Eduardo Siegle

An extensive reef system at the Amazon River mouth

A novel Amazonian reef biome was discovered, encompassing large rhodolith and sponge beds under low light, low oxygen, and high POC. Large rivers create major gaps in reef distribution along tropical shelves. The Amazon River represents 20% of the global riverine discharge to the ocean, generating up to a 1.3 × 106–km2 plume, and extensive muddy bottoms in the equatorial margin of South America. As a result, a wide area of the tropical North Atlantic is heavily affected in terms of salinity, pH, light penetration, and sedimentation. Such unfavorable conditions were thought to imprint a major gap in Western Atlantic reefs. We present an extensive carbonate system off the Amazon mouth, underneath the river plume. Significant carbonate sedimentation occurred during lowstand sea level, and still occurs in the outer shelf, resulting in complex hard-bottom topography. A permanent near-bottom wedge of ocean water, together with the seasonal nature of the plume’s eastward retroflection, conditions the existence of this extensive (~9500 km2) hard-bottom mosaic. The Amazon reefs transition from accretive to erosional structures and encompass extensive rhodolith beds. Carbonate structures function as a connectivity corridor for wide depth–ranging reef-associated species, being heavily colonized by large sponges and other structure-forming filter feeders that dwell under low light and high levels of particulates. The oxycline between the plume and subplume is associated with chemoautotrophic and anaerobic microbial metabolisms. The system described here provides several insights about the responses of tropical reefs to suboptimal and marginal reef-building conditions, which are accelerating worldwide due to global changes.

Wave refraction and longshore transport patterns along the southern Santa Catarina coast

Based on the wave climate for the southern Brazilian coast, wave refraction has been modelled in order to obtain the refracted wave heights and directions along the southern Santa Catarina coast, providing the needed information for potential longshore drift estimates. According to its coastline orientation, different sectors of the coast present varying longshore drift patterns. Estimates have been made for the yearly-averaged wave climate as well as for each season, showing thereby the longshore drift patterns along the year. Based on the results of the potential longshore drift intensities and directions and on the shoreline outline in plan, it has been possible to identify a strongly drift-dominated coast in the south turning to a mixed drift and swash dominated coast towards the north of the studied area. Contrasting patterns of longshore drift between the southern and northern portion of the coastline indicate a sediment surplus in the central portion, making sediment available for cross-shore transport processes, either on- or offshore. Considering long-term aspects, the longshore drift patterns are in agreement with the coastal infilling process which has mainly been driven by persistent surplus from littoral sediment drift.

The dynamics of a frictionally-dominated Amazonian estuary

A hidrodinâmica, morfologia e sedimentologia do estuario do Taperacu foram investigadas. Este e um entre varios estuarios do litoral amazonico que integram a maior extensao continua de manguezais do mundo, apresentando uma descarga de agua doce muito reduzida, atipica para a regiao. Os resultados revelam grandes bancos arenosos que ocupam em grande parte a porcao central do estuario. Areias muito finas e bem selecionadas de origem marinha prevalecem. Fases de enchente mais curtas, com velocidades de corrente substancialmente mais altas, sao observadas na porcao superior do estuario, como esperado para um estuario raso e dominado por friccao. Por outro lado, uma vazante mais intensa poderia ocorrer como resultado de grandes areas de manguezais associadas e intenso preenchimento estuarino, sendo que ambas as condicoes sao observadas no Taperacu. Neste caso, a prevalencia da enchente parece estar associada a ausencia de uma descarga fluvial efetiva. Alem disso, alguns canais de mare conectam o Taperacu com seu vizinho estuario do Caete, o que contribuiria para o dominio de enchente. Como um todo, os resultados demonstram uma complexa interacao de aspectos de configuracao (friccao, drenagem fluvial, conexoes com estuarios vizinhos, preenchimento e grandes areas intermareais) na determinacao dos padroes hidrodinâmicos, contribuindo para o entendimento dos estuarios da regiao amazonica.

Hidrodinâmica e transporte de material particulado em suspensão sazonal em um estuário dominado por maré: Estuário de Caravelas (BA)

The objective of the present paper is to characterize the hydrodynamics and the suspended particulate matter (SPM) in the Caravelas Estuary under different tidal and river regimes. Four hydrographic campaigns were carried out comprising a complete semi-diurnal tidal cycle, comprising neap and spring tidal phases, and during dry and wet seasons. Water level, current speed and direction, salinity, temperature and turbidity were recorded in an anchored station near to the estuarine mouth. The residual SPM flux and the mechanisms of transport were calculated. The higher SPM concentrations were recorded during the spring tidal phase. During neap tides, the net SPM flux was up-estuary, although with low scale. During spring tides the tidal currents are strongly ebb dominant, and the net flux can be either up- or down-estuary. The estuary was classified as well mixed or weakly stratified. The hydrography and the sediment balance are firstly modulated by the tidal range, and the fresh water inflow is negligible.

Hydrodynamics and water properties at the entrance of Araçá Bay, Brazil

Current meter and hydrographic properties at the entrance of the Araçá Bay (AB), an intertidal flat adjacent to the São Sebastião channel, were collected between July 2013 and February 2014. These data sets show two different hydrographic periods, marked by a sharp change in the temperature and salinity values, clearly caused by the arrival of the South Atlantic Central Water (SACW). The first period is characterized by small variabilities on both properties with the dominance of coastal water, with relatively low salinity values. The second period shows a strong increase in the average salinity values and a much larger variability of temperature. This change in the hydrographic characteristics seems to be caused by anomalous winds, capable of displacing the SACW toward the coast. On the other hand, current meter data shows that the dynamics is mainly driven by the large-scale wind and is not impacted by the arrival of the SACW. Also, the currents are dominated by the barotropic mode, independently of the stratification differences that are observed between the beginning and end of the observational period.

Shoreline variability from days to decades: Results of long-term video imaging

The present work characterizes the time-space scales of variability and forcing dependencies of a unique 26 year record of daily to hourly shoreline data from a steep beach at Duck, North Carolina. Shoreline positions over a 1500 m alongshore span were estimated using a new algorithm called ASLIM based on fitting the band of high light intensity in time exposure images to a local Gaussian fit, with a subsequent Kalman filter to reduce noise and uncertainty. Our findings revealed that the shoreline change at long times scales dominates seasonal variability, despite that wave forcing had only 2% variance at interannual frequencies. The shoreline response presented 66% of the variance at interannual scales. These results were not expected since from wave forcing it would have been expected that the shoreline response should similarly lack interannual variability, but we found it to be dominated by this scale. The alongshore-mean shoreline time series revealed no significant annual cycle. However, there are annual oscillations in the shoreline response that are coherent with wave forcing and deserves further explanations. The pier was found to have a significant influence on shoreline behavior since restricts the seasonal longshore transport between the sides, resulting in a seasonally reversing sediment accumulation. Thus, there is a significant annual peak in shoreline variability that is coherent with the annual forcing but becomes insignificant in the longshore-average.

HYDRODYNAMICS AND SUSPENDED SEDIMENT TRANSPORT IN THE CAMBORIÚ ESTUARY - BRAZIL: PRE JETTY CONDITIONS

Estuarine hydrodynamics is a key factor in the definition of the filtering capacity of an estuary and results from the interaction of the processes that control the inlet morphodynamics and those that are acting in the mixing of the water in the estuary. The hydrodynamics and suspended sediment transport in the Camboriu estuary were assessed by two field campaigns conducted in 1998 that covered both neap and spring tide conditions. The period measured represents the estuarine hydrodynamics and sediment transport prior to the construction of the jetty in 2003 and provides important background information for the Camboriu estuary. Each field campaign covered two complete tidal cycles with hourly measurements of currents, salinity, suspended sediment concentration and water level. Results show that the Camboriu estuary is partially mixed with the vertical structure varying as a function of the tidal range and tidal phase. The dynamic estuarine structure can be balanced between the stabilizing effects generated by the vertical density gradient, which produces buoyancy and stratification flows, and the turbulent effects generated by the vertical velocity gradient that generates vertical mixing. The main sediment source for the water column are the bottom sediments, periodically resuspended by the tidal currents. The advective salt and suspended sediment transport was different between neap and spring tides, being more complex at spring tide. The river discharge term was important under both tidal conditions. The tidal correlation term was also important, being dominant in the suspended sediment transport during the spring tide. The gravitational circulation and Stokes drift played a secondary role in the estuarine transport processes.

Mud in the surf: Nature at work in a Brazilian bay

Massive discharge of mud from coastal rivers is a well-documented phenomenon. However, in areas with limited historical and instrumental records it is often difficult to assess the nature and history of the process. This article looks at Tijucas Bay, in southern Brazil (Figure 1a) (an area that was the landfall region in March 2004 for South Americas first recorded hurricane [Bossack, 2004]), to examine the time frame for extensive deposition of fluid muds in the nearshore (Figure 1b). The new geological data suggest that whereas recent human activities (e.g., massive sand mining) along the Tijucas River may be important in increasing the suspended sediment discharge, the shift to a mud-dominated regime was part of the natural evolution of this coastal plain.

Modelling Water Surface Topography at a Complex Inlet System – Teignmouth, UK

ABSTRACT Accurate water surface topography data and its spatial and temporal variability provide information about the interaction of physical processes acting in coastal regions. At the inlet system in Teignmouth, UK, these data complement methods for the extraction of nearshore morphology using remotely sensed video techniques. The video methods normally assume that the water surface is horizontal over the region, an assumption that is often invalid in shallow water. The study area is a complex macro-tidal inlet system bounded by a rocky headland and a 2 km-long beach. In order to predict the water surface topography and its response to different tide, wave and river discharge conditions, a calibrated and validated numerical model (MIKE21 HD, NSW) was applied. The water surface topography at the inlet and adjacent coast exhibits high spatial and temporal variability, mainly related to the tidal phase. It is the interaction between the tidal phase and the sandbar morphology, defining the velocity field in the channels, which drives the water surface topography distribution across the region. Since a small, unaccounted, difference in water level may result in significant deviations of the horizontal shoreline position, this study highlights the importance of using numerical modelling in conjunction with the video image techniques for the extraction of nearshore morphology.

Environmental Evolution of the Caravelas Estuary (Northeastern Brazilian Coast, 17° S, 39° W) Based on Multiple Proxies in a Sedimentary Record of the Last Century

ABSTRACT Sousa, S.H.M.; Amaral, P.G.C.; Martins, V.; Figueira, R.C.L.; Siegle, E.; Ferreira, P.A.L.; Silva, I.S.; Shinagawa, E.; Salaroli, A.; Schettini, C.A.F.; Santa-Cruz, J., and Mahiques, M.M., 2014. Environmental evolution of the Caravelas Estuary (northeastern Brazilian coast, 17° S, 39° W) based on multiple proxies in a sedimentary record of the last century, A sediment core (∼100 cm long) taken from the upper Caravelas estuary was interpreted based on sedimentological, microfaunal, geochemical, and historical data for the last 70 years. Outcomes are a significant contribution to the understanding of the relationship between anthropogenic impact and natural ecosystem change. The results obtained for the period between the 1930s and the beginning of the 1970s do not reveal any evidence of natural and/or anthropogenic impacts in the estuary. This period is characterized by a higher percentage of sandy sediments, lower pollution load index (PLI) values, and a mixed source of organic matter (C3 land plants and marine dissolved organic carbon [DOC]). During the period from the 1970s to the 1990s, widening of the inlet and the entrance of marine water initiated a rapid natural environmental transformation of the area, increasing the amount of mud and producing an abundance of open marine–dwelling ocean foraminifera species, including Pararotalia cananeiaensis, and a decrease of foraminifera density and diversity values. The slight increase of PLI values and the predominance of C3 land plant organic matter observed in this period might be the result of anthropogenic activities related to the soil occupation in the area, which are also reflected in the abundance of Ammonia tepida. During the last decade, dredging operations close to the inlet channel may have contributed to increased transport and deposition of marine sediments into the estuary.