Light scattering and Secchi depth in an oligotrophic lake: Insights from an individual particle analysis technique

Abstract

Light-scattering attributes of particles of the upper waters of oligotrophic Skaneateles Lake, New York, were characterized monthly for the spring - fall interval of 2 years with scanning electron microscopy interfaced with automated image and X-ray analyses (SAX). SAX results were used to make forward estimates of the particulate scattering coefficients associated with 2 components (bx) of the overall particle population, minerogenic particles (bm) and diatoms (bdi), through Mie theory calculations. Estimates of the third component, scattering by organic particles other than diatoms (bo/nd), were calculated as the residual of bulk measurements of the particulate scattering coefficient (bp) and the summation of bm and bdi. Regulation of Secchi depth (SD) in the lake by bp was documented, in the form of a strong (R2 = 0.73) linear SD-1 - bp relationship. Variations in the summation (bm + bdi) explained 66% of the dynamics of bp and 56% for SD-1. A 2-component partitioning of bp that incorporates the SAX-Mie-based values of bm, and estimates of the contribution of all organic particulates with an empirical chlorophyll-a-based bio-optical model, failed to predict the bp dynamics but closed well on average for the entire study period. The SD-1 - bp relationship and the reported partitioning of bp were used to predict: (1) changes in bx from the study conditions that are consistent with a robust range of historic observations of SD, and (2) scenarios of management interest of levels of SD expected from hypothetical changes in bx.