Modeling the Impact of Extreme River Discharge on the Nutrient Dynamics and Dissolved Oxygen in Two Adjacent Estuaries (Portugal)

Influence of climate change and extreme weather events on an estuarine fish community

Extreme weather events are becoming more frequent as a result of climate change, and the increasing frequency of these events may lead to significant changes in fish assemblages. In this sense, this work aimed to study the effects of climate change and extreme weather events on fish assemblages in the Rio Minho estuary (Portugal). Between 2010 and 2019, continuous weekly sampling with fyke nets was carried out to assess the dynamics of fish assemblages in the estuary. In addition, temperature and precipitation data were obtained from satellite information to assess the relationship between climatic variables and fish composition, structure, and diversity. Fish populations changed significantly over time, becoming less diverse and largely dominated by a few, mostly invasive species (e.g., carp, goldfish, pumpkinseed, and tench), while the abundance of most native species declined over the years (e.g., panjorca, stickleback, and shad). High temperatures and low precipitation negatively affected native species, while the invasive species benefited from increased temperatures and extreme weather events (droughts and floods).

Dynamics of Two Anadromous Species in a Dam Intersected River: Analysis of Two 100-Year Datasets

Long-term time-series datasets are key for assessing the population dynamics of fish species with economic interest. This study examines two 100-year datasets for sea lamprey Petromyzon marinus and allis shad Alosa alosa from the Minho River. This basin on the Iberian Peninsula is home to one of the largest populations at the southern distribution limit of these critically endangered anadromous fish species. Besides assessing the importance of the environmental drivers of fish capture data (temperature, salinity, upwelling, precipitation, and climatic oscillation), this study also assesses how dam construction affected these populations using intervention analysis—A statistical tool to detect significant breakpoints in time series data. Results showed contrasting trends between sea lamprey and allis shad, with the number of captured fish from the first progressively improving from 1914 to 2017, and the latter recording a significant decline over time. Although no significant correlations were detected between fishing data and environmental variables, some of the identified breakpoints in the time series data matched the dates when the major dams in Minho River were built. Other historical activities associated with mining might explain the notable changes detected in the trends, while issues associated with illegal, unreported, and unregulated capture data are also discussed.

Modeling Dynamic Processes of Mondego Estuary and Óbidos Lagoon Using Delft3D

Estuarine systems currently face increasing pressure due to population growth, rapid economic development, and the effect of climate change, which threatens the deterioration of their water quality. This study uses an open-source model of high transferability (Delft3D), to investigate the physics and water quality dynamics, spatial variability, and interrelation of two estuarine systems of the Portuguese west coast: Mondego Estuary and Óbidos Lagoon. In this context, the Delft3D was successfully implemented and validated for both systems through model-observation comparisons and further explored using realistically forced and process-oriented experiments. Model results show (1) high accuracy to predict the local hydrodynamics and fair accuracy to predict the transport and water quality of both systems; (2) the importance of the local geomorphology and estuary dimensions in the tidal propagation and asymmetry; (3) Mondego Estuary (except for the south arm) has a higher water volume exchange with the adjacent ocean when compared to Óbidos Lagoon, resulting from the highest fluvial discharge that contributes to a better water renewal; (4) the dissolved oxygen (DO) varies with water temperature and salinity differently for both systems. On the one hand, for Mondego Estuary during winter the DO levels mainly fluctuate with salinity. On the other hand, for Óbidos Lagoon, DO distribution is determined by both water temperature and salinity. During summer, the high residence time and water temperature limit the DO levels in both systems. The high transferability and superior stability of Delft3D make this model a foundation for realistic simulation and research of distinct estuarine systems, giving support to their maintenance and restoration.

Three-dimensional numerical modelling of transport, fate and distribution of microplastics in the northwestern Arabian/Persian Gulf

Marine plastic litter has been a major concern over the past decade particularly in semi-enclosed seas such as the Arabian/Persian Gulf, which are likely to impose a relatively higher threat to ecosystem and human health. In this work, we have focused our efforts on the transport features of marine surface microplastics (MPs) in the Gulf. The assessment utilizes a 3D hydrodynamic model of the northern Gulf which was coupled with a particle tracking model. We have considered five release locations and investigated two dominant wind conditions by applying different numerical scenarios. The results revealed that the northerly winds result in high dispersion and seaward transport of MPs in the open coastal zones, while in semi-enclosed regions they result in high trapping and beaching verified by visual investigation. The study shows that further detailed field investigations are warranted to enable the models to better parameterize the fate and distributions of MPs.

Physico-Chemical Characterization of Two Portuguese Coastal Systems: Ria de Alvor and Mira Estuary

The present study aims to research the physico-chemical processes in two under-researched coastal systems located on the west and south Portuguese coast—Mira Estuary and Ria de Alvor—through the development and exploitation of dedicated coupled physical and water quality models. Both systems are highly dynamic, supporting a wide range of biological diversity; however, they are characterized by distinct environmental and oceanographic conditions, enhancing the importance of a comparative approach. In this context, the Delft3D modeling suite was implemented and successfully calibrated and validated for both systems, accurately reproducing their hydrodynamic, hydrographic, and chemical features. A broad characterization of Mira Estuary and Ria de Alvor was carried out. Results show that the tidal wave interacts differently with the different geomorphology of each estuary. The tidal wave amplitude decreases as it propagates upstream for both estuaries; however, the magnitude for Ria de Alvor is higher. A flood tidal dominance was found for Mira Estuary, allowing the transport of well-oxygenated water into the estuary, contributing to high residence times in the middle estuary and therefore to poor dissolved oxygen (DO) replenishment. Ria de Alvor shifts from ebb dominance at the central area to flood dominance upstream with low residence times, allowing an effective exchange of water properties. Results also reveal that the water temperature is the dominant driver of seasonal dissolved oxygen variations in both estuaries, with the lowest levels occurring during the late summer months when the water temperature is highest. In addition, pH is influenced by biological activity and freshwater inflow.