A record of recent change in terrestrial sedimentation in a coral-reef environment, La Parguera, Puerto Rico: A response to coastal development?

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Abstract

Increased sediment flux to the coastal ocean due to coastal development is considered a major threat to the viability of coral reefs. A change in the nature of sediment supply and storage has been identified in a variety of coastal settings, particularly in response to European colonization, but sedimentation around reefs has received less attention. This research examines the sedimentary record adjacent to a coastal village that has experienced considerable land-use change over the last few decades. Sediment cores were analyzed to characterize composition and sediment accumulation rates. Sedimentation rates decreased seaward across the shelf from 0.85 cm y−1 in a nearshore bay to 0.19 cm y−1 in a fore-reef setting. Data reflected a significant (up to 2×) increase over the last ∼80 years in terrestrial sediment accumulating in the back-reef setting, suggesting greater terrestrial sediment flux to the area. Reef health has declined, and increased turbidity is believed to be an important impact, particularly when combined with additional stressors.

Introduction

Enhanced terrestrial sedimentation in a coral-reef environment may be a sign of important changes in environmental conditions, serving as an additional stress to organisms and leading to structural and functional changes in the ecosystem. The direct (e.g., smothering) and indirect (e.g., decreased light availability) impacts of sedimentation that can affect the viability of corals are well documented (Fabricius, 2005 and references therein).

European settlement and more recent coastal development are recognized as major factors altering terrestrial sediment loading to the coastal ocean (e.g., Meade, 1990, Syvitski et al., 2005). Previous studies of sediment accumulation in marine environments have used radionuclides (i.e., 210Pb, 137Cs) to document changes within temperate estuaries (Plater et al., 2000, Cooper et al., 2004, Mizugaki et al., 2006) and on continental shelves (Hart et al., 1998, Alexander and Vernherm, 2003). However, few studies have applied these radionuclide tools to examine sedimentation in a coral reef setting (Draut et al., 2006, Field et al., 2007).

The objective of this research was to evaluate changes in the nature and/or rate of sediment accumulating in a coral-reef setting adjacent to a coastal community (La Parguera, Puerto Rico) which has experienced modest tourism-related growth. This case study examines an area receiving little terrestrial sediment and can be considered closer to a minimal-impact end-member; locations with greater fluvial sediment supply can be expected to show more pronounced impacts. This study quantified the nature of sedimentation across the shelf using a combination of high-resolution radiochemical, geochemical, and sedimentological techniques. It is hypothesized that a notable amount of terrestrial sediment accumulating in the La Parguera coral-reef environment has increased as a result of land modification over the last several decades.

Section snippets

Materials and methods

La Parguera is situated in southwest Puerto Rico (Fig. 1). Like many coastal villages adjacent to coral-reef communities, it has experienced considerable coastal development within the last century (Fig. 2). The climate of southwest Puerto Rico is warm (average temp = 27 °C) and dry (average rainfall = 75 cm y−1). Direct terrestrial sediment supply to the coast is thought to be primarily in the form of sheet runoff from precipitation as no sizable rivers drain into the immediate area (García et al.,

Carbonate patterns in mixed settings

The most conspicuous variation in the nature of sediments across the shelf was the trend of carbonate content (Fig. 3). Sediments in the nearshore core had a moderately high carbonate content (50 ± 0.9%, mean ± SD) and microscopic investigation revealed a composition of pelecypods, ostracods, and porcellaneous foraminifera. The back-reef site landward of Corral Reef had an average carbonate percentage of 90 ± 0.5%, and sediments were composed of a variety of carbonate constituents such as

Summary

This research in La Parguera gives valuable insight into recent change in the terrestrial sedimentation changes in a coral reef area, and this is widely cited as critical stress to coral reefs although it is not well documented. These data suggest that modern local terrestrial sediment accumulation may be nearly double that which occurred in the early 20th century. Although the cause for this change cannot be unequivocably established, land-use change in the watershed (i.e., coastal

Acknowledgements

The authors thank Richard Appeldoorn for his considerable efforts in coordinating the Puerto Rico research in the NOAA-Coral Reef Ecosystem Studies program. Funding for this work was provided from a subcontract from the University of Puerto Rico-Mayaguez, with additional student support from NSF Grant (OCE-0452166) to J.P. Walsh. Mike Dail is thanked for his efforts with the field work and lab analyses. Also, this work could not have been accomplished without help from Francisco Pagan, Mike

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