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Excess evaporation within the Persian (also referred as the Arabian) Gulf induces an inverse-estuary circulation. Surface waters are imported, via the Strait of Hormuz, while saltier waters are exported in the deeper layers. Using output of a 1/12-Degree horizontal resolution ocean general circulation model, the spatial structure and time variability of the circulation and the exchanges of volume and salt through the Strait of Hormuz are investigated in detail. The model's circulation pattern in the Gulf is found to be in good agreement with observations and other studies based on numerical models. The mean export of salty waters in the bottom layer is of 0.26±0.05Sv (Sverdrup = 1.0 × 106 m3 s−1 ). The net freshwater import, the equivalent of the salt export divided by a reference salinity, done by the baroclinic circulation across that vertical section is decomposed in an overturning and a horizontal components, with mean values of 7.2±2.1 × 10−3 Sv and 5.0±1.7 × 10−3 Sv respectively. An important, novel finding of this work is that the horizontal component is confined to the deeper layers, mainly in the winter. It is also described for the first time that both components are correlated at the same level with the basin averaged evaporation minus precipitation (E-P) over the Persian Gulf. The highest correlation (r2 = 0.59) of the total freshwater transport across 26˚N with E-P over the Gulf is found with a one-month time lag, with E-P leading. The time series of freshwater import does not show any significant trend in the period from 1980 to 2015. Power spectra analysis shows that most of the energy is concentrated in the seasonal cycle. Some intraseasonal variability, likely related to the Shamal wind phenomenon, and possible impacts of El-Nino are also detected. These results suggest that the overturning and the horizontal components of freshwater exchange across the Strait of Hormuz are both driven by dynamic and thermodynamic processes inside the Persian Gulf.

This study was undertaken to determine the relationships between the biomass, morphometry, and density of short shoots (SS) of the tropical seagrass Thalassia testudinum and the physical-environmental forcing in the region. Seasonal sampling surveys were undertaken four times in Bahia de la Ascension, a shallow estuary in the western Mexican Caribbean, to measure plant morphology and environmental variables. The estuary has a fresh water-influenced inner bay, a large central basin and a marine zone featuring a barrier reef at the seaward margin. Leaf size was positively correlated with increasing salinity, but total biomass was not, being similar across most of the sites. Aboveground biomass exhibited seasonal differences in dry and rainy seasons along the bay, most markedly in the brackish inner bay where an abrupt decline in biomass coincided with the rainy season. The relationship between nutrients and biomass indicates that the aboveground/belowground biomass ratio increases as nutrient availability increases. Areal cover was inversely correlated with SS density during both dry and rainy seasons. Maximum SS recruitment coincided with the rainy season. Peaks in SS density were recorded in the freshwater-influenced inner bay during an ENSO cold phase in 2007 (\"La Niña\") which is associated with a wetter dry season and following a strong storm (Hurricane Dean). The onset of the rainy season influences both shoot density and T. testudinum biomass by controlling the freshwater input to the bay and thus, the system's salinity gradient and external nutrients supply from the coastal wetland.

Marine and coastal ecosystems occupy the dynamic interface where land, water, and atmosphere interact and constantly are modified by natural events and human actions, causing the most immediate effects of environmental changes, habitat destruction, and biodiversity loss. They are rich in biodiversity and of great economic importance by providing multiple uses and resources for over half of the population currently living in coastal areas. Also, coastal wetlands (mangrove forests, salt marshes, and seagrass meadows) constitute the blue carbon ecosystems and are among the most efficiently sequesters of carbon). The pressure that marine and coastal ecosystems across the coasts of the world now face is not new and highly varied, resulting in far-reaching implications for the ocean and adjacent areas. Nevertheless, apart from the known responses of marine environments to climate changes on their functioning and structure, significant knowledge gaps on the potential effects of catastrophic natural events and intense anthropogenic stress and destruction on ecosystem dynamics are still needed, requiring new approaches to solve this problem. This Research Topic aimed to contribute to the potential responses of marine and coastal ecosystems, when subject to extreme chronic stressors or catastrophic events, either by natural or anthropogenic actions, providing a broad overview of assessment and monitoring tools from case studies from different continents.

Coastal ecosystems and the livelihoods they support are threatened by stressors acting at global and local scales. Here we used the data produced by the Caribbean Coastal Marine Productivity program (CARICOMP), the longest, largest monitoring program in the wider Caribbean, to evidence local-scale (decreases in water quality) and global-scale (increases in temperature) stressors across the basin. Trend analyses showed that visibility decreased at 42% of the stations, indicating that local-scale chronic stressors are widespread. On the other hand, only 18% of the stations showed increases in water temperature that would be expected from global warming, partially reflecting the limits in detecting trends due to inherent natural variability of temperature data. Decreases in visibility were associated with increased human density. However, this link can be decoupled by environmental factors, with conditions that increase the flush of water, dampening the effects of human influence. Besides documenting environmental stressors throughout the basin, our results can be used to inform future monitoring programs, if the desire is to identify stations that provide early warning signals of anthropogenic impacts. All CARICOMP environmental data are now available, providing an invaluable baseline that can be used to strengthen research, conservation, and management of coastal ecosystems in the Caribbean basin.

The Persian Gulf is a semi-enclosed marine system surrounded by eight countries, many of which are experiencing substantial development. It is also a major center for the oil industry. The increasing array of anthropogenic disturbances may have substantial negative impacts on marine ecosystems, but this has received little attention until recently. We review the available literature on the Gulf’s marine environment and detail our recent experience in the United Arab Emirates (U.A.E.) to evaluate the role of anthropogenic disturbance in this marine ecosystem. Extensive coastal development may now be the single most important anthropogenic stressor. We offer suggestions for how to build awareness of environmental risks of current practices, enhance regional capacity for coastal management, and build cooperative management of this important, shared marine system. An excellent opportunity exists for one or more of the bordering countries to initiate a bold and effective, long-term, international collaboration in environmental management for the Gulf.

[This corrects the article DOI: 10.1371/journal.pone.0188564.].

Palm Jumeirah is the most completely developed of several man-made coastal island megaconstructions in Dubai, United Arab Emirates. The palm-shaped island, surrounded by an elliptical breakwater, was developed 7 y ago, has an overall footprint of 23 km2, of which the constructed island surface area is 7.9 km2, and is connected to shore via a 5-km-long spine from the mainland to the crescent tip. Time-series observations of hydrographic variables and currents within the interior of the development (Palm Jumeirah Lagoon) during 30 d in April–May 2008 were utilized to examine current flow, tide variability, water budget, vertical mixing, and turnover time within this megastructure. Currents within Palm Jumeirah Lagoon varied between stations; however, similar water temperatures and salinities were apparent throughout all the stations. Palm Jumeirah Lagoon tides were mixed and mainly semidiurnal, with spring and neap tidal ranges measuring 116 and 56 cm, respectively, and no difference in amplitude or phase throughout Palm Jumeirah Lagoon. There were substantial differences in water discharge between the east and west entrances, with high discharge on average exiting the eastern entrance and low discharge exiting the western entrance. These results indicate that the eastern and western halves of Palm Jumeirah Lagoon are flushed unequally and show differences in residence times (1.2 and 42 d, respectively), due to differences in tidal currents, wind influence, and variability of the bathymetric contour. Previous numerical modeling studies of water residence time within Palm Jumeirah Lagoon did not capture this difference, which could be associated with the exclusion of bathymetric variability in the previous modeling. Due to the strong shear and weak saline stratification, the water column throughout Palm Jumeirah Lagoon remained instable, with vertical mixing present during the spring-neap tidal cycle and well-mixed conditions predominating throughout the lagoon system.

The Cayman Crown is a recently identified, highly resilient coral reef ecosystem in the deep-water channel between Belize and Guatemala. We collected single and multi-beam sonar data to map bathymetry and describe the unique reef morphology and high resilience. The morphology is the result of the setting of the reef along the edge of the North American tectonic plate at the southwestern apex of the Cayman Trench. Our data were used to support the creation of newly declared marine reserves.

The characteristics and forcing mechanisms of high-frequency flow variations (periods of minutes to days) were investigated near Gladden Spit, a reef promontory off the coast of Belize. Direct field observations and a high-resolution (50-m grid size) numerical ocean model are used to describe the flow variations that impact the initial dispersion of eggs and larvae from this site, which serves as a spawning aggregation site for many species of reef fishes. Idealized sensitivity model experiments isolate the role of various processes, such as internal waves, wind, tides, and large-scale flow variations. The acute horizontal curvature and steep topography of the reef intensify the flow, create small-scale convergence and divergence zones, and excite high-frequency oscillations and internal waves. Although the tides in this area are relatively small (∼10-cm amplitude), the model simulations show that tides can excite significant high-frequency flow variations near the reef, which suggests that the preference of fish to aggregate and spawn in the days following the time of full moon may not be coincidental. Even small variations in remote flows (2–5 cm s −1 ) due to say, meso-scale eddies, are enough to excite near-reef oscillations. Model simulations and the observations further suggest that the spawning site at the tip of the reef provides initial strong dispersion for eggs, but then the combined influence of the along-isobath flow and the westward wind will transport the eggs and larvae downstream of Gladden Spit toward less turbulent region, which may contribute to enhanced larval survival.

Salt marsh ecosystems are maintained by the dominant macrophytes that regulate the elevation of their habitat within a narrow portion of the intertidal zone by accumulating organic matter and trapping inorganic sediment. The long-term stability of these ecosystems is explained by interactions among sea level, land elevation, primary production, and sediment accretion that regulate the elevation of the sediment surface toward an equilibrium with mean sea level. We show here in a salt marsh that this equilibrium is adjusted upward by increased production of the salt marsh macrophyte Spartina alterniflora and downward by an increasing rate of relative sea-level rise (RSLR). Adjustments in marsh surface elevation are slow in comparison to interannual anomalies and long-period cycles of sea level, and this lag in sediment elevation results in significant variation in annual primary productivity. We describe a theoretical model that predicts that the system will be stable against changes in relative mean sea level when surface elevation is greater than what is optimal for primary production. When surface elevation is less than optimal, the system will be unstable. The model predicts that there is an optimal rate of RSLR at which the equilibrium elevation and depth of tidal flooding will be optimal for plant growth. However, the optimal rate of RSLR also represents an upper limit because at higher rates of RSLR the plant community cannot sustain an elevation that is within its range of tolerance. For estuaries with high sediment loading, such as those on the southeast coast of the United States, the limiting rate of RSLR was predicted to be at most 1.2 cm/yr, which is 3.5 times greater than the current, long-term rate of RSLR.