Senior Theses 2007
Leslie Wallace (2007). Anthropogenic influence on coastal change: St. John, U. S. Virgin Islands
Faculty Advisor: Gregg Brooks
Sediment cores collected in salt ponds along the coast of St. John, USVI, show that anthropogenic activities have accelerated the rate of coastal evolution. Previous phases of this research indicate that at least some salt ponds form by coral reef growth cutting off open embayments. Over time, ponds fill with island-derived sediments. Rates of sediment accumulation have been shown to increase where anthropogenic activities are present. The objective of this study is to quantify the magnitude of increase in sediment accumulation rates in salt ponds and correlate this to the degree of anthropogenic influence within the associated watershed.
Sediment cores were collected from five salt ponds, chosen primarily on the basis of degree of anthropogenic development within the watershed. Based on short-lived isotope analyses (210Pb, 137 Cs, 7Be for the last 100 years, and 14C for long-term accumulation) we determined accumulation rates using two methods. Linear accumulation rates were calculated based simply on rates of sediment accumulation. Compacted linear accumulation rates take into consideration compaction of sediments over time. Results show that sediment accumulation rates increase by l.4x under moderate anthropogenic influence (Fish Bay). Under high anthropogenic influence, sediment accumulation rates increase by lax. (Coral Bay). These increases may be applied to other salt ponds on St. John to predict the change is sediment accumulation rate and the change in total infill time of salt ponds assuming moderate and high anthropogenic influence. Thus, results show a progressive increase in sediment accumulation rate linked to increasing levels of anthropogenic activities within the associated watershed. Likewise, increasing levels of anthropogenic activities are linked to decreases in total infilling time, which may lead to premature infilling of salt ponds. Once ponds are completely filled, sediments will empty directly into the marine environment, increasing the potential for degradation of nearby marine ecosystems.