Trophic ecology of midwater zooplankton along a productivity gradient in the Southeast Pacific

Abstract

The environmental regulation of planktonic food web structure is a key determinant for the efficiency of energy transfer through trophic levels, with direct implications for the amount of energy that is available to top-consumers and deep-sea communities. Yet, the complex trophic interplay between the different components of plankton communities under contrasting environmental conditions remains unresolved. Here, we present a comprehensive field study based on compound-specific isotope analysis of amino acids that aims to explore depth-resolved changes in the trophic strategies of various size-classes of mesozooplankton from distinct habitats across the Southeast Pacific. According to the δ15N values of the so-called source amino acids such as phenylalanine, we first identified a clear shift in nitrogen sources for biological production, from nitrate in the productive upwelling waters of the Humboldt Current System to increased inputs of diazotroph-N within the ultra-oligotrophic South Pacific gyre. These shifts in the productivity and in the source of N at the base of the food web were not accompanied by significant changes in the trophic position (TP) of mesozooplankton, which were mainly omnivores (TP = 2.8 ± 0.3, n = 65) in all ecoregions. However, although the planktonic food web length remained relatively unaltered along the productivity gradient, the microbial contribution to mid-trophic level consumers appeared to be more important at relatively high productivities, likely due to more intense remineralization processes in a late successional stage of the upwelling throughout the coastal-transition zone. Altogether, this research work will contribute to a better understanding of the food web functioning in one of the least explored marine regions of the world’s oceans.

Trophic lengthening triggered by filamentous, N2‐fixing cyanobacteria disrupts pelagic but not benthic food webs in a large estuarine ecosystem

“…In the central Baltic Sea, the trigger for massive microbial systems are FNCs. However, massive microbial systems can also be caused by other factors such as eutrophication (Dähnke et al., 2008 ) or oxygen minimum zones (Fernández‐Urruzola et al., 2023 ).…”

“…In the central Baltic Sea, the trigger for massive microbial systems are FNCs. However, massive microbial systems can also be caused by other factors such as eutrophication(Dähnke et al, 2008) or oxygen minimum zones(Fernández- Urruzola et al, 2023).In flounder, there were no significant differences or trends in the mean TPs between the two areas and the scatter of the individual TPs of flounder was lower than in cod. This suggests that flounder has a narrower diet composition and is decoupled from trophic lengthening in the pelagic food web.…”

Investigating the Plankton Diversity and Physicochemical Properties in a Temple Pond in Tamil Nadu, India: Taxonomic Composition, Seasonal Variations, and Ecological Implications

“…Zooplanktons play a crucial role in the transfer of energy at the secondary level in aquatic food webs, bridging the gap between autotrophs and heterotrophs [3]. Beyond their role in the complex balance of ecosystem components, zooplanktons serve as vital indicators of water quality and contamination, owing to their high sensitivity to environmental changes, Planktons, comprising a heterogeneous group of organisms, actively float and passively drift along the course of water currents in aquatic environments, making them invaluable indicators of environmental health [4].…”

A missing trophic link: Contribution of the microbial loop to the estimation of the trophic position of pelagic consumers

“…Actually, the existence of these "scattered" changes (i.e., differential trophic isotopic fractionation of trophic amino acids) is the basis of the reworking index (ΣV, McCarthy et al 2007), which quantifies the heterotrophic microbial re-synthesis of the original autotrophic amino acid pool (Hannides et al 2013;Doherty et al 2021). A few studies have observed that this differential isotopic fractionation of glutamic acid and alanine in the microbial food web is further traceable in natural assemblages of zooplankton (Decima and Landry 2020;Fern andez-Urruzola et al 2023) and micronektonic fish (Bode et al 2021a,b). Based on these studies, the differential fractionation observed in alanine and glutamic acid through microbial metabolic processes can be used as an indicator of the contribution of bacteria and heterotrophic protists (i.e., number of trophic steps through protistan consumers) to the trophic position of metazoan consumers.…”