Leaky Lagoons Research Articles

Seagrass macrobenthic biodiversity does not vary in conformity with a leaky-lagoonal confinement gradient

Coastal lagoon ecology often changes on progression from the open, well-flushed mouth region to the depositional zone furthest from the open sea. This is generally considered consequent on increasing ‘confinement’ and associated features, rather than on the often co-occurringly decreasing salinity. The 12 km Rainbow Channel connecting part of Moreton Bay, a microtidal leaky lagoon, to the adjacent Pacific provides a gradient of increasing confinement without any significant salinity change, i.e. a tenfold increase in water residence time for a salinity decrease of <1. Macrobenthic faunal assemblages characterising intertidal Zostera seagrass at strategic points along its length were compared to test whether their nature changed in conformity with confinement models. Results suggest that it does not; faunal abundance, species richness, evenness and composition remaining effectively unchanged along the gradient. Seagrass systems may constitute a special case because they decouple renewal times of the overlying water and local organic enrichment/decomposition; as may leaky lagoons because of their high tidal velocities.

Anthropocene Trails on Geomorphology of Meghalayan Chilika Basin Odisha

The Chilika gulf in the pre-Meghalayan era has transformed into Greater Chilika, later a leaky lagoon and now downsizing. During early Anthropocene epoch (1950) became a restricted lagoon with the stable southern half of the barrier spit and fragile the northerly half. The sediment balance governed by the inland riverine flood flows from south Mahanadi delta (SMD), and through diurnal tides during other periods. The present anthropogenic development are the emerging Bhubaneswar capital (1948), rising population, and peripheral growth of urbans. Chilika, have deteriorated its eco-bio-hydro system and its associated delta. The anthropogenic interventions to flow by dams, barrages, new mouth dredging, Gobakund cut and natural sea-level changes, frequent storms, high waves, and longshore drift, have altered the geomorphology of the area. The present search envisages the physical changes that occurred in about 6000km2 of study area. The Remote Sensing and GIS technology and the ERDAS software tool were in use to concoct for the years 1984, 1994 and 2022, and compared the basic geophysical features geospatially (1975 to 2022). The erratic changes have developed over the lacustrine area, tidal inlets variability, increase in stable barrier spit and ecosystem with the land slamming of storms. Excess tourism activities, exploitation of fishery, motorized communications, poaching and increase of stakeholders have lost its forests, hills, solitude, and tranquility within.

SEDIMENTOLOGY AND PALAEOENVIRONMENTAL RECONSTRUCTION OF AG-1 CORE OF AYVALIK REGION, NW TURKEY

This study presents sedimentological and palaeontological data retrieved from a coastal lagoon near Ayvalik in NW Turkey. The objective of this study is to interpret the depositional environments and reconstruct the evolution of the study area during the late Holocene. 42 sediment samples have been retrieved from a 13m core. Sedimentological analyses such as grain size analysis, moment measures of mean, sorting, skewness and kurtosis, definition of total organic carbon (T.O.C.), total nitrogen (T.N.), colour and CaCO3 (%) were employed. Moreover, macro and microfossils were collected to reconstruct the depositional environments. The palaeoenvironmental - palaeoecological analyses indicated a closed shallow shelf environment (around 4000 to 2400 yr BP) that gradually turned into a leaky lagoon (around 2400 yr BP to present).

Do physiography and hydrology determine the physico-chemical properties and trophic status of coastal lagoons? A comparative approach

Coastal lagoons are very heterogeneous systems covering a wide range of physiographical and hydrological characteristics. The hypothesis of the present investigation was that physiographical and hydrological characteristics of coastal lagoons affect their physico-chemical properties and their buffering capacity against nutrient enrichment. We compared data collected during a complete annual cycle in six coastal lagoons which were representative of the different lagoon types proposed by Kjerfve (1986) and were subjected to different anthropogenic pressures. The greater exchange of water with the sea in the leaky lagoon of Ria Formosa (type 3) reduced the seasonal differences in the physico-chemical variables and increased the buffer capacity against nutrient enrichment when compared to the restricted (type 2) and chocked (type 1) lagoon types. The lagoon types also showed important differences regarding nutrient limitation, with types 1 and 2 being P-limited and type 3 being N-limited. Our approach can be applied to other coastal lagoons representative of transitional water types in order to provide a general framework for the rational management and policy strategies of coastal lagoons.

The importance of habitat-type for defining the reference conditions and the ecological quality status based on benthic invertebrates: The Ria Formosa coastal lagoon (Southern Portugal) case study

Coastal lagoons are complex systems, with considerable habitat heterogeneity and often subject to high temporal dynamics, which constitutes a great challenge for ecological assessment programs. For defining reference conditions for benthic invertebrates, under the EU Water Framework Directive objectives, historical data from the Ria Formosa leaky lagoon (wet surface area of about 105km2) located in Southern Portugal was used. The influence of habitat features, such as channel depth, sediment type and seagrass cover, on the expression of these biological communities, was inferred by analyzing subtidal data collected at stations with different environmental characteristics. Such heterogeneity effect was analyzed at the community compositional and structural levels, and also for three indices in the multimetric Benthic Assessment Tool (BAT). This tool for the assessment of ecological status includes the Margalef index, Shannon–Wiener diversity index, and AZTI's Marine Biotic Index (AMBI). Significant variations associated with environmental features were reflected on specific reference conditions at four habitats in the lagoon. After habitat calibration, the Benthic Assessment Tool (BAT) revealed that, in general and for the period of time covered by this historical data set, the status of the lagoon corresponded to a good ecological condition, which is mainly due to its high water renewal rate. Such classification is in accordance with the majority of studies at the lagoon. However, at punctual sites with human induced high water residence times, significantly lower BAT values were registered. Such community degradation can be associated with physical stress due to salinity increase and to a degradation of water quality, with occurrence of occasional dystrophic crisis, triggered by low water renewal. Habitat differentiation was a crucial step for a correct evaluation of the ecological condition of invertebrate communities across the lagoonal system.

A lagoonal interlude with occasional hypersalinity in the deposition of the Early–Middle Miocene Brasso Formation of Trinidad

Abstract An outcrop of the Early–Middle Miocene age Brasso Formation along the Guaico-Tamana Road of Trinidad was logged and sampled for 20 m at 1 m intervals. The log revealed two sand- and silt-rich successions separated by a claystone-rich interval with thinner sands. One horizon yielded gypsum that is concluded to indicate that conditions occasionally became hypersaline. With the exception of one laminated siltstone interval ∼1 m thick, all the beds lacked primary sedimentary structures, but many contained much mollusc debris. The lack of primary structures is thought to reflect bioturbation. Pyrite was common throughout much of the section, but replaced by hematite in two intervals. The pyrite is indicative of anoxic conditions in the sediment below the bioturbated layer, while the hematite shows that conditions occasionally became strongly oxic. The development of low-diversity benthonic foraminiferal communities dominated by porcellaneous-walled Quinqueloculina seminulangulata supports a model of occasional hypersalinity. These conditions are concluded to have developed in a leaky lagoon situated between the carbonate bioherms of the Tamana Formation of Central Trinidad and the mountainous allochthon of the Northern Range. Occasional high numbers of planktonic foraminifera are suggested to result from onshore winds blowing surface seawater into the lagoon. The lagoonal interlude came at the end of a tectonically-induced transgressive–regressive cycle. The influence of tectonics on the deposition of the Brasso Formation complicates at least for the Early–Middle Miocene any attempt to tie the sequence stratigraphy of Trinidad to a global sea-level model. It remains to be seen how much this is true for the remainder of the Neogene succession.

Tidal and residual circulations in coupled restricted and leaky lagoons

Finite element numerical modelling based on field data is used to study the tidal and tidally induced residual circulation dynamics of a coupled “restricted” and “leaky” coastal lagoon system located in the Magdalen Islands, Gulf of Saint-Lawrence. Havre-aux-Maisons Lagoon (HML) is of a “restricted” nature with a neutral inlet in terms of tidal asymmetry. Grande-Entrée Lagoon (GEL) is of a “leaky” nature with a marked ebb dominance at the inlet due to direct interactions between the main astronomical tidal constituents. The imbalance caused by the different tidal filtering characteristics of both inlets combines with the internal morphological asymmetries of the system to produce a residual throughflow from HML to GEL. The residual circulation is also characterized by strongest values at both inlets, very weak residual currents in HML deep basin and a dipole of residual eddies over the deeper areas of GEL. Further investigations including numerical tracer experiments will be necessary to achieve a full understanding of the long term circulation of this lagoonal system.

Geoarchaeology of Sidon's ancient harbours, Phoenicia

Geoarchaeological data from Sidon's ancient harbour areas elucidate six evolutionary phases since the Bronze Age. (1) At the time of Sidon's foundation, during the third millennium BC, medium sand facies show the city's northern and southern pocket beaches to have served as proto-harbours for Middle to Late Bronze Age societies. (2) Towards the end of the Late Bronze Age and the Early Iron Age, expanding international trade prompted coastal populations into modifying these natural anchorages. In Sidon's northern harbour, transition from shelly to fine-grained sands is the earliest granulometric manifestation of human coastal modification. The lee of Zire island was also exploited as a deep-water anchorage, or outer harbour, at this time. (3) Although localised sediments evoke developed port infrastructure during the Phoenician and Persian periods, high-resolution reconstruction of the northern harbour's Iron Age history is problematic given repeated dredging practices during the Roman and Byzantine periods. (4) Fine-grained silts and sands in the northern harbour are coeval with advanced Roman engineering works, significantly deforming the coastal landscape. Bio- and lithostratigraphical data attest a leaky lagoon type environment, indicative of a well-protected port. (5) The technological apogee of Sidon's northern harbour is recorded during the late Roman and Byzantine periods, translated stratigraphically by a plastic clays unit and brackish lagoon fauna. (6) A final semi-abandonment phase, comprising coarse sand facies, concurs silting up and a 100–150 m progradation of the port coastline after the seventh century AD. We advance three hypotheses to explain these stratigraphic data, namely cultural, tectonic and tsunamogenic. Finally, our results are compared and contrasted with research undertaken in Sidon's sister harbour, Tyre.