego estuary (Portugal).Biological trait analysis, in addition to traditional taxonomic analysis provided a more comprehensive understanding of the functioning within this type of ecosystem. Some of the most important outcomes are: (i) the trait “salinity preference” was the most important trait that distributed the species along the estuary, (ii) the central part of the estuary appeared to be under higher environmental stress levels than the other areas, as suggested by a dominance of some “opportunistic” traits (e.g. small short-lived species), (iii) the ratio between functional diversity (FD) and Shannon–Wiener diversity (H′) indicated lower functional redundancy at the upper reaches of the estuary. Our results, suggest that the ratio (FD/H′) might be a helpful tool to visualize this functional attribute and could potentially be applied to different communities from distinct environments. Using the traditional taxonomic analysis alone, this last functional aspect would not be detectable. Therefore, the inclusion of biological traits analysis is recommendable for estuarine ecological studies.     

Do fish larvae have advantages over adults and other components for assessing estuarine ecological quality?

Using a novel approach to the assessment of ecological quality status of estuarine ecosystems, this study hypothesizes that compared to adult fishes and other components, the younger fish stages will be more sensitive and act as an early warning and will reflect more effectively the ecological status of estuaries. Larval stages of fishes were used to assess the ecological quality status (EQS) of four NW Portuguese estuaries, with different types and magnitudes of human pressures. The larval fish assemblages, together with water column characteristics and pollution indicators (faecal contamination and nutrient load) were sampled in the Lima, Cávado, Ave and Douro estuaries, during spring and autumn 2009. The four estuaries were classified in terms of human pressures by a global pressure index that identified the Cávado estuary as the least impacted estuary, followed by the Ave and Lima, both classified as moderately impacted system, while the Douro was classified as a highly impacted system. The Ave emerged as the most polluted system, carrying the highest nutrient load and sewage contamination. Larval fish assemblages included estuarine species, marine migrants, marine stragglers and the larger estuaries had higher species richness. Compared to adult fishes, three multimetric fish-based indices classified the Cávado, Ave and Douro estuaries with a lower ecological status when fish larvae were used. Similarly, the EQS assessed by macroinvertebrates were equal or higher when compared with fish larvae results. The EQS assessed by fish larvae was negatively correlated with sewage contamination and nitrogen nutrients, but did not reflect other anthropogenic pressures expressed by the global pressure index, which was only detected by adult fish. Fish larvae assessments were able to detect short-time events of hydrological manipulations observed in the Cávado estuary, as well as a seasonal decrease of water quality especially evident in the Ave estuary. The indices used denoted some limitations to the use of fish larvae data, thus emphasising the need for new indices to test the observed tendency for lower EQS given by fish larvae. The advantages and disadvantages of using fish larvae as more sensitive and accurate bioindicators of ecosystem integrity is also discussed as a means of providing strategically important information for improved estuarine management.     

Fishes as indicators of environmental and ecological changes within estuaries: a review of progress and some suggestions for the future

Estuarine habitats, and the fish assemblages associated with them, are potentially impacted upon by many anthropogenic influences which can have a direct influence on the food resources, distribution, diversity, breeding, abundance, growth, survival and behaviour of both resident and migrant fish species. The direct and indirect coupling between ichthyofaunal communities and human impacts on estuaries reinforces the choice of this taxonomic group as a biological indicator that can assist in the formulation of environmental and ecological quality objectives, and in the setting of environmental and ecological quality standards for these systems. This review examines the rationale and value of selecting fishes as bio-indicators of human induced changes within estuaries, using examples from both the northern and southern hemispheres. The monitoring of estuarine 'health' using fish studies at the individual and community level is discussed, with an emphasis on the potential use of estuarine fishes and their monitoring and surveillance in national and international management programmes. In illustrating the above concept, examples are presented of the way in which fishes are threatened by anthropogenic impacts and of the way in which teleosts can contribute to a monitoring of estuarine ecosystem health.     

Zonation of intertidal macrobenthos in the estuaries of Schelde and Ems

Based on data, collected in 1980–1990, the intertidal benthic macrofauna of the Schelde and Ems estuaries was compared. The spatial occurrence of the benthic macrofauna along the salinity gradient, including the freshwater tidal area was emphasized. Both estuaries appeared to have a very similar species composition, especially at genus level. The higher number of species observed in the Schelde estuary was probably due to a greater habitat diversity. In both estuaries species diversity decreased with distance upstream. The total density did not vary along the estuarine gradient, whereas biomass is highest in the polyhaline zone.In both estuaries distinct intertidal benthic communities were observed along the salinity gradient: a marine community in the polyhaline zone, a brackish community in the mesohaline zone, and a third community in the oligohaline and freshwater tidal zones of the estuary. These three communities were very similar between both estuaries. Their main characteristics were discussed together with the occurrence and distribution of the dominant species.For the Schelde estuary and to a lesser extent also for the Ems estuary, there was evidence that anthropogenic stress had a negative effect on the intertidal macrobenthic communities of the oligohaline/freshwater tidal zone. Only Oligochaeta were dominating, whereas the very euryhaline and/or true limnetic species were missing. In the mesohaline zone, the Schelde estuary was dominated by large numbers of short-living, opportunistic species, whereas in the Ems estuary relatively more stable macrobenthic communities were observed. A comparison with some other European estuaries showed in general similar trends as those observed for the Schelde and Ems estuaries.     

Is polychaete family-level sufficient to assess impact on tropical estuarine gradients?

Regular, robust monitoring programs set up to assess the environmental conditions of aquatic systems often target different biological groups. And, of these, macroinvertebrate communities and particularly the class Polychaeta are frequently used. Identifying these organisms takes time, money and specialized expertise to ensure correct identification to the lowest possible taxonomic level. Identification errors can lead to an erroneous assessment. The concept of taxonomic sufficiency has been proposed both to minimize errors and to save time and money. This study tested the usefulness of this concept in tropical estuaries in northeast Brazil. We selected two transitional systems with different degrees of human impact due to different land uses and different conservation systems: the Mamanguape estuary, which is in an environmental conservation unit for sustainable use, and the highly impacted, urban Paraíba do Norte estuary. The results clearly showed that nutrient concentrations were markedly higher in the Paraíba do Norte estuary in the dry season and that the composition of the polychaete assemblages differed between the two estuaries as well as along the spatial gradient of each estuary. The use of either genus or family level led to equivalent representation in each system in terms of taxon richness and both the Margalef and Shannon-Wiener diversity indices. Both taxonomic levels described similar changes in the polychaete assemblage along the estuarine gradients. Based on our findings, the use of a coarser taxonomic level (i.e., family) is a good option when the aim is to implement a monitoring program in tropical estuaries with the polychaete assemblages as one of the target groups. This time-efficient taxonomic resolution can help improve sampling designs and allow long-term monitoring studies without losing much vital information.     

Short and long-term movement and site fidelity of juvenile Haemulidae in back-reef habitats of a Caribbean embayment

In the context of a main project that aims to recover modern data on diatom distribution applicable to paleosalinity reconstructions in coastal areas of Southern South America, the composition and distribution of dead diatom assemblages in the littoral zone of the Quequén Salado estuary (Argentina) were studied. Diatom zones were defined along the estuarine gradient by cluster analysis and related to the salinity range and sediment composition by Canonical Correspondence Analysis. Four diatom zones were identified. A mixture of marine, brackish and freshwater diatoms, probably allochthonous, characterized the inlet (zone I). Marine/brackish taxa, represented mainly by Paralia sulcata dominated zone II, characterized by polyhaline conditions and sandy sediments. Zone III was characterized by mesohaline conditions, muddy sediments and the dominance of the estuarine diatom Amphora helenensis. Brackish/freshwater and freshwater diatoms dominated the headwaters (zone IV), where salinity was always below 5‰. The comparison of Quequén Salado diatom assemblages with previous results from the Quequén Grande estuary showed a similar taxonomic composition between both estuaries. However, differences in the salinity ranges of the estuaries (related to differences in the degree of human impact and tidal range) lead to a displacement in their spatial distribution along the longitudinal estuarine axis. This paper contributes to the knowledge of the ecological requirements of South American estuarine diatoms and provides useful data for paleosalinity reconstructions in the region. Electronic Supplementary Material Supplementary material is available in the online version of this article at and is accessible for authorized users Handling editor: J. Padisak     

Environmental metabarcoding reveals heterogeneous drivers of microbial eukaryote diversity in contrasting estuarine ecosystems

Assessing how natural environmental drivers affect biodiversity underpins our understanding of the relationships between complex biotic and ecological factors in natural ecosystems. Of all ecosystems, anthropogenically important estuaries represent a ‘melting pot'' of environmental stressors, typified by extreme salinity variations and associated biological complexity. Although existing models attempt to predict macroorganismal diversity over estuarine salinity gradients, attempts to model microbial biodiversity are limited for eukaryotes. Although diatoms commonly feature as bioindicator species, additional microbial eukaryotes represent a huge resource for assessing ecosystem health. Of these, meiofaunal communities may represent the optimal compromise between functional diversity that can be assessed using morphology and phenotype–environment interactions as compared with smaller life fractions. Here, using 454 Roche sequencing of the 18S nSSU barcode we investigate which of the local natural drivers are most strongly associated with microbial metazoan and sampled protist diversity across the full salinity gradient of the estuarine ecosystem. In order to investigate potential variation at the ecosystem scale, we compare two geographically proximate estuaries (Thames and Mersey, UK) with contrasting histories of anthropogenic stress. The data show that although community turnover is likely to be predictable, taxa are likely to respond to different environmental drivers and, in particular, hydrodynamics, salinity range and granulometry, according to varied life-history characteristics. At the ecosystem level, communities exhibited patterns of estuary-specific similarity within different salinity range habitats, highlighting the environmental sequencing biomonitoring potential of meiofauna, dispersal effects or both.     

Effects of natural hydrological variability on fish assemblages in small Mediterranean streams: Implications for ecological assessment

Small Mediterranean streams are shaped by predictable seasonal events of flooding and drying over an annual cycle, and present a strong inter and intra-annual variation in flow regime. Native fish assemblages in these streams are adapted to this natural environmental variability. The distinction of human-induced disturbances from the natural ones is thus a crucial step before assessing the ecological status of these streams. In this aim, the present study evaluates the effects of natural hydrological variability on fish assemblages from disturbed and least disturbed sites in small intermittent streams of south Portugal. Data were collected over the last two decades (1996–2011) in 14 sites located in the Guadiana and Sado river basins. High variability of fish assemblages was strongly dependent on human-induced disturbances, particularly nutrient/organic load and sediment load, and on natural hydrological variability. Natural hydrological variability can act jointly with anthropogenic disturbances, producing changes on fish assemblages structure of small intermittent streams. In least disturbed sites, despite the natural disturbances caused by inter-annual rainfall variations (including drought and flood events), fish assemblages maintained a long-term stability and revealed a high resilience. On the contrary, disturbed sites presented significantly higher variability on fish assemblages and a short and long-term instability, reflecting a decrease on the resistance and resilience of fish assemblages. Under these conditions, fish fauna integrity is particularly vulnerable and the ecological assessment may be influenced by natural hydrological variations. High hydrological variability (especially if it entails high frequency of dryer years and meaningful cumulative water deficit) may affect the impact of the human pressures with significant and consistent consequences on fish assemblage composition and integrity. In this study, fish metrics that maximize the detection of human degradation and minimize the response to natural variability were based on the relative abundance of native species (insectivorous species, eurytopic species, water column species, native lithophilic species), relative abundance of species with intermediate tolerance and relative number of exotic species. Results highlight the importance of assessing temporal variability on stream biomonitoring programs and emphasize the need to improve the assessment tools, accounting for long-term changes in fish assemblages, namely by selecting the most appropriate fish metrics that respond to anthropogenic disturbances but exhibit low natural temporal variability, essential both in the characterization of the biological reference conditions and in the development of fish indexes in intermittent streams.     

A benthic perspective in assessing the ecological status of estuaries: The case of the Mondego estuary (Portugal)

In transitional waters the process of defining reference conditions (in the scope of the WFD) must account for the natural great variability of such environments. Therefore, stretches reflecting different physical–chemical and biological conditions throughout the system should be defined in order to correctly establish benthic specific reference conditions. Both salinity and sediment structure are major factors controlling physical–chemical conditions and therefore organisms’ distribution within an estuary. These environmental variables (salinity, sediment grain size composition and organic matter content) patterns were studied in the Mondego estuary and some clear gradients emerged. Also, ecological indices (AMBI, Margalef and Shannon-Wiener) were applied to subtidal benthic communities of the Mondego estuary and, generally, there was not only evidence of a decrease in diversity in the estuary from the downstream section towards its inner parts, but also differences were found between areas of distinct sediment composition. After comparing environmental patterns with biodiversity trends, the information was used to define homogeneous sectors along a temperate estuary in Portugal. In the Mondego estuary six zones, covering the main physical gradients affecting benthic communities, were defined: four in the northern arm and two in southern arm. Zones established will allow future determination of benthic reference conditions adjusted for each of the sectors, according to their characteristics, and consequently the conditions they provide for benthic assemblages settlement.     

Scavenging Rate Ecoassay: A Potential Indicator of Estuary Condition

Monitoring of estuary condition is essential due to the highly productive and often intensely impacted nature of these ecosystems. Assessment of the physico-chemical condition of estuaries is expensive and difficult due to naturally fluctuating water quality and biota. Assessing the vigour of ecosystem processes is an alternative method with potential to overcome much of the variability associated with physico-chemical measures. Indicators of estuary condition should have small spatial and temporal variability, have a predictable response to perturbation and be ecologically relevant. Here, we present tests of the first criterion, the spatio-temporal variability of a potential ecoassay measuring the rate of scavenging in estuaries. We hypothesised that the proposed scavenging ecoassay would not vary significantly among A) sites in an estuary, B) trips separated by weeks, or C) days in a trip. Because not all habitats are present in all estuaries, this test was undertaken in two habitats. When conducted over bare substrate there were occasional significant differences, but no discernible patterns, within levels of the experiment. When conducted over vegetated substrate, days within a trip did not vary significantly, but later trips experienced greater scavenging. This scavenging ecoassay shows potential as a tool for assessing the condition of estuarine ecosystems, and further exploration of this protocol is warranted by implementation in estuaries across a gradient of anthropogenic stress.     

Landscape transformation alters functional diversity in coastal seascapes

The ecological impacts of landscape modification and urbanisation have transformed the composition of plant and animal assemblages, and altered the condition of ecosystems globally. Landscape transformation influences the spatial distribution of species and ecological functions by selecting for generalist species with wide ecological niches, which can adapt to opportunities in highly-modified environments. These effects of landscape modification can shape functional diversity on land, but it is not clear whether they have similar functional consequences in the sea. We used estuaries as a model system to test how landscape transformation alters functional diversity in coastal seascapes, and measured how variation in level of urbanisation, catchment modification and habitat loss influenced fish diversity across thirty-nine estuaries in eastern Australia. Fish were surveyed with baited remote underwater video stations and functional diversity was indexed with three metrics that describe variation in the functional traits and niche space of assemblages. The extent of landscape transformation in the catchment of each estuary was associated with variation in the functional diversity of estuarine fish assemblages. These effects were, however, not what we expected as functional diversity was highest in modified estuaries that supported a large area of both urban and grazing land in their catchments, were bordered by a small area of natural terrestrial vegetation and that contained a moderate area of mangroves. Zoobenthivores and omnivores dominated assemblages in highly-modified estuaries, and piscivorous fishes were common in natural waterways. Our results demonstrate, that the modification and urbanisation of ecosystems on land can alter functional diversity in the sea. Intense landscape transformation appears to select for abundant generalists with wide trophic niches, and against species with specialised diets, and we suggest that these changes might have fundamental consequences for ecosystem functioning in estuaries, and other highly modified seascapes.     

Seasonal fluctuations in taxonomic and functional diversity in assemblages of catfishes in the Venezuelan Arauca River Floodplain

The Arauca River harbors a considerable fish biodiversity, yet it remains poorly sampled and the knowledge about its fish communities is incomplete. We studied the taxonomic and functional composition and diversity of catfish assemblages in one lotic and one lentic habitat of a segment of this floodplain during one hydrological cycle, from August 2014 to August 2015, comprising six samplings using gillnets, and identified the relationships between catfish traits and the environment based on five limnological variables. The functional structure was studied through 11 morphological measures and the species diets. We identified 31 catfish species and nine trophic groups. There were significant differences in species composition and diversity between habitats and months, but the functional structure of the catfishes was only determined by the temporal variability. The functional structure of the catfish assemblages was ordered by morphological traits related to locomotion, habitat use, and trophic ecology and these traits were correlated with the limnological variables, supporting the existence of environmental filters. Integrating the taxonomic and functional perspectives can provide a more complete picture of the ecological patterns of freshwater fish assemblages in the Neotropics and can help us predict how these assemblages might change in response to anthropogenic alterations of the environment.     

Composition, temporal changes and ecological guild classification of the ichthyofaunas of large European estuaries – a comparison between the Tagus (Portugal) and the Elbe (Germany)

Fish species densities from 1025 hauls obtained with beam trawl and stow net catches from the southern Tagus and northern Elbe estuaries were analysed for two different periods of time between 1978 and 1997. The data were related with environmental factors to compare species composition and densities, contributions of different ecological guilds, structuring environmental key factors, intra‐annual cyclicity and interperiodical stability of ichthyofaunas in large European estuaries at different latitudes. Although the total number of species in both the Tagus and Elbe estuaries (Tagus: 57 species, Elbe: 58 species) was very similar, significant differences were estimated in species compositions, species densities and number of species, and individuals of the different ecological guilds between both estuaries. A total of 15 species occurred in both estuaries but only two of these species, the catadromous Anguilla anguilla and the marine estuarine opportunist Pleuronectes flesus, contributed substantially to the total abundance. Marine species together contributed more than 5% of the total number of species in the Tagus and Elbe estuaries, whereas their contribution in number of individuals amounted to 22% in the Tagus and only 9% in the Elbe. In terms of number of individuals, the ichthyofauna of the Tagus estuary was dominated by estuarine species (especially Pomatoschistus spp.) contributing 68% of the total abundance. In contrast, the fish fauna of the Elbe estuary was dominated by high numbers of anadromous species (88%) individuals, especially Osmerus eperlanus. Canonical corresponding analyses (CCA) indicated that, besides intra‐annual variables, salinity and mouth distance were two of the most important environmental factors structuring the fish communities in both estuaries. Moreover, water depth was found to be an important environmental factor in the Tagus estuary, whereas the water temperature substantially affected the ichthyofaunal composition in the Elbe estuary. In both estuaries, contributions by number of species and number of individuals of the different ecological guilds were not statistically different between the two periods considered, 1978–80 and 1995–97 for the Tagus, and 1981–86 and 1989–95 for the Elbe. Nevertheless, from 1981–86 until 1989–95 a remarkable increase of about 4.5 times in the densities of O. eperlanus was observed in the Elbe estuary, probably related to improved water quality, especially of oxygen conditions in the nurseries since 1990. This was also supported by the results of CCA, suggesting important influences of the years 1989 and 1990 on the Elbe estuary fish assemblage. Climate change as a result of global warming may be evident for the observed changes of the densities of P. flesus which decreased in the southern Tagus estuary and, in contrast, increased in the northern Elbe estuary during comparable periods of time. It is recommended to use the ecological guild classification scheme which was applied for the Tagus and Elbe estuaries for further studies of European estuarine fish assemblages.     

Do nematode and macrofauna assemblages provide similar ecological assessment information?

Do nematode and macrofauna assemblages provide similar ecological assessment information? To answer this question, in the summer of 2006, subtidal soft-bottom assemblages were sampled and environmental parameters were measured at seven stations covering the entire salinity gradient of the Mondego estuary. Principal components analysis (PCA) was performed on the environmental parameters, thus establishing different estuarine stretches. The ecological status of each community was determined by applying the Maturity Index and the Index of Trophic Diversity to the nematode data and the Benthic Assessment Tool to the macrofaunal data. Overall, the results indicated that the answer to the initial question is not straightforward. The fact that nematode and macrofauna have provided different responses regarding environmental status may be partially explained by local differentiation in microhabitat conditions, given by distinct sampling locations within each estuarine stretch and by different response-to-stress times of each benthic community. Therefore, our study suggests that both assemblages should be used in marine pollution monitoring programs.     

The Influence of Selected Environmental Variables on Fish Assemblage Structure in a Range of Southeast African Estuaries

This study compared the ichthyofaunal assemblages in 10 Eastern Cape Province, South Africa, estuaries to identify whether there were any structural variations between the assemblages and to determine the environmental and physical factors linked to these variations. Species belonging to both the marine migrant and estuarine resident groups, by consistent representation within each estuary grouping, were responsible for the separation of fish assemblage structures between the different types of systems. The data further revealed that fish assemblages in the three main types of estuaries investigated were distinct, linked primarily to estuary mouth status and estuary size.     

The relative influence of catchment, riparian corridor, and reach-scale anthropogenic pressures on fish and macroinvertebrate assemblages in French rivers

This study compares the relative influences of physiography and anthropogenic pressures on river biota at catchment, riparian corridor, and reach scales. Environmental data, catchment and riparian corridor land use, anthropogenic modifications and biological data were compiled for 301 French sites sampled from 2005 to 2008. First, relationships between anthropogenic pressures and fish and macroinvertebrate assemblages were analysed using redundancy analysis. Second, the influences of physiography and the three scales of human pressures on biological assemblages were measured using variance partitioning. Distributions of fish and macroinvertebrate taxa along the pressure gradients agreed with bio-ecological knowledge. At the reach scale, assemblage variability among the 301 French sites was related to the presence of an impoundment and to poor water quality, while at larger scales it was linked to a gradient from forest to agricultural covers. In addition, a large proportion of the explained variability in assemblage composition was related to complex interactions among factors (~40%) and to physiographic variables (~30%). Furthermore, our results highlight that catchment land use better reflects local water quality impairments than hydromorphological degradations. Finally, this study supports the idea that human pressure effects on river communities are linked at several spatial scales and must be considered jointly.