Factors Affecting Terrigenous Sedimentation in Coastal Bays with Coral Reefs: Implications for Monitoring the Effectiveness of Watershed Restoration
Date of Award
Thesis: Open Access
MS Marine Science
Environmental and Ocean Sciences
In the Caribbean, sedimentation has been identified as a serious threat to coral reef communities. Although land-based sediment delivery to coastal waters harboring coral reefs occurs under natural conditions, human activities in the watersheds above reefs increases the erosion and delivery of terrigenous sediment to the reefs. Delivery of terrigenous sediment into marine areas below developed watersheds affects sedimentation rates, alters the composition and texture of sediments that are suspended in the water column, and/or sediments that are deposited on the sea floor and on corals.
St. John, U.S. Virgin Islands is an ideal location to study the effects of rainfall and human development on sedimentation on coral reefs. From a management perspective, there is a need on St. John for studies that examine how watershed development and watershed restoration activities affect marine sedimentation. The island is surrounded by fringing coral reefs and over half the area (56%) of the island’s landmass is largely protected from development by the Virgin Islands National Park (VINP) or the Virgin Islands Coral Reef National Monument (CRNM). The presence of minimally developed watersheds within the VINP or the CRNM makes it possible to compare sedimentation in bays below adjacent developed and minimally developed watersheds simultaneously.
Building on previous sedimentation studies by our research group, the objectives of this project were to examine how marine sedimentation varied in eastern St. John as a function of: (a) different rainfall parameters and wave activity over several seasons (4-5), (b) location with respect to shoreline runoff inputs and coral reefs, and (c) degree of watershed development.
Sediment traps (13) were deployed over ~26-day sampling periods in shore and offshore reef sites below developed and minimally developed watersheds over five rainy seasons between 2007-2012. Sediments collected in traps were analyzed to determine: (a) the proportion of terrigenous sediment (%T); (b) total sediment accumulation rate (ΣAR), terrigenous sediment accumulation rate (TAR), and silt accumulation rate (SAR) in mg/cm2/d; and (c) sediment grain size. Rainfall data collected from recording rain gauges in Coral Bay were used to determine mean daily rainfall, mean rainfall intensity, maximum daily rainfall, and an antecedent precipitation index (API) for each ~26-day sampling period. Wave height data were collected from NOAA buoys near St. John or St. Croix (USVI). The relationships between TARs and parameters of rainfall (mean daily rainfall, mean rainfall intensity, and maximum daily rainfall) and wave height (mean, median and maximum) were tested using regression analyses.
The 2007 to 2012 study period included storm events characterized by a wide variety of total rainfall amounts and swell. Tropical Storm Otto during October of 2010 was the greatest rain-producing storm historically. One to two major storms (storms characterized by 100 mm or more of rainfall) occurred each year from 2007 – 2012, all of which occurred between May and December. Based on the cumulative rainfall per storm, Hurricane Earl (9/2010) only ranked 18th but was notable due to the high wind and ocean swells produced.
TAR and %T were normally greater during periods of greater rainfall and varied more with rainfall intensity and maximum daily rainfall than with mean rainfall intensity. At most sites, the greatest TARs were recorded during the sampling period when Tropical Storm Otto occurred. Because of the ephemeral nature of runoff on St. John, terrigenous sediment delivery only occurred during periods when there was enough rainfall to saturate the soil and produce saturation overland flow. Increased wave activity was sometimes associated with high rainfall, thus, sediment resuspension from the seafloor in addition to runoff from rainfall contributed to TAR during some rainfall/storm events. Resuspension caused by waves can lead to the entrapment of both terrigenous and carbonate grains, explaining why wave activity simultaneously increased TAR and lowered %T, even during periods during both runoff and non runoff sampling periods. For example, resuspension during both Tropical Storm Otto (characterized by high runoff) and T.S. Earl (characterized by little to no runoff) produced %Ts lower than the study period means. Some resuspension contributed to sediment accumulated in most traps during periods with moderate to low rainfall and wave activity.
%T and TAR were greatest nearest to the ephemeral stream outfalls, where most terrigenous sediments are presumed to be deposited following delivery to the bay. Because there is greater terrigenous sedimentation near shore and greater carbonate production offshore, mean grain sizes were normally finer near shore compared to at the offshore reef locations, as terrigenous grains are in general finer than carbonate grains.
Consistent with (a) GIS-based modeling and watershed erosion studies at our study sites that predicted 3 to 10 times greater sediment delivery below developed compared with minimally developed watersheds, and (b) previous sediment trap studies, terrigenous sedimentation was on average approximately four times greater below developed compared with minimally developed watersheds. When API (a proxy for soil moisture) was considered in the regression model, rainfall better predicted TAR below the minimally developed but not the developed watershed. High density of exposed and compacted surfaces such as unpaved roads, where sediments are more easily eroded and where compaction of soil favors runoff over infiltration, may have contributed to greater terrigenous sedimentation overall below developed watersheds. Because there is greater terrigenous sediment below developed watersheds, mean grain size was normally (but not always) finer for sediment collected below developed compared with minimally developed watersheds.
Based on the comparisons between our total (ΣAR) and silt accumulation rates (SARs) with published levels related to coral stress, corals near our study locations were most likely subjected to greater stress during (and immediately following) study periods of high rainfall, near shore, and below developed watersheds. SARs exceeding 4 mg/cm2/d were measured approximately twice as frequently (86% of sampling periods) at the shore sites with patch reefs below developed compared to the minimally developed watersheds (42% and 32% of sampling periods). Similarly, SARs exceeding 4 mg/cm2/d more commonly occurred at the offshore reef below the developed watershed (40% of sampling periods) than below the minimally developed watershed (11% of sampling periods).
As building and human development continues along tropical and subtropical coastlines, the input of terrigenous sediment is likely to increase on coral reefs. The outcomes of this study have shown that sediment traps are an effective way to monitor general temporal and spatial patterns in terrigenous sedimentation. This study is the first marine sediment trap study to capture the natural variability in storms, rainfall, and wave activity over a study period longer than two years that also monitored distinct near shore and offshore areas below both developed and minimally developed watersheds simultaneously. Monitoring simultaneously across distinct areas over a long, 5-year study period made it possible to examine the relationship between terrigenous sedimentation, rainfall and wave activity statistically, and identify areas most likely to be subjected to the greatest sediment-related coral stress. The results of this study have generated potentially useful data related to watershed land management in tropical, coastal areas and will help inform future studies that will assess the effect of watershed restoration efforts on terrigenous sediment delivery to marine areas harboring coral reefs.
Digital USD Citation
Sears, Whitney, "Factors Affecting Terrigenous Sedimentation in Coastal Bays with Coral Reefs: Implications for Monitoring the Effectiveness of Watershed Restoration" (2015). Theses. 7.
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