Zooplankton Biomass Depletion Event Reveals the Importance of Small Pelagic Fish Top-Down Control in the Western Mediterranean Coastal Waters

Lidia Yebra,Pablo León,Jesús M Mercado,Francisco Gómez-Jakobsen,Pedro Torres,Ana Giráldez,Sébastien Putzeys,Soluna Salles,Estefanía Espejo

doi:10.3389/fmars.2020.608690

Abstract

The influence of hydrochemistry and trophic conditions on the coastal zooplankton community’s biomass and metabolic activities was investigated along the Spanish Mediterranean coastal waters, from Algeciras Bay to Barcelona, from autumn 2011 to autumn 2012. Two hydrographic regions were differentiated: NW Alboran (ALB) and W Mediterranean (MED). Zooplankton metabolism was assessed from measurements of the electron transport system (ETS) and aminoacyl-tRNA synthetases (AARS) activities, as proxies for potential respiration and somatic growth, respectively. Zooplankton showed three to fivefold higher biomass in ALB than in MED during autumn 2011 and spring 2012. However, in autumn 2012, a drastic decrease in biomass standing stock was observed in ALB, with no significant differences between the two regions. This biomass depletion event was not associated with environmental variables, food availability or zooplankton metabolic rates, but coincided with a twofold peak of Sardina pilchardus landings in ALB. A reduced standing stock coupled with high zooplankton growth rates suggests mortality by predation as the main cause for the low zooplankton biomass typically observed in MED, and in ALB during autumn 2012.

Highlights

The Mediterranean Sea is characterized by a high biodiversity compared to its size (Bianchi and Morri, 2000), and is mainly considered an oligotrophic basin (Siokou-Frangou et al, 2010)
Changes in hydrology modifying the mesoscale dynamics in the western Mediterranean waters would affect the distribution and abundance of the zooplankton communities via direct effects on their metabolism and/or by affecting different trophic levels of the food web. Such variations may be responsible for the increased frequency of jellyfish blooms (Bernard et al, 2011; Brotz and Pauly, 2012; Falkenhaug, 2014) or the decrease in the small pelagic fish (SPF) stocks observed in the Alboran and NW Mediterranean (MED) basins (Van Beveren, 2015; Brosset et al, 2016, 2017; Albo-Puigserver et al, 2020), with notable socioeconomic impacts on tourism and fisheries
Based on salinity and temperature, we defined two zones within our study area: ALB and MED. This coastal zonation is consistent with previous studies showing significant differences in plankton productivity between NW and NE Alboran coastal waters (Mercado et al, 2016; Yebra et al, 2017b; Quintanilla et al, 2020)

Summary

Introduction

The Mediterranean Sea is characterized by a high biodiversity compared to its size (Bianchi and Morri, 2000), and is mainly considered an oligotrophic basin (Siokou-Frangou et al, 2010). Several studies have revealed a strong coupling between the composition and metabolism of phytoplankton and zooplankton and the presence of upwelling, eddies, and fronts in the Spanish Mediterranean waters (Gaudy and Youssara, 2003; Alcaraz et al, 2007; Mercado et al, 2014; Yebra et al, 2018) Biotic factors such as prey availability and abundance of predators determine zooplankton population dynamics, modulating their growth and mortality (Lin et al, 2013; Maar et al, 2014). Changes in hydrology modifying the mesoscale dynamics in the western Mediterranean waters would affect the distribution and abundance of the zooplankton communities via direct effects on their metabolism and/or by affecting different trophic levels of the food web. No previous surveys analyze the coastal zooplankton communities across multiple basins of the Mediterranean Sea at once

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Conclusion