Estuaries and Coasts最新文献

We investigate how the physical forcing factors of river discharge and winds affect sediment delivery to, and retention within, mangrove-lined coastal regions. We use an idealized numerical model, broadly similar to the Firth of Thames deltaic system in New Zealand, to isolate and explore the underlying processes without some of the complexities of the real system. Total sediment transport and the relative contributions of riverine and bed-sourced sediment into the forest are assessed using a transect along the edge of the forest region. The model results demonstrate that both river discharge and winds alter the distribution of sediment transport, and that the spatial patterns relate to different regions of the river plume. At the river mouth (the near-field region), irrespective of the discharge employed, sediment fluxes are directed into the mangrove forest, indicating an accretionary environment consistent with satellite observations. Here, contributions from the riverine and bed-sourced sediments are similar. For small to medium discharge scenarios (up to (sim) 280 m(^{3}) s(^{-1}), flow speeds (sim) 0.6 m s(^{-1})), mass loads increase with river discharge. However, in the case of large discharge events, the high momentum in the near-field region allows the river plume to effectively transport sediment through the full width of forested region and out of the forest front. In the mid- and far-field regions of the plume, tidal influences also play a stronger role. Suspended sediment is primarily composed of bed-sourced material and transported out of the forest. Weaker winds are found to affect the far- and mid-field regions of the river plume. Stronger winds are able to reshape the entire plume structure, also including the near-field, such that sediment deposition is enhanced when winds are directed towards the forest.

As global climate change alters the magnitude and rates of environmental stressors, predicting the extent of ecosystem degradation driven by these rapidly changing conditions becomes increasingly urgent. At the landscape scale, disturbances and stressors can increase spatial variability and heterogeneity — indicators that can serve as potential early warnings of declining ecosystem resilience. Increased spatial variability in salt marshes at the landscape scale has been used to quantify the propagation of ponding in salt marsh interiors, but ponding at the landscape scale follows a state change rather than predicts it. Here, we suggest a novel application of commonly collected surface elevation table (SET) data and explore millimeter-scale marsh surface microtopography as a potential early indicator of ecosystem transition. We find an increase in spatial variability using multiple metrics of microtopographic heterogeneity in vulnerable salt marsh communities across the North American Atlantic seaboard. Increasing microtopographic heterogeneity in vulnerable salt marshes mirrored increasing trends in variance when a tipping point is approached in other alternative stable state systems — indicating that early warning signals of marsh drowning and ecosystem transition are observable at small-spatial scales prior to runaway ecosystem degradation. Congruence between traditional and novel metrics of marsh vulnerability suggests that microtopographic metrics can be used to identify hidden vulnerability before widespread marsh degradation. This novel analysis can be easily applied to existing SET records expanding the traditional focus on vertical change to additionally encapsulate lateral processes.

Phragmites australis is a common helophyte, covering much of the sheltered and shallow soft bottoms along the coasts of the Baltic Sea. Despite the expansion of P. australis over the past decades, there is little information on aquatic macroinvertebrates within P. australis beds. In this study, we examined the effect of large-scale (wave exposure, nutrients) and small-scale (distance from the seaward edge, live and dead stalk density, epiphyte and rhizome biomass) drivers on the density, taxa richness, diversity, and community structure of epifauna and infauna in monospecific P. australis beds around the Åland Islands and the Archipelago Sea. We found that higher wave exposure and nutrient levels generally supported higher epi- and infauna abundance and taxa richness. The effects on Shannon–Wiener diversity were less evident apart from an increase of the infauna diversity in the Archipelago Sea with increasing nutrient levels. On a local scale, the distance from the seaward edge, live and dead stalk density, and epiphyte biomass had varying effects on both epi- and infauna communities in the different regions. Rhizome biomass had no effect on either the epi- or infauna abundance, taxa richness, or diversity. Furthermore, according to existing studies, other habitats, e.g., Zostera marina meadows, Fucus vesiculosus belts, and vegetated soft-bottomed shallow bays, are generally characterized by more abundant fauna, except for the infauna, which had a higher density in P. australis beds than in vegetated soft-bottomed shallow bays. P. australis are a widespread, expanding, and understudied habitat with an important role in supporting coastal biodiversity.

Sea-level rise rates are predicted to surpass rates of wetland vertical adjustment in the coming decades in many areas, increasing the potential for wetland submergence. Information on where wetland migration is possible can help natural resource managers for planning land acquisition or enhancing habitat connectivity to bolster adaptation of coastal wetlands to rising seas. Elevation-based models of wetland migration are often hampered with uncertainty associated with ground surface elevation, current water levels (i.e., tides and extreme water levels), and future water levels from sea-level rise. Here, we developed an approach that involved digital elevation model error reduction and the use of Monte Carlo simulations that utilize uncertainty assumptions regarding elevation error, contemporary water levels, and future sea levels to identify potential wetland migration areas. Our analyses were developed for Duvall and Nassau Counties in northeastern Florida (USA). We focus on the migration of regularly oceanic-flooded wetlands (i.e., flooded by oceanic water daily) and irregularly oceanic-flooded wetlands (i.e., flooded by oceanic water less frequently than daily). For two relative sea-level rise scenarios based on the 0.5 m and the 1.5 m global mean sea-level rise scenarios, we quantified migration by wetland flooding frequency class and identified land cover and land use types that are vulnerable to future exposure to oceanic waters. The variability in total coverage and relative coverage of wetland migration from our results highlights how topography and accelerated sea-level rise interact. Our wetland migration results communicate uncertainty by showing flooding frequency class as probabilistic outputs.

Subtropical and tropical islands are undergoing rapid urbanization as the human population expands globally. Urbanization disrupts coastal ecosystems through several pathways—including the replacement of natural habitats with concrete structures that increase runoff pollution—but it remains difficult to isolate and characterize specific impacts of urbanization on marine ecosystems. The historical gradient in urbanization on the subtropical island of Okinawa, Japan, sets up a natural laboratory to study urbanization effects on nearshore ecosystems. Physicochemical parameters and bacterial community composition were assessed every 2 weeks for 1 year at two nearshore sites adjacent to watersheds with > 70% urban land use and two nearshore sites adjacent to watersheds with > 70% rural land use. Urbanization increased freshwater input and nutrient loading—indicated by decreased salinity and elevated nitrate + nitrite, ammonium, and phosphate at urban sites—despite the urban sites being more open to flushing due to land reclamation projects filling in the coral lagoon. Urbanization significantly altered microbial community composition by increasing diversity through the addition of fecal indicator and pathogenic bacteria—eight orders of bacteria were only detected in urban samples, whereas only Verrucomicrobiales was unique to rural samples. The change in microbial community composition at urban sites persisted throughout the seasonal cycle, suggesting a regime change or sustained disturbance. The altered physicochemical conditions and microbial communities at urban sites could degrade nearby coral reefs and their ecosystem services, highlighting the importance of coastal land management in marine conservation efforts.

At a global scale, deltas are vital economic hubs, in part due to the combination of their access to inland regions via river systems with their proximity to sea. However, with the sea in close vicinity also comes the threat of freshwater contamination by saline seawater, especially during droughts. This study explores the potential of a mitigation measure to estuarine salt intrusion, namely the construction of a (temporary) earthen sill—a measure implemented in the Lower Mississippi River near New Orleans (LA, USA). This study suggests design guidelines on how a sill can be used to mitigate estuarine salt intrusion: the design should focus on the longitudinal placement and the height of the sill, and the mitigating efficiency of the sill reduces with increasing tidal range. Overall, a (temporary) sill has great potential to reduce salt intrusion in salt wedge estuaries if there is sufficient water depth available.

Emergent marsh and open water have been identified as alternate stable states in tidal marshes with large, relative differences in hydrogeomorphic conditions. In the Florida coastal Everglades, concern has been raised regarding the loss of non-tidal, coastal peat marsh via dieback of emergent vegetation and peat collapse. To aid in the identification of alternate stable states, our objective was to characterize the variability of hydrogeomorphic and biologic conditions using a field survey and long-term monitoring of hydrologic and geomorphic conditions across a range of vegetated (emergent, submerged) and unvegetated (open water) communities, which we refer to as “ecosystem states,” in a non-tidal, brackish peat marsh of the coastal Everglades. Results show (1) linear relationships among field-surveyed geomorphic, hydrologic, and biologic variables, with a 35-cm mean difference in soil surface elevation between emergent and open water states, (2) an overall decline in soil elevation in the submerged state that was related to cumulative dry days, and (3) a 2× increase in porewater salinity during the dry season in the emergent state that was also related to the number of dry days. Coupled with findings from previous experiments, we propose a conceptual model that describes how seasonal hydrologic variability may lead to ecosystem state transitions between emergent and open water alternate states. Since vegetative states are only moderately salt tolerant, as sea-level rise pushes the saltwater front inland, the importance of continued progress on Everglades restoration projects, with an aim to increase the volume of freshwater being delivered to coastal wetlands, is the primary management intervention available to mitigate salinization and slow ecosystem state shifts in non-tidal, brackish peat marshes.

The coast of Cameroon, located at the bottom of the Gulf of Guinea, is confronted with coastal hazards whose magnitude, distribution, and consequences are currently largely underestimated if not poorly understood. This study aims to fill this gap by proposing an integrated approach to coastal vulnerability assessment, combining simple traditional methods, multicriteria AHP (analytic hierarchy process) analysis, and machine learning techniques. Using geospatial data, field observations, and numerical models, we assessed the 402-km Cameroon coastline, taking into account interactions between physical, geological, and socio-economic factors. The results highlight geomorphology, slope, coastal erosion, and population density as the main contributors to vulnerability. The Integrated Coastal Vulnerability Index (IVCI) calculated by the simple method shows variable levels of vulnerability, with a predominance of “very low” and “low” in the northern sectors (S1 = 58%, S2 = 99%, and S3 = 87%) and “high” and “very high” in the south (S4 = 58% and S5 = 61%). The AHP method reveals a more balanced distribution of vulnerability levels, highlighting a sector (S3 = 96%) at “very strong” and “strong” risk. The application of six machine learning algorithms shows good predictive capabilities for ICVI, with the exception of the support vector machine (SVM). The artificial neural network (ANN) algorithm stands out for its superior accuracy, with an F-score of 0.9, ability to explain data variance (R = 0.95), accurate predictions (RMSE = 0.2), and excellent ability to distinguish classes (kappa coefficient of 0.9 and ROC AUC of 0.9). This study emphasizes the magnitude and complexity of interactions as indicators of the susceptibility of coastal populations to vulnerability.

Seagrass meadows are known as hot spots for carbon accumulation, but there is limited field data on the variability of sediment accumulation across time and space. We developed a method to assess spatial and temporal heterogeneity in net sediment accumulation in seagrass meadows using small, inexpensive samplers, allowing for over 200 unique measurements across multiple transects within our study site. Using this method, we assessed sediment accumulation across seagrass meadow edges, and in varying weather conditions. We found the greatest accumulation of sediment 5 m outside of seagrass meadow edges, with sediment accumulation rates averaging just under 100 g m⁻² day⁻¹, though rates were highly variable. Carbon accumulation from settled sediment was generally greater outside of seagrass meadow edges than within the bed, especially at sites undergoing recent expansion. Measurements made during tropical storms showed both scouring of sediment away from sites, and increased accumulation, depending on site properties as well as individual tropical storm characteristics. In the storm that had a measurable storm surge, scouring of sediment was a more dominant mechanism, whereas deposition dominated in the storm that had high winds but no associated storm surge. Our data demonstrate the necessity of including measurements that characterize both spatial and meteorological variability to develop a more holistic understanding of the movement of sediment and particulate organic carbon associated with seagrass meadows, especially as meadow area becomes increasingly fragmented with human activity and global change.

Suspended oysters employ physiological strategies to adjust their metabolic needs with the available food resources. Using the biodeposition method, the feeding and processing behavior of Pacific oysters (Crassostrea gigas) was investigated with a field study comparing four periods (May, July, October, and December 2016) with different upwelling intensities in the coastal lagoon of San Quintin Bay (Mexico). We calculated physiological feeding responses throughout the culture cycle, including the clearance rate (CR), filtration rate (FR), net organic ingestion rate (NOIR), net organic absorption rate (NOAR), net organic selection efficiency, net absorption efficiency, and the ammonium excretion rate (AER). The dietary quality predictors showed large fluctuations in terms of total particulate material, organic fraction of seston, and chlorophyll concentration. Unlike the pumping activity, FR, NOIR, and NOAR were related to upwelling conditions, and C. gigas removed twofold, ingested fourfold, and assimilated fivefold more of the organic suspended material during the upwelling season compared with periods of weak upwelling. C. gigas showed the potential of depositing nearly twice the organic biodeposits to the sediments during the intense upwelling events. The highest AER was recorded in July and October, suggesting that seasonal temperature variation is the most important exogenous factor regulating nitrogen metabolism, even in a subtropical environment. Also, mechanistic models incorporating dietary quality predictors to the feeding and processing response functions of C. gigas were performed. We conclude that coastal upwelling plays an important bottom-up control on oysters’ feeding and processing activity, and our results facilitate further studies of the carrying capacity of embayments influenced by eastern boundary current systems.