Observations and measurements of planktonic bioluminescence in and around a milky sea

Abstract

The Indian Ocean and Arabian Sea have been observed to exhibit surface bioluminescent displays unparalleled in intensity and spatial extent. In July 1985, we conducted bioluminescence measurements in the surface waters of the western Arabian Sea during the period of the southwest monsoon and to identify the causative plankton for these displays. While an intense stimulable bioluminescence signal was always present (range: 1 − 5 × 10 8 photons per second per cubic centimeter of turbulently flowing seawater, measured by an onboard underway photometer system), a unique type of bioluminescence display, known as “milky sea”, was observed on the ocean surface for 3 days. Luminous dinoflagellates, Zooplankton, and bacteria were isolated and tested in a shipboard laboratory photometer system for bioluminescent potential. Their light output values, together with abundance of luminous species present in collected plankton samples, indicated a stimulable bioluminescence field superimposed on a milky sea. The stimulable bioluminescence field was dominated by luminescent dinoflagellates, primarily Pyrocystis spp. and Protoperidinium spp. The Zooplankton contribution to the overall light budget was estimated at < 50% for the samples examined and was dominated at times by euphausiid furcilia and Pleuromamma spp. copepods. Luminous larvaceans, ostracods, siphonophore nectophores, and radiolarians were intermittently present in the samples. First observations of bioluminescence were recorded in the calanoid copepod Pleuromamma quadrungulata (Dahl) and the Calycophorae siphonophores Chelophyes contorta (Lens et Van Riemsdijk), Abylopsis tetragona (Otto), and A. eschscholtzi. Phaeocystis colonies glowed continuously and appeared to act as a substratum for the colonizing luminous bacteria Vibrio harveyi. This bacteria is hypothesized to be the source of the luminescent “milky sea.”