Abstract
In tide-dominated estuaries, maximum-turbidity zones (MTZs) are common and prominent features, characterized by a peak in suspended-sediment concentration (SSC) associated with estuarine processes. The Brazilian Amazon coast includes many estuaries, experiencing macrotidal conditions. MTZs are expected to occur and are crucial for sediment delivery to the longest continuous mangrove belt of the world. The area is under influence of the Amazon River plume (ARP), the main SSC source, as local rivers do not deliver substantial sediment supply. To assess the processes that allow the ARP to supply sediment to the estuaries and mangrove belt along the Amazon coast, the results from previous individual studies within five Amazon estuaries (Mocajuba, Taperaçu, Caeté, Urumajó and Gurupi) were compared with regards to SSC, salinity, morphology and tidal propagation. This comparison reinforces that these estuaries are subject to similar regional climate and tidal variations, but that their dynamics differ in terms of distance from the Amazon River mouth, importance of the local river sediment source, and morphology of the estuarine setting. The Urumajó, Caeté and Gurupi are hypersynchronous estuaries where perennial, classic MTZs are observed with SSC > 1 g·L−1. This type of estuary results in transport convergence and MTZ formation, which are suggested to be the main processes promoting mud accumulation in the Amazonian estuaries and therefore the main means of mud entrapment in the mangrove belt. The Mocajuba and the Taperaçu estuaries showed synchronous and hyposynchronous processes, respectively, and do not present classic MTZs. In these cases, the proximity to the ARP for the Mocajuba and highly connected tidal channels for the Taperaçu estuary, assure substantial mud supply into these estuaries. This study shows the strong dependence of the estuaries and mangrove belt on sediment supply from the ARP, helping to understand the fate of Amazon River sediments and providing insights into the mechanisms providing sediment to estuaries and mangroves around the world, especially under the influence of big rivers.
Highlights
Estuaries have been defined and classified based on different factors that reflect the diversity of coastal areas and scientific interests [1]
These estuaries show an unbalanced condition between flow and morphology, which is the product of inherited morphology for the Mocajuba, and lack of fluvial discharge for the Taperaçu estuary [42]
It highlights other processes that are relevant to the suspendedsediment dynamics at the regional level, i.e., the contribution of the Amazon River plume (ARP) and the role of connectivity between estuaries
Summary
Estuaries have been defined and classified based on different factors that reflect the diversity of coastal areas and scientific interests [1]. The baroclinic circulation has little relevance where the tidal variations are intense and the turbulence generates strong mixing of the estuarine waters, and the estuaries can be classified as vertically homogeneous, well mixed, or tide-dominated [11], in contrast to wave-dominated estuaries [3] or river-dominated estuaries [12]. These patterns are substantially influenced by friction, which in this context is mostly a product of the estuarine morphology (e.g., depth) [13,14]. Shallow depths in relation to the tidal range favor turbulence and mixing [15]
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