Determination of zooplankton absorption spectra and their potential contribution to ocean color

Abstract

Zooplankton are keystone organisms that provide a critical link between primary production and higher-order predators in the marine food web, as well as facilitating the sequestration of carbon within the ocean. In this context, there is considerable interest in the detection of zooplankton swarms from satellite ocean color signals. However, for this to be possible, accurate inherent optical property characterization of key zooplankton groups is first required. In this study, spectral absorption properties of six epipelagic zooplankton groups have been measured using what we believe to be a novel serial addition technique carried out with a Point Source Integrating Cavity Absorption Meter. The measured absorption spectra were used to model the impact of each group on remote sensing reflectance signals and determine a concentration threshold that would generate a distinguishable signal from ocean color data. Results indicate that the spectral shape of absorption did not vary much between species, with most organisms showing a peak at around 480 nm, characteristic of the pigment astaxanthin. Conversely, the magnitude of absorption did vary considerably between species, with larger organisms typically producing stronger absorption signals than smaller species. Thus, detection thresholds also varied for each group measured and were additionally influenced by background constituents within the water column. The calculated concentration thresholds indicate the feasibility of identifying zooplankton from ocean color, but owing to the spectral similarity in absorption properties, knowledge of in situ populations would be required to determine species abundances from satellite signals.