Semi-enclosed Basin Research Articles

Wave–current interaction: Effect on the wave field in a semi-enclosed basin

The effect on waves of the Wave–Current Interaction (WCI) process in the semi-enclosed Gulf of Venice (northern region of the Adriatic Sea) was investigated using the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modeling system. COAWST relies on the ocean model ROMS (Regional Ocean Modeling System), the wave model SWAN (Simulating WAves Nearshore), and the CSTMS (Community Sediment Transport Modeling System) routines. The two-way data transfer between circulation and wave models was synchronous via MCT (Model Coupling Toolkit), with ROMS providing: current field, free surface elevation, and bathymetry to SWAN. For coupling, the 3-D current profiles were averaged using a formulation which integrated the near-surface velocity over a depth controlled by the spectral mean wavenumber. COAWST system was implemented on a parent grid (with horizontal resolution of 2.0km) covering the whole Adriatic Sea with one-way nesting to a child grid resolving the northern area (Gulf of Venice) at a resolution of 0.5km. The meteorological forcings provided by the operational meteorological model COSMO-I7 (a mesoscale model developed in the framework of the COSMO Consortium) were used to drive the modeling system in the period bracketing September 2010–August 2011. The adopted winds and the simulated waves were compared with observations at the CNR-ISMAR Acqua Alta oceanographic tower, located off the Venice littoral. Wave heights and sea surface winds were also compared with satellite-derived data. The analysis of WCI was performed on the child grid over the winter season (January–March 2011) with particular focus on the waves generated by prevailing and dominant winds blowing on the Adriatic Sea: Bora and Sirocco. Due to the variable wind direction with respect to the ocean current direction different effects on WCI were depicted, showing that within the northern Adriatic Sea the ocean–wave interactions are strongly dependent on the wind forcing direction. Further investigations reveal that, when applied to intense storms, the effect of coupling on waves results in variations of significant wave height up to 0.6m, with some areas experiencing significant increase/decrease of wave spectral energy for opposite/following currents respectively.

Wave-current interaction effect on sediment dispersal in a shallow semi-enclosed basin

Sclavo, M., Benetazzo, A., Carniel, S., Bergamasco, A., Falcieri, F.M., and Bonaldo, D., 2013. Wave-current interaction effect on sediment dispersal in a shallow semi-enclosed basinThe shallow northern Adriatic Sea (namely Gulf of Venice) serves as a model for exploring the interaction of surface gravity waves and oceanic currents and how they influence bottom sediment dispersal and bathymetry evolution. This wave-current interaction effect is investigated using the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modeling system. COAWST relies on the Regional Ocean Modeling System (ROMS), the Simulating WAves Nearshore (SWAN) model, and the CSTMS (Community Sediment Transport Modeling System) models and routines. The 2-way data transfer between wave and ocean circulation models is synchronous via MCT (Model Coupling Toolkit), with ROMS providing to SWAN the 2-D current field, free surface elevation, and bathymetry. COAWST modeling system is implemented on two computational grids at different horizontal resolution: a parent grid (with resolution of 2.0 km) covering the whole Adriatic Sea and a child grid resolving the Gulf of Venice at a resolution of 0.5 km. Simulated waves and currents are validated against in-situ observations at the CNR-ISMAR Acqua Alta oceanographic tower, located 15 km off the Venice lagoon. The analysis of wave-current interaction effect on sediment dispersal and sea bottom evolution are performed over the 2011 winter season (January-March) with particular focus on the waves generated by dominant and prevailing winds blowing on the Adriatic Sea: Bora and Sirocco. Results show that while the effects on bottom stress may vary depending on wave propagation and current direction, the effects on advective dynamics may become dominant particularly in presence of severe storms with parallel wave propagation and global circulation, which is the case of Bora storms in northern Adriatic Sea.

Storm surges in the Singapore Strait due to winds in the South China Sea

Among the semi-enclosed basins of the world ocean, the South China Sea (SCS) is unique in its configuration as it lies under the main southwest-northeast pathway of the seasonal monsoons. The northeast (NE) monsoon (November–February) and southwest (SW) monsoon (June–August) dominate the large-scale sea level dynamics of the SCS. Sunda Shelf at the southwest part of SCS tends to amplify Sea Level Anomalies (SLAs) generated by winds over the sea. The entire region, bounded by Gulf of Thailand on the north, Karimata Strait on the south, east cost of Peninsular Malaysia on the west, and break of Sunda Shelf on the east, could experience positive or negative SLAs depending on the wind direction and speed. Strong sea level surges during NE monsoon, if coincide with spring tide, usually lead to coastal floods in the region. To understand the phenomena, we analyzed the wind-driven sea level anomalies focusing on Singapore Strait (SS), laying at the most southwest point of the region. An analysis of Tanjong Pagar tide gauge data in the SS, as well as satellite altimetry and reanalyzed wind in the region, reveals that the wind over central part of SCS is arguably the most important factor determining the observed variability of SLAs at hourly to monthly scales. Climatological SLAs in SS are found to be positive, and of the order of 30 cm during NE monsoon, but negative, and of the order of 20 cm during SW monsoon. The largest anomalies are associated with intensified winds during NE monsoon, with historical highs exceeding 50 cm. At the hourly and daily time-scales, SLA magnitude is correlated with the NE wind speed over central part of SCS with an average time lag of 36–42 h. An exact solution is derived by approximating the elongated SCS shape with one-dimensional two-step channel. The solution is utilized to derive simple model connecting SLAs in SS with the wind speeds over central part of SCS. Due to delay of sea level anomaly in SS with respect to the remote source at SCS, the simplified solutions could be used for storm surge forecast, with a lead time exceeding 1 day.

Wave climate in the Arkona Basin, the Baltic Sea

Abstract. The basic features of the wave climate in the Southwestern Baltic Sea (such as the average and typical wave conditions, frequency of occurrence of different wave parameters, variations in wave heights from weekly to decadal scales) are established based on waverider measurements at the Darss Sill in 1991–2010. The measured climate is compared with two numerical simulations with the WAM wave model driven by downscaled reanalysis of wind fields for 1958–2002 and by adjusted geostrophic winds for 1970–2007. The wave climate in this region is typical for semi-enclosed basins of the Baltic Sea. The maximum wave heights are about half of those in the Baltic Proper. The maximum recorded significant wave height HS =4.46 m occurred on 3 November 1995. The wave height exhibits no long-term trend but reveals modest interannual (about 12 % of the long-term mean of 0.76 m) and substantial seasonal variation. The wave periods are mostly concentrated in a narrow range of 2.6–4 s. Their distribution is almost constant over decades. The role of remote swell is very small.

Erratum to: Spatial and Temporal Heavy Metal Concentration (Cu, Pb, Zn, Hg, Fe, Mn, Ni) in Sediments of the Mar Piccolo in Taranto (Ionian Sea, Italy)

The Mar Piccolo (surface area of 20.72 km2) is located in the Northern area of the Taranto town (Ionian Sea, Italy). It is an inner, semi-enclosed basin with lagoon features connected with the Mar Grande through two channels which are very important for water exchange. Mar Piccolo basin is subjected to urbanization, industry, agriculture, aquaculture and commercial fishing. Hence, it is important to have a temporal picture both of heavy metal content and of organic carbon and their distribution (hydrophobic fraction, hydrophilic fraction, humic compounds) to check the progress of pollution in time. Three sediment cores collected on the basis of the pollution sources have been analyzed. Both heavy metal and organic carbon concentrations underline the fact that the anthropogenic input is different in the three sites, and that in time the amount of pollutants coming into the Mar Piccolo have changed in different ways. The high amount of organic hydrophilic compounds present in sediments, both due to the small water depth and to the high accumulation rate, make the sediment site high in oxygen consumption due to a considerable chemical and biochemical transformations of organic matter.

Modal analysis of semi-enclosed basins

This paper presents a novel technique for the computation of eigenvalues and eigenvectors of partially enclosed basins such as harbours and bays. The procedure makes use of the finite element approximation of the linear shallow water equations, and converts the time-depending problem into an eigenvalues one. The main point of novelty of this research is the mathematical condition used at the boundary that separates the computational domain from the open sea. While classical techniques impose a zero surface elevation (i.e. a nodal line), here an approximate radiation condition is applied. The use of a radiation condition at the open boundary gives a quadratic eigenvalue problem that admits as solution complex eigenvalues and eigenvectors, thus describing the flow in terms of both standing and progressive waves. The new method is applied to an idealized long and narrow harbour, for which an analytical solution of long wave resonance exists, and to the harbour of Marina di Carrara (Italy), for which measurements and previous numerical computation results are available. In both cases the results show good agreement with the available data.

The species complex of Diamysis Czerniavsky, 1882, in fresh waters of the Adriatic basin (NE Mediterranean), with descriptions of D. lacustris Băcescu, 1940, new rank, and D. fluviatilis sp. nov. (Mysida, Mysidae)

Most populations of Diamysis Czerniavsky, 1882, living in freshwater tributaries of the Adriatic Sea, belong to D. fluviatilis sp. nov., which is distinguished from its congeners mainly by the peculiar distribution of fringes on the carapace of males. A small number of freshwater bodies is inhabited by the more euryhalobious D. mesohalobia heterandra Ariani & Wittmann, 2000. A first description is given for adults of both sexes of D. bahirensis mod. lacustris Bacescu, 1940, formerly known only from one subadult female taken in the near-coastal freshwater lake Scutari (SE Adriatic). This taxon is raised to species level as D. lacustris, based on the shallow telson cleft and the undulate margins of the scutellum paracaudale in both sexes, and on peculiar features of the male. All freshwater records of Diamysis species so far confirmed for the Mediterranean are restricted to tributaries of the Adriatic basin. This is possibly related to transgressive-regressive events during the Pliocene-Pleistocene, causing alternations between marine and freshwater-terrestrial phases in this semi-enclosed basin, particularly in the area where the sedimentary basin of the Po river evolved.

The forced and free response of the South China Sea to the large-scale monsoon system

Non-tidal sea level anomalies (SLAs) can be produced by many different dynamical phenomena over many time scales, and they can induce serious damages in coastal regions especially during extreme events. In this work, we focus on the SLAs in the South China Sea (SCS) to understand whether and how they can be related to the large-scale, seasonal monsoon system which dominates the SCS circulation and dynamics. We have two major objectives. The first one is to understand whether the NE (winter) and SW (summer) monsoons can be responsible for the persistent SLAs, both positive and negative, observed at the SCS ends along the main monsoon path. The second objective is to understand the SCS response as a free system upon onset/relaxation or sudden changes in the forcing wind. It is well known that sudden changes in the forcing mechanism induce free oscillations, or seiches, in closed, semi-enclosed basins and harbors, and we want to identify the possible seiche modes of the SCS. To our knowledge, these two objectives have not been previously addressed. We address these objectives both through observational analysis and modeling simulations. Multi-year tide-gauge data from stations along the coastal regions of the SCS are analyzed examining their spatial correlations. Strong negative correlations are found between the northeast and southwest stations at the two ends of the SCS under the path of the NE/SW monsoons. They correspond to wind-induced positive/negative sea level set-ups lasting for the entire monsoon season and changing sign from winter to summer. Short periods of negative correlations are also found between the SLAs at eastern and western stations during El Nino years in which the monsoons are weaker and have an enhanced E/W component inducing corresponding sea level set-ups. The tide-gauge station at Tanjong Pagar at the southwest SCS end near Singapore is chosen to study four extreme SLAs events in the observational record during 1999. Modeling simulations are carried out to reproduce them. The observed and modeled extreme SLAs agree quite well, both in the amplitude of the highest peak and in phase. Three main peaks are identified in the observational energy spectrum of the de-tided SLAs at the same station in 1999. Using Merian’s formula to evaluate the periods of seiches in idealized basins Wilson (Adv Hydrosci 8:1–94, 1972) the first two peaks (24.4 h and 11.9 h) are found to correspond to the first two seiche modes in the direction of the main, longer axis of the SCS. The third peak (8.5 h) is found to correspond to the seiche in the transversal, shorter axis. Finally, modeling simulations are carried out by suddenly dropping a circular bump of water in the quiescent basin at different locations to excite the seiches. The periods of the modeled peaks agree quite well with the observational ones, the first two periods being actually identical.

Microcystin producing cyanobacterial communities in Amvrakikos Gulf (Mediterranean Sea, NW Greece) and toxin accumulation in mussels (Mytilus galloprovincialis)

Various cyanobacterial species have the capacity to produce different types of toxins. Microcystins, the most prominent cyanotoxins are considered health hazards because of their potential hepatotoxic effects. They are well known to contaminate freshwater ecosystems but their presence in marine ecosystems has been reported only occasionally. We investigated seasonal changes of microcystin concentrations both in water and in the edible species of mussels Mytilus galloprovincialis collected from Amvrakikos Gulf (salinity ranging from 30‰ to 34‰), the biggest semi-enclosed basin in Greece. The microcystin concentrations in the water ranging from 0.003 to 19.8ngl−1, were below the World Health Organization (WHO) upper limit for recreational activities. In contrast, we found that microcystin concentrations in M. galloprovincialis mussels (ranging from 45±2 to 141.5±13.5ngg−1ww) exceeded the upper limit of the tolerable daily intake (TDI) of microcystin as determined by WHO.Genotype composition of the total cyanobacterial community of the Gulf was analyzed by using denaturing gradient gel electrophoresis (DGGE) profiling of the rRNA internal transcribed spacer region (rRNA-ITS). The cyanobacterial community was found to be dominated almost exclusively by the cosmopolitan species Synechococcus –Synechocystis. In order to determine genes involved in the production of microcystins, a range of both specific and degenerate molecular primers against microcystin synthetase gene cluster (mcyS) was used.To our knowledge this is the first report of the presence of the hepatotoxic microcystins in the Mediterranean Sea, the first study on the accumulation of these toxins in mussels from a Mediterranean marine ecosystem and one of the few published works suggesting a potential association of microcystins with Synechococcus and/or Synechocystis cyanobacteria.The importance of our study is strengthened by the fact that Amvrakikos Gulf is among the most productive Greek “seafood” areas and a Mediterranean wetland of international significance according to Ramsar Convention.

Ecosystem modelling: Towards the development of a management tool for a marine coastal system part-II, ecosystem processes and biogeochemical fluxes

Pagasitikos gulf is a semi-enclosed basin highly influenced both by anthropogenic activities (inflow of nutrients at the north and west parts) as well as by water exchange between the gulf and the Aegean Sea at its south part (Trikeri channel) resulting in the development of functional sub-areas within the gulf. Thus the inner part is characterised by eutrophic conditions with sporadic formation of harmful algal blooms whilst the central part acts as a buffer with mesotrophic characteristics influenced by the outer area. In a companion paper, the circulation fields and the development of water masses in the Pagasitikos gulf were explored. The aim of this study is to investigate the interactions between the physical and biogeochemical systems in the Pagasitikos gulf by coupling advanced hydrodynamic and ecological models. The simulation system comprises two on-line coupled sub-models: a three-dimensional hydrodynamic model based on the Princeton Ocean Model (POM) and an ecological model adapted from the European Regional Seas Ecosystem Model (ERSEM) for this specific ecosystem. After a model spin up period of 10years, the results from an annual simulation are presented. Emphasis is given to the description of the spatial and temporal variability of the ecosystem parameters as well as to the relationship between physical forcing and the evolution of the ecosystem along with other factors affecting the nutrient cycling and primary production. A cost function is used for the validation of model results with field data. Simulation results are in good agreement with in-situ data illustrating the role of the physical processes in determining the evolution and variability of the ecosystem, as well as highlighting the potential usefulness of the model as an operational tool to support environmental management decisions.

Integrated use of biomarkers in the mussel Mytilus galloprovincialis for assessing off-shore gas platforms in the Adriatic Sea: Results of a two-year biomonitoring program

Despite a large number of gas platforms existing in the Adriatic Sea, which is a semi-enclosed basin characterized by a slow turnover rate and increasing industrial as well as other anthropogenic activities, the effects of these structures on the aquatic ecosystem require further investigation. Since 1998, multidisciplinary studies have been performed by CNR-ISMAR to comply with legislation and to support the development of protocols for the monitoring of offshore activities in the Adriatic Sea. The present study was developed to implement a biomonitoring plan to assess the ecotoxicological effects of the extraction activities of an off-shore gas platform. Biomarkers were evaluated in mussels collected from the platform in relation to physiological stress, DNA damage, cellular damage, oxidative stress and exposure effects. Organic contaminants and trace element bioaccumulation were also assessed in the soft body of the mussels to correlate bioaccumulation of pollutants with biomarker responses. The results indicate an absence of platform-related environmental stress.

A box model of the Late Miocene Mediterranean Sea: Implications from combined 87Sr/86Sr and salinity data

Under certain conditions the strontium isotope ratio in the water of a semi‐enclosed basin is known to be sensitive to the relative size of ocean water inflow and river input. Combining Sr‐isotope ratios measured in Mediterranean Late Miocene successions with data on past salinity, one can derive quantitative information on the Mediterranean hydrological budget at times before and during the Messinian Salinity Crisis (MSC). Previous studies obtained this hydrological budget by inverting the salinity and strontium data with steady state solutions to the conservation equations of salt, strontium and water. Here, we develop a box model with a time‐dependent set of equations to investigate the coeval evolution of salinity and Sr ratios under different water budgets, gateway restrictions and riverine Sr characteristics. Model results are compared with the salinity and strontium ratio data from the Mediterranean. With a present‐day water budget, strontium ratios in the Mediterranean never reach the observed Messinian values regardless of gateway restriction and water budget. However, a model with tripled river input, as inferred for the Late Miocene, is able to reproduce the Sr ratios observed. The onset of the MSC can be explained with a simple restriction of the gateway(s) between the Mediterranean and Atlantic. Lower Evaporite gypsum formed in a basin with less outflow to the Atlantic than modeled in previous studies because of the large Late Miocene river input. Evaporite thicknesses predicted by our model and consistent with the Messinian Sr ratios are on the low end of the thickness range inferred from seismics.

The role of gateways in the evolution of temperature and salinity of semi-enclosed basins: An oceanic box model for the Miocene Mediterranean Sea and Paratethys

Marine gateways are important for semi-enclosed basins as they control exchange flows and influence water circulation. During much of the Early and Middle Miocene (~ 23–13 Ma), the Paratethys (of which at present only the Black Sea and Caspian Sea remain) and the Mediterranean Sea were connected to both the Atlantic and Indian Oceans. The gateways to the Indian Ocean were closed ultimately in the Middle Miocene. Here, we apply an oceanic 4-box model to determine the temperature, salinity and exchange flows for the Paratethys and the Mediterranean Sea before and after closure of the Indian Ocean gateways. Our analysis forms a novel way of linking tectonics, climate and basin evolution. We investigate whether changes observed in the geological record of Paratethys are caused by changes in gateway configuration or in climate. We mainly focus on Paratethys which, because of the availability of a large variety of geological studies, presents an outstanding opportunity for studying the evolution of semi-enclosed basins and the impact of gateway closure. Moreover, Paratethys is considered to have played an important role in the evolution of Eurasian climate. Our analysis explores various values of the model parameters such as net evaporation and surface heat flux. Our main conclusions are: (1) Paratethys became more responsive to climate change after closure, (2) closure of the gateways to the Indian Ocean probably caused cooling of Paratethys and, in particular, accounted for the enigmatic Mid-Burdigalian cooling in Paratethys, (3) closure induced a change in salinity of Paratethys which is dependent on the net evaporation and size of the gateway connecting Paratethys to the Mediterranean, (4) Paratethys is sensitive to closure of both the gateway between Paratethys and the Indian Ocean, and the gateway between the Mediterranean Sea and the Indian Ocean, (5) Paratethys is influenced by the water properties of the Mediterranean Sea, (6) the water properties of Paratethys have only very limited influence on the Mediterranean Sea, and (7) in the advanced stage of the closure, the Mediterranean/Paratethyan temperature and salinity became more responsive to the restriction of the eastern gateways.

The processes of semi-enclosed basin–ocean water exchange across a tidal mixing zone

To assess each of the two possible mechanisms responsible for the fortnightly modulation of semi-enclosed basin–ocean water exchange (‘density tides’), a set of numerical experiments is carried out using a vertically two-dimensional numerical model with realistic situations in Puget Sound in mind. It is found that, although the localized vertical viscosity (or the localized vertical diffusivity) enhanced in the entrance sill region primarily controls the bottom-water transport (or the bottom-water density) during spring tides, it does not lead to any appreciable variations of bottom-water density (or bottom-water transport). This indicates that the fortnightly modulation of vertical viscosity and that of vertical diffusivity both play important roles in creating density tides. In the real ocean, the vertical viscosity and diffusivity are enhanced simultaneously during spring tides, so that it is difficult to discriminate between both effects on density tides. This causes the widespread misunderstanding that density tides are mainly caused by the decreased advection of dense bottom-water from outside due to the enhanced vertical viscosity during spring tides.

Benthic foraminifer palaeoecology of the Late Quaternary continental outer shelf of a landlocked marine basin in central Aegean Sea, Greece

North Evoikos Gulf, Greece, constitutes a landlocked, epicontinental and semi-enclosed basin, connected to the central-west Aegean Sea through a strait, probably not allowing full exchange of intermediate and deep water masses. Benthic foraminifer analyses were carried out on a Late Quaternary core from the outer shelf of the basin with the aim of tracing palaeoceanographic changes.The benthic foraminifer species percentage per sample was used to calculate two main biological proxies: the Benthic Foraminifer Oxygen Index (BFOI) and Benthic Foraminifer High Productivity (BFHP) proxy estimated by grouping benthic foraminifer taxa which are known to flourish under high Corg flux. The estimated values of BFHP and BFOI were controlled mainly by the abundance of bolivinid species which are generally considered as opportunistic and shallow infaunal species, tolerant to low oxygen conditions. The statistical analysis (Q-mode cluster analysis) singled out four time intervals, each characterized by certain levels of oxygenation and food availability. Time interval 1 (8513–8300calBP) is characterized by a mixed fauna of oxyphilic taxa and taxa that live in suboxic environment, indicating the existence of a relatively eutrophic system with a well-oxygenated sediment–water interface. Time interval 2 (8300–8100calBP) records a deterioration of conditions on sea floor, testified by significant oxygen and biotic diversity decrease. Benthic foraminifer composition in Time interval 3 (8100–7000calBP) depicts environmental conditions still highly variable, yet with faunal indication of more stable conditions than during the previous interval. Finally, the benthic foraminifer assemblage of Time interval 4 (7000–3500calBP) is related to the high food supply and good bottom circulation.The data suggest a strong linkage between the quantity and quality of the organic matter supplies, and the diversity and composition of the benthic foraminifer faunas. Moreover, this study clearly demonstrates that vertical tectonic movements need to be taken into account when reconstructing the late Quaternary palaeoceanography of this region.

Evaluating the impact of atmospheric deposition on dissolved trace-metals in the Gulf of Aqaba, Red Sea

Trace metals in the ocean act as both essential micro-nutrients and as toxins. There are relatively few multi-element studies of dissolved trace metals in the ocean, and none from the Gulf of Aqaba, Red Sea. This semi-enclosed basin surrounded by desert is a natural laboratory for studying the impact of atmospheric dry deposition of trace metals on the ocean surface. We have combined measurement of dissolved metals in seawater with measurements of the flux of metals associated with dry deposition. The total dissolved trace metal concentrations in Gulf of Aqaba water are generally higher (Fe, Cu, Zn, Co, Mn, Pb) or similar (Ni, Al, Cd, Mo) to those measured in the open North Atlantic Ocean. The concentrations of elements that are highly enriched in aerosols relative to Al (e.g. Cd, Pb, Zn and Cu) are not necessarily proportionally enriched in surface seawater when compared to Al, indicative of the high reactivity of these elements in seawater. Iron concentrations in the Gulf of Aqaba are high relative to Al, despite the fact that the aerosols are not more enriched in Fe relative to Al. There may be additional sources of dissolved iron to the Gulf of Aqaba, not associated with Al. Alternatively, intense photochemically-driven redox cycling may act to enhance Fe dissolution from aerosols, or may otherwise increase the lifetime of Fe in the water column, relative to Al. Copper concentrations in the Gulf of Aqaba are close to the value found to be a threshold for Cu toxicity in this region. A surface maximum in Cd:P is found in the Gulf of Aqaba, in contrast to the more typical surface minimum in this ratio observed in other locations. The surface maximum appears to be driven by atypically low uptake of Cd relative to P. A low Cd:P uptake ratio for this region is consistent with known environmental determinants of low Cd:P uptake, such as high concentrations of dissolved Zn and Fe, and a predominance of small phytoplankton including cyanobacteria.

Longitudinal variability of the biogeochemical role of Mediterranean aerosols in the Mediterranean Sea

Abstract. The Mediterranean Sea is a semi-enclosed basin characterized by a strong thermal stratification during summer during which the atmosphere is the main source of new nutrients to the nutrient-depleted surface layer. From aerosol sampling and microcosm experiments performed during the TransMed BOUM cruise (June–July 2008) we showed that: (i) the Mediterranean atmosphere composition (Al, Fe, P) was homogeneous over ~28° of longitude and was a mixture with a constant proportion of anthropogenic contribution and a variable but modest contribution of crustal aerosols. This quite stable composition over a one month period and a long transect (~2500 km) allowed to define the Mediterranean atmospheric "background" that characterizes the summer season in the absence of major Saharan event and forest fires, (ii) primary production significantly increased at all tested stations after aerosols addition collected on-board and after Saharan dust analog addition, indicating that both additions relieved on-going (co)-limitations. Although both additions significantly increased the N2 fixation rates at the western station, diazotrophic activity remained very low (~0.2 nmol N L−1 d−1), (iii) due to the presence of anthropogenic particles, the probable higher solubility of nutrients associated with mixed aerosols (crustal + anthropogenic contribution), conferred a higher fertilizing potential to on-board collected aerosol as compared to Saharan dust analog. Finally, those experiments showed that atmospheric inputs from a mixed atmospheric event ("summer rain" type) or from a high-intensity Saharan event would induce comparable response by the biota in the stratified Mediterranean SML, during summer.

Impact of mega-scale sand extraction on tidal dynamics in semi-enclosed basins: An idealized model study with application to the Southern North Sea

Long-term considerations of repeated and increasing sand extraction on the Netherlands Continental Shelf (North Sea) may lead to the creation of a mega-scale extraction trench in front of the Dutch coast (length hundreds of km, width over 10 km, depth several m). We investigate the impact of such a huge topographic intervention on tidal dynamics, which is a key aspect in hydrodynamics, and indirectly also affecting morphodynamics and ecology. Because of the unprecedented extent of the extraction scenario, we follow a generic approach aimed at understanding the physical mechanisms behind the changes, the orders of magnitude and area of influence. Hence, rather than applying an existing numerical model, we develop an idealized model for tide propagation in semi-enclosed rectangular basins. The model geometry consists of three adjacent compartments with a realistic cross-basin depth profile, the trench being located in the second compartment, while assuming along-basin uniformity of depth within each compartment. The problem is forced by an incoming Kelvin wave. The solution in each compartment, satisfying the linear depth-averaged shallow water equations on the f plane including bottom friction, is written as the superposition of semi-numerically obtained wave solutions. A collocation technique is employed to satisfy no-normal flow across the basin's closed end as well as continuity of elevation and flux across the interfaces between the compartments. The results indicate changes in tidal range, phase and currents throughout the whole basin, which shows the importance of a model domain on the scale of the basin. Changes in coastal tidal range show zones of decrease and increase (order cm). Changes in coastal shore-parallel tidal current amplitudes are of the order of cm s − 1 . A sensitivity analysis for various trench geometries shows that the hydrodynamic impact generally increases with increasing extraction volume, being more sensitive to trench depth and width than trench length. The changes in tidal currents may structurally alter sediment transport rates with several percents. As the bathymetry and coastal morphology result from subtle balances in long-term sediment transport, the trench may indirectly affect various user functions and values (coastal safety, navigation, marine ecology, cables and pipelines) and, hence, be of concern to coastal management.

Synthesis of a Tsunami Spectrum in a Semi-Enclosed Basin Using Its Background Spectrum

We explained spectra of distant tsunamis observed in enclosed basins by applying the synthesis method based on joint analysis of tsunami and background spectra from a number of stations. This method is the generalization of the method proposed by Rabinovich (J. Geopys. Res. 102, 12663-12676, 1997) to separate source and topography effects in recorded tsunami waves. The source function is extracted by inversion of the tsunami/background spectra averaged from many observational sites. The method is applied to the 2009 Papua tsunami observed at the Owase tide station in southwest Japan, a region with complicated topography and numerous bays and inlets. The synthesized spectrum explains the observed spectral amplitudes for each frequency component. It is shown that averaging of tsunami and background from various tide gauge stations in semi-enclosed basins is an efficient approach to extract the source function.

AN IDEALIZED MODEL OF TIDAL DYNAMICS IN SEMI-ENCLOSED BASINS: THE EFFECTS OF A MEGA-SCALE SAND EXTRACTION TRENCH IN THE NORTH SEA

We investigate the effects of a mega-scale sand extraction trench (length ~200 km, width ~10 km, depth ~10 m, located in front of the Dutch coast) on tidal dynamics in the Southern North Sea. To this end, an idealized modeling approach is adopted, extending the classical problem of Kelvin wave reflection in rectangular semi-enclosed basins to account for lateral depth variations and the presence of a trench. The model results indicate changes in tidal range (zones of decrease and increase of the order of several cm), phase and currents (order cm/s) throughout the whole basin, which emphasizes the importance of a model domain on the scale of the basin. The changes in currents may structurally change sediment transport rates by several percents, which is relevant to coastal morphology. It is argued that this idealized model can be used to support a companion study carried out with a more detailed numerical engineering model. This is because of its ability to (1) provide insight into the physical mechanisms, (2) reveal the order of magnitude and area of influence of the effects and (3) allow for a quick sensitivity analysis with respect to trench geometry.