Complete zooplankton size spectra re-constructed from « in situ » imaging and Multinet data in the global ocean

Abstract

Plankton size spectra are important indicators of the ecosystem state, as they illustrate the quantity of organisms available for higher marine food web and reflect multiple size-dependent processes. Yet, such measurements are typically biased by the available sampling methods, either disrupting fragile organisms or lacking good resolution (in size and/or time and space). In this study, we combined two of the most common approaches to measure zooplankton Normalized Biomass/Biovolume Size Spectra (NBSS) to calculate a complete zooplankton distribution for organisms larger than 1 mm. The reconstructed NBSS slopes appeared steeper and closer to those measured by the UVP5 (+7.6%) and flatter than those of the Multinet (-20%) particularly in tropics and temperate latitudes. The overall gain in polar biomass was relatively small for reconstructed biomass compared to bulk estimates from Multinet (+0.24 mgC/m3 or +4.25%) and high from the UVP5 (+2.0 mgC/m3 or +53%). In contrast, in the tropical and temperate ecosystems, the gain in biomass was small for UVP5 (+0.67 mgC/m3 or +30.44% and +0.74 mgC/m3 or +19.59% respectively) and high for Multinet (+1.66 mgC/m3 or +136% and +3.4 mgC/m3 or +309% respectively). Given these differences, we suggest here to combine in situ imaging sensors and net data in any comprehensive study exploring key living players in the ocean ecosystem and their contributions to the biological pump.

First release of the Pelagic Size Structure database: Global datasets of marine size spectra obtained from plankton imaging devices

“…This size range is supplemented by UVP and scanner datasets, which include predominantly living microplankton (20-200 µm), mesoplankton (200-2,000 µm) and macroplankton (2,000-20,000 µm). The UVP additionally samples fragile organisms and non-living particles, which are disrupted by the net collection (e.g., Biard et al, 2016;Soviadan et al, 2023). As a result, the UVP-specific spectra were consistently higher than the scanner spectra.…”

“…The latter are key for pooling datasets obtained from multiple imaging devices deployed in spatial and temporal proximity. In some cases, merging imaging datasets integrated over specific depth layers (e.g., net-collected datasets) with profiling or towed datasets is facilitated by simply integrating observations using the lowest sampling resolution (Soviadan et al, 2023); but merging discrete (e.g., surface-only) and integrated observations is 2). Prior to PSSdb, efforts to set guidelines and best practices for obtaining plankton observations with imaging instruments (see Lombard et al, 2019;Neeley et al, 2021) had yet to establish protocols on harmonizing these datasets across platforms, given the large variability between sampling strategies, instrument detection limits, size estimates, organisms targeted, and classification schemes.…”

“…Prior to PSSdb, efforts to set guidelines and best practices for obtaining plankton observations with imaging instruments (see Lombard et al, 2019;Neeley et al, 2021) had yet to establish protocols on harmonizing these datasets across platforms, given the large variability between sampling strategies, instrument detection limits, size estimates, organisms targeted, and classification schemes. We hope to build upon this first data release and recent work from (Soviadan et al, 2023) to provide merged data products, that will effectively span the five orders of magnitude that can be captured by commercially available plankton imagers (Lombard et al, 2019).…”