Showing papers by "Michael H. Bothner published in 2009"

Supply and dispersal of flood sediment from a steep, tropical watershed: Hanalei Bay, Kaua’i, Hawai’i, USA

Abstract: In contrast to many small, mountainous watersheds in temperate coastal regions, where fl uvial discharge and wave energy commonly coincide, deposition and reworking of tropical fl ood sediment can be seasonally decoupled, and this has important implications for coral-reef ecosystems. An understanding of the interaction between tropical fl ood sedimentation and wave climate is essential to identifying and mitigating effects of watershed changes on coral reefs as urbanization and climate change proceed. Sedimentary facies and isotopic properties of sediment in Hanalei Bay, on the island of Kaua’i, Hawai’i, USA, were used to assess deposition and reworking of fldeposits from the Hanalei River in a case study demonstrating the potential ecosystem effects of runoff from a steep, tropical watershed. In Hanalei Bay, the youngest and thickest terrigenous sediment was consistently present near the river mouth and in a bathymetric depression that acted as at least a temporary sediment sink. During this 2 yr study, the largest fl ood events occurred in late winter and spring 2006; substantial terrestrial sediment delivered by those fl oods still remained in the bay as of June 2006 because oceanic conditions were not suffi ciently energetic to transport all of the sediment offshore. Additional sediment was deposited in the bay by a summer 2006 fl ood that coincided with seasonal low wave energy. In most years, fl ood sediment accumulating in the bay and on its fringing reefs would be remobilized and advected out of the bay during winter, when the wave climate is energetic. Turbidity and sedimentation on corals resulting from late spring and summer fl oods during low wave energy could have a greater impact on coral-reef ecosystems than fl oods in other seasons, an effect that could be exacerbated if the incidence and sediment load of tropical summer fl oods increase due to urbanization and climate change.

Sources of land-derived runoff to a coral reef-fringed embayment identified using geochemical tracers in nearshore sediment traps

Abstract: Geochemical tracers, including Ba, Co, Th, 7Be, 137Cs and 210Pb, and magnetic properties were used to characterize terrestrial runoff collected in nearshore time-series sediment traps in Hanalei Bay, Kauai, during flood and dry conditions in summer 2006, and to fingerprint possible runoff sources in the lower watershed. In combination, the tracers indicate that runoff during a flood in August could have come from cultivated taro fields bordering the lower reach of the river. Land-based runoff associated with summer floods may have a greater impact on coral reef communities in Hanalei Bay than in winter because sediment persists for several months. During dry periods, sediment carried by the Hanalei River appears to have been mobilized primarily by undercutting of low 7Be, low 137Cs riverbanks composed of soil weathered from tholeiitic basalt with low Ba and Co concentrations. Following a moderate rainfall event in September, high 7Be sediment carried by the Hanalei River was probably mobilized by overland flow in the upper watershed. Ba-desorption in low-salinity coastal water limited its use to a qualitative runoff tracer in nearshore sediment. 210Pb had limited usefulness as a terrestrial tracer in the nearshore due to a large dissolved oceanic source and scavenging onto resuspended bottom sediment. 210Pb-scavenging does, however, illustrate the role resuspension could play in the accumulation of particle-reactive contaminants in nearshore sediment. Co and 137Cs were not affected by desorption or geochemical scavenging and showed the greatest potential as quantitative sediment provenance indicators in material collected in nearshore sediment traps.