Long-term trends (1986-2019) in the physicochemical properties of the Sado Estuary (Portugal) driven primarily by changes in river flow and influenced by marine upwelling.

Estuaries are among the most sensitive systems to climate change. Previous studies have suggested that the Sado Estuary (Portugal) has decreasing trends for water temperature, which is uncommon in a global warming scenario. However, no long-term analysis regarding water column conditions has been conducted on this estuary to date. Therefore, the main aim of this study is to understand if and how the properties of this estuary have changed, considering a 34-year trend analysis and an assessment of their main drivers. Water temperature significantly decreased (up to 0.04 °C/year) along with chlorophyll a. Salinity increased and nutrients displayed an overall decrease. These trends were potentially driven by higher influence of upwelled water combined with a reduction of the river flow. Sado appears to have a lower vulnerability to climate change than other estuaries as it does not show a high sensitivity to changes in its water column physicochemical properties.

Effects of climate change on aquaculture site selection at a temperate estuarine system.

Aquaculture is one of the food industries that most evolved in recent years in response to increased human demand for seafood products, which has led to a progressive stock threat in nature. With a high seafood consumption per capita, Portugal has been exploring its coastal systems to improve the cultivation of fish and bivalve species with high commercial value. In this context, this study aims to propose the use of a numerical model as a tool to assess the impact of climate change on aquaculture site selection in a temperate estuarine system (Sado estuary). Therefore, the Delft3D model was calibrated and validated, showing good accuracy in predicting the local hydrodynamics, transport, and water quality. Furthermore, two simulations for the historical and future conditions were performed to establish a Suitability Index capable of identifying the most appropriate sites to exploit two bivalve species (one clam and one oyster), considering both winter and summer seasons. Results suggest that the estuary's northernmost region presents the best conditions for bivalves' exploitation, with more suitable conditions during summer than winter due to the higher water temperature and chlorophyll-a concentrations. Regarding future projections, the model results suggest that environmental conditions will likely benefit the production of both species due to the increase in chlorophyll-a concentration along the estuary.

40 years of changes in sea surface temperature along the Western Iberian Coast.

Climate change is causing mean sea surface temperatures (SST) to increase worldwide. However, this increase has not been temporally or spatially uniform, with variations observed depending both on the period considered and the geographic region. In this context, this paper aims to quantify relevant changes in SST along the Western Iberian Coast over the last four decades, through the calculation of trends and anomalies of long-term time series of in situ observations and satellite-derived data. Potential drivers of SST changes were considered using atmospheric and teleconnections time series. Changes in the seasonal cycle of SST were also evaluated. We show that SST has increased since 1982, with regional variations between 0.10 and 0.25 °C per decade, with an increase in air temperature appearing to drive the SST trends along the Iberian coast. In the near-shore area, no significant trends or changes in the seasonal cycle of SST were observed, which is likely due to a buffer effect caused by the seasonal upwelling that characterizes the region. Recent decades show a slowdown in the increase rate of SST along the Western Iberian Coast. An upwelling intensification could justify this observation, along with the effect of teleconnections on the regional climate, such as the North Atlantic Oscillation (NAO) and the Western Mediterranean Oscillation Index (WeMOI). Our results suggest that the WeMOI plays a more important role in coastal SST variability than the other teleconnections. The present study quantifies regional changes in SST and enhances knowledge of the role of ocean-atmosphere interactions in regulating climate and weather conditions. Moreover, it provides a relevant scientific context to the development of regional adaptative and mitigation actions in response to climate change.

Climate change in estuarine systems: Patterns and gaps using a meta-analysis approach.

Although regional studies and projections suggest the deterioration of estuaries as a consequence of climate change, it is still difficult to fully understand the importance of such changes in estuarine systems. This limitation is particularly important considering their high dynamism and the lack of temporally extended in situ databases with a good spatial coverage for these systems worldwide. Furthermore, contradictory patterns have been observed across the globe. Motivated by these issues, in this study we question the availability of in situ observational evidence of climate change in estuarine systems through a detailed meta-analysis of existing publications. A topic-related search considering the outputs of the Web of Science library was conducted in order to obtain a characterization of the existing studies on climate change in estuarine systems. Results confirmed that climate change has increasingly been studied since 2000 and that marine climate change constituted the focus of 9.69 % of those studies. From these, only 9.30 % encompassed estuarine studies and just 1.13 % used in situ observations from estuarine systems (i.e., 0.11 % of the total climate change publications). Reanalysis products were the most used tools to assess changes in estuarine systems and sea temperature was the most analyzed variable. These results highlight the need to further address such questions using in situ observational data and to implement long-term observatories to fully identify evidence of climate change in estuarine systems, supporting modelling approaches and promoting the development of effective mitigation plans.

Evaluation of future estuarine floods in a sea level rise context.

Reliable predictions of future inundation extent within estuaries require a precise evaluation of future extreme sea levels and the application of accurate numerical models that account for the physical processes driving estuarine hydrodynamics. In this study, a methodology that integrates the estimation of local extreme sea levels with high-resolution numerical modeling was applied to assess the future inundation extent in five estuarine systems located on the Portuguese Coast. The main findings obtained were compared with available results from the popular bathtub approach, that disregards the physical processes driving estuarine hydrodynamics and therefore provide imprecise predictions of inundation extent and associated socio-economic impacts. The inundation extent is revealed to be highly dependent on the extreme sea levels and on the estuarine geomorphology, which controls the propagating long-wave. As the long-wave height is highly attenuated within estuaries that have adjacent low-lying areas, restricted inlets, or extensive tidal flats, the results of this study revealed that the extent of inundation is considerably smaller than that obtained by the bathtub approach. The uncertainties associated with mean sea level rise and the estuarine geomorphological evolution constitute the greatest difficulty in assessing the extent of flooding, posing major challenges to the efficient and sustainable management of estuaries.

NW Iberian Peninsula coastal upwelling future weakening: Competition between wind intensification and surface heating.

Climate change will modify the oceanographic future properties of the NW Iberian Peninsula due to the projected variations in the meteorological forcing, that will intensify local winds and promote surface heating. The Delft3D-Flow model forced with atmospheric conditions provided within the framework of the CORDEX project under the RCP 8.5 greenhouse emission scenario was used to analyse changes in upwelling. Numerical experiments were conducted under high-extreme upwelling conditions for the historical (1976-2005) and future (2070-2099) period. This study also innovates through the exploitation of a numerical modelling approach that includes both shelf and estuarine processes along the coastal zone. Coastal upwelling will be less effective in the future despite the enhancement of upwelling favorable wind patterns previously predicted for this region. Upwelling weakening is due to the future sea surface warming that will increase the stratification of the upper layers hindering the upward displacement of the underlying water, reducing the surface input of nutrients.

Subtidal variability of the Tagus river plume in winter 2013.

The subtidal variability of surface plume response to local wind and river runoff is studied off central Portugal using a hindcast model for the Tagus Estuary (TE) plume. In addition to the model, the main patterns of variability of the TE plume were extracted in an automated mode using Self Organizing Maps (SOM). Three characteristic spatial patterns are extracted in a 3 × 2 SOM array: in synoptic time scales, northward and southward propagation of the plume and transitional periods. Results suggest that river runoff and wind induce strong variability in the coastal ocean, and that the TE plume has a rapid response to their changes. Under high river runoff there is a southern propagation of the plume, even during downwelling favorable winds, revealing bi-directional patterns due to the combined effect of estuarine outflow, wind direction and coastline in establishment the plume direction. The same is found during transitional periods, revealing that the TE plume has significant influence both north and south of the estuary mouth. SOM was applied for the first time to extract spatial and temporal features of the TE plume and prove to be a reliable tool to study large quantities of data in addition to the model results analysis.

Why coastal upwelling is expected to increase along the western Iberian Peninsula over the next century?

Former studies about coastal upwelling along the Western Iberian Peninsula (WIP) using historical data indicated contradictory results, showing either its strengthening or reduction, while previous studies using Global Climate Models (GCMs) indicated that global warming is likely to intensify this phenomenon although predicting different rates and not justifying the patterns found. Taking advantage of the recent high spatial resolution Regional Climate Models (RCMs) projections from EURO-CORDEX project (Representative Concentration Pathway, RCP 8.5), detailed higher accuracy estimations of the spatio-temporal trends of Upwelling Index (UI) along the WIP coast were performed in this study, integrating the coastal mesoscale effects within the framework of climate change. Additionally, this research brings new insights about the origin of the WIP coastal upwelling intensification over the next century. These new projections clarified the upwelling strengthening rates predicted along the coast of the WIP from 2006 to 2099 revealing more prominent changes in the northern limit of the region (25-30m3s-1km-1 per decade between 41.5 and 42.5°N). Trends observed at high latitudes of the region were found to be induced by the displacement of the Azores High, which will intensify (0.03hPa per decade) and drift northeastward (10km per decade) during the 21st century.

Unusual circulation patterns of the Rias Baixas induced by Minho freshwater intrusion (NW of the Iberian Peninsula).

The Minho River, situated 30 km south of the Rias Baixas, is the most important freshwater source flowing into the Western Galician coast (NW of the Iberian Peninsula). The buoyancy generated by the Minho estuarine plume can reverse the normal circulation pattern inside the Rias Baixas affecting the exchange between the Rias and the ocean, changing the input of nutrients. Nevertheless, this inversion of the circulation patterns is not a well-monitored phenomenon. The only published results based on in situ data related to the presence of the Minho River plume inside the Rias de Vigo and Pontevedra correspond to an event measured on spring 1998. In this case unexpectedly higher inflow surface current velocities were found at the Ria de Pontevedra, located further away from Minho River. Thus, the main aim of this study is to research the main factors inducing this unusual pattern on the circulation of the Rias de Vigo and Pontevedra. A numerical model implementation of MOHID previously developed, calibrated, and validated for this coastal area was used. Several scenarios were performed in order to explain the individual effect of the Minho River, rivers discharging into each Rias, and estuarine morphology changes. According to the model results, the Minho River discharge is a key factor in the establishment of the negative circulation, while small rivers inside the Rias slightly attenuate this circulation. The negative circulation was stronger in Ria de Pontevedra independently of the distance of this coastal system from the Minho River mouth, showing that morphologic estuarine features are the main factor justifying the different local circulation patterns.

A numerical study of local variations in tidal regime of Tagus estuary, Portugal.

Tidal dynamics of shallow estuaries and lagoons is a complex matter that has attracted the attention of a large number of researchers over the last few decades. The main purpose of the present work is to study the intricate tidal dynamics of the Tagus estuary, which states as the largest estuary of the Iberian Peninsula and one of the most important wetlands in Portugal and Europe. Tagus has large areas of low depth and a remarkable geomorphology, both determining the complex propagation of tidal waves along the estuary of unknown manner. A non-linear two-dimensional vertically integrated hydrodynamic model was considered to be adequate to simulate its hydrodynamics and an application developed from the SIMSYS2D model was applied to study the tidal propagation along the estuary. The implementation and calibration of this model revealed its accuracy to predict tidal properties along the entire system. Several model runs enabled the analysis of the local variations in tidal dynamics, through the interpretation of amplitude and phase patterns of the main tidal constituents, tidal asymmetry, tidal ellipses, form factor and tidal dissipation. Results show that Tagus estuary tidal dynamics is extremely dependent on an estuarine resonance mode for the semi-diurnal constituents that induce important tidal characteristics. Besides, the estuarine coastline features and topography determines the changes in tidal propagation along the estuary, which therefore result essentially from a balance between convergence/divergence and friction and advection effects, besides the resonance effects.

Salt fluxes in a complex river mouth system of Portugal.

Measurements of velocity and salinity near the mouth and head of the Espinheiro channel (Ria de Aveiro lagoon, Portugal) are used to study the local variation of physical water properties and to assess the balance, under steady conditions, between the seaward salt transport induced by river discharge and the landward dispersion induced by various mixing mechanisms. This assessment is made using data sampled during complete tidal cycles. Under the assumption that the estuarine tidal channel is laterally homogeneous and during moderate tidal periods (except for one survey), currents and salinity data were decomposed into various spatial and temporal means and their deviations. Near the channel's mouth, the main contributions to the salt transport are the terms due to freshwater discharge and the tidal correlation. Near the channel's head, this last term is less important than the density driven circulation, which is enhanced by the increase in freshwater discharge. The remaining terms, which are dependent on the deviations from the mean depth have a smaller role in the results of salt transport. The computed salt transport per unit width of a section perpendicular to the mean flow is in close agreement to the sum of the advective and dispersive terms (within or very close to 12%). An imbalance of the salt budget across the sections is observed for all the surveys. Considerations are made on how this approach can inform the management of hazardous contamination and how to use these results to best time the release of environmental flows during dry months.

Relation between bacterial activity in the surface microlayer and estuarine hydrodynamics.

Bacterial communities of the surface microlayer (SML) of the estuary Ria de Aveiro (Portugal) were characterized in terms of abundance and activity during a 2-year survey at two sites with distinct hydrodynamic properties (marine and brackish water zones). The hydrodynamic conditions were simulated using a bidimensional numerical model and related to the microbiological observations. The pattern of variation of bacterial biomass productivity (BBP) was distinct between the two sampling sites. At the outer site, BBP was significantly lower at the SML, whereas at the inner site, it was significantly enhanced at the SML. Although the total bacterial abundance was similar in the SML and underlying water (UW), the fraction of cells attached to particles was significantly higher at the SML (two to three times). The integration of microbiological results with environmental and hydrological variables shows that strong currents in the marine zone promote the vertical mixing, inhibiting the establishment of an SML bacterial community distinct from that of UW. In contrast, in the brackish water zone, lower current velocities provide conditions for enhancing the bacterial activity in the enriched SML. Estuarine dynamics influence the distribution and activity of microorganisms at the SML and in the water column, with anticipated impacts for the carbon cycle in the estuarine environment.

Crystal structure of the 16 heme cytochrome from Desulfovibrio gigas: a glycosylated protein in a sulphate-reducing bacterium.

Sulphate-reducing bacteria have a wide variety of periplasmic cytochromes involved in electron transfer from the periplasm to the cytoplasm. HmcA is a high molecular mass cytochrome of 550 amino acid residues that harbours 16 c-type heme groups. We report the crystal structure of HmcA isolated from the periplasm of Desulfovibrio gigas. Crystals were grown using polyethylene glycol 8K and zinc acetate, and diffracted beyond 2.1 A resolution. A multiple-wavelength anomalous dispersion experiment at the iron absorption edge enabled us to obtain good-quality phases for structure solution and model building. DgHmcA has a V-shape architecture, already observed in HmcA isolated from Desulfovibrio vulgaris Hildenborough. The presence of an oligosaccharide molecule covalently bound to an Asn residue was observed in the electron density maps of DgHmcA and confirmed by mass spectrometry. Three modified monosaccharides appear at the highly hydrophobic vertex, possibly acting as an anchor of the protein to the cytoplasmic membrane.