Bioluminescence of the tropical Indian Ocean: a multiple‐scale variation

Abstract

Large‐scale surveys represented by 5800 bathymetric casts in the western Indian Ocean (0–22oN, 54–58oE), elucidated the 10‐fold variation of the bioluminescent potential (BP) in the upper mixed layer, during the winter (north‐east) monsoon season. The mesoscale survey in February 2017 consisted of 26 drift stations (4oN–3oS, 65–68oE) on which 5–10 bathymetric casts were deployed down to 60 m. The maximal BP was associated with the periphery of a cyclonic eddy. The two‐fold to three‐fold variation of BP characterized the spatial heterogeneity modulated by a detected eddy. High‐frequency casts on drift stations resembled the fine‐scale heterogeneity in which the three‐fold variation was observed within the BP maximum at a 37 ± 13 m depth. The latter one was located above the deep chlorophyll maximum at a 80 m depth. A general decline of the BP variance from the large scale through mesoscale to fine scale, fits that of the zooplankton biomass.

Bioluminescent eddies of the World Ocean

“…For example, dinoflagellates drive the BP in many regions of the Mediterranean basin, [4] whereas zooplankton (small-sized copepods and chaetognaths) are major contributors in tropical waters of the Atlantic and Indian Oceans. [2][3][4] On the other hand, mesoscale eddies redistribute the plankton biomass and induce its patchy distribution across eddy fields. [23,24] Through the turbulent diffusivity, the mean EKE moderates the BP spatial heterogeneity along the vertical component of the field.…”

“…Black dots stand for oceanographic stations. [ 3 ] (c) Vertical distribution of BP (10 −12 W cm −2 L −1 .) and temperature (°C, right panel) across the Indian Ocean western transect (65°E), on February 20–23, 2017.…”

“…The historical database on bathy-photometric surveys carried out by the Former Soviet Union and (later on) by the Russian Academy of Sciences was employed (https://www.bodc.ac.uk/resources/ inventories/edmed/report/1586/), in order to seek for the expeditions (with oceanographic station grids) resembling the mesoscale variability. Initially, we dealt with the data potential represented by 24,000 vertical profiles of bioluminescence intensity which contributed to a published annual World Ocean map [3,14] based on 71 expeditions deployed in 1966-2022 (Figure 1, Table gov/LAS) designed to provide access to geo-referenced data and complemented by CTD profiles from the aforementioned expeditions. The total kinetic energy is separated into the mean kinetic energy of the flow and EKE.…”

“…In terms of the spatial distribution of bioluminescent plankton, the studies focused on mechanically stimulated bioluminescence which gained intensity over years and enabled the basin scale spatial patterns to be elucidated. [2][3][4] Less research has been focused on mesoscale variability. The latter one is gradually important because the kinetic energy of geostrophic currents of the World Ocean is channeled predominantly through the mesoscale variability in which cyclonic and anticyclonic eddies with sea surface height anomalies of up to 25 cm are characteristic events.…”

“…(b) Sea surface temperature ( C). Black dots stand for oceanographic stations [3]. (c) Vertical distribution of BP (10 À12 W cm À2 L À1 .)…”