HR: 0800h
AN: H51C-1133    [Abstracts]
TI: Terrestrial Sediment and Nutrient Discharge, and Their Potential Influence on Coral Reefs, Puerto Rico
AU: * Larsen, M C
EM: mclarsen@usgs.gov
AF: U.S.Geological Survey, 436 National Center, Reston, VA 20192
AU: Webb, R M
EM: rmwebb@usgs.gov
AF: U.S.Geological Survey, Denver Federal Center Box 25046, MS 413, Denver, CO 80225
AU: Warne, A G
AF: U.S.Geological Survey, GSA Center 651 Federal Drive , Guaynabo, PR 00965
AB: Sediment and nutrient discharge to the insular shelf of Puerto Rico (18 degrees latitude), augmented by anthropogenic activity, is believed to have contributed to widespread degradation of coral reefs of Puerto Rico during the 20th century. Sediment deposition degrades coral reefs because it reduces the area of sea floor suitable for growth of new coral, diminishes the amount of light available for photosynthesis by symbiotic algae that live within individual coral animals, and in extreme cases, buries coral colonies. Land-use history and data from 30 water-discharge, 9 daily and 15 intermittent sediment-concentration, and 24 water-quality gaging stations were analyzed to investigate the timing and intensity of terrestrial sediment and nutrient discharge into coastal waters. Watersheds in Puerto Rico generally are small (10's to 100's of square km), channel gradients are steep, and stream valleys are deeply incised and narrow. Major storms are usually brief (<24 h) but intense such that the majority of the annual sediment discharge occurs in a few days. From 1960 through 2000 the highest mean daily discharge for a water year (October - September) accounted for 20 to 60 percent of the total annual sediment discharge. Major storms, with a return frequency of approximately a decade, were capable of discharging up to 30 times the median annual sediment-discharge volume. Prior to agricultural and industrial development, coastal waters are believed to have been relatively transparent, with strong currents and seasonal high-energy swells assisting corals in the removal of minor amounts of sediment deposited after storms. Land clearing and modification, first for agriculture and later for urban development, have increased sediment and nutrient influx to the coast during the 19th and 20th centuries. Although forest cover has increased to approximately 30 percent of the surface of Puerto Rico during the past 60 years, sediment eroded from hillslopes during the agricultural period is still being episodically transported from upland valleys to downstream floodplains and the coast. In response to better land management, the quality of water has improved significantly since the 1980s. Nitrogen and phosphorous concentrations in river waters are now well within regulatory limits, although current concentrations are as much as 10 times the estimated pre-settlement levels. Concentrations of pathogens also are improved but continue to be near or above regulatory limits. Unlike sediment discharge, which is episodic and intense, the discharge of river-borne nutrients and pathogens is a less intense but chronic stressor to coral reefs located near the mouths of rivers.
UR: http://pr.water.usgs.gov/
DE: 1803 Anthropogenic effects
DE: 1815 Erosion and sedimentation
DE: 1860 Runoff and streamflow
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting