Original Research
Small river basin and estuarine sediment fluxes: The magnitude necessary for coastal dispersion and siltation effects on a coral reef

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Abstract

Increasing continental suspended sediment influx to coral reefs is an example of land-sea coupling that requires the identification of sources, magnitude of transport, and controlling processes. In Brazil, a small coastal basin (Macaé River) was identified as a source of suspended sediment to a coral reef on the coast of Cape Armação dos Búzios. Biannual suspended sediment loads were measured at the basin as were fluxes within the estuary and towards the coast during eight tidal cycles. Particle load and yield from this basin were typical of small coastal basins, showing high to moderate slopes and transitional land management. However, the magnitude of the river loads was lower than the sediment transport within the estuary, indicating that the estuary amplifies river fluxes and sustains the transference of suspended sediment alongshore to the coral reef. Nonetheless, the estuary displays both suspended particle retention and export capacity and, therefore, fluxes to the coast and the coral reef occur as episodic events.

Introduction

Increased continental runoff of sediment particles, mainly from river basins impacted by human activities such as deforestation and river channelization, which, in estuaries, are modulated by hydrodynamic processes, may affect distant coral reefs (Fabricius, 2005, Mclaughlin et al., 2003). Suspended sediment comprising organic and mineral particles delivered from drainage basins to oceans have been estimated worldwide. However, inventories should always be revisited to provide further information in this regard, including fluxes from small basins that, when grouped, can significantly contribute to global inventories (Milliman and Meade, 1983, Milliman and Syvitski, 1992). In addition, on a local scale, small rivers can also affect coastal ecosystems but little information is available on the magnitude of basin particle yield, estuarine control on fluvial transport to coastal waters, and possible coastal effects, including coastal reefs (Callaway et al., 2014, Godiva et al., 2010, Milliman and Syvitski, 1992).

Godiva et al. (2010) identified particle sources to Siderastrea stellata Verril 1868 coral colonies at Cape Armação de Búzios, located on the southeastern Brazilian coast. Geochemical and aerogammaspectrometry surveys indicated that sedimentary material collected from the Macaé River basin was similar to the material obtained from the surface of the coral colonies, suggesting efficient coastal particle transport across 44 km from the river mouth and subsequent reef deposition. However, the transport from river basins to estuaries and subsequent transference from the coast to coral reef environments in several ecologically relevant regions in the southwest Atlantic are still poorly described, especially concerning small coastal basins (Godiva et al., 2010).

The aforementioned coastal river basin (Macaé River basin) is included in the Southeast Atlantic Basin group, Brazil (22°S - 29°S), comprising many small to medium basins (10−1 to 104 km2) draining towards the Atlantic Ocean. Sediment transport from the Macaé River and its estuary illustrate the magnitude and other aspects of sediment transport in Southeast Atlantic Basins. An effective particle transport is expected, considering the high to moderate drainage basin slope. In addition, possible intensification of such fluxes is expected in view of the artificial elimination of river meanders and land use changes across the river basin (Marçal et al., 2017, Molisani et al., 2015). Thus, considering that more information on sediment fluxes from small rivers and related estuaries is required for international inventories, and that few studies identify the origin of particles deposited onto coral reefs, the current study investigated the magnitude and dynamics of particle fluxes from a small river basin and its estuarine modulation necessary to sustain coastal dispersion and proven siltation effects onto the coral reef.

Section snippets

Study area

The study site is located on the central-north coast of the state of Rio de Janeiro, Brazil. The coastal line comprises, four small river basins from north to south, draining into the sea and the Cape Armação dos Búzios, where coral colonies are located (Fig. 1). The rivers Macaé, Rio das Ostras, São João, and Una comprise drainage areas of 1765, 157, 2160, and 626 km2, respectively.

The cape is composed of embayed beaches distributed along the coastline, influencing waves and, consequently,

Results

Discharges from 9.0 to 213 m3/s and concentrations from 10 to 175 mg/L were obtained during the two-year measurement of monthly water discharges and suspended sediment from the Macaé River basin to the estuary, with higher discharges and concentrations occurring in the rainy season (Fig. 2 and Table 2). Biannually, particulate loads ranged from 0.092 kg/s (July 2012) to the highest peak of 15.4 kg/s (March 2013). On average, higher values occurred in the rainy season (2.43 kg/s) compared to the

Discussion

According to Milliman and Syvitski (1992), small mountainous rivers are more likely to discharge larger percentages of their sediment loads directly into the coast compared to larger rivers, mainly episodically. Like many Southeast Atlantic Basins, the small Macaé River basin, with a high to moderate slope until the coastal plain, suggests efficient sediment transport to the ocean, mainly due to deforestation and the straightening of the lower river. However, when comparing the Macaé River

Conclusions

This study indicates that the small Macaé River basin displays marked seasonal and episodic water fluxes, as well as sediment particle fluxes and yield ranging from 9.0 to 213 m3/s; 0.09–15 kg/s; and 1.7–286 ton/km2/yr, respectively, with a marked sediment load control by water discharge. However, suspended sediment particle loads were mostly retained within the ebb-tide dominated estuary, including sediment loads induced by sea storm events. Nonetheless, during episodic rainfall-driven events

Acknowledgments

This work was supported by the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) (E-26/112.565/2012), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (476497/2010-8), and Comitê de Bacias Hidrográficas Macaé/Ostras. P.S. Issackson and A. Machado were students from the Programa de Educação Tutorial (PET) from Ministério da Educação. Thanks to the Iate Clube de Macaé where the samplings were based.

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