Mediterranean Estuary Research Articles

Feeding behavior of the mussel Mytilus galloprovincialis (L.) in a Mediterranean estuary: A field study

The feeding behavior of the mussel Mytilus galloprovincialis was investigated in the field on top of a mussel raft in Alfacs Bay, NW Mediterranean Sea. The experiments were performed in November 2006 and February, April and July 2007 using a flow-through filter feeding device. Total particulate matter (TPM), particulate organic matter (POM) and particulate inorganic matter (PIM) were calculated for the bay water, as well as the feces and pseudofeces of the mussels. These were used with the biodeposition method to estimate several feeding-physiological parameters, such as the clearance rate (CR), rejection rate through pseudofeces production (RP), organic ingestion rate (OIR), absorption rate (AR) and absorption efficiency (AE). The results showed that the characteristics of available suspended matter for mussels in terms of particle concentration (TPM: mg/l) and organic content ( f defined as POM/TPM) ranged 1.03–2.30 mg/l, and 0.48 to 0.73 respectively throughout the study period. This indicates a rather stable feeding environment of low concentrations of high organic content particles despite the wide range of temperatures recorded (from 10 to 26 °C). However, a characteristic of such a variation pattern of particle suspension (TPM and f) was that short-term variations (in the course of days) covered the whole range of annual variation. Accordingly, physiological parameters characterizing both food acquisition and absorption in mussels were found to respond to short-term variations in food regime. Pseudofeces production in mussels was low (less than 5% in most cases) and they tended to reduce their clearance rate instead of increasing their pseudofeces production in response to rising particle concentration. The absorption efficiency was positively related to the organic content of the seston particles. There was also a positive correlation between clearance rate and absorption efficiency. The reduction of clearance, ingestion and absorption rates obtained in July highlights a negative influence of high water temperatures upon feeding and digestive processes of mussels.

Nutrient limitation of phytoplankton growth in the freshwater tidal zone of a turbid, Mediterranean estuary

Identification of the limiting nutrient(s) is a requirement for the rational management of eutrophication. Here, we present the first experimental analysis of nutrient limitation of phytoplankton growth and its seasonal variation in the Guadiana estuary (SE Portugal-SW Spain). Ten microcosm experiments were performed during 2005 and 2008, using water samples collected in the freshwater tidal zone of the Guadiana estuary. Nitrate, phosphate and silicate were added in a single pulse, alone and in combinations. Experimental treatments were incubated for 4 days under controlled laboratory conditions. Phytoplankton response to nutrient enrichment was evaluated through changes in biomass (Chl a), and abundance of specific phytoplankton groups. Overall, phytoplankton growth seemed to be nitrogen-limited throughout the productive period, especially green algae in 2005 and diatoms in 2008. In the summer 2008, cyanobacteria and the harmful dinoflagellate Kryptoperidinium foliaceum responded to N enrichment in the absence of Si. Indeed, the presence of K. foliaceum was observed for the first time in the freshwater tidal reaches of the Guadiana estuary, where dinoflagellates were usually absent or rare. The significant increase on dinoflagellates and cyanobacteria growth in response to N enrichment in the absence of Si is alarming, because anthropogenic nutrient enrichments usually increase N and P, but not Si. Furthermore, relatively high N concentrations, up to 22 μM, were found to be limiting to phytoplankton growth. These results should therefore be used as a management tool when establishing nutrient criteria and nutrient loading budgets to estuarine waters.

Environmental Forcing of Phytoplankton in a Mediterranean Estuary (Guadiana Estuary, South-western Iberia): A Decadal Study of Anthropogenic and Climatic Influences

Phytoplankton seasonal and interannual variability in the Guadiana upper estuary was analyzed during 1996–2005, a period that encompassed a climatic controlled reduction in river flow that was superimposed on the construction of a dam. Phytoplankton seasonal patterns revealed an alternation between a persistent light limitation and episodic nutrient limitation. Phytoplankton succession, with early spring diatom blooms and summer–early fall cyanobacterial blooms, was apparently driven by changes in nutrients, water temperature, and turbulence, clearly demonstrating the role of river flow and climate variability. Light intensity in the mixed layer was a prevalent driver of phytoplankton interannual variability, and the increased turbidity caused by the Alqueva dam construction was linked to pronounced decreases in chlorophyll a concentration, particularly at the start and end of the phytoplankton growing period. Decreases in annual maximum and average abundances of diatoms, green algae, and cyanobacteria were also detected. Furthermore, chlorophyll a decreases after dam filling and a decrease in turbidity may point to a shift from light limitation towards a more nutrient-limited mode in the near future.

First evidence of fiberglass ingestion by a marine invertebrate (Mytilus galloprovincialis L.) in a N.W. Mediterranean estuary

Alfacs Bay is a N.W. Mediterranean estuary important for mussel (Mytilus galloprovincialis) aquaculture. During studies at the site, fiberglass particles were detected. The presence of fiberglass occurred naturally in the water throughout the study period (November 2006 to July 2007). An investigation was undertaken into its role in the feeding behavior of the local mussels. Fiberglass was present in all types of mussel samples. Rejection, which we would have expected for the whole study period, was only evident during the second season studied. To our knowledge, this is the first report of the ingestion of fiberglass by a marine organism. Our novel finding indicates the need to investigate fiberglass ingestion by marine organisms at different levels of the food web and the possible implications for human health and the health of the organisms themselves. In addition, we propose the use of mussels as sentinel organisms to monitor fiberglass contamination in marine ecosystems.

Mechanisms of land–sea interactions – the distribution of metals and sedimentary organic matter in sediments of a river-dominated Mediterranean karstic estuary

This paper examines disposal of metals and the origin, characteristics, and distribution of sedimentary organic matter (SOM) in a Mediterranean karstic estuary in the north-eastern Adriatic. This environment offers a real-time, small model system for studies of geochemical processes in microtidal Mediterranean estuaries that are infilling with sediments and classified as river-dominated disequilibrium estuaries. The results have shown that the longitudinal distribution of heavy metals in sediments follows the sedimentation dynamics and deposition pattern of river-borne, clay mineral particles. The highest concentration of metals was found in the restricted upper part of the estuary, characterized by rapid deposition of clay particles and terrestrial sedimentary organic matter, and decreases toward the open sea. The vertical distribution of metals in sediment cores depends on the prevailing pH and Eh conditions. Significant increases of the concentrations of metals in the uppermost strata are the result of recent anthropogenic inputs. The share of the terrestrial component in SOM, estimated by N/C org atomic ratios and δ 13C values, decreases with distance from the river mouth. The small vertical variation in δ 13C values of SOM indicates that a fast sedimentation rate overrides the diagenetically determined decomposition. The results obtained indicate that river-borne inorganic particles, natural terrigenous organic material, and anthropogenic metal loads are trapped in sediments of the estuarine system. Under the prevailing conditions, there is negligible transport towards the open sea.

Juvenile bottlenecks and salinity shape grey mullet assemblages in Mediterranean estuaries

Previous research has suggested that competitive bottlenecks may exist for the Mediterranean grey mullets (Osteichthyes, Mugilidae) at the fry stage with the exotic Cyprinus carpio (Osteichthyes, Cyprinidae) playing a central role. As a consequence, the structure of grey mullet assemblages at later stages is thought to reflect previous competition as well as differences in osmoregulatory skills. This paper tests that hypothesis by examining four predictions about the relative abundance of five grey mullet species in 42 Western Mediterranean estuary sites from three areas (Aiguamolls de l'Empordà, Ebro Delta and Minorca) differing in the salinity level and occurrence of C. carpio. Field data confirmed the predictions as: (1) Liza aurata and Mugil cephalus were scarce everywhere and never dominated the assemblage; (2) Liza saliens dominated the assemblage where the salinity level was higher than 13; (3) Liza ramado always dominated the assemblage where the salinity level was lower than 13 and C. carpio was present; and (4) Chelon labrosus dominated the assemblage only where the salinity level was lower than 13 and C. carpio was absent. The catch per unit effort of C. labrosus of any size was smaller in the presence of C. carpio than where it had not been introduced, which is in agreement with the juvenile competitive bottleneck hypothesis. Discriminant analysis confirmed that the assemblage structure was linked to the salinity level and the occurrence of C. carpio for both early juveniles and late juveniles as well as adults. The data reported here reveal that the structure of grey mullet assemblages inhabiting Mediterranean estuaries is determined by salinity and competitive interactions at the fry stage.

Habitat selection by grey mullets (Osteichthyes: Mugilidae) in Mediterranean estuaries: the role of salinity

A stratified study of microhabitat use by grey mullet on the island of Minorca (Balearic archipelago, western Mediterranean) showed that the distribution of all the species was dramatically affected by salinity. Sites with a salinity level under 15 were positively selected in spring and summer by those species whose growth performance was the best in oligomesohaline water (Liza ramado and Mugil cephalus) but also by a species whose growth was not affected by salinity (Chelon labrosus). Liza aurata concentrated in polyhaline and euhaline sites, where growth was improved, a pattern also exhibited by Liza saliens. Both species avoided fresh water sites all year round. As a consequence, community structure was correlated with salinity. The above reported electivity patterns often disappeared in autumn, when most grey mullets migrate off-shore.

On the effects of wind and tides on the hydrodynamics of a shallow mediterranean estuary

A study is made of the effect of wind and tides on the hydrodynamics of the shallow inner basins of mediterranean estuaries. The paper includes a case study of Harvey Estuary in southwestern Australia where salinity and temperature data exist for 11 years during the 1980s and 1990s when that estuary experienced massive annual blue-green algal blooms. An analysis is made of salt exchange through the channels that join estuarine basins of this class to either the ocean or, as in the case of Harvey Estuary, to another shallow estuarine basin. A detailed three-dimensional numerical model is also implemented for the basin of Harvey Estuary. It is concluded that exchange through the channel is dominated by the (mainly diurnal) tides, despite the general micro-tidal nature of this class of estuary, although the efficiency of this process is found to be controlled by the length of the channel. Wind set-up in the basin also produces channel exchange and for Harvey Estuary this is about 20% of the exchange due to tides. Baroclinic flow through the channel is also capable of producing significant exchange but this is suppressed by the tidal currents in the channel except immediately after riverflow. Salt transport along the basins of this class of estuary is mainly driven by the longitudinal density gradient and the strength of this process is controlled by vertical mixing from the wind. However, there is also significant salt transport from wind-induced advection, the effect of which changes seasonally with the direction of the salt gradient.

Inter-annual Variability of Bottom Hypoxia in Shallow Mediterranean Estuaries

Shallow mediterranean estuaries receive most of their riverflow during winter, and ocean exchange is usually sufficiently weak that estuarine salinities remain depressed below ocean values throughout spring so creating vertical stratification. These are ideal conditions for bottom hypoxia during spring and an analytical model is presented for the time duration of this hypoxia. It shows that spring hypoxia only occurs if the winter salinity lies below a threshold value which depends both on sediment oxygen demand and physical parameters of the basin. The propensity of a shallow mediterranean estuary to become hypoxic is related not only to vertical mixing, but depends strongly on the vertical shear of currents and this is a critical factor for water quality in basins that are very shallow and microtidal. For one such basin, Harvey Estuary in south-west Australia, the modelled hypoxia threshold corresponds to the winter threshold salinity for blooms of Nodularia spumigena observed during the 1980s. The model is further supported by taking the annual values of observed organic phosphorus content in those blooms and dividing by the number of days of modelled hypoxia for each year. These calculated values of phosphorus content per day are all within a factor of 2 of the published values for the independently measured daily phosphorus release from sediments in the estuary when subjected to hypoxic conditions.

Life history pattern of the endangered Valencia toothcarp, Valencia hispanica (Actinopterygii: Valenciidae) and its implications for conservation

The life history pattern of the most northern population of an endangered endemic fish from the Iberian Peninsula, Valencia hispanica, was studied in Catalonia. Maximum observed ages were 4+ in females, and 3+ in males. The seasonal growth period started in June and continued until October. Females lived longer and reached larger sizes than males. The largest sizes found were 71 mm versus 67 mm for females and males, respectively. V. hispanica is a multiple spawner that releases small batches of oocytes (varying from 10 to 30) between April and July. The release of oocytes is greatest in May and June. After reproduction, the number of parental individuals decreased. The relationships between fecundity (F) and total length (L t , mm), total weight (W t , g) and gonad weight (W g , g), were: F = 15.735 L t -- 432.91; F = 111.70 W t + 101.17; F = 717.36 W g + 220.80. To examine the drastic regression of this species, its life history tactics were compared with those of another protected endemic cyprinodontid species (Aphanius iberus) which still inhabits areas once occupied by the two species. The biology of the Valencia toothcarp is characterised by slow growth, low reproductive effort, relatively late maturity, high longevity and multiple spawning. This seems to be an inappropriate life history strategy for small fish species in unstable environments with high human pressure, such as Mediterranean estuaries (where the species was once abundant) which are characterised by high, variable or unpredictable adult mortality. Some conservation measures and suggestions for a recovery programme are discussed.

Arsenic and antimony release from sediments in a Mediterranean estuary

An ancient realgar mine is responsible for the contamination of a riverine system in central Corsica. Nearby the mine, high As and Sb concentrations are found (up to 2–3 mg l-1 and 2–3 μg l-1 respectively). A significant increase of the As and Sb concentrations in water is found in the water of the mixing zone of river and sea (Bravona estuary; chlorinity 7000–7800). In the same area, a decrease of As and Sb concentrations in sediments is observed. Fe and Mn exhibit the same behaviour. The possible involvement of redox processes is discussed. However, owing to insufficient reducing capacity and insignificant variation of redox potential values along the various estuarine sites (from 320 down to 280 mV), this scenario is rather doubtful. Desorption from FeOOH particles is the most likely process in the Bravona estuary. The impact of these contaminated waters on the marine environment is discussed. Owing to decrease by dilution, as well as the reduced water flow of the estuary, the net supply of As and Sb to the Tyrrhenian Sea is approximately 5 and 1.5 t y-1 for As and Sb respectively. The possible harmful influence of the Bravona river should be of local concern only, and the emphasis of this study is, therefore, on a better knowledge of in situ processes.

Application of the Stommel model to shallow Mediterranean estuaries and their characterization

During summer, the basins of shallow estuaries in mediterranean climates are usually vertically mixed, hypersaline, and sometimes inverse (in the sense of their densities being elevated above ocean values). These estuaries are commonly controlled by heat and salt exchange with the ocean. Because of the compensating effect of estuarine heating, the magnitude of the density inversion (due to hypersalinity) depends on the net surface buoyancy flux. The surface heat and evaporative fluxes are functions of the estuarine temperature and so the estuarine state (and its associated surface buoyancy flux) depends on the strength of the ocean exchange processes. A simple Stommel model of a summer mediterranean estuary is presented here in which this exchange is controlled by density‐driven processes. The Stommel model has two possible stable stationary states and it is shown that the estuary can be characterized by four types of steady state. They include the familiar classical and inverse estuaries plus a solution, called “quasi‐neutral”, in which the estuary is close to neutrality but just marginally inverse; that is, the net buoyancy flux is small and negative. This quasi‐neutral solution resembles the steady state in Tomales Bay (California) in late summer and shows the characteristic high level of thermal compensation of the density inversion from hypersalinity. It also has the observed very long flushing time which is responsible for some of the environmental fragility of this class of estuary. This appears to be the first detailed confirmation of the applicability of the Stommel model to estuaries. The fourth type of estuarine state arises from the simultaneous existence of both stable states. It is here called “intermittent” and contains both the classical and a quasi‐neutral Stommel solution. The type of estuarine state achieved by a narrow basin in autumn is highly dependent on its length and depth. This produces extreme sensitivity to changes in basin geometry from human influences.

The Summer Buoyancy Dynamics of a Shallow Mediterranean Estuary and Some Effects of Changing Bathymetry: Tomales Bay, California

A numerical study is presented of the summer buoyancy dynamics of Tomales Bay (California, U.S.A.) which is an example of a shallow mediterranean estuary. Tomales Bay lies along the line of the San Andreas fault, and the resultant unidirectionality of the estuary greatly simplifies its hydrodynamics. The numerical model hindcasts the temperature and salinity of Tomales Bay from 1987 to 1993 using detailed hydrological and meteorological data, and the results are compared with field observations made during that 7-year period. A second simulation is made over the same 7-year period but using the bathymetry for 1861 when the estuary was longer and deeper. This allows an analysis to be made of the effects on the summer buoyancy dynamics of anthropogenic bathymetry changes. The model predicts that the historical estuary experienced greater negative buoyancy inputs during the late summer periods, and became more inverse with faster dispersion near the head of the basin.

Trace metals transport and behaviour in the Mediterranean estuary of Acheloos river

The concentration levels and the chemical behaviour of trace metals during their transport, were studied in the estuary of the Acheloos river, in western Greece. It is a typical, non tidal and stratified Mediterranean estuary which, although is not heavily polluted, is affected by a lot of human activities (dams, agriculture, fishing, etc.) that, in combination with its hydrological and morphological characteristics, affect also the distribution of trace metals. The concentration levels of metals in the estuary are lower than in polluted Greek gulfs and rivers. In the Acheloos case, in contrast to polluted estuaries, the dissolved metal concentrations are increased seawards whereas the particulate metal concentrations and the metal content of the particles are decreased. Suspended particulate matter and especially particles having diameter < 8 μm were found to play an important role in the processes related to trace metal mobility. These processes include desorption of particulate metals seawards, adsorption of dissolved metals on increased particulate loads, non conservative behaviour of both dissolved and particulate metals and association of particulate metals with particulate Al, Fe and Mn. The behaviour of metals show that Cu, Pb and Mn are possible dangerous pollutants for the system.

Life-History Pattern of the Iberian Toothcarp Aphanius Iberus (Pisces, Cyprinodontidae) From a Mediterranean Estuary, the Ebro Delta (Spain)

The life-history pattern was studied of an endangered endemic cyprinodontid from the Iberian peninsula, Aphanius iberus (Cuvier & Valenciennes, 1846), found in a Mediterranean estuary, the Ebro delta. Maximum ages observed were 1 + in both sexes. The seasonal growth period started in March and continued until September. The females reached larger sizes than the males, as they have a higher rate of growth and live longer. The largest sizes found were 45 mm versus 38 mm. A. iberus is a multiple spawner that releases batches of eggs between May and August but with few eggs each time (varying from 10 to 30). Despite this continuous release of eggs two moments appeared to stand out as more important. After the reproduction period the parental cohort reduced drastically. The relationship between fecundity (F) and total length (mm) was represented by the formula: F = 8.358 x 10 -9 x L t 6.343 . The biology of the Iberian toothcarp is characterized by fast growth, early maturity, high reproductive effort, multiple spawning and reduced longevity. This may be a good life strategy for small fish species in unstable environments, such as estuaries, where adult mortality is high, variable or unpredictable. It allows them to exploit favourable environmental conditions, which occur in a specific, reduced period; practically all the population is renewed during this period.

Trace metals in sediments of a Mediterranean estuary affected by human activities (Acheloos river estuary, Greece)

Trace metals were studied in the sediments of the ecologically, economically and scientifically important estuary of the Acheloos river, in western Greece. Human activities (dams, agriculture, traffic, etc.) influence the estuarine system of Acheloos and in combination with the hydrological, mineralogical and morphological characteristics of the estuary affect the chemical behaviour and the distribution patterns of trace metals in its sediments. The large scale disturbance of the system is imminent in the near future as it is planned to divert ∼50% of the river water. A study of the distribution patterns of trace metals revealed that in the estuary there are zones with different metal levels. The concentrations of most metals (Al, Fe, Cu, Ni, Zn) are elevated in three of these zones (upstream, sill, seawards). A different behaviour was observed for Mn due to its association with carbonates that were observed in significant concentrations throughout the estuarine zone. A sequential extraction procedure, applied to the sediments, indicated low percentages of easily exchangeable metals, increased mobility of Cu and Zn and increased association of Ni, Cr and Fe with the aluminosilicate lattice. Although the river is not considered to be heavily polluted, some metals have shown an enrichment in the surface sediments as a result of general anthropogenic activities not derived from point sources.

Implications of a multi-layered nepheloid system for hydrocarbon distributions and transport in a microtidal Mediterranean estuary

Abstract The origin and fate of non-aromatic hydrocarbons (NAH) and polycyclic aromatic hydrocarbons (PAH) were studied at the land-sea boundary in the microtidal estuary of the Rhone River. The sampling strategy was based on the collection of particles in the multi-layered nepheloid system which develops off the river mouth and extends towards the continental shelf. After separation by normal phase HPLC, hydrocarbons were analyzed by gas chromatography and gas chromatography/mass spectrometry. The distributions of NAH and PAH among the various nepheloid layers are discussed in terms of abundances, compositional patterns and related diagnostic criteria. Advection within the benthic nepheloid layer ( bnl ) appeared to constitute an important transport pathway for hydrocarbons, and particularly for PAH. This layer showed a preferential enrichment in fossil fuel related PAH, and to a lesser extent in n-alkanes, whereas pyrolytic inputs were essentially transported within the surface layer. The qualitative decoupling of hydrocarbons between the two layers strongly supported the hypothesis that processes such as flocculation, aggregation and interactions between different phases are the dominant mechanisms leading to the formation of the bnl . A comparison between the hydrocarbon distributions in the BNL and in the underlying surface sediments confirmed the distinct character of the BNL and suggested that resuspension does not significantly contribute to the BNL material.

Trace metal behaviour in a highly stratified Mediterranean estuary: the Krka (Yugoslavia)

The distribution of total dissolved trace metals (Pb, Cd, Cu, Ni and Fe) in the Krka River estuary was studied in May 1988 during the high-productivity period. The dissolved metal concentrations in the freshwater end-member are: Pb 0.08 nM; Cd 0.044 nM; Cu 1.78 nM; Ni 2.47 nM; and Fe 24.1 nM. These concentrations are of the same order of magnitude as those observed in unpolluted rivers throughout the world and are in accordance with the carbonate nature of the catchment of the Krka River, which is, further, located in a national park. The trace metals in the plume show low reactivity during estuarine mixing. In contrast to most estuarine data, dissolved trace metal concentrations are generally lower in the river water than in seawater. However, the bottom saline waters of the estuary are inhomogeneous with regard to dissolved trace metal concentrations, which increase with water age. This may be due to degradation of organic matter. There is no clear evidence for accumulation of dissolved trace metals at the freshwater-seawater interface. This is dependent on the physico-chemical properties of the metals involved and may be more readily detected at downstream sites.

Researches On Spatial Distribution of Amphipods, Isopods and Tanaids (Peracarida) in a Mediterranean Estuary (River Dese, Lagoon of Venice)

[L'Adriatique du nord est la seule partie de la Mediterranee ou la presence de cours d'eau importants (fig. 1), et d'une maree significative (fig. 3) permet la formation de veritables estuaires. L'Auteur etudie la distribution spatiale des Peracariens dans l'estuaire de la riviere Dese, qui traverse la partie septentrionale de la Lagune de Venise (figs. 1 et 2). Trente-neuf especes de Peracariens ont ete rencontrees, dont 28 Amphipodes, 8 Isopodes et 3 Tanaidaces (tableau I). Ce peuplement est mis en relation avec l'amplitude moyenne des salinites (fig. 4), les temperatures de l'eau en surface (fig. 5) et les rapports Ca++/Mg++ au cours d'une maree de vive eau particulierement importante (fig. 6). Cette methode permet de reconnaitre trois groupes d'especes, a savoir: I) un faible stock d'especes oligohalines, dans la partie superieure de l'estuaire; II) un nombreux contingent d'especes marines dans la partie inferieure; III) un groupe plus euryhalin dans le secteur central., L'Adriatique du nord est la seule partie de la Mediterranee ou la presence de cours d'eau importants (fig. 1), et d'une maree significative (fig. 3) permet la formation de veritables estuaires. L'Auteur etudie la distribution spatiale des Peracariens dans l'estuaire de la riviere Dese, qui traverse la partie septentrionale de la Lagune de Venise (figs. 1 et 2). Trente-neuf especes de Peracariens ont ete rencontrees, dont 28 Amphipodes, 8 Isopodes et 3 Tanaidaces (tableau I). Ce peuplement est mis en relation avec l'amplitude moyenne des salinites (fig. 4), les temperatures de l'eau en surface (fig. 5) et les rapports Ca++/Mg++ au cours d'une maree de vive eau particulierement importante (fig. 6). Cette methode permet de reconnaitre trois groupes d'especes, a savoir: I) un faible stock d'especes oligohalines, dans la partie superieure de l'estuaire; II) un nombreux contingent d'especes marines dans la partie inferieure; III) un groupe plus euryhalin dans le secteur central.]