Determining factors that influence Turbidity in Kaneohe Bay, Oahu

Abstract

In recent years, much attention has been given to suspended particulate matter (SPM) concentrations, or suspended load, due to its potential impact on reef ecosystems. Solids are transported through the ocean and can have both detrimental (through sedimentation) and beneficial (supplying nutrients) effects on a coral reef community. This project seeks to identify the marine physical mechanisms that influence SPM concentrations in the reef environment of Kaneohe Bay. An acoustic Doppler current profiler (ADCP) was used to measure a number of water properties over the coral reef that extends across the mouth of Kaneohe Bay. The benefit of the ADCP is that it allows researchers to not only measure water current velocities throughout the entire water column, but it also enables this sampling to occur over extended periods of time. Sampling occurred during three separate deployments. The third ADCP deployment occurred in conjunction with field sampling with a Niskin bottle in hopes of calibrating measured ADCP values of acoustic backscatter with SPM concentration from filtered samples. Analysis of the ADCP data showed significant variation in the echo intensity (EI) signal, which can be used as an analogue for turbidity, between the ADCP sites. Current speed and wave height exhibited a covariant relationship with EI data, with higher EI values typically associated with higher waves and faster currents. Surprisingly, Sites 0-2 exhibited a diurnal cycle in EI levels, suggesting that marine biota may constitute a significant portion of the scatterers responsible for the EI data. It was not possible to confirm this hypothesis however, due to minimal data regarding the actual composition of the SSC at the time of deployments. The strongest relationship was found to be the EI response to vertical current velocities, themselves exhibiting a diurnal cycle. This relationship was not present at Site 3. The mechanism for vertical currents appears to be related to the incoming tide. There was a poor correlation between EI, as measured by the ADCP, and the filtered water samples from the third experiment. This was likely due to high spatial variations in reef characteristics and the inherent difficulty in measuring relatively small changes in suspended load.