Coastal Circulation Research Articles

Advances on Coastal and Estuarine Circulations Around the Changjiang Estuary in the Recent Decades (2000–2020)

Advances on the circulation in the Changjiang Estuary and adjacent East China Sea (ECS) and Yellow Sea (YS) coastal waters in the recent decades (2000–2020) are synthesized in this review. The circulation over the complicated bathymetry in the region is locally driven by winds, tides, as well as riverine discharge, and is remotely influenced by shelf currents between the 50 and 100-m isobaths through the cross-shelf exchanges. The interchange of the momentum and the freshwater pathway inside the Changjiang Estuary are jointly determined by tides and seasonally varying discharge and winds over the shelf. The buoyant waters are trapped inside the bulge that forms and expands over the shelf to the west of the 30-m isobath in the vicinity of Hangzhou Bay and the Changjiang Estuary. These buoyant waters are exported offshore by the shelf current, tidal mixing, and variations of wind patterns, forming the Changjiang River plume, which shows notable seasonality due to the reversal of both winds and shelf currents in the ECS and YS. Extensive spatial irregularities in the form of freshwater patches are present along its pathway to the Tsushima Strait in summer and to the Taiwan Strait in winter, respectively. Tides and the bathymetry irregularity have recently been found to play critical roles in determining the cross-shelf exchanges of water mass and momentum along the pathway of the ECS coastal current, and along this pathway, a year-round upslope intrusion of shelf waters appears in both summer and winter. Tides also play an important role in altering the expansion of the Changjiang River plume, cross-shelf extrusion of waters, and variation in the Yellow Sea Coastal Current over the shallow Subei Shoal.

Analiz dinamicheskikh i energeticheskikh kharakteristik tsirkulyatsii vod u beregov Zapadnogo Kryma na osnove assimilyatsii dannykh nablyudeniy v chislennoy modeli dinamiki Chernogo morya

Purpose. The study is aimed at evaluating effectiveness of the procedure of the observational data assimilation using the Kalman filter algorithm as compared to sequential analysis of the hydrophysical fields based on the optimal interpolation method, and at analyzing the mesoscale features of coastal circulation near the western Crimea coast and in the Sevastopol region. Methods and Results. Based on the hydrodynamic model adapted to the Black Sea coastal zone conditions including the open boundary and on the temperature and salinity data from the hydrological survey in 2007, the dynamic and energy characteristics of the Black Sea coastal circulation were calculated with high spatial resolution (horizontal grid is ~1.6×1.6 km and 30 vertical horizons). The hydrophysical fields were reconstructed using two algorithms of data assimilation: the sequential optimal interpolation and the modified Kalman filter. The kinetic energy changed mainly due to the wind action, vertical friction and the work of pressure forces; the potential energy – due to the potential energy advection and the horizontal turbulent diffusion. The following circulation features were reconstructed: the anticyclonic eddy with the radius about 15 km in the Kalamitsky Bay in the water upper layer, the anticyclonic eddy with the radius about 15 km between 32.2 and 32.6° E in the whole water layer, the intense current near Sevastopol and along the Crimea western coast directed to the north and northwest, and the submesoscale eddies of different signs of rotation in the upper layer. Conclusions. It is shown that having been taken into account, heterogeneity and non-isotropy of the error estimates of the temperature and salinity fields relative to the correlation function lead to qualitative and quantitative differences in the hydrodynamic fields (amplification of currents, change of the currents’ direction and eddy formations were better pronounced). At the same time, the mean square errors of the thermohaline fields’ estimates decreased. Formation of the anticyclonic eddy with the radius about 15 km in the Kalamitsky Bay could be related to the current shear instability. Submesoscale eddies with the diameters less than 5 km were formed when the current flowed around the coastline and the bottom topography inhomogeneities

South Caspian vertical circulation during rapid sea level rise and fall

The Caspian Sea as the largest land-locked water body is very sensitive to the environmental changes. The rise and fall of the Caspian Sea level has significant impact on the environment and coastal communities. Using extensive deep water measurements in the south Caspian Sea, we have examined the role of sea level changes on the vertical water exchange. While the deep water ventilation happened during sea level fall (70s of 20 century), coastal lagoon expansion and marine stagnant circulation state were the major impacts during rapid sea level rise (1980-1995). The marine environment however benefits the rapid sea level fall. During sea level fall, vertical circulation penetrates deeper and faster which distributes oxygen and nutrient more effectively in the water column. It provides the condition for enhanced bio-productivity. However, the current global warming could prevent the impact of lowstand to faster circulation. The present rapid sea level fall displays new impacts on the marine environment, reflecting as hypoxia.

Necessary Conditions for Warm Inflow Toward the Filchner Ice Shelf, Weddell Sea

AbstractUnderstanding changes in Antarctic ice shelf basal melting is a major challenge for predicting future sea level. Currently, warm Circumpolar Deep Water surrounding Antarctica has limited access to the Weddell Sea continental shelf; consequently, melt rates at Filchner‐Ronne Ice Shelf are low. However, large‐scale model projections suggest that changes to the Antarctic Slope Front and the coastal circulation may enhance warm inflows within this century. We use a regional high‐resolution ice shelf cavity and ocean circulation model to explore forcing changes that may trigger this regime shift. Our results suggest two necessary conditions for supporting a sustained warm inflow into the Filchner Ice Shelf cavity: (i) an extreme relaxation of the Antarctic Slope Front density gradient and (ii) substantial freshening of the dense shelf water. We also find that the on‐shelf transport over the western Weddell Sea shelf is sensitive to the Filchner Trough overflow characteristics.

The structure of incipient coastal counter currents in South Portugal as indicator of their forcing agents

The alongshore subtidal water circulation along the South Portugal inner shelf is characterized by the temporal alternation of equatorward (i.e., broadly eastward) flows related to coastal upwelling processes and poleward (i.e., broadly westward) Coastal Counter Currents (CCCs). The objective of this study is to get insights about the main drivers of CCCs based on kinematic parameters describing the structure of the flow at the moment it changes direction. The parameters are derived from an extensive bottom-mounted ADCP dataset (16 deployments; 34,121 hourly records) collected at a single mooring (23 m water depth). Results show that the so-called incipient flows present contrasted general patterns whether they turn from equatorward to poleward or the opposite. Complementary observations at a nearby station indicate that these characteristics are spatially consistent along the studied area. Although 70% of CCCs are generated under favourable wind conditions (Levanter), these flows generally develop through the bed layer, in particular in summer. Hence, the Levanter wind - expected to promote flow setup through the surface layer - is not the main driver of CCCs in most cases. The general structure of incipient CCCs strongly suggests that the dominant force competing with the wind stress is an alongshore pressure gradient (APG). Furthermore, the maximum equatorward flow magnitude before CCCs setup is significantly correlated with the following (poleward) acceleration of incipient CCCs near the bed. Such relation is consistent with the development of CCCs due to the unbalance of an APG (produced during active upwelling) when wind relaxes. This process is further supported by an analysis of the depth-averaged momentum equation which suggests that the coastal circulation is mainly driven by linear dynamics in the region.

Historical and contemporary factors affect the genetic diversity and structure of Laguncularia racemosa (L.) Gaertn, along the western Atlantic coast

Laguncularia racemosa (L.) Gaertn (Combretaceae) is one of the eight tree mangrove species of the Atlantic-Caribbean-Eastern Pacific region (AEP). This species is widely distributed throughout the Americas, Caribbean and the West Coast of Africa. Understanding genetic structure allows us to elucidate the main historical and contemporary mechanisms that influenced the distribution of natural populations. The aim of the present study was to evaluate the genetic diversity and genetic structure in populations of L. racemosa in the western Atlantic coast. Seven loci of nuclear microsatellites were used to genotype 102 individuals in eight populations. Laguncularia racemosa populations presented a hierarchical island model of genetic structure most likely resulting from propagule dispersal mediated by coastal superficial climatological circulation (Near-Surface Ocean Currents). Moreover, equatorial populations presented greater genetic variability than the tropical and subtropical populations, possibly related to the existence of refuges in the equatorial region during the last glacial period, followed by stepping-stone recolonization of tropical and subtropical mangroves in the Holocene.

Multiresolution Satellite-Derived Bathymetry in Shallow Coral Reefs: Improving Linear Algorithms with Geographical Analysis

da Silveira, C.B.L.; Strenzel, G.M.R.; Maida, M.; Araujo, T.C.M., and Ferreira, B.P., 2020. Multiresolution satellite-derived bathymetry in shallow coral reefs: Improving linear algorithms with geographical analysis. Journal of Coastal Research, 36(6), 1247–1265. Coconut Creek (Florida), ISSN 0749-0208.Bathymetric maps are one of the first steps for most hydrological and ecological studies of the seascape, as depth is a determinant factor in the distribution of organisms, patterns of wave exposure, and coastal circulation. The coral reefs of Tamandare-Brazil, located at Costa dos Corais marine protected area (MPA), encompass a mosaic of interconnected habitats of complex geomorphology, including coral reefs, algal, and seagrass beds. These coastal habitats are subjected to chronic impacts such as sedimentation, reef erosion, and increasing human use, leading to habitat loss. Despite their social and ecological importance and the conservation measures in place, bathymetric and habitat maps of this coast are lacking. Indeed, in situ surveys are not always feasible in shallow coral reef areas. The present study offers a detailed bathymetric mapping of the area using multiresolution satellite imagery. One Landsat-8 (December 2016) and one WorldView-03 (February 2017) imagery were used to derive medium (30-m) and high-resolution (2-m) bathymetry of the study area. Single-beam echo sounder surveys were performed to obtain field data to calibrate the depth-retrieving algorithms: linear model, ratio band model, principal component analyses of the transformed bands, and geographically weighted regressions (GWR). For both resolution datasets, results showed that the algorithms' RMS were significantly improved by the GWR technique (RMS > 0.9) because of adaptability to the bottom heterogeneity found in complex areas such as coral reefs. Specific geomorphological reef zones were recognizable in the resulting bathymetric maps, such as intrareef lagoon, reef crest, fore-reef, and reef flat. This research concluded that affordable methods such as single-beam data coupled with satellite imagery through GWR can provide the required inputs for mapping shallow areas with complex relief. Such results may be further used to habitat mapping, necessary to inform the multiple-use zonation foreseen in MPA management plans.

Three-Dimensional Unstructured Grid Finite-Volume Model for Coastal and Estuarine Circulation and Its Application

We developed a three-dimensional unstructured grid coastal and estuarine circulation model, named the General Ocean Model (GOM). Combining the finite volume and finite difference methods, GOM achieved both the exact conservation and computational efficiency. The propagation term was implemented by a semi-implicit numerical scheme, the so-called θ scheme, and the time-explicit Eulerian–Lagrangian method was used to discretize the nonlinear advection term to remove the major limitation of the time step, which appears when solving shallow water equations, by the Courant–Friedrichs–Lewy stability condition. Because the GOM uses orthogonal unstructured computational grids, allowing both triangular and quadrilateral grids, considerable flexibility to resolve complex coastal boundaries is allowed without any transformation of governing equations. The GOM was successfully verified with five analytical solutions, and it was also validated when applied to the Texas coast, showing an overall skill value of 0.951. The verification results showed that the algorithm used in GOM was correctly coded, and it is efficient and robust.

Mapping coastal circulations using moving vehicle acoustic tomography.

With the increased availability of highly maneuverable unmanned surface/underwater vehicles, abundant ocean data can now be collected. This study uses tomographic techniques to extend the survey area covered by moving vehicles. An acoustic reciprocal transmission experiment was conducted using three tomographic sensors installed on an autonomous underwater vehicle, a fishing ship, and a buoy. The distributed sensing method is applied for currents estimation. The estimated currents near the ship show consistent results with the direct measurements. In particular, an anticyclonic circulation was revealed. Further, a general least-squares method is employed to improve the estimate of this vortical structure.

Izmenchivost' kharakteristik pribrezhnogo techeniya u Yuzhnogo berega Kryma v 2017–2019 godakh

This paper aims at specifying peculiarities of the coastal water circulation according to the current monitoring data obtained at the stationary oceanographic platform of the Black Sea hydrophysical sub-satellite testing area of the MHI RAS near the Southern coast of Crimea in 2017–2019. The monitoring was performed using the technology of continuous measurements with a vertically distributed (from the surface to bottom layer) complex of Euler vector-averaging meters of the horizontal current component. The new results are matched to those of the coastal current monitoring obtained for the previous period from 2008 to 2016. For the entire observation period (2008–2019), the annual focused current of coastal waters kept the west-south-western direction, with its vertical structure remaining stable. In 2017–2019, a verifiable decrease was registered in the annual longshore current velocity, on average by 10 % throughout the depth up to 7.8 cm/s at a depth of 5 m and up to 6.4 cm/s at a depth of 20 m. Also, the relevant characteristics of bimodal distribution of direction repeatability within original (nonaveraged) data remained unchanged. Further, for 12 years, the oscillation intensity of the coastal current with periods of 10–13 days or less remained reliably unchanged. While analyzing the vertical structure of averaged for 2017–2019 kinetic energy spectra in the inertial-gravitational fluctuation range, two separate regions are reliably detected of regular oscillations with daily (24 h) and local inertia (about 17 h) periods. The obtained new research results can be used while performing validation calculations of local models of the coastal water circulation in the water area of the Southern coast of Crimea. This includes also planning measures to provide crisis monitoring of the coastal ecotone of the sea due to pollution of the coastal and shelf waters

Investigations into Synoptic Spatiotemporal Characteristics of Coastal Upper Ocean Circulation Using High Frequency Radar Data and Model Output

Numerical models and remote sensing observation systems such as radars are useful for providing information on surface flows for coastal areas. Evaluation of their performance and extracting synoptic characteristics are challenging and important tasks. This research aims to investigate synoptic characteristics of surface flow fields through undertaking a detailed analysis of model results and high frequency radar (HFR) data using self-organizing map (SOM) and empirical orthogonal function (EOF) analysis. A dataset of surface flow fields over thirteen days from these two sources was used. A SOM topology map of size 4 × 3 was developed to explore spatial patterns of surface flows. Additionally, comparisons of surface flow patterns between SOM and EOF analysis were carried out. Results illustrate that both SOM and EOF analysis methods are valuable tools for extracting characteristic surface current patterns. Comparisons indicated that the SOM technique displays synoptic characteristics of surface flow fields in a more detailed way than EOF analysis. Extracted synoptic surface current patterns are useful in a variety of applications, such as oil spill treatment and search and rescue. This research provides an approach to using powerful tools to diagnose ocean processes from different aspects. Moreover, it is of great significance to assess SOM as a potential forecasting tool for coastal surface currents.

Ensemble 4DVAR (En4DVar) data assimilation in a coastal ocean circulation model. Part II: Implementation offshore Oregon–Washington, USA

The ensemble four-dimensional variational (En4DVar) data assimilation (DA) system introduced in Part I (Pasmans and Kurapov, 2019) is tested in the coastal waters offshore Oregon and Washington, U.S. West coast, during the spring and summer of 2011. The background error covariance B is derived from the forecast ensemble. Satellite sea-surface temperature (SST), sea-surface height (SSH), and daily-averaged radial surface currents from high-frequency radars (HFRs) are assimilated. The performance of the En4DVar system is compared with a “traditional” 4DVAR system using a static B. It is found that the presence of the Columbia River plume has a profound impact on the ensemble B. Near the plume front the SST–SSS covariance can be up to a factor 20 larger in magnitude than in the static B. This introduces large spatial and temporal variability in the ensemble B. The En4DVar system is more successful than the 4DVAR with the static B preserving the temperature–salinity properties when compared to glider data. The En4DVar system also produces more accurate forecasts and analyses for temperature in the subsurface below 30 m at a buoy location on the continental shelf. In comparisons with other surface and subsurface observations En4DVar shows consistent, albeit not significant, improvement over traditional 4DVAR. Large surface temperature–salinity covariances in combination with the episodic occurrence of large-scale errors in the SST observations lead to erroneous freshening in the centre of the model domain. Adding constraints on the surface salinity corrections based on the prior model reduces this effect.

Typhoon-induced megarips as triggers of turbidity currents offshore tropical river deltas

Tropical cyclones impose stresses on narrow and shallow continental shelves. The interaction of strong wind- and wave-induced currents with the local topography near the shore gives rise to complex flow and sediment transport patterns. Considerable uncertainty remains on the initiation mechanisms of turbidity currents, particularly in coastal oceanic settings subject to extreme weather events. Here we use state-of-the-art numerical models to investigate the implications of tropical cyclone-induced coastal circulation patterns for the generation of turbidity currents. In our simulations tropical cyclones induce megarip currents associated with shoreline curvature and rotation of incoming wave directions. These currents flush water and sediment towards submarine canyons, ultimately triggering turbidity currents into deep waters. Evidence of sediment-laden underflows, which resulted in subsea pipeline displacements, supports our hypothesis that tropical cyclone-induced megarip currents can trigger turbidity currents offshore from tropical river deltas.

Impacts of surface gravity waves on a tidal front: A coupled model perspective

A set of realistic coastal coupled ocean-wave numerical simulations is used to study the impact of surface gravity waves on a tidal temperature front and surface currents. The processes at play are elucidated through analyses of the budgets of the horizontal momentum, the temperature, and the turbulence closure equations. The numerical system consists of a 3D coastal hydrodynamic circulation model (Model for Applications at Regional Scale, MARS3D) and the third generation wave model WAVEWATCH III (WW3) coupled with OASIS-MCT at horizontal resolutions of 500 and 1500 m, respectively. The models were run for a period of low to moderate southwesterly winds as observed during the Front de Marée Variable (FroMVar) field campaign in the Iroise Sea where a seasonal small-scale tidal sea surface temperature front is present. Over the 2 day period considered, long fetch waves grow gradually propagating north east and east.Contrasting a stand-alone ocean run with a coupled ocean-wave run shows that waves move the Ushant front offshore by up to 4 kilometres and cool the offshore stratified side of the front by up to 1.5°C. The analysis of the temperature budget shows that the change in advection is the dominant factor contributing to the frontal shift while the contribution of wave enhanced vertical temperature diffusion is secondary. Temperature, considered to be a tracer, is advected in the coupled run by the Lagrangian current resulting from the quasi-Eulerian and Stokes drift. Although the Stokes drift is directed shorewards, changes in the quasi-Eulerian current lead to a more offshore advection in the coupled than the stand-alone run. The quasi-Eulerian current is reduced (enhanced) during the ebb (flood) flow which correspond to periods of wave-following (-opposing) currents. This is due to wave breaking enhanced vertical mixing acting on the positive vertical gradient present in the quasi-Eulerian current during both ebb and flood tides. Partially coupled runs reveal that it is the surface flux of TKE associated to wave breaking that is key rather than the changes in the surface stress. They further elucidate the role of other modelled wave related processes. Although the contribution of the Stokes–Coriolis force and the wave breaking induced enhancement in vertical mixing to the quasi-Eulerian current are of similar magnitude and sign, it does not contribute significantly to the frontal shift. This is because it partially counters the Stokes drift advection which pushes the front shorewards. All Stokes drift related processes combined thus only lead to a very slight displacement of the front.

Estimating the Annual Exceedance Probability of Water Levels and Wave Heights from High Resolution Coupled Wave-Circulation Models in Long Island Sound

Accurately estimating the probability of storm surge occurrences is necessary for flood risk assessments. This research models Long Island Sound using a coupled coastal circulation and wave model (FVCOM-SWAVE) to hindcast the 44 highest storms between 1950–2018 and fitted Poisson-GPD distributions to modelled water levels and wave heights. Floodwater elevations and significant wave heights for 10% (1/10), 3% (1/30), 2% (1/50), and 1% (1/100) annual exceedance probabilities are provided for all Connecticut coastal towns. The results show that both water levels and their corresponding return intervals are higher along the western coast of Connecticut than the eastern coast, whereas significant wave heights increase eastward. Comparing our model results with those from the North Atlantic Coast Comprehensive Study (NACCS) shows that the mean NACCS results are higher for water levels and lower for significant wave heights for longer return periods. Likewise, the Federal Emergency Management Agency (FEMA) results in large errors compared to our results in both eastern and western coastal Connecticut regions. In addition to evaluating historical risks, we also added a sea-level height offset of 0.5 m for 2050 estimates in order to examine the effect of rising sea-levels on the analysis. We find that sea-level rise reduces the return period of a 10-year storm to two years. We advise periodically updating this work as improved sea-level rise projections become available.

Spatio-temporal variability and ENSO modulation of turbid freshwater plumes along the Oregon coast

The influence of climate variability on the Northeast Pacific (NEP) ocean is mainly linked to the impact of El Niño Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and North Pacific Gyre Oscillation (NPGO) on the modulation of coastal circulation, coastal upwelling, and ecosystem response. The impact of climate variability on the interannual variability of freshwater river plumes has been largely unstudied. Here, 14.5 years of ocean color satellite imagery were used to study the interannual variability of turbid freshwater plumes off the Oregon coast. EOF analysis reveals two dominant modes associated with (i) the winter plumes of coastal rivers merged along the entire Oregon shelf (EOF1) in the downstream (northward) direction as coastal-attached buoyancy-driven plumes, and (ii) the offshore Columbia River plume occupying most of the coastal ocean off Oregon (EOF2) as result of its southward and offshore transport during spring-summer upwelling. Large plumes corresponded mainly with certain aspects of ENSO cycles. However, extended ocean time series are needed to better evaluate the influence of PDO and NPGO because of their dominant decadal variability. Anomalously large coastal plumes (EOF1) lagged the canonical El Niño primarily during fall and La Niña during winter. The largest offshore Columbia River plume events occurred after persistent La Niña conditions (e.g. 2008, 2011, 2014).

Impact of Freshwater Inflow From the Volturno River on Coastal Circulation

The coastal area located in front of the Volturno river’s estuary (Gulf of Gaeta, central-eastern Tyrrhenian Sea) has been synoptically sampled during seven surveys, between June 2012 and October 2014. The vertical profiles of temperature and salinity have been acquired on a high resolution nearly-regular grid, in order to describe the spatial and temporal variability of the water masses characteristics. Moreover, to provide a first assessment of the steady circulation at small scale, the three-dimensional velocity field associated to each survey has been obtained through the full momentum equations of the Princeton Ocean Model. The data analysis has shown the entire water column characterized by an evident thermal cycle and a vertical thermohaline structure dominated by three types of water masses: the freshwater of river’s origin, the saltier coastal Tyrrhenian waters and transitional waters, resulting from the mixing of them. The freshwater inflow strongly affected the density distribution, causing a strong temporal variability in the upper layer. Its impact was more evident in winter inducing also a temperature inversion along the water column. In case of rainy events, its effect was clear also in condition of high vertical temperature stratification, forming a surface-trapper layer characterized by high density gradients. Together with wind forcing, they contributed to the formation of small scale shallow features, such as longshore currents, cyclonic and anticyclonic eddies. The latter influenced the vertical stratification and modify the coastal circulation preserving the transitional waters from the surrounding saltier ones.

Sub-Mesoscale Circulation Features along the Andhra Pradesh Coast, Bay of Bengal: Observations from HF Radars

Mandal, S.; Pramanik, S.; Sil, S.; Arunraj, K.S., and Jena, B.K., 2020. Sub-mesoscale circulation features along the Andhra Pradesh coast, Bay of Bengal: Observations from HF radars. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 132-138. Coconut Creek (Florida), ISSN 0749-0208.This work presents the coastal circulation variability along the Andhra coast in western Bay of Bengal (BoB) using hourly high frequency radar (HFR, 4.4 MHz) derived ocean surface currents during 2015. The comparison of daily averaged HFR surface currents with satellite derived currents shows high correlations (0.86, 0.70) and low errors (0.28 m s-1, 0.25 m s-1) for the zonal and meridional components. The HFRs captured and quantified the dominant circulation features associated with the reversal of boundary currents. The northward (southward) currents are observed from mid-March till May end (October to November with highest current speed of ∼1.58 (1.24) m s-1 along the Andhra coast (between 15-17°N). In addition, the sub-mesoscale features associated with dominant circulation features are observed, like, the propagation of an anticyclonic eddy (81.80°E, 15.25°N) during March and cyclonic eddy (82.25°E, 15.40°N) during November. The mean vorticity pattern from the HFR currents shows the dipole nature during April - May to indicate the near coast upwelling signatures which are also supported by chlorophyll observations from satellite. The tidal driven currents, extracted from the HFRs, show that the semi-diurnal tidal constituent, M2 dominates during both the seasons with less intensity in March than in November. The distributions of the M2 tidal ellipses match well with the broad-scale current pattern in the offshore region. They follow the bathymetry in the nearshore region to indicate that it is driven by tides resulting in the generation of the shallow water tidal constituents (M3, M4, MS4 and M8) with significant amount of amplitudes.

Coupling and Decoupling of High Biomass Phytoplankton Production and Hypoxia in a Highly Dynamic Coastal System: The Changjiang (Yangtze River) Estuary

The global increase in coastal hypoxia over the past decades has resulted from a considerable rise in anthropogenically-derived nutrient loading. The spatial relationship between surface phytoplankton production and subsurface hypoxic zones is often explained by considering the oceanographic conditions associated with basin size, shape, or bathymetry, but this is not the case where nutrient-enriched estuarine waters merge into complex coastal circulation systems. We investigated the physical and biogeochemical processes that create high-biomass phytoplankton blooms and hypoxia off the Changjiang (Yangtze) Estuary in the East China Sea (ECS). Extensive in situ datasets were linked with a coupled Regional Ocean Modelling Systems (ROMS) and Carbon, Silicate and Nitrogen Ecosystem (CoSiNE) model to explain the connect and disconnect of phytoplankton production and hypoxia. Diatoms were the major contributor of carbon export, and even though phytoplankton concentrations generally were three times greater above the hypoxic zones, high-biomass distributions during the summer-fall period did not closely align with that of the hypoxic zones. A major cause for this decoupling was the non-uniform offshore advection and detachment of segments of water underlying the Changjiang river plume, which carried organic-rich subsurface water northeast of the river mouth. The remineralization of this dissolved organic matter during transit created offshore patches of hypoxia spatially and temporally independent of the nearshore high biomass phytoplankton blooms. The absence of high phytoplankton biomass offshore, and the 1–8 weeks time-lag between the inshore diatom production and offshore hypoxia, made it otherwise difficult to mechanistically explain the in situ observations. The findings here highlight the value of developing integrated physical and biogeochemical models to aid in forecasting the dynamics of coastal hypoxia, under both contemporary and future coastal ocean conditions.

Integration of biophysical connectivity in the spatial optimization of coastal ecosystem services

Ecological connectivity in coastal oceanic waters is mediated by dispersion of the early life stages of marine organisms and conditions the structure of biological communities and the provision of ecosystem services. Integrated management strategies aimed at ensuring long-term service provision to society do not currently consider the importance of dispersal and larval connectivity. A spatial optimization model is introduced to maximise the potential provision of ecosystem services in coastal areas by accounting for the role of dispersal and larval connectivity. The approach combines a validated coastal circulation model that reproduces realistic patterns of larval transport along the coast, which ultimately conditions the biological connectivity and productivity of an area, with additional spatial layers describing potential ecosystem services. The spatial optimization exercise was tested along the coast of Central Chile, a highly productive area dominated by the Humboldt Current. Results show it is unnecessary to relocate existing management areas, as increasing no-take areas by 10% could maximise ecosystem service provision, while improving the spatial representativeness of protected areas and minimizing social conflicts. The location of protected areas was underrepresented in some sections of the study domain, principally due to the restriction of the model to rocky subtidal habitats. Future model developments should encompass the diversity of coastal ecosystems and human activities to inform integrative spatial management. Nevertheless, the spatial optimization model is innovative not only for its integrated ecosystem perspective, but also because it demonstrates that it is possible to incorporate time-varying biophysical connectivity within the optimization problem, thereby linking the dynamics of exploited populations produced by the spatial management regime.