Fine-scale variability in phytoplankton community structure and inherent optical properties measured from an autonomous underwater vehicle

The relationship between inherent optical properties (IOPs), phytoplankton community structure and the abundance of suspended particles in the size range 3–500 μm was studied near the Isles of Scilly (UK) in May 2000. Autosub, an autonomous submersible vehicle specifically designed for science missions, was used as an instrument-positioning platform. It carried a CTD system, an ac-9+ dual tube spectrophotometer, a prototype submersible flow cytometer and an Aqua-monitor water sampler. The vehicle made a 10-km transect at constant depth across a boundary between water masses with contrasting remote sensing reflectance, which was located using SeaWiFs ocean colour imagery. This boundary corresponded to a sharp (1 km) transition between one phytoplankton community consisting of coccolithophores, flagellates and dinoflagellates, and a second community dominated by diatoms and flagellates. Inherent optical properties measured along the autonomous underwater vehicle (AUV) track showed marked changes in magnitudes, ratios, spectral shapes and fine-scale spatial variability. These changes were well correlated with variations in the composition of the suspended particle assemblage measured by microscopy and in situ flow cytometry.