Golbuu, Y., Victor S., Wolanski, E., Richmond R.H. (2003) Trapping of fine sediment in a semi-enclosed bay, Palau, Micronesia. Estuarine, coastal and shelf science, 57, 941-949.
The purpose of this study was to collect data regarding the high sediment loading in Airai Bay that is smothering corals and creating a habitat shift from coral dominance to algae dominance. Airai Bay is semi-enclosed on the southeast coast of Palau, which is protected from tidal currents and the local winds are not strong enough to re-suspend the settled sediment. Environmental degradation like poor land use practices and destroying mangroves surrounding Airai Bay could be having a major impact of this coral reef system that is very ecologically and economically important.
The researchers developed five stations that were used as testing sites in Airai Bay from the end of the Ngerikiil River to the mouth of the bay. They measured the vertical profile of the water column including salinity, temperature, and suspended sediment concentration at all the stations. At station one the mangrove swamp, they set up sediment traps on the river banks to determine how much sediment was being captured within the mangroves. Rainfall amounts were provided by the National Weather Service and during the time of the study two floods occurred.
At station 1 during river floods, the SSC (suspended sediment concentration) reading showed that the waters were very turbid as they exceeded the nephelometers saturation level. SSC was also higher at rising tide then during low runoff periods showing that sediment is being brought into the bay and settling in the mangroves. Following a river flood the river plume, freshwater exiting the estuary, could be traced though each stations readings. The readings at each station suggested that the sediment was being diluted, by lowering SSC and higher salinity levels. Based on the study’s results only about 2% of sediment is being exported out of the bay.
Significance for Palau:
Airai Bay is accumulating about 98% of the riverine sediment, which is an extremely high sediment loading. This is smothering corals and increasing turbidity which further stresses the system due to low light access and decreasing larval recruitment. These stressors are causing ecologically issues which is now effecting the local fisheries economy. In order for the ecosystem to restore itself the stressors would need to be lowered. The mangroves that are being cleared are exceptional at trapping sediment, lowering the sedimentation rate about 15-30% before the river plume continues throughout the bay. In order for the people of Palau to conserve this coral reef system, mangroves need to be stop being cleared and actually expanded. If the mangrove swamp had a higher land area, it would be able to trap more sediment. The next step in conservation efforts would be to educate and develop better land use practices. Perhaps during the nine-month period of heavy rainfall agricultural fields could be required to have a cover crop to prevent soil erosion. Lowering soil erosion without impacting the agricultural economy would be another important step to conservation and finding a balance between agriculture and fisheries.