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Terrigenous sediment dynamics in a small, tropical, fringing-reef embayment

  • Author(s): Messina, Alex Thomas
  • Advisor(s): Biggs, Trent W
  • et al.
Abstract

Anthropogenic watershed disturbance by industry, agriculture, deforestation, roads, and urbanization alters the timing, composition, and mass of sediment loads to coral reefs, causing enhanced sediment stress on corals near the outlets of impacted watersheds (Syvitski et al., 2005; West and van Woesik, 2001). Few studies have developed an integrated understanding of sediment sources, transport processes, and deposition in small, reef-fringed embayments (Bartley et al., 2014; Draut et al., 2009; Wolanski et al., 2003) and many are outside the scope of local environmental managers in remote islands like at the study site, Tutuila, American Samoa. Ridge-to-Reef studies on sediment dynamics have three general components, which are reflected in the three chapter structure of this dissertation: watershed inputs, hydrodynamic circulation over the reef, and how they interact to govern spatiotemporal distribution of sediment accumulation on the reef. This dissertation provides an example of how a scientific, process-oriented Ridge to Reef study of sediment dynamics can answer critical scientific questions about the source, transport, and fate of sediment in the near-shore environment, and how answers to the scientific questions can support local coral management.

Data on suspended sediment yield (SSY) from small, steep, tropical watersheds is limited, and assessments of sediment mitigation projects have been hindered by interannual climatic and sediment source variability. Chapter 1 used an event-wise approach to compare SSY from disturbed and undisturbed subwatersheds from storms of the same size, estimate total SSY to Faga'alu Bay, and estimate annual SSY to compare to other watersheds. It was unknown what the dominant sediment source was in Faga'alu watershed, and what potential management solutions were available. The sediment budget developed in the first chapter of this dissertation showed the quarry was a significant source, compared to natural background, and so local managers focused on reducing sediment discharge from the quarry. Continued monitoring presented in the third dissertation chapter showed SSY to the Bay was significantly reduced following sediment mitigation at the quarry.

The fate of suspended sediment once it enters the marine environment is difficult to predict, but is strongly controlled by hydrodynamic conditions and circulation patterns. Computer models of hydrodynamic circulation require detailed forcing data, bathymetric data, and computer resources that are often unavailable to local managers. Chapter 2 of the dissertation used a simple approach combining Lagrangian GPS-logging drifters and Eulerian acoustic current profilers to determine dominant water circulation patterns under the most common conditions that characterize forcing in the Bay: calm, high onshore winds, and high waves.

Measuring sediment accumulation on the reef is a contested area of research and the most common method, using tube traps, has some weaknesses. Others argue flat surfaces should be used to show net sediment accumulation. Chapter 3 presents results from both tubular sediment traps and flat-surfaced sediment pods to show gross vs net monthly sediment accumulation over one year. While many studies deploy traps haphazardly, or just below stream outlets, here sediment traps were arranged to observe spatial patterns between the north and south sections of the reef, as a result of prevailing currents and distance from the stream outlet.

Integrating SSY and water circulation from Chapters 1 and 2 with observations of sediment accumulation in Chapter 3 showed that the predominant water circulation patterns deflect the storm-supplied terrigenous sediment from the stream over the northern reef where it caused enhanced sediment stress on corals. Temporal patterns of sedimentation were complex, and only the site nearest the stream outlet correlated with monthly SSY from the watershed, whereas nearly all sites showed increased carbonate sedimentation with increased wave energy. Sediment accumulated in traps and on sediment pods was mostly similar to surrounding benthic sediment, and correlated with wave energy, showing most sediment transport over the reef was from wind and wave-forced resuspension of carbonate sediment.

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