Variability of Currents and Water Column Structure in a Temperate Estuarine System (Sado Estuary, Portugal)

Abstract

The circulation in estuaries promotes the transport of organisms, nutrients, oxygen and sediments. Simultaneously, the mixture of fresh and salt water leads to variations of the physicochemical and biological components of the region. Therefore, it is important to further understand the hydrodynamic patterns of an estuary as one of the bases to understand the whole dynamic of these systems, ecologically important regions that must be preserved. However, little is known about the hydrodynamics of some estuarine systems. In order to bridge the knowledge gap about the Sado estuary, sampling was conducted with the purpose of evaluating some circulation patterns of the estuary and classifying it according to the stratification of the water column. The campaigns were conducted to collect monthly data on the intensity and direction of the currents, and on the temperature and salinity of the water column, between September 2018 and September 2019. The data indicated that water circulation in the Sado estuary, occurred through the two main navigation channels (North and South), according to the tidal regime. Both the temperature and the salinity were homogeneous along the water column, revealing little stratification. The analysis suggests possible hydrodynamic changes of the estuary in the past years.

Distribution patterns of microplastics in subtidal sediments from the Sado river estuary and the Arrábida marine park, Portugal

“…In fact, when low-density particles (known to ultimately contribute for the pool of MPs found in sediments) enter in the marine environment, they will not sink for several weeks, due to the lack of biofouling (Ye and Andrady, 1991;Lobelle and Cunliffe, 2011;Kaiser et al, 2017). Moreover, the lack of such relation in this study may be also related with an increase of the current velocity at the Sado estuary from late summer to February (Vale et al, 1993;Martins et al, 2002;Biguino et al, 2021) preventing a faster settling of the new MPs inputs in the system.…”

“…However, despite the potential for MPs to accumulate in stations 4 and 5, this was only verified in st1. Such strong retention of MPs inside the estuary may be greatly attributed to its reported slow flow rate (Vale et al, 1993;Cunha et al, 2007;Biguino et al, 2021) and to flocculation (Andersen et al, 2021;Laursen et al, 2022). The typical aggregation of suspended particulate matter in the estuary water column (Meade, 1972;Eisma, 1986;Manning and Dyer, 1999) enhanced by the mixture between freshwater and seawater, is suggested to transport MPs from the water surface into the estuarine sediments due to their incorporation in such flocs (Andersen et al, 2021;Laursen et al, 2022).…”

Spatial and Temporal Variations of Cockle (Cerastoderma spp.) Populations in Two Portuguese Estuarine Systems With Low Directed Fishing Pressure

“…Tidal currents dominate the estuarine dynamics given the limited freshwater discharge of the Sado river (Freitas et al, 2008). Consequently, the salinity remains close to normal marine water in most of the estuary, and is highly influenced by tidal currents and seasonal variations (ICNF, 2007;Biguino et al, 2021). This estuary has an average tidal range of 2.7 m, varying between 1.3 m at neap tides to 3.5 m at spring tides (Freitas et al, 2008).…”

“…This estuary has an average tidal range of 2.7 m, varying between 1.3 m at neap tides to 3.5 m at spring tides (Freitas et al, 2008). It comprises a wide bay characterized by two channels, the Northern channel with weaker residual currents and a highly dynamic Southern channel separated from the North channel by sand banks (Caeiro et al, 2005;Biguino et al, 2021). This estuary is also characterized by large areas of salt marshes and intertidal flats, and sandy sediments of marine origin, in the area closest to the mouth.…”

Environmental Flow Requirements of Estuaries: Providing Resilience to Current and Future Climate and Direct Anthropogenic Changes

“…For estuaries, process-based models have been proven to be particularly useful for exploring a range of flow regimes, including reference conditions, target hydrological conditions and extreme events (Beilfuss and Brown, 2010;Van Niekerk et al, 2019b). Most models focus on simulating the prevailing hydrodynamics and thermodynamics (Duarte et al, 2014;Biguino et al, 2021), fewer examine geomorphological evolution (Deng et al, 2017), and even fewer consider ecological states (Panda et al, 2015;Weng et al, 2020). As flow volumes to estuaries are naturally stochastic (Gillanders and Kingsford, 2002), model simulations need to include a range of relevant temporal variations that conform to the historical and future variance distribution in terms of flow exceedance likelihoods (Beilfuss and Brown, 2010;Van Niekerk et al, 2019b).…”