Semi-enclosed Basin Research Articles

Minimising harbour siltation—findings of PIANC Working Group 43

Modern trends, increases in ship size, improved cargo handling capability, sea level rise and erosion threats to low-lying land, undesirable attributes of certain traditional mud dredging and disposal practices, coupled with pressure from environmental legislation, are all driving a need to manage coastal waters and fine sediment in a more sophisticated manner. Wide-ranging review of world “best practice” has highlighted five fully evolved northwest European generic sediment management systems (SMS) to address these challenges. These are being applied to universally recurring types of port facilities: impounded docks, lock entrances, semi-enclosed basins, channels and fairways. They are applicable to large and small, existing and new facilities in rivers, estuaries, open sea coasts, even in some cases to lakes. They can be passive or active. The functioning of each of these five is explained with comment on economic benefit. What emerges strongly is that the applied academic disciplines of physical and chemical oceanography, plus marine microbiology, are being drawn together to supplement or replace traditional maritime engineering. Following on from a Permanent International Association of Navigation Congresses review, these technologies are spreading beyond the northwest European conception area, as well as beginning to be adopted by international dredging companies. The complexity, scale of potential benefit, numbers of poorly understood issues highlighted and embryonic way these are beginning to spread all suggest that this is potentially an important area for future scientific endeavour.

Influence of topography on tide propagation and amplification in semi-enclosed basins

An idealized model for tide propagation and amplification in semi-enclosed rectangular basins is presented, accounting for depth differences by a combination of longitudinal and lateral topographic steps. The basin geometry is formed by several adjacent compartments of identical width, each having either a uniform depth or two depths separated by a transverse topographic step. The problem is forced by an incoming Kelvin wave at the open end, while allowing waves to radiate outward. The solution in each compartment is written as the superposition of (semi)-analytical wave solutions in an infinite channel, individually satisfying the depth-averaged linear shallow water equations on the f plane, including bottom friction. A collocation technique is employed to satisfy continuity of elevation and flux across the longitudinal topographic steps between the compartments. The model results show that the tidal wave in shallow parts displays slower propagation, enhanced dissipation and amplified amplitudes. This reveals a resonance mechanism, occurring when the length of the shallow end is roughly an odd multiple of the quarter Kelvin wavelength. Alternatively, for sufficiently wide basins, also Poincaré waves may become resonant. A transverse step implies different wavelengths of the incoming and reflected Kelvin wave, leading to increased amplitudes in shallow regions and a shift of amphidromic points in the direction of the deeper part. Including the shallow parts near the basin’s closed end (thus capturing the Kelvin resonance mechanism) is essential to reproduce semi-diurnal and diurnal tide observations in the Gulf of California, the Adriatic Sea and the Persian Gulf.

Periodic Wind-Driven Circulation in an Elongated and Rotating Basin

AbstractAn idealized model is developed for the three-dimensional response of a coastal basin (e.g., lagoon, bay, or estuary) to time-periodic wind stress. This model handles basins that are deeper and/or shallower than an Ekman depth with wind forcing frequencies ranging from subinertial to superinertial. Here the model is used to describe how the response (current and sea level) of a basin deeper than one Ekman depth depends on the wind forcing frequency. At low subinertial frequencies, the response is similar to the steady wind case and is hence called “quasi steady.” There is a near-surface Ekman transport to the right of the wind balanced by a return flow at depth. Lateral bathymetric variations introduce an along-basin circulation that decays with increasing frequency and sets the extent of the quasi-steady response in the frequency domain. At the inertial frequency, the wind forces a damped resonant response with large vertical shear and weak depth-integrated flow. This result is potentially important for coastal basins located near ±30° latitude and forced by a diurnal breeze. At superinertial frequencies, the response becomes irrotational and is amplified near seiche frequencies. The response to a sudden onset of wind is computed in the time domain and confirms that the slow growth of the along-basin circulation controls the spinup process. The response of basins shallower than an Ekman depth, and the validity of the model inside semienclosed basins, are also discussed.

Mercury and methylmercury contamination in Mytilus galloprovincialis from Taranto Gulf (Ionian Sea, Southern Italy): Risk evaluation for consumers

Total mercury (THg) and methylmercury (Me-Hg) concentrations were determined in mussels ( Mytilus galloprovincialis) from 10 stations located in the Mar Piccolo of Taranto (Ionian Sea, Taranto Gulf) an important semi-enclosed basin in Italy, devoted to mussel culture activities. The obtained results show that THg and Me-Hg concentrations ranged from 0.236 to 0.559 μg g −1 d.w. and from 0.066 to 0.155 μg g −1 d.w., respectively. Consequently, the Me-Hg/THg ratios ranged from 17% to 49%. The dietary intake of THg and Me-Hg were studied among children and adults from Taranto (Southern Italy). The estimated weekly intake for THg and Me-Hg was below the Provisional Tolerable Weekly Intake (PTWI) established by European Food Safety Autority (EFSA) for all sampled mussels, though their consumption provides a THg intake in children near the PTWI.

Paleoceanographic changes at the northern Tethyan margin during the Cenomanian–Turonian Oceanic Anoxic Event (OAE-2)

The late Cenomanian–early Turonian Oceanic Anoxic Event (OAE-2) represents major paleoceanographic and faunal perturbations. Samples from the northern Tethyan margin (Rehkogelgraben, Eastern Alps) were investigated in order to trace the paleoceanographic processes. Paleoecologic conditions were reconstructed by combining the results of assemblage counts of indicative microfossil groups (foraminifera, and radiolaria). Assemblages, size distributions and abundances show a tripartite subdivision for surface and bottom waters: 1) Oligotrophic surface conditions and oxic bottom waters with a reasonably high food supply for the late Cenomanian interval. 2) An OAE period with black shales characterized by very low numbers but high diversities and a lack of high-productivity indicators among planktic foraminifera. Low abundances of small sized benthic foraminifera indicate low oxic–dysoxic conditions at the seafloor. 3) Post-OAE assemblages are characterized by mesotrophic planktic species and benthic foraminifera suggest oxic bottom waters. It took about 300 ky to re-establish a pelagic carbonate-producing regime. The semi-enclosed basin situation of the Penninic Ocean is thought to be responsible for differences between the high productivity in the world ocean during the OAE-2 and the overall absence of high-productivity indicators and high foraminiferal diversities at Rehkogelgraben. The Penninic Ocean may have even served as a refuge during the environmental crisis.

Seasonal variation of biomarkers in Mytilus galloprovincialis sampled inside and outside Mar Piccolo of Taranto (Italy)

The aim of this work was to study spatial and temporal variation in biomarkers in autochthonous Mytilus galloprovincialis sampled inside and outside Mar Piccolo of Taranto, a typical polluted semi-enclosed basin of the Mediterranean Sea characterised by scarce hydrodynamism. Mar Piccolo of Taranto represents a site of Italian National Interest because of the high level of pollution. A battery of biomarkers (lysosomal destabilisation, catalase, metallothioneins, acetylcholinesterase, air survival) was applied to assess pollution-induced stress effects in authoctonous mussels. The responses were analysed comparatively in two different seasons, summer and winter, in order to assess possible changes in the pollutant-induced stress syndrome throughout the year. No significant difference inside and outside Mar Piccolo was observed for metallothioneins. By contrast, the dramatically decreased acetylcholinesterase values and strongly increased catalase activity in organisms taken from Mar Piccolo in winter indicate an increased risk of exposure to anticholinesterase compounds during this season. The results suggest the importance of temporal variability in biomarker responses throughout the year for monitoring possible seasonal changes in the pollutant-induced stress syndrome of organisms living in a certain environment and, in turn, more properly detecting changes in ecotoxicological risks.

Experimental data on growth, mortality and reproduction of Ostrea edulis (L., 1758) in a semi-enclosed basin of the Mediterranean Sea

Biometry, growth, survival and mortality rates as well as reproduction of O. edulis have been evaluated in the Taranto Sea, a semi-enclosed basin of the North-Western Ionian Sea (Mediterranean Sea). A crossed experimental design with 3 factors (container, depth and stocking density) was defined to investigate their effect on the sizes, growth performance, survival and mortality rates. Moreover, the reproduction was studied to better understand the life cycle of the species in the basin. The investigated experimental conditions caused significant differences in both size changes and growth parameters. In particular, the density was the main factor influencing both biometry and growth. The oysters cultivated at low density showed L ∞ (155.46 ± 46.38 mm DVL), k (0.42 ± 0.25 year − 1 ) and Φ′ (2.00) values significantly higher than those reared at high density L ∞ (134.28 ± 36.13 mm DVL), k (0.35 ± 0.22 year − 1 ) and Φ′ (1.80). The winter point (WP) occurred during winter and the strength of the seasonal oscillation (C) ranged between 0.10 and 0.37. No significant differences between the experimental rearing conditions were observed in either survival (S) or mortality (Z) rates. Oysters reared at low and high density showed mean S values equal to 0.88 ± 0.03 and 0.89 ± 0.03, respectively as well as mean Z values equal to 0.13 ± 0.03 and 0.12 ± 0.04. Maturation of gonads occurred continuously during the year showing a slackening during summer when the highest temperatures were recorded in the water column. In addition, the highest percentages of fluent gonads were observed during winter. Histological analysis confirmed the macroscopic observation of gonads. In particular, gamete differentiation was observed during late autumn and early winter. An inverse correlation between the condition index and the mean temperature in the water column was observed throughout the study period. The present results indicate an optimal crop age of about 2 years, corresponding to the commercial size of 8–9 cm dorsal–ventral length and weight of 55–60 g.

Natural and human-induced hypoxia and consequences for coastal areas: synthesis and future development

Abstract. Hypoxia has become a world-wide phenomenon in the global coastal ocean and causes a deterioration of the structure and function of ecosystems. Based on the collective contributions of members of SCOR Working Group #128, the present study provides an overview of the major aspects of coastal hypoxia in different biogeochemical provinces, including estuaries, coastal waters, upwelling areas, fjords and semi-enclosed basins, with various external forcings, ecosystem responses, feedbacks and potential impact on the sustainability of the fishery and economics. The obvious external forcings include freshwater runoff and other factors contributing to stratification, organic matter and nutrient loadings, as well as exchange between coastal and open ocean water masses. Their different interactions set up mechanisms that drive the system towards hypoxia. Coastal systems also vary in their relative susceptibility to hypoxia depending on their physical and geographic settings. It is understood that coastal hypoxia has a profound impact on the sustainability of ecosystems, which can be seen, for example, by the change in the food-web structure and system function; other influences include compression and loss of habitat, as well as changes in organism life cycles and reproduction. In most cases, the ecosystem responds to the low dissolved oxygen in non-linear ways with pronounced feedbacks to other compartments of the Earth System, including those that affect human society. Our knowledge and previous experiences illustrate that there is a need to develop new observational tools and models to support integrated research of biogeochemical dynamics and ecosystem behavior that will improve confidence in remediation management strategies for coastal hypoxia.

Upper Albian planktic foraminifera and radiolarian biostratigraphy (Nebeur – northern Tunisia)

Preliminary lithostratigraphic and biostratigraphic data recovered from Upper Albian pelagic successions of the Jebel Srassif area (Tunisian trough) reveal the presence of a moderately well preserved radiolarian fauna. This fauna is fossilized in blackshale beds of Rotalipora subticinensis, R. ticinensis and the lower part of the R. appenninica foraminifer zones. Thirty-five radiolarian morphotypes were identified. A correlation established with foraminifer planktic biochronology allows us to distinguish three radiolarian assemblages. The close association between organic-rich deposits and the radiolarian bloom can be interpreted as productivity proxy triggered by semi-enclosed basin configuration and an intensified upwelling currents provided by a possibly halokinetic-induced hydrothermalism.

Sheltered sandy beaches of southwestern Australia

Abstract Four beach types were identified from field surveys of beach profiles on low-energy sandy beaches in Cockburn Sound, a micro-tidal, semi-enclosed basin in southwestern Australia. These were delineated by an exposure factor (Ef), which provided a surrogate for the relationship between incident wave energy and attenuation as a result of structures, banks and shoals. The four beach profiles identified were exponential, segmented, concave-curvilinear and convex-curvilinear in form. Whether the different profile shapes occurred in environments subject to different degrees of sheltering and protection under the same tidal regime was open to question. Beach profiles were examined for Como Beach and Princess Royal Harbour in Western Australia and compared with observations from Cockburn Sound to test this proposition. Cockburn Sound and Princess Royal Harbour are subject to minimal penetration of swell waves whereas Como Beach, in the Swan River Estuary, is wholly fetch limited. Profile forms identified in each environment were consistent with those described from Cockburn Sound. Similar profile types had similar exposure values for all sites such that Ef<1 was characteristic of exponential profiles, 1–1.5 of segmented, 1.5–2 of concave-curvilinear, and Ef>2 of convex-curvilinear. Although this approach requires further testing in low-energy environments subject to different tidal ranges and fluctuations in sea level, use of the exposure factor for delineation of low-energy sheltered beach types appears to be a robust procedure.

Do major neogene hiatuses in the Ciscaucasian semi-enclosed basin (Eastern Paratethys, southwestern Russia) record eustatic falls?

Hiatuses in semi-enclosed basins can be caused by either eustatic falls or local tectonic uplifts. The Ciscaucasian basin is located in the south of European Russia. In the Neogene, it belonged to the Eastern Paratethys domain. On the basis of available stratigraphic data, four major hiatuses are traced in this basin as erosional surfaces or lengthy sedimentation breaks, namely the Tarkhanian, Middle/Upper Sarmatian, Sarmatian/Maeotian, and Kimmerian hiatuses. They are documented in most of the areas of the study basin. The three earlier hiatuses mark short-term and nearly isochronous, basinwide sedimentation breaks, whereas the latter hiatus is diachronous, embracing more than 2 myr. All reported hiatuses record the eustatic falls. Consequently, we argue that eustatic processes controlled sedimentation in the Ciscaucasian basin throughout the entire Neogene. This means the basin was connected to the open ocean throughout this period, with important consequences for our understanding of watermass history in the Mediterranean and Paratethyan basins further west and south.

Subsurface Chlorophyll a Maxima in the Boka Kotorska Bay

ABSTRACTSubsurface chlorophyll maximum (SCM) are common phenomena in variety of environmental, from fresh water, brackish water, estuaries, shelf areas, the coastal zone to open ocean. It is now known that the chlorophyll maximum at the subsurface layer occurs widely in water where the water column is stratified. In permanent stratified tropical water, therefore, the SCM is expected to be a year round phenomenon. In higher latitudes, where there are occasional vertical mixing of the water column due to seasonal surface cooling or strong winds, the SCM will be restricted during a particular period of the year, such as in the summer or between spring and autumn. The Boka Kotorska Bay is semi-enclosed basin, located in the south-easter Adriatic Sea (Mediterranean Sea). The weekly changes in chlorophyll a and physico-chemical parameters were investigated in the period September 2008 to March 2009. Maximum phytoplankton biomass (11.13 mg m−3 chlorophyll a) was observed on 2m depth in February nearly the pycnocline. The good correspondence of SCM with occurrence of nitrate clearly indicates that the phytoplankton that make up the SCM depended highly on nitrate, which is supplied mainly by diffusional process from depth below the SCM.

On the response of Black Sea level to external forcing: altimeter data and numerical modelling

In this work, we address the Black Sea setup of Nucleus for European Modelling of the Ocean (NEMO), and in particular some model enhancements associated with the most important characteristic of ocean dynamics in this semi-enclosed basin, that is the sea-level variability and its relationship with water cycles and wind. Forcing data are presented in detail and compared with previously used coarser-resolution data. One emphasis in this paper is on the statistical analyses of forcing data and outputs from simulations with a focus on the sea level and its change. Numerical simulations are carried out as free run, and alternatively, altimeter data assimilation based on displacement of water properties in the pycnocline is used. Comparisons between the two runs identify the robustness of circulation driven by water balance and winter intensification. Problems in the model to replicate the redistribution of water properties between the two sub-basins in free-run mode are also discussed, which are observed during years with extreme climatic conditions.

Biodiversity patterns of free-living marine nematodes in a tropical bay: Cienfuegos, Caribbean Sea

Spatial and temporal biodiversity patterns of free-living marine nematodes were studied in Cienfuegos Bay, a tropical semi-enclosed basin in the Caribbean Sea. Taxonomic (to species level) and functional (biological trait) approaches were applied for describing the assemblage structure and relating it to abiotic environment based on a sampling scheme in six subtidal stations and three months. Biological trait approach added relevant information to species pattern regarding relationships between diversity patterns and the abiotic environment. The most common morphotypes were deposit feeding nematodes, with colonising abilities of 2–3 (in a scale from 1 to 5), tail conical cylindrical or filiforme and body slender; and their abundance were correlated with depth, organic matter and silt/clay fraction. In spite of a high turnover of species, functional diversity of assemblages did not change notably in space and time. A result probably due to sampling of the habitat pool of species and to low heterogeneity of the studied muddy bottoms. Chemical pollution (organic enrichment and heavy metals) and hydrodynamic regime possibly drove the biodiversity patterns. Spatial distribution of assemblages support the existence of two well differentiated basins inside the bay, the northern basin more polluted than the southern one. The low hydrodynamic regime would determine a poor dispersion of nematodes resulting in high spatial variance in the assemblage structure; and also the associated hypoxic conditions and pollutants in sediments can explain the dominance of tolerant nematode species such as Daptonema oxycerca, Sabatieria pulchra, Terschellingia gourbaultae, and Terschellingia longicaudata. A comparison of spatial–temporal patterns of biodiversity between Cienfuegos Bay and other semi-enclosed bays in temperate regions suggests several similarities: nematode assemblages are strongly influenced by anthropogenic disturbance, temporal trends are weak or overridden by spatial ones, and few cosmopolitan genera/species tolerant to pollution and hypoxic conditions are dominant.

Climatic circulation in the Gulf of Trieste (northern Adriatic)

The climatic circulation of the Gulf of Trieste, which is a shallow semienclosed basin in the closed northeastern end of the northern Adriatic, is studied with a numerical model. In all seasons there is a general inflow into the Gulf of Trieste at its lower, deeper part. This inflow makes a cyclonic turn centered in the southern part during average winter conditions. This turn is enhanced during spring and closes in an elongated cyclonic gyre during average summer conditions. In spring and summer, the cyclonic gyre is coupled with an anticyclonic gyre near the closed eastern part of the gulf. A “dome”‐like density profile across the gulf's axis in the inner part of the gulf above the bottom appears with this circulation during spring and summer. In climatic autumn there is a smaller anticyclonic gyre on its southern side. Near the sea surface there is an outflow during winter, which is driven by the dominant “bora” wind blowing along the gulf's axis. This outflow, however, is detached from the southern coastline to the right, and crosses the gulf diagonally, merging with the belt of freshwater outflow along the northern coastline. This is shown to be a consequence of the balance between the pressure gradient force caused by elevation piled up in the direction out of the gulf, the Coriolis force, and vertical friction between layers near the sea surface. During the stratified season the surface of the gulf is occupied by an anticyclonic gyre due to the inertial plume of the Isonzo River.

Colour variation is incongruent with mitochondrial lineages: cryptic speciation and subsequent diversification in a Gulf of California reef fish (Teleostei: Blennioidei)

The Gulf of California endemic reef fish, Acanthemblemaria crockeri (Blennioidei, Chaenopsidae), reportedly has two colour morphs, one with melanic lateral spots ('Gulf' morph) and one with orange spots ('Cape' morph). In this study, we recorded colour morph in both males and females and collected mitochondrial DNA sequence data for cytochrome c oxidase I (COI) and tRNA-Pro/D-loop of specimens from throughout the Gulf to explore the genetic basis of the colour morphs. Two highly divergent (HKY + I distance = 11.9% for COI), reciprocally monophyletic lineages were identified, consistent with the presence of two parapatric species. A 30-km gap between the distributions of mitochondrial lineages roughly corresponds to a hypothesized former seaway across the Baja California peninsula north of La Paz, although the estimated divergence time (1.84 million years ago) is more recent than the hypothetical seaway (3-4 million years ago). Surprisingly, the distribution of mitochondrial species is not congruent with the distribution of either male or female colour morphs. Our analysis also revealed significant population differentiation within both species and no shared haplotypes among populations. The northern Gulf species includes four populations (NB, CB, NM and CM) corresponding to northern and central Baja and northern and central mainland sites, while the Cape species includes two populations (SB and SM) corresponding to the Baja and mainland sides of the southern Gulf. The NB/CB division corresponds to a hypothesized Plio-Pleistocene mid-peninsular seaway. The level of genetic divergence documented in this lineage is extraordinary for a marine fish with a pelagic larval stage within a semi-enclosed basin.

Risk assessment of polycyclic aromatic hydrocarbons in a Mediterranean semi-enclosed basin affected by human activities (Abu Qir Bay, Egypt)

The contamination of sediments and mussels sampled from Abu Qir Bay by polycyclic aromatic hydrocarbons (PAHs) was investigated. Concentrations of PAHs recorded in the bay sediments ranged from <MDL to 2660 μg/kg dw. In general, concentrations of PAHs in mussels were higher than their corresponding sediment concentrations reflecting their great bioavailability (242–3880 μg/kg dw). The highest concentration was observed in the western part of the bay, a location affected by intensive shipping activities. The distribution pattern of PAHs was similar for mussels and sediments, particularly for sediments characterized by high contamination level, and they were dominated by the high molecular weight PAHs (4–6-rings). Applying different PAHs ratios, it was found that PAHs originated predominantly from the pyrogenic source either from the combustion of grass, wood and coal (majority of the samples) or from petroleum combustion (harbour area). The output of a Screening Level Ecological Risk Assessment (SLERA) on the bay sediments revealed that adverse ecological effects to benthic organisms are expected to occur in only one sample, and thus PAHs are not considered as contaminants of concern in Abu Qir Bay. Also adverse health effects are not expected to occur from the consumption of the investigated mussels with respect to PAHs in Abu Qir Bay.

Spatial and temporal variability in the pelagic ecosystem of the East Sea (Sea of Japan): A review

The East Sea (Sea of Japan) is a unique marginal sea because it exhibits features of oceanic dynamics of much larger ocean basins. This semi-enclosed basin may be considered as a model or microcosm for understanding of how biological processes and distributions in pelagic ecosystem are interacting with physical processes in highly dynamic ocean regions. This overview summarizes the recent progresses concerning spatial and temporal variability of pelagic ecosystem components form an interdisciplinary point of view. Spatial characteristics of physical environments and biogeography in the region are distinguished mainly by the subpolar front. It was also found that long-term changes in biomass and community structure as well as those in the physical and biological environments are associated with climate variability in the region. We conclude by identifying main needs for the information and researches, particularly regular and long-term sampling, and permanent monitoring if possible.

Wind wave measurements and modelling in a fetch-limited semi-enclosed lagoon

The south-west reef lagoon of New Caledonia is a semi-enclosed basin where, on first approximation, dominating sea state component corresponds to locally generated wind waves. This study aims to evaluate the ability of the wave model WAVEWATCH III to simulate wind wave distribution in this particular fetch-limited context, with a given parameterisation. In order to evaluate the consistency of the simulation results, wave parameters were measured in situ by a wave and tide recorder (WTR9 Aanderaa) and by an acoustic Doppler velocimeter (ADV Sontek). This study underlines specific constrains for the deployment of instruments to assess the characteristic parameters of low amplitude and high frequency wind-waves. Special care was taken in the comparison step as, on one hand the wave model did not simulate the propagation of low-frequency oceanic waves inside the lagoon, and on the other hand the measured spectra bear an intrinsic limitation for high frequencies. The approximation of a sea state dominated by wind waves is verified on the study site. The accuracy of the simulation results is discussed with regards to the wind forcing applied to the model.

Spatial and seasonal distribution of ascidians in a semi-enclosed basin of the Mediterranean Sea

A total of 25 species of ascidians were collected in the Mar Piccolo of Taranto, a semi-enclosed Mediterranean basin. Three are non-indigenous for the Mediterranean Sea: Microcosmus squamiger, Polyandrocarpa zorritensis and Distaplia bermudensis. The substrate features, season and depth affect the distribution of ascidians in the study area. Some species, such as Pyura dura and Pyura microcosmus, were found only on artificial substrates, while Ascidiella aspersa was almost exclusively recovered on natural bottoms. Seasonal variation in the ascidian distribution and abundance seems to be due mainly to their biological cycles, larval recruitment and adaptation. During the autumn and winter the most abundant species were Clavelina phlegraea and Ciona intestinalis, while A.aspersa was particularly abundant during spring. Depth and more directly light intensity play an important role in ascidian distribution. In the upper few metres the shallow-water species Polyandrocarpa zorritensis was abundant due to its photopositive larvae. Even though the distribution and abundance changed significantly between substrates, seasons and depths, the most abundant species in the Mar Piccolo of Taranto were Clavelina phlegraea, Ciona intestinalis and Styela plicata all of which are able to tolerate the variations in environmental conditions, low rate of water renewal and continuous silting of this semi-enclosed sea. Assuming the role that the above mentioned species have as marine pollution indicators and the abundance recorded for some of them, a high degree of environmental stress can be confirmed for the Mar Piccolo of Taranto. A comparative list of the ascidians recorded in this and previous studies is also reported.