Time series of the vertical distribution of particles during and after a spring phytoplankton bloom in a coastal basin

Abstract

Bottom nepheloid layer formation was observed through a time series study of the vertical distribution of suspended particles conducted during and after a phytoplankton bloom in a small coastal basin, Bedford Basin, Nova Scotia, Canada. In this study, we measured the suspended particle distribution over a 6 month period using a backward-recording (30°) light-scattering meter. Although the profiles of the particle distribution reflected episodic events such as fresh water river input and storm mixing, through the period of the study three general stages were observed. During the peak of the spring bloom in March, the water column was characterized by a high particle load in the surface mixed layer (0–8 m) with a smaller but uniform particle load in the middle and bottom water. The second stage (mid-April–July) featured bottom nepheloid layer formation (45–65 m) and a high particle load in the surface water with minimum light-scatter at mid-depth. In late July–August and bottom nephloid layer thinned and the mid-water became progressively clearer. The light-scatter measurements revealed a patchiness in the vertical particle distribution. We collected water samples from some of the depths corresponding to light-scatter peaks for measurements of particulate organic carbon (POC) concentration and mass-volume concentration of suspended matter (SPM), and for microscope observation to identify particle characteristics. These observations demonstrated a change of particle characteristics with depth as well as with time. As a bottom nepheloid layer developed, particles in this layer changed from predominantly intact diatom chain assemblages with high POC concentrations to small and degraded aggregates with low POC concentration.