Mediterranean Estuary Research Articles

The Transmission Effectiveness of the Dissolved Nutrients (N, P, Si) Through a South Mediterranean Estuary (Seybouse, Algeria) Under Large Anthropogenic Forcing

The Seybouse Estuary (SE) is the downstream of the second great river in Algeria draining one of the main country’s watersheds (6471 km2, 1.5 million inhabitants). This study highlights the effect of the estuarine part on the distribution and the flux of dissolved nitrogen (N), phosphorus (P) and silicon (Si) through the SE. Surface water sampling was taken and analysed in the dry (September 2016) and wet (April 2017) seasons. Physical parameters and water discharge data were jointly recorded in parallel to sampling dates. Very low flows varying between 0.3 and 4.7 m3 s-1 were recorded at the outlet in dry and wet seasons, respectively. The SE outlet waters were 2.6 and 10 folds higher than upstream in dissolved inorganic nitrogen (DIN) and phosphates (PO4) levels, respectively. However, the silicates (SiO4) levels decreased by about 1/2 in the outlet waters. In terms of flux, the SE delivered to the coast 84 t·yr-1, 9 t·yr-1 and 56 t·yr-1 of DIN, PO4 and SiO4, respectively. The high DIN:PO4 = 137 and low SiO4:DIN = 0.52 molar ratios classify SE as one of the strong anthropised estuaries. The estuarine part amply modifies cycles and budgets of riverine nutrients introduced to the coast.

Monitoring Sand Spit Variability Using Sentinel-2 and Google Earth Engine in a Mediterranean Estuary

Estuarine degradation is a major concern worldwide, and is rapidly increasing due to anthropogenic pressures. The Mediterranean Guadiaro estuary, located in San Roque (Cadiz, Spain), is an example of a highly modified estuary, showing severe negative effects of eutrophication episodes and beach erosion. The migration of its river mouth sand spit causes the closure of the estuary, resulting in serious water quality issues and flora and fauna mortality due to the lack of water renewal. With the aim of studying the Guadiaro estuary throughout a 4-year period (2017–2020), the Sentinel-2 A/B twin satellites of the Copernicus programme were used thanks to their 5-day and 10 m temporal and spatial resolution, respectively. Sea–land mapping was performed using the Normalized Difference Water Index (NDWI) in the Google Earth Engine (GEE) platform, selecting cloud-free Sentinel-2 Level 2A images and computing statistics. Results show a closure trend of the Guadiaro river mouth and no clear sand spit seasonal patterns. The study also reveals the potential of both Sentinel-2 and GEE for estuarine monitoring by means of an optimized processing workflow. This improvement will be useful for coastal management to ensure a continuous and detailed monitoring in the area, contributing to the development of early-warning tools, which can be helpful for supporting an ecosystem-based approach to coastal areas.

Environmental influences on the recruitment dynamics of juvenile European eels, Anguilla anguilla, in a small estuary of the Tyrrhenian Sea, Sardinia, Italy

Abstract The European eel, Anguilla anguilla, is a catadromous and migratory species of commercial importance. Its complex life cycle results in its exposure to many risk factors, which have resulted in stock declines across all life stages since the 1970s. The temporal recruitment dynamics of juvenile eels (glass eels and elvers) were investigated in a small Mediterranean estuary (Sardinia, Italy). The composition of the population and the monthly and seasonal variations in the abundances of juvenile eels was assessed over 78 sampling events (from February 2017 to February 2018). Furthermore, the effects of abiotic variables on the abundances of glass eels and elvers were investigated using generalized additive models (GAMs). Glass eels had the greatest abundance during the winter months, whereas elvers had the greatest abundance during spring. Modelling revealed that the abundance of glass eels was mostly explained by the combined effects of water temperature (12.3–14.5 °C), tidal coefficient (40–110 cm), moon phase, season, and river mouth condition, whereas the abundance of elvers was associated with water temperature (14–21 °C), dissolved oxygen content (>7 mg/L), and season. These results suggest that the annual recruitment of juvenile eels occurs throughout the year, with clear seasonal migration dynamics. The use of multiple statistical approaches allowed us to identify the importance of several environmental variables in regulating the recruitment dynamics, providing useful information for conserving eel stocks through the restoration of the natural flow regime and the connectivity between freshwater habitats and the sea.

Episodic Biogeochemical Variability in a Low-Flow Mediterranean Estuary

To investigate to what extent episodic physical processes regulate nutrient availability and phytoplankton assemblages of the Mahon estuary (Minorca Island), we carried out an intensive field study during 2010–2011. During the study period, environmental conditions spanned from intense stratification to a continuous mixing and from lack of riverine inflow to intense runoff. Our data reveals a sequence of biogeochemical states of the estuary that result from the interplay between runoff, other non-periodic forcings (winds, sea level oscillations), and variations in water renewal. Seasonal runoff was revealed as a major driver of winter circulation and of the influx of inorganic nutrients, in particular nitrate. However, because of the combination between runoff and flushing time, the effects of floodwater events on phytoplankton are short-lived (days). Conversely, during summer, when freshwater influx declines, water renewal relies on pulsed atmospheric forcing that may be of local or remote origin. As depicted from the low nitrate concentrations ( 1 μM), this change in circulation and external loads carries nutrient assimilation within the estuary head and forces the use of remnant nutrients through regenerating pathways to sustain an enhanced phytoplankton biomass at the lower estuary. Episodic variability represented between 52 and 65% of the annual chlorophyll variance. Despite the fact that episodic pulses represented intense departures from base biogeochemical state of the estuary, at time scale larger than weeks, the phytoplankton community composition and dynamics was largely regulated by the integrated effect of these episodes and other environmental drivers associated with seasonality rather than by individual storm events only. Our results suggest that even though the system presents good recovery capacity to individual storm episodes, it may be more vulnerable to increased nutrient fluxes during summer, as well as to changes in episode timing and frequency.

Bacterial diversity across a highly stratified ecosystem: A salt-wedge Mediterranean estuary

Highly stratified Mediterranean estuaries are unique environments where the tidal range is low and the tidal currents are almost negligible. The main characteristics of these environments are strong salinity gradients and other environmental parameters. In this study, 454 pyrosequencing of the 16S rRNA gene in combination with catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) was used to estimate the bacterial diversity across the Krka estuary in February and July 2013. The comparison of the data derived from these two techniques resulted in a significant but weak positive correlation (R=0.28) indicating a substantial difference in the bacterial community structure, depending on the applied method. The phytoplankton bloom observed in February was identified as one of the main factors shaping the bacterial community structure between the two environmentally contrasting sampling months. Roseobacter, Bacteroidetes and Gammaproteobacteria differed substantially between February and July. Typical freshwater bacterial classes (Actinobacteria and Betaproteobacteria) showed strong vertical distribution patterns depending on the salinity gradient. Cyanobacteria decreased in abundance in February due to competition with phytoplankton, while the SAR11 clade increased its abundance in July as a result of a better adaptation toward more oligotrophic conditions. The results provided the first detailed insight into the bacterial diversity in a highly stratified Mediterranean karstic estuary.

Benthic macrofaunal dynamics and environmental stress across a salt wedge Mediterranean estuary

The spatial distribution of benthic macroinvertebrate community in relation to environmental factors was studied along the Ebro Estuary (NE Iberian Peninsula), a salt wedge Mediterranean estuary. Both ordination methods and generalized additive models were performed to identify the different benthic assemblages and their relationship to abiotic factors. Our results showed a strong relationship between macrofaunal assemblages and the predominant environmental gradients (e.g. salinity); thus revealing spatial differences in their structure and composition. Two different stretches were identified, namely the upper (UE) and the lower Ebro Estuary (LE). UE showed riverine characteristics and hence was colonized by a freshwater community; whereas LE was influenced by marine intrusion and sustained a complex marine-origin community. However, within each stretch, water and sediment characteristics played an important role in explaining species composition differences among sampling stations. Moreover, outcomes suggested a total species replacement pattern, instead of the nestedness pattern usually associated with well-mixed temperate estuaries. The sharp species turnover together with the estuarine stratification point out that the Ebro Estuary is working, in terms of ecological boundaries, under an ecotone model. Finally, despite obvious differences with well mixed estuaries (i.e. lack of tidal influence, stratification and species turnover), the Ebro Estuary shares important ecological attributes with well-mixed temperate estuaries.

FISH DIVERSITY AND DISTRIBUTION IN THE AKSU RIVER ESTUARY (ANTALYA-TURKEY), IN RELATION TO ENVIRONMENTAL VARIABLES

Seasonal changes in abundance, species composition, and life-style categories of the fish assemblage in a shallow Mediterranean estuary of southwestern Turkey are described. Fishes were sampled monthly by different nets in Aksu River Estuary from October 2006 and March 2008. In total, 3347 individuals from 26 species were captured. Mugil cephalus, Vimba vimba and Liza aurata were the dominant species and comprised over 50% of the total number. The fish comprised 10 marine, 14 freshwater and 2 migrant. The index of species richness, Shannon-wiener diversity, and evenness each showed a consistent seasonal pattern. Canonical correspondance analysis indicated that salinity correlated with the seasonal pattern in the fish assemblage.

Composition of Eukaryotic and Prokaryotic rRNA Gene Phylotypes in Guts of Adults and Fingerlings of Mugil cephalus, Inhabiting an Egyptian Mediterranean Estuary

To understand the contribution of gut microflora to the feeding and health of Mugil cephalus in a Mediterranean estuary, we have explored and evaluated the diversity of rRNA genes in gut contents of both of adults and fingerlings. Bulk DNAs were extracted from the fish guts, followed by PCR, cloning and sequencing of eukaryote 18S rRNA gene; bacterial and archaeal 16S rRNA genes. Rarefaction analyses recorded 11, 18 and 13 phylotype groups of rRNA genes for eukaryotes, bacteria and archaea, respectively, in the adult guts. The fingerling guts contained 6 and 11 phylotype groups of rRNA genes for eukartyotes and bacteria, respectively, while lacked archaea. Adult and fingerling guts were dominated by zooplankton-like 18S rRNA gene phylotypes, belonging to those of rotifers, genus Brachionus and copepods, genus Apocyclops. Both of diatoms, genus Cyclotella- and fish-like rRNA gene phylotypes were recorded only in adult guts. Phylotypes, which belonged to uncultured proteobacteria and cyanobacteria, covered most of the bacterial 16S rRNA gene composition of adult guts. Bacteroidetes-like phylotypes characterized the fingerling guts. The study showed the first record of uncultured thaumarchaeote-like 16S rRNA genes in the fish guts, opening a window for exploring thaumarchaeote biological significances in fish metabolism. The fish guts represented a reservoir for geographic distant species and reflected the plankton ecosystem variability at the studied estuary. The pollution in the estuary could shape a fraction of the fish gut microbiota. To our knowledge, this work presented the first complete profile of rRNA gene diversities for all known microbiome domains in fish guts.

Species-specific variation in the phosphorus nutritional sources by microphytoplankton in a Mediterranean estuary

We investigated the species-specific phosphorus (P) nutrition sources in the microphytoplankton community in the Mahon estuary (Minorca, Western Mediterranean) in 2011, under two contrasting hydrographic scenarios. Estuarine flow, nutrient concentrations, phytoplankton community composition and enzyme-labeled fluorescence (ELF) were measured in June and October, corresponding to the beginning and the end of summer. Dissolved inorganic nitrogen (DIN) and inorganic phosphate (Pi) exhibited enhanced concentrations in the inner estuary where N:P molar ratios suggested P-limitation in both surveys. Pi was low and variable (0.09±0.02 μmol•l-1 in June and 0.06±0.02 μmol•l-1 in October), whereas organic phosphorus remained a more reliable P source. Even though ambient Pi concentrations were slightly higher on June, when the microphytoplankton assemblage was dominated by dinoflagellates, the percentage of cells expressing ELF labeling was notably higher (65% of total cells) than in October (12%), when the presence of diatoms characterized the microphytoplankton community. ELF was mainly expressed by dinoflagellate taxa, whereas diatoms only expressed significant AP in the inner estuary during the June survey. A P-addition bioassay in which response of AP to Pi enrichment was evaluated showed remarkable reduction in AP with increasing Pi. However, some dinoflagellate species maintained AP even when Pi was supplied in excess. We suggest that in the case of some dinoflagellate species AP is not as tightly controlled by ambient Pi as previously believed. AP activity in these species could indicate selective use of organic phosphorus, or slow metabolic response to changes in P forms, rather than physiological stress to low Pi availability. We emphasize the importance of identifying the links between the different P sources and the species-specific requirements, in order to understand the ecological response to anthropogenic biogeochemical perturbations.

Towards a suitable ecological status assessment of highly stratified mediterranean estuaries: A comparison of benthic invertebrate fauna indices

Biotic indices developed to assess the ecological status of coastal waters according to the European Water Framework Directive (WFD) often show discrepancies when they are applied in transitional environments. Although several indices have been widely used in transitional waters throughout Europe, there is still a lack of knowledge about their suitability assessing ecological status. We evaluated the performance of most common used biotic indices and community parameters (e.g. Multivariate AZTI's Marine Biotic Index (M-AMBI), BENTIX, Benthic Opportunistic Polychaetes Amphipods index (BOPA), diversity indices, species richness, abundance) that have been proposed in the scope of WFD, using data of macroinvertebrate community coming from a special case of transitional water body, the highly stratified Ebro estuary. Additionally, we tested their ability to respond to the main pressures affecting the Ebro estuary, the hydrological alteration due to regulation and the pollution pressure due to nutrient enrichment. Estimation of hydrological alteration was based on flow historical data (period from 1913 to 1963), that we assumed as 'hydrological reference conditions' for Ebro estuary. Pollution pressure was estimated by means of PCA analysis including organic and nutrient enrichment related variables, expressed as a synthetic index by PCA factor scores extraction. All the community parameters were able to detect changes in macrofauna composition along the estuarine gradients and were able to differentiate between the impoverished stations and the healthier ones. Regarding indices, the ratings were contradictory and only M-AMBI classified the stations in the correct way. Strong significant correlations were found between indices and metrics and the calculated pressures; nevertheless, these correlations showed a paradoxical result, since increasing hydrological alteration benefited the macrofauna, achieving great complexity. Other identified limitations of biotic indices were the opposite classifications, overestimation of ecological status and low resolution ability. We conclude that for transitional water ecosystems, where each water body has particular characteristics, is difficult the use of 'common biological' assessment tools as the results of this study, among others (more details in discussion section), have demonstrated. Nevertheless, M-AMBI seemed to work in the correct way, so further investigation about its use for transitional waters is necessary. The development of new strategies such as the use of historical data, the use of metrics as a complement for the assessment could be a reliable alternative.

Freshwater scarcity effects on the aquatic macrofauna of a European Mediterranean-climate estuary

In the Mediterranean-climate zone, recurrent drought events and increasing water demand generally lead to a decrease in freshwater input to estuaries. This water scarcity may alter the proper function of estuaries as nursery areas for marine species and as permanent habitat for estuarine species. A 12-year data set of the aquatic macrofauna (fish, decapod and mysid crustaceans) in a Mediterranean estuary (Guadalquivir estuary, South Spain) was analysed to test if water scarcity favours the nursery function of regional estuaries to the detriment of permanent estuarine inhabitants. Target species typically displayed a salinity-related distribution and estuarine salinisation in dry years resulted in a general upstream community displacement. However, annual densities of marine species were neither consistently higher in dry years nor estuarine species during wet years. Exceptions included the estuarine mysid Neomysis integer and the marine shrimp Crangon crangon, which were more abundant in wet and dry years, respectively. High and persistent turbidity, a collateral effect of water scarcity, altered both the structural (salinity-related pattern) and functional (key prey species and predator density) community characteristics, chiefly after the second drought period of the analysis. The observed high inter-year environmental variability, as well as species-specific effects of water scarcity, suggests that exhaustive and long-term sampling programmes will be required for rigorously monitoring the estuarine communities of the Mediterranean-climate region.

Tidal transformation and resonance in a short, microtidal Mediterranean estuary (Alfacs Bay in Ebre delta)

Tidal and subtidal waves are analyzed with sea-level data and numerical modeling in a short and micro-tidal embayment, Alfacs Bay (NW Mediterranean Sea). Data analysis exhibits tidal wave amplification and seiching (characteristic period of 3.5 h) along the bay. Numerical results show an eight-fold increase in quarter-wave resonant wave amplitudes from the mouth to the head of the bay. This amplification follows the classical description of a standing wave. Moreover, resonant wave velocities measured and computed at the bay mouth (node location) are about one order of magnitude higher than tidal currents. Analysis of astronomic tidal propagation in the bay reveals similar behavior for diurnal and semidiurnal constituents. Tidal waves amplify along the bay by 3% for diurnal and 10% for semidiurnal constituents. Numerical simulations conducted with different domains indicate that geometric effects dominate over frictional influences in causing the wave behavior. This behavior is consistent with the existence of a quasi-steady standing wave within the bay.

Catchment controls and human disturbances on the geomorphology of small Mediterranean estuarine systems

Geographic signatures are physical and human-induced characteristics or processes that identify comparable or unique features of estuaries along latitudinal gradients. In Mediterranean areas, the microtidal regime and the strong seasonal and inter-annual contrasts cause an alternation between relatively high runoff and arid conditions. Furthermore, the long history of human settlement also increases the complexity in the study of these estuarine systems. This study investigates these signatures of the estuaries located within the Mallorcan eastern coast, which are geomorphologically homogeneous because of a similar bedrock geology and Holocene history. A multi-method approach focused on the integration of geomorphometry, hydraulics, historical sources and statistics was used. We explore the role played by catchment morphometric parameters, severe flash flood events and human disturbances in controlling the geomorphology of 10 beach-barrier enclosed, fluvial incised lagoons. Most of the lagoons discharge into ‘calas’, ranging in size from 1345 to 17,537 m2 and their related catchments are representative of the Mediterranean hydrological systems. Multiple regression models illustrate that the size, slope and drainage network development of the catchments explain the variance in length (r2 = 0.67), volume (r2 = 0.49), area (r2 = 0.64), circularity (r2 = 0.72) and average width (r2 = 0.81) of the lagoons. Depending on these catchment morphometric variables, the shape of the lagoons is also determined by the occurrence of catastrophic flash floods, which cause scouring and dredging, whereas the ordinary flood events and sea storms promote refilling and sedimentation. A historical analysis since 1850 documented 18 flood events, 5 of which were catastrophic with destructive effects along the catchments and large morphological changes in coastal lagoons. High intensity rainfall (up to 200 mm in 2 h), the geomorphometry of the catchments and the massive construction of terraces and transverse walls are involved in the generation of catastrophic flood events. Additionally, the lagoons were altered considerably by human intervention for flood control and to allow for an increased amount of human activities within the surrounding areas, although the high recurrence of catastrophic flood events causes a persistent difficulty in the human battle to dominate these ecosystems. Therefore, the area occupied by lagoons increased between 1956 and the present time from 31,981 m2 to 63,802 m2 because of the high recurrence of catastrophic flood events. Furthermore, tourism demand and a social conservation consciousness have promoted restoration and preservation since the 1990s. This study has improved the geomorphological knowledge of small Mediterranean estuaries affected by human disturbances in the high-energy environment found in Mallorca.

Understanding the spatio-temporal variability of phytoplankton biomass distribution in a microtidal Mediterranean estuary

Understanding the spatio-temporal variability of phytoplankton in aquaculture zones is necessary for the prevention and/or prediction of harmful algal bloom events. Synoptic cruises, time series analyses of physical and biological parameters, and 3D modeling were combined to investigate the variability of phytoplankton biomass in Alfacs Bay at basin scale. This microtidal estuary located in the NW Mediterranean is an important area of shellfish and finfish exploitation, which is regularly affected by toxic outbreaks. Observations showed the existence of a preferential phytoplankton accumulation area on the NE interior of the bay. This pattern can be observed throughout the year, and we show that it is directly linked to the physical forcing in the bay, in particular, the interplay between freshwater input and wind-induced turbulence. Both drivers affect the strength of the estuarine circulation, explaining nearly 75% of the variability in phytoplankton biomass. More cells are retained when stratification is weakened and the estuarine circulation reduced, while flushing rates are higher during times of increased stratification and stronger estuarine flow. This has been confirmed by using a 3D hydrodynamic model with Eulerian tracers. Nutrients, while important to support phytoplankton populations, have been found to play only a secondary role in explaining this variability at basin scale.

Observations on the life cycle and ecology of Acanthoica quattrospina Lohmann from a Mediterranean estuary.

Observations on the life cycle and ecology of <i>Acanthoica quattrospina</i> Lohmann from a Mediterranean estuary.

Positive trends between salinity and chromophoric and fluorescent dissolved organic matter in a seasonally inverse estuary

Estuaries present high content of chromophoric and fluorescent dissolved organic matter (CDOM, FDOM) affecting the attenuation of ultraviolet and blue radiation. In temperate and tropical estuaries, the main CDOM source is riverine input that generally mixes conservatively with the oceanic waters leading to negative relationships between CDOM and salinity. In Mediterranean estuaries, riverine inputs are more limited or absent during the dry season producing a negative hydrological budget and, consequently, hypersalinity and an inverse circulation pattern. Much remains to be understood about CDOM and FDOM dynamics during rainy and dry periods in these last estuaries. In this study, we determined DOM absorbance and fluorescence in the Bay of Cádiz, Spain during a rainy (high riverine inputs) and dry (hypersaline conditions) period. We determined the absorption coefficients at 355 nm (a355) and acquired excitation–emission matrices (EEMs). The EEMs showed two fluorescence peaks associated with terrestrial humic-like components (peaks A and C), one peak considered to be a marine humic-like component (peak M), and two peaks considered to be amino acid-like components (peaks T and B). The a355 values ranged from 0.30 to 1.99 m−1 with the highest values during the dry season. From the rainy season to the dry season, the overall fluorescence intensity increased and fluorescent peaks M, B, and T increased by greater than two orders of magnitude, whereas the fluorescence intensities of peaks A and C changed less than one order of magnitude. Unlike temperate and tropical estuaries, the a355 values and fluorescence of the five peaks were positive and significantly related to salinity, but with different slopes. The inverse nature of the Bay of Cádiz during the dry season could be responsible for these positive trends between CDOM/FDOM and salinity. The slopes of the humic-like peaks A and C were lower than the a355 slope suggesting their preferential losses, likely due to photobleaching. By contrast, the slopes of the amino acid-like peaks B and T were higher than the a355 slope suggesting an in situ production of these fluorophores, likely due to diffusion from salt marsh and coastal sediments.

A Regional Survey of the Extent and Magnitude of Eutrophication in Mediterranean Estuaries of Southern California, USA

The magnitude and extent of eutrophication was assessed at 27 segments in 23 estuaries in the Southern California Bight (SCB) between October 2008 and 2009. We applied thresholds from the existing assessment frameworks from both the European Union and the U.S. National Eutrophication Assessment to measurements of three indicators [macroalgae biomass and cover, phytoplankton biomass, and dissolved oxygen (DO) concentration] to categorize eutrophic condition in each estuary. Based on these frameworks, a large fraction of segments had moderate or worse eutrophic condition—78 % based on macroalgae, 39 % for phytoplankton, and 63 % for DO. Macroalgal biomass exceeding 70 g dw m−2 and 25 % cover was found at 52 % of sites during any sampling event and in 33 % of segments for 8 weeks or longer, a duration found to negatively impact benthic infauna. Duration of hypoxic events (DO < 4 mg L−1) was typically short, with most events less than one day; although 53 % of segments had at least one event longer than 24 h. Assessment frameworks of eutrophic condition are likely to evolve over time as the body of literature on eutrophication grows, including aspects such as the applicability of indicators in specific habitat types, indicator thresholds, and how event frequency and duration are incorporated. This paper informs this debate by discussing how eutrophic conditions in SCB estuaries are categorized using different indicators and thresholds. To this end, categorization of estuarine eutrophic condition was found to be very sensitive to the type of threshold, how data are integrated to represent duration or spatial extent, and how indicators are used as multiple lines of evidence.

The effects of hydrological dynamics on benthic diatom community structure in a highly stratified estuary: The case of the Ebro Estuary (Catalonia, Spain)

This study of the distribution of benthic diatom assemblages and their relationship with environmental factors in a highly stratified Mediterranean estuary, i.e. the Ebro Estuary, shows the importance of hydrological dynamics to explain the features of the diatom community in such an estuary, where river flow magnitude and fluctuations imply strong physicochemical variability especially in sites close to the sea. Eight sites along the estuary were sampled during 2007–2008 both at superficial and deep water layers, in order to gather both horizontal and vertical estuarine physicochemical and hydrological gradients. Canonical Variates Analysis and Hierarchical Cluster Analysis segregated diatom community in two assemblages depending on the dynamics of the salt-wedge. The diatom assemblages of riverine conditions (i.e. without salt-wedge influence) where characterised by high abundances of Cocconeis placentula var. euglypta and Amphora pediculus, meanwhile high abundances of Nizschia frustulum and Nitzschia inconspicua were characteristic of estuarine conditions (i.e. under salt-wedge influence). Redundancy Analysis showed that both diatom assemblages responded seasonally to Ebro River flows, especially in estuarine conditions, where fluctuating conditions affected diatom assemblages both at spatial and temporal scale.

Review of the Monograph by Yu.N. Gurskii Geochemistry of the Lithohydrosphere of Interior Seas Volume 1: Methods of the Investigation and the Processes of the Formation of the Chemical Composition of the Interstitial Waters in Deposits of the Black, Azov, Kaspian, White, and Baltic Seas (Moscow, GEOS, 2003.). Volume 2: Interstitial Waters of the Red and Mediterranean Seas and Estuary

Review of the Monograph by Yu.N. Gurskii Geochemistry of the Lithohydrosphere of Interior Seas Volume 1: Methods of the Investigation and the Processes of the Formation of the Chemical Composition of the Interstitial Waters in Deposits of the Black, Azov, Kaspian, White, and Baltic Seas (Moscow, GEOS, 2003.). Volume 2: Interstitial Waters of the Red and Mediterranean Seas and Estuary

Community structure of benthic macroinvertebrates inhabiting a highly stratified Mediterranean estuary

SUMMARY: The community composition and spatial distribution of benthic macroinvertebrates were studied along the Ebro estuary, a highly stratified estuary located in the NE Iberian Peninsula. During the last decade the oligotrophication process occurring in the lower Ebro River and its estuary has allowed a complex benthic macroinvertebrate community to become established; these results contrast with the poor community found there in the early nineties. A total of 214 taxa were identified, and polychaetes dominated the community both in abundance and species richness. The results showed spatial differences in the structure and composition of macroinvertebrates, which suggests that there are two distinct communities along the estuary. Each community was found in a specific stretch (upper and lower estuary) in function of the presence of the salt wedge. The macrobenthos of the upper estuary was dominated by freshwater taxa, but some euryhaline species were also found. The lower estuary showed a marine community typical of shallow Mediterranean environments. The transition between these two communities fits an ecotone model. The highest abundances, richness and diversities were recorded at the lower estuarine stations, especially those closer to the river mouth, whereas the lowest values corresponded to the stations adjacent to the tip of the salt wedge.