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.

Effect of illegal glass eel (Anguilla anguilla) fishery on estuarine fish stocks: a case study in the Mondego Estuary, Portugal

Illegal fishing is a critical activity that affects a wide range of species, leading to a decline in total abundance and the overexploitation of target and non-target species. In Portugal, despite fishing for eel juveniles not being allowed, a substantial amount of illegal fishing takes place during the migratory season (October–March). The aim of this study was to investigate the effects of glass eel fishing in the Mondego estuary on target and non-target species. Samples were collected between November 2011 and March 2013 during the autumn and winter (dry and regular respectively with regard to hydrological characteristics) in 2 years. No differences in glass eel catches were observed between fishing seasons; however, a higher number of non-target species was observed in 2012–13 (regular year). There was higher species diversity in the regular year, as well as the highest number of individuals. Higher catches of the most abundant species were also observed in the regular year, compared with the dry year, suggesting that the magnitude of the effect of these illegal practices is related to hydrological cycles. Despite the low replication (2 years), the present study shows that non-selective practices have considerable effects on the species captured, as well as on the functioning of the estuarine ecosystem.

Feeding niche preference of the mudsnail Peringia ulvae

The feeding behaviour of Peringia ulvae was studied with the aim of assessing its preferential feeding niche (habit) as well as its potential role in controlling or preventing the occurrence of macroalgal blooms. Both these aspects were studied in the Mondego estuary because of the clear long-lasting eutrophication process that favours the growth of two opportunistic macroalgae, Gracilaria spp. and Ulva spp., over the local beds of the macrophyte Zostera nolteii, which is another potential food source. All three species, both with and without periphyton, were used as a single food source in feeding experiments to assess different feeding parameters of the gastropod. Sediment with microphytobenthos was used together with the latter three species in feeding niche preference experiments. None of the food types provided were resistant to grazing. Higher levels of grazing occurred on Ulva spp. (apical parts) and Z. nolteii, both with periphyton. The presence of periphyton did not change grazing rates. When a choice between each food type was offered, the sediment was clearly the feeding niche preferred by the snails. As P. ulvae spent more time on the sediment (preferred feeding niche), the hypothesis that it has a role in preventing macroalgal blooms was not supported.

Halophytes as sources of metals in estuarine systems with low levels of contamination

Heavy metal concentrations present in the above- and beowground tissues of Scirpus maritimus L., Spartina maritima (Curtis) Fernald and Zostera noltii Hornem were analysed seasonally in the Mondego Estuary, Portugal. The sediments of the estuary were confirmed to contain only low concentrations of heavy metals. The belowground tissues of all three species showed higher heavy metal concentrations than the aboveground tissues. Although the sediments only contained low levels of contamination, because the area occupied by S. maritimus and Z. noltii was large, significant quantities of heavy metals were accumulated and exported to the surrounding water bodies. In contrast with observations of highly contaminated estuaries, it was found that in spite of the low level of contaminants in the sediments of the Mondego Estuary, aquatic vegetation functioned as a source of metals for nearby systems.