Univ. of Puerto Rico Sea Grant College Interim Progress Report (09/30/17)
Interim Research Progress Report University of Puerto Rico Sea Grant College Reporting Period: 2/01/17-7/31/17 Assessment of the Impact of Watershed Development and Restoration on Marine Sediment Dynamics, St. John, USVI Sarah C. Gray (Dept. of Environmental and Ocean Sciences, University of San Diego); Carlos E. Ramos-Scharrón (Dept. of Geography, University of Texas, Austin); Gregg R. Brooks (Dept. of Marine Science, Eckerd College) -Results and findings Terrestrial monitoring. Rainfall total for Jan-17 through Apr-17 was 171 mm or 69% of normal and it was distributed over 91 events (Fig. 1). At Shipwreck Ghut, only nine of those events generated runoff for a total of 9.9 mm or 5.7 % of rainfall. Only one event triggered a total of 0.002 mm of runoff at Johnny Horn Ghut and this is only 0.001% of rainfall. Rain-runoff analyses for data spanning back to Nov-13 (Shipwreck) and Aug-14 (Johnny Horn) showed that runoff occurs about twice more frequently at Shipwreck and that it produces roughly 7.2 times more runoff than at Johnny Horn. Johnny Horn is operating almost like an undisturbed site although it contains a number of roads, but Shipwreck’s response is anomalous and is due to a high density of roads. Marine Monitoring. Greater terrigenous sediment accumulation was measured below developed compared to minimally developed watersheds in sediment traps (Fig. 2A). Though statistically significant decreases in sedimentation rates were not detected post restoration due to f resuspension, we observed significant decreases (17-41% decrease) in trap and benthic % clay and %terrigenous sediment post-restoration at all developed shore sites (Fig. 2B). Since there were no significant differences in % clay post-restoration at minimally developed sites in either the trap or the benthic samples we may conclude that this decrease could be an outcome of restoration. Both near shore patch reefs and coral reefs below the developed watersheds were under stress from terrigenous and finegrained sedimentation for over half of the sampling periods (Fig. 2C). Nephelometer turbidity and deposition values averaged over ~26-day sediment trap deployment periods were significantly correlated with sediment trap accumulation rates at most sites. Short-lived radioisotope (SLR) analyses. Activities of 7Be (t1/2=53 days) were present in almost all sediment trap and adjacent bottom samples (Fig. 3), (though higher in the traps than the benthic samples) suggesting active deposition of island-derived sediment within the past year. This indicates that some terrigenous sediment is rapidly transported to the reefs and the outer part of Coral Harbor and beyond (Fig. 5). Monthly time series data show similar temporal patterns in 7Be activities. 7Be activities decreased from August through October, and then increased in November. During the next reporting period these data will be integrated with rainfall and runoff data to further determine linkages among rainfall, runoff, sediment distribution patterns and island-derived sediment residence times within the Coral Bay basin.
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