Coastal lagoons are areas of ecological and economic importance with a high spatial and temporal variability, which makes them complex systems of high productivity in most cases [1
]. Their economic importance, not only depends on the existence of fisheries and related activities [3
], but also the various tourism and recreational activities that have appeared over recent years, increasing the stress on the existing ecosystems [4
]. Pumping and dredging activities related to coastal works devoted to increase tourist facilities in lagoons can have negative consequences on water and sediment quality, and therefore on biological assemblages, and eventhough these were intended to be improved for tourist appeal [8
]. At the same time, the hydrodynamic behaviour of coastal lagoons plays a crucial role in their functioning, not only in terms of water quality conditions, but also in terms of environmental range for species inhabiting the lagoons, species connectivity, and fishing capacities [3
]. Therefore, it is necessary to consider carefully any measure with potential impacts on the hydrodynamics of the lagoons, such as dredging, pumping sand, building new structures, or any other human intervention of this kind.
The issues of hydro-sedimentary processes in lagoons and enclosed bays are important in terms of the coastal management and, as such, they have been often treated so in the literature. Some examples can be found in the works by Wu et al. [12
], who applied a three-dimensional Finite Volume Community Ocean Model (3D FVCOM) model together with empirical models of the sediment transport and measurements of the surface sediment concentrations in a macro-tidal environment, the Bay of Fundy (Canada). They found that the sediment transport in the upper part of the Bay of Fundy (Minas Basin, Minas Passage and Chignecto Bay) is strongly influenced by the shear stress parameters. A previous study by Davidson-Arnott et al. [13
] also focused on the Bay of Fundy and analysed the hydro-sedimentary processes at Chignecto Bay and Cumberland Basin through field measurements of flow and sediments concentration as well as surveys. The authors found that flows at the saltmarsh depended on the bathymetry and marsh surface, and that a large amount of sediments moved through the marsh margin with sediment deposition controlled by waves. The authors mentioned the ecological importance of the saltmarshes vegetation in the primary productivity of waterfowl habitats as well as their relevance for sediment exchange and accumulation of contaminants. Regarding areas with high pressures from human activities, the study by Suanez & Bruzzi [14
] analysed the impacts of different kinds of engineering structures (soft and hard) used for the dune restoration and coastal protection in the Rhone Delta, which hosts salt industries and tourism activities. It was found that the efficiency of each system mainly depended on the sediment supply at each location. The authors also proposed three management scenarios and analysed the associated issues in terms of hydrosedimentary processes. Grifoll et al. [15
] applied the three-dimensional Regional Ocean Modeling System (3D ROMS) numerical model to evaluate the water renewal conditions in Bilbao Harbour in terms of the cleaning capacity for the water contamination, highlighting the importance of the hydrodynamics in understanding the water renewal processes due to their close relationships. Water renewal was computed by the local flushing time. An integrated index of risk for each pollutant was calculated, considering the different activities at the harbour, such as oil and chemicals activities, construction, shipyard, and dredging. It was highlighted that the use of 3D models provides a more realistic representation in this case, where the renewal patterns are affected by the changes in the vertical currents structure.
However, most of these works are carried out in estuarine systems or with strong tidal ranges in which the tidal forces or the flow of fresh water introduces a marked unidirectional character in the flows and physico-chemical and biological gradients. In fact, it is precisely that uni-versus multidirectional character and the restrictions on communication with the adjacent sea imposed by physical barriers that are among the main characteristics that determine the differences between estuaries and coastal lagoons [16
In the case of coastal lagoons, numerical models have been used to provide a better understanding of the underlying physical trends governing the effects caused by changing their main features, including their isolation degree. For example, numerical models have been widely applied to describe the water exchange of the lagoons with the open sea or other coastal bodies, given its importance for different biogeochemical and ecological processes.
In a study by Umgiesser et al. [17
] the three-dimensional (3D) Shallow water hydrodynamic finite element model (SHYFEM) model was used to compare 10 Mediterranean lagoons in terms of water exchange and mixing behaviour. The authors found that the exchange with the open sea highly influenced the transport scale times, although the wind can also enhance the exchange mechanisms in lagoons with more than one inlet. Based on this model, Ferrarin et al. [18
] assessed the impact of climate change on temperature, salinity, as well as water exchange and mixing efficiency. The study predicts that choked lagoons, like the Mar Menor, would have shorter renewal times because the exchange with the Sea increased due to climate change. The increase of the lagoon volume would not be as large as the increase in the exchange. Therefore, choked lagoons will shift towards restricted types. On the other hand, leaky types of lagoons will move towards restricted types too, because the water exchange will not increase as much as the volume of the lagoon, resulting in a higher water renewal time (WRT). They conclude that there will be a homogenisation of the lagoon types in terms of their hydrological conditions, resulting in changes to the biogeochemical conditions and a loss of biodiversity. More recently, Umgiesser et al. [19
] performed a study to analyse the dependency of the variability of water exchange on different hydrographical and climatic factors in the Curonian lagoon. The study analyses the seasonal variations of the water renewal and it demonstrates that there is no influence of density-driven circulations on the water exchange.
What concerns the impact of dredging scenarios on water renewal times, a study at the Obidos lagoon has been carried out in [20
], in which the simulations were done using the three-dimensional Modelo Hidrodinâmico (3D MOHID) model and included a sand transport module. This study delimits the range of changes in the WRT due to the dredging, which can reach up to a 50% reduction is some areas. Jeyar et al. [21
] developed a two-dimensional (2D) model of the Nador lagoon through the UFV-SWM model in order to analyse changes in the water renewal times due to the relocation of the inlet as a measure of the improvement of water quality in the lagoon. One recent study is the work carried out by Teatini et al. [22
], where a numerical model of the Venice lagoon was used together with various techniques (Computational Fluid Dynamics (CFD) models, field measurements) in order to analyse the effects of dredging the channels on the water exchange from the aquifer existing underneath the lagoon and the consequences for water quality in terms of pollutants transport. They also analysed the effects of the depression waves produced by vessels in groundwater pumping.
This study focuses on the Mar Menor as a representative case due to the serious environmental issues that have arisen over recent decades. One of the more important transformations was linked to the hydrodynamic changes due to the enlargement of the El Estacio inlet in 1972 to construct a navigable channel up to 30 m wide and 5 m deep. This provoked an increase in water renewal rates, reduction of salinity, and extreme temperatures, thus allowing colonisation by new species, in a process of “Mediterranisation” of the lagoon with important effects on sediment characteristics and a drop in fishing yields [23
]. More recently, and correlated with changes in agricultural practices in the drainage basin with the introduction of intensively irrigated crops, the alteration of the nutrient entry regime has produced a series of changes that has affected the trophic status and the ecological integrity of the lagoon [26
]. Dredging the channels connecting the lagoon with the open sea has been proposed as a possible mitigation measure for the improvement of the water quality, but the effectiveness and potential impacts of such a measure has to be analysed cautiously. Therefore, the aim of this study is to analyse the impact of dredging activities on the hydrodynamic conditions in the Mar Menor, in order to determine the environmental consequences of such changes.
An earlier modelling study of the Mar Menor was carried out by De Pascalis et al. [27
], in which the 2D SHYFEM model was used to analyze the effects of climate change on the hydrodynamic, salinity and temperature conditions. Ghezzo et al. [28
], considered the Mar Menor, as well as part of the Mediterranean Sea, using the 2D SHYFEM model coupled with a Lagrangian particle tracking module in order to assess the species connectivity between the lagoon and the sea and inside the lagoon. More recently, the thesis by López-Castejón [29
] has given insight into the hydrodynamic mechanisms in the Mar Menor in terms of the spatial and temporal variability of water levels, currents, water exchange with the Sea and water renewal times using the 3D ROMS model.
The novelty of this study is the use of the 3D SHYFEM model as a tool for a detailed analysis of the impacts of dredging interventions in the channels, covering a range of depths and extensions, on the currents regime, the water exchange volumes and the water renewal times, as well as the potential effects on the ecosystem of the Mar Menor.
5. Final Remarks: Actions on the Inlets as a Management Tool
From this study, it can be concluded that the impacts of dredging the channels on the hydrodynamics of a coastal lagoon increase with the magnitude of the dredging activities, that is, their maximum depth and extension. In this sense, there is a threshold for the magnitude of these activities beyond which important environmental effects can be expected.
Although the current speeds are small in the Mar Menor, which has been chosen as a representative case study, it can be remarked that the effects of dredging on the current modules are noticeable and could affect the sedimentation processes, as well as the sand and mud transport. The currents modules are generally decreased, particularly in the areas around the shoreline inside the Mar Menor, with consequences, not only for the communities of species living in those areas, but also for beach stability and dynamics. Besides this, dredging can affect the fish movements in the fishing areas. Unlike the rest of the lagoon, the velocity modules are increased in and around the area of El Estacio channel.
The spatial distribution of the effects of dredging over the velocity modules varies seasonally and the most severe dredging scenarios would lead to a change in the global circulation patterns inside the lagoon. The deeper dredging scenarios involving the Encañizadas channel show different behavior for the vertical stratification of the currents in the Northern area with respect to the Undisturbed state, except for the maximum wind conditions. Therefore, it is necessary to highlight that these changes can affect significantly the fishing yield in the Mar Menor, among other environmental factors.
The dredging of the channels also has an important effect on the maximum fluxes in and out the lagoon through the channels as well as the water exchange volumes with the Mediterranean Sea. This aspect has to be carefully taken into consideration in conjunction with the expected global warming effects on sea level rise.
Although the dredging scenarios based on partial maintenance or shallower depths show a reduction of the water renewal times closer to the usual ranges of variability of this factor in the lagoon, for the largest activities the reduction could reach half of the values of those of the undisturbed situation. This would have important consequences for the general regime of the lagoon and the species connectivity. Further study of this aspect will be carried out in the future.
Changes of the water renewal times spread over the whole lagoon in the case of dredging the Encañizadas channel whereas the effect is localised over the South sub-basin for those cases involving the Marchamalo channel only. The combination of dredging both channels, in general, leads to a homogenisation of the water renewal times along the lagoon, in contrast with the undisturbed situation, where the difference between the water renewal in the North and South is noticeable. This homogenisation could affect species biodiversity.
As a general remark, it can be said that maintenance of the exchange between coastal lagoons and the sea through inlets is a frequent management action [80
], and is considered important for the biological functioning of a lagoon [81
], avoiding dystrophic crises [82
], and facilitating the migration of target fishing species [3
]. However, the effects of dredging or building new communication channels may vary considerably from one lagoon to the other, and the ways in which inlets are managed must be carefully analysed [3
Depending on the initial geomorphological and hydrodynamic conditions, dredging the channels can in some cases avoids excessive isolation of the lagoon, facilitating an active gradient in the physical conditions of the lagoon, as well as fish migration, and improvement of fisheries, whereas in other cases it can excessively increase communication with the sea, homogenizing the system and reducing the intensity of gradients, leading to a fall in fishing yields.
These practices, whether or not linked to fishing activity, can also lead to significant changes in the environmental conditions, the introduction of new species, lagoon biological productivity, the distribution of benthic macrophytes, and spatio-temporal heterogeneity in hydrological and environmental conditions, having strong consequences on lagoon communities and homeostatic capabilities [9
Therefore, these complex effects of modifying the geomorphology of the inlets on the general biodiversity and structure of lagoon communities should be taken into account in any restoration or integrated management plan for these ecosystems.
This study demonstrates that numerical models provide a suitable tool for evaluating the effects of human interventions on the sensitive environments of the coastal lagoons, as they can integrate all the different physical parameters involved in the regime of these systems in an accurate way. Predictive models of this kind will be essential for coastal lagoon management under the pressures of climate change. The analyses included in this research, some of which have been treated in an unprecedented way for the Mar Menor lagoon, can be applied generically to other locations.