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  • Recent evolution of the tidal prisms at the inlets of the western sector of the Ria Formosa, south coast of Portugal
    Publication . Jacob, J.; Cravo, Alexandra
    The Ria Formosa is a coastal lagoon comprising six tidal inlets. The western sector encompassing three inlets, Ancao, Faro-Olhao and Armona is the most important one since it represents approximately 90% of the total tidal prism. The Ancao inlet is characterized by a cyclic eastward migration pattern, but until now the knowledge of the real impact of its dynamics in the last 20 years in terms of tidal prisms and interconnectivity between these three inlets is scarce. This work presents the most exhaustive data set acquired so far, comprising 25 field surveys led in the period 2004-2013 on the western sector of Ria Formosa. This provides a long-term analysis of the recent temporal evolution of the tidal prisms from the three inlets during a single migration cycle (18 years) of the Ancao inlet. It determines the relative contribution of the three inlets to the total prism of the western sector of Ria Formosa before the last artificial opening of the Ancao inlet. Results show that at Ancao inlet the tidal prism decreased in spring tide mainly due to its eastward migration. The other natural inlet, Armona inlet, depict a progressive narrowing responsible for the decrease of the tidal prism in neap tide while during the same tidal phase, at the Faro-Olhao inlet, the most important one in terms of exchanges, the tidal prism increased. Those alterations at the three inlets could be related to morphodynamic changes and morphological evolution and confirmed the strong interconnectivity between them to compensate the variations of their relative contributions to the total prism. The tidal prisms errors were low for all cases (< 10%) and confirm that, globally, there was an increase of the prisms with time.
  • Impact of outflow from the Guadiana River on the distribution of suspended particulate matter and nutrients in the adjacent coastal zone
    Publication . Cravo, Alexandra; Madureira, Pedro Miguel; Felícia, Helena; Rita, F.; Bebianno, Maria
    In this study we collected surface water samples from the coastal area adjacent to the Guadiana estuary during winter for 3 consecutive years to assess the impact of the Guadiana outflow upon the concentration and spatial distribution of suspended particulate matter and nutrients (nitrate, phosphate, and silicate). Deeper water samples were also collected near the river mouth in water greater than 10 m in depth. Our results indicate that the maximal surface influence of the Guadiana outflow is close to the mouth of the Guadiana River, at the 10-m isobath, where the highest concentrations of suspended particulate matter (SPM) and nutrients were recorded, as well as the lowest salinity. SPM and nutrient concentrations decrease with increased water depth, while salinity increased. Beyond the 10-m isobath, toward the ocean, nutrient concentrations decreased gradually with increasing salinity. Nutrient concentrations showed a conservative behaviour only during the last of the three sampling periods. The impact of Guadiana outflow was especially low when river discharge was low, however, after periods of peak rainfall the river outflow increased enormously and the impact of SPM and nutrients (more than an order of magnitude higher than normal) was observed, particularly around the mouth of the estuary. This impact involved the development of a fingerprint plume that represents a net export of SPM and nutrients to the coastal area. This plume had a width of about 10e15 km, and despite being centred slightly east of the mouth of the Guadiana River, tended to migrate westward. The increase in N compounds was more significant than increases in P and Si, is reflected in high N:P and N:Si nutrient ratios. In water depths in excess of 10 m, the effect of the Guadiana outflow was most evident until 5 m depth. It is expected that with the completion of the biggest dam in Europe along the Guadiana River, the outflow of the river will be markedly reduced, especially during summer if climatic change continues to produce significant periods of dry weather. Under such conditions, nutrient concentrations will be reduced even further and impart a negative impact on nutrient biogeochemical cycles and productivity of the coastal zone.
  • Temporal assessment of the water quality along an urban estuary (Tagus estuary, Portugal)
    Publication . Rodrigues, Marta; Cravo, Alexandra; Freire, Paula; Rosa, Alexandra; Santos, Daniela
    Understanding the natural dynamics of estuaries and their response to changes in the human-related or climatic drivers is fundamental to guarantee their environmental quality. This study aims to contribute to a better understanding of the water quality dynamics in an urban estuary, the Tagus estuary (Portugal), across different temporal scales. This estuary supports diverse uses and activities and its ecological value is well recognized. Three field campaigns were performed covering distinct seasons (Spring, Summer and Autumn - 2018). Physical, chemical and biological data were measured along the estuary. The acquired data was then integrated into a set of indicators and compared with historical data, to assess the long-term evolution of the water quality in the estuary. Results showed marked spatial gradients, typically with larger concentrations of chlorophyll a, nitrate and silicate upstream. Silicate and nitrate presented a conservative behavior. Ammonium and phosphate were not conservative, which suggests an anthropogenic pressure along the estuary. Seasonally, the highest nutrients and suspended solids concentrations were found in Autumn, after a period of rainfall, pointing out to the relevance of land runoff for material supply into the estuary. These conditions were favorable for phytoplankton development upstream, once chlorophyll a was maximum during this campaign (24 mu g/L in the upper estuary). Regarding the nutrients classification, results suggest that the upper estuary is at risk with a "Medium" status. The middle and downstream areas presented a "High" status, with the exception of the middle-right margin that is also at risk ("Medium" status). The assessment of the historical data suggests that high loads of nutrients have been reaching the Tagus estuary over time, with a decreasing trend in recent years. The trophic index (TRIX) suggests a "Moderate" trophic status in the middle and upstream areas of the estuary. As for nutrients, a "Poor" classification was obtained in the middle-right margin of the estuary, mainly due to ammonium concentrations, confirming an intensification of anthropogenic pressure at this site. Similarly to the nutrients status, TRIX suggests some improvement of the water quality in the Tagus estuary in recent years compared to the 1980s. These results provide further insight regarding the water quality dynamics in the Tagus estuary and are useful to support management.
  • Nutrient concentrations in coastal waters: impact of the Guadiana River
    Publication . Cravo, Alexandra; Madureira, M. A.; Rita, F.; Silva, A. J.; Bebianno, Maria João
    Water samples were collected over an area of 900 km(2) adjacent to the outflow of the Guadiana River in January 1999 to characterize the concentrations and spatial distribution of nutrients (ammonium, nitrite, nitrate, phosphate and silicate) in the water column. The purpose of the study was to characterize a winter situation of low discharge in terms of nutrient concentrations in the coastal area adjacent to the Guadiana outflow, before the reduction of the river flow due to the completion of the Alqueva Dam. The results show that the maximum influence of the Guadiana outflow was close to the mouth of the Guadiana River, where the highest nutrient concentrations and the minimum of salinity were registered. At the surface, the nutrient concentrations decreased gradually as the distance from the coast increased. The influence of the Guadiana outflow at the surface, despite being small, was evident in the area of about 90 km(2) that extends westwards from the mouth of the river. In this area, the increase in N compounds was more significant than in P and Si. The vertical influence of the outflow of the Guadiana River, until over the isobath lower than 30 m, was reflected in nutrient concentrations that decreased with the increase in depth. However, when the depth of the water column was greater than 30 m, the nutrient concentrations increased with the increase in depth. It is expected that with the completion of the Alqueva Dam, the outflow of the Guadiana River will be reduced even further. Nutrient concentrations will also be reduced and the influence of the river in the coastal zone will be even smaller. This could have a negative impact on the nutrient biogeochemical cycles and on the overall productivity of the area.
  • Effects of climate change and anthropogenic pressures in the water quality of a coastal lagoon (Ria Formosa, Portugal)
    Publication . Rodrigues, Marta; Rosa, Alexandra; Cravo, Alexandra; Jacob, J.; Fortunato, André B.
    Understanding how climatic and anthropogenic drivers will influence coastal lagoons is fundamental to guarantee their preservation and sustainability. The Ria Formosa (coastal lagoon, South coast of Portugal) is a very important ecosystem that supports diverse economic activities in the region. The 3D coupled hydrodynamic-biogeochemical model SCHISM was validated and used to assess the influence of climate change and anthropogenic pressures on the water quality of the Ria Formosa. Five scenarios were simulated: reference scenario (S0), mean sea level rise (SLR) of 0.5 m (S1), increase of the air temperature of 1.68 °C (S2), increase of the outflow from the wastewater treatment plants (WWTP) by 50% (S3) and a combined scenario (S4). Results suggest that SLR of 0.5 m promotes an increase of 0.5-3 in the salinity near the area of influence of the WWTP. SLR decreases the inorganic nutrient concentrations in these areas by about 40-60%, due to an increase of the dilution. In contrast, the increase of the outflow from the WWTP by 50% increases the nutrients concentrations by about 20-40%. The increase of the air temperature alone by 1.68 °C increases the water temperature by 0-1 °C. The combined scenario suggests antagonist effects in the nutrient concentrations. Overall, the trophic index (TRIX) of the lagoon calculated for the scenarios exhibits only minor differences relative to the reference scenario, except in some areas near the WWTP discharges. In these areas, TRIX tends to increase with the increase of the outflow from the WWTP in scenario S3. These results provide further insight into the response of coastal lagoons, and the Ria Formosa in particular, to future changes and contribute to support their management.
  • Export of dissolved and suspended matter from the main estuaries in South Portugal during winter conditions
    Publication . Correia, C.; Torres, A. F.; Rosa, A.; Cravo, Alexandra; Jacob, J.; de Oliveira Junior, L.; Garel, Erwan
    Estuaries are essential drivers of primary productivity in coastal waters through the export of dissolved and suspended material. This study aims at quantifying this fertilization process at the two main estuaries in South Portugal, where typical low river discharge conditions occur in winter due to strong flow regulation by upstream dams and seldom-local rain events. Data were collected across the channel of the lower Arade (10/01/2018) and Guadiana (04/02/2019) estuaries during a semi-diurnal tidal cycle with intermediate tidal ranges (1.5 m and 2.2 m, respectively). Current velocity records were completed with hourly water quality measurements (temperature, salinity, pH, dissolved oxygen, suspended solids, chlorophyll a and nutrients) for determination of the along channel fluxes and residual mass transport of dissolved and suspended matter across the channel sections. The results show that the two estuaries were characterized by distinct water circulation patterns and tidal properties (e.g., standing wave at the Arade, mixed wave at the Guadiana). The concentration of dissolved and particulate matter displayed a general similar anti-phase pattern of variations with tidal height at both sites. The corresponding fluxes were generally strongest during the ebb phase. Thus, the residual mass transport was seaward, indicating that both estuaries contribute to fertilize adjacent coastal waters by exporting nutrients (order of kg), chlorophyll a (order of kg) and suspended solids (up to tons). Moreover, transport differences at both estuaries reflect specific discharge flow and environmental conditions as well as contamination sources close to the study areas. In particular, the signatures of a discharge event from a water treatment plant at the Arade and of coastal upwelling at the Guadiana were identified.
  • Integrating physical and biogeochemical processes and oceanic exchanges at a coastal lagoon in Southern West Europe
    Publication . Cravo, Alexandra; Jacob, José; Rosa, Alexandra; Correia, Cátia
    Coastal lagoons are highly productive systems and the quantification of mass fluxes, which is of paramount importance for the sustainable management of these systems, remains poorly studied. In this context, a detailed study was conducted to better understand the exchanges between the productive coastal lagoon Ria Formosa (South-West Europe) and the ocean. The exchanges of water, nutrients, chlorophyll-a and suspended solids between the main inlets (Faro-Olhao inlet - BFO; Armona inlet - BAR; and Ancao inlet - BAN) and adjacent channels (Faro - CF and Olhao - CO) and the adjacent ocean were estimated along complete semidiurnal tidal cycles, under extreme fortnightly tidal ranges and different seasonal and environmental/oceanographic conditions. The net tidal prism was highest during spring tides. Among the three inlets, BFO was the most important in terms of exchange, followed by BAR and BAN. Net transport at BFO was lowest during the Summer campaign, although it exported material that fertilised the adjacent coast. The persistent net export of suspended solids and ammonium suggests the higher biological productivity of Ria Formosa compared to that found in coastal waters. In the Winter campaign, after a period of rainfall and increased land runoff, there was a remarkable export of matter, on which, ammonium and suspended particles exported can exceed 0.3 times and almost 0.9 times, respectively, those imported from coastal water. However, the import of phosphate and nitrate can be attributed to a weak coastal upwelling event, as well to low consumption and nitrification at this period of low temperature. During the Spring and Autumn campaigns, the Ria Formosa was fertilised either by upwelling events or due to rapid consumption of nutrients by phytoplankton in this shallow system. BFO and the other two inlets of the western sector of Ria Formosa are interconnected by CF and CO. The higher nutrient transport was recorded at CF, despite the highest nutrients concentrations was recorded at CO. The data show the strong link between physical and biogeochemical processes with meteorological/oceanographic factors. The study showed that associated biological processes are superimposed on the tidal effect in this system. Data from this study could be used as a reference, particularly important for management of Ria Formosa, a productive system where bivalves production depends deeply on water quality. In addition, the nutrient concentrations and mass exchanges resulting from the different processes can be used as a reference for other lagoon systems where shellfish production is practised.
  • Water quality of a southwest Iberian coastal lagoon: Spatial and temporal variability
    Publication . Rosa, Alexandra; Cravo, Alexandra; Correia, Cátia; Jacob, José
    The present work aims to characterize Ria Formosa water quality considering its spatial, and temporal variability at two scales: short-term (among seasons) and long-term to evaluate its evolution over the past 40 years, by comparing six historical datasets with data obtained in this study. To attain these goals, four field surveys under different seasons and/or weather conditions were conducted between 2017 and 2019 at seven sites along the Ria Formosa, covering the water bodies specified for this system. In situ measurements (temperature, salinity, pH and dissolved oxygen) and water sampling for determination of nutrients, chlorophyll a and suspended solids were taken every 2 h at each site, during complete semidiurnal tidal cycles. Moreover, these data were complemented with in situ data acquired at a high frequency (every 15 min) by a real time observational station deployed at an inner area, close to a main channel, where the anthmpogenic pressure is more intense. Data analysis clearly depict a spatial variability pattern along the Ria Formosa, as well as a temporal heterogeneity, influenced by the contribution of precipitation, sediments, wind and water exchanges with the adjacent ocean. Between sampling sites, the lowest variability of water quality parameters occurred at the boundary coastal station, at the main inlet, in permanent connection with the ocean, while the maximum variability was found at both the lagoon edges, mainly due to the shallowness of the water column. Temporally, the highest concentrations of nutrients were obtained during the Wet/rainy conditions survey, under the influence of runoff. The lowest concentrations of nutrients were attained during the Summer, except for phosphate, due to consumption by phytoplankton. Although the sampling frequency along time has been limited, Ria Formosa water quality data from the last 40 years shows a decreasing trend in nutrients concentration and a marginal increase of dissolved oxygen, suggesting a water quality improvement over time, in contrast with other coastal lagoons that are showing a water quality deterioration due to an increasing anthropogenic pressure. Altogether, these are relevant aspects to consider regarding Ria Formosa present and future management, including climate change and anthropogenic pressures susceptibility assessment and to use them within an international context by comparison with other similar systems.