Physical dynamics of the reef flat, channel, and fore reef areas of a fringing reef embayment: An oceanographic study of Pago Bay, Guam

Abstract

Long-term observations of oceanographic patterns and processes provide necessary context for integrative ecological studies and for assessing and mitigating anthropogenic impacts to coastal ecosystems. The oceanographic patterns and processes of Pago Bay, Guam, a tropical coral reef system with a small estuary, were observed for one year. An array of 50 sensors including current profilers, temperature and dissolved oxygen loggers, and water quality sensors were deployed throughout Pago Bay from 0–26 m depth. Water flow was primarily driven by wave height in the reef flat and channel. In addition, wind and wave directions that were directly across-shore contributed to faster flow speeds throughout the bay. On the fore reef, wave height was the strongest predictor of current speed, but wind direction had a strong influence on current direction. Flow in the channel and on the reef flat was more tightly tied to environmental factors of wind and waves than flow on the fore reef. Extreme daily temperature variations on the reef flat were observed. In deeper water, cold pulses were observed, indicating that large internal waves may propagate around the slopes of Guam. Cold pulses may deliver nutrients to shallower ecosystems and periodically cool deeper areas of the fore reef. In response to heavy rain events, the estimated backscatter in the channel increased sharply along with significant changes in other water quality parameters. Rain events of this magnitude and associated backscatter increases occur frequently. Along with soil-destabilizing land use practices in the Pago River watershed, the current flow and water quality observations presented here indicate that sedimentation may be a risk for the Pago Bay fore reef coral habitat. The understanding of circulation, thermal environment, and rainstorm impacts in this fringing reef system provides a baseline for understanding the ecology, coral reef resilience, and future oceanographic changes in Pago Bay.