Paleoenvironment changes in Juag Lagoon, Philippines based on sedimentology, bulk geochemistry and stable isotopes and their implication to nutrification

摘要

In recent decades, eutrophication has been identified as a major problem in coastal and estuarine systems. Eutrophication can be due to cultural activities but can also be due to natural factors. This study explores the possible role of geomorphic change in the eutrophication history of Juag Lagoon, which is one of the sites with harmful algal blooms in the Philippines. Surface and sediment cores were acquired for this study to trace the paleoenvironmental changes that brought about the present day lagoon. Radiocarbon dates and ~(210)Pb-based estimates of sedimentation rates provided age control. Sedimentological indicators and stable isotope signatures of organic matter were utilized to assess changes in the source of organic matter deposited in the lagoon whereas XRF-derived bulk sediment geochemistry was used as proxy for sediment input. Three phases of geomorphic state were identified over the lagoon history. Prior to 700 A.D., bulk geochemistry and isotopic measurements indicate a pre-lagoon environment. Subsequent drop in sea level led to the development of the lagoon, which persisted until 1100 A.D. The lagoonal condition transitioned to increased marine influence towards 1200 A.D., through several sandbar-breaching events that may have persisted in recent years. The present day lagoon maintains a continuous water exchange through a small tidal channel. The closing of the lagoon and the limited flushing of lagoon waters could have led to the nutrification of the lagoon. Moreover, anthropogenic impact through forest denudation, mariculture and fertilizer use in the neighboring islands is reflected, which could have also affected the nutrification of the lagoon in the last 100 years.