Seasonal variability in non-consumptive mortality of Arctic zooplankton.

Malin Daase,Janne E Søreide,Marja Koski

doi:10.1093/plankt/fbab042

Malin Daase, Janne E Søreide + Show 1 more

Open Access

https://doi.org/10.1093/plankt/fbab042

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Abstract

Recent observations from high-latitude marine ecosystems indicate that non-consumptive mortality may be particularly high in Arctic zooplankton during the polar night. Here we have estimated the contribution of dead organisms to the mesozooplankton community in the high Arctic (Svalbard 78–81oN) during the polar night (January), in spring (May) and in late summer (end of August). To identify in situ dead organisms, we used Neutral Red Stain. The dead zooplankton fraction consisted mainly of copepods, while the contribution of dead non-copepods was low in all seasons. The absolute abundance of dead copepods varied little between seasons; however, the relative contribution of dead copepods was highest in January with 11–35% of the copepods classified as dead, in contrast to 2–12% in spring and summer. Furthermore, there were species-specific differences: copepods of the genus Calanus contributed more to the dead fraction of the copepod community during the polar night compared to spring and summer, leading to a higher “dead” biomass in winter. We conclude that non-consumptive winter mortality is considerable in calanoid copepods in the Arctic and an important but so far neglected component of the passive carbon flux, providing carbon in larger portions for higher trophic level consumers during the low-productive winter.

Highlights

Not all zooplankton are alive in the natural environment, but the contribution of dead individuals to zooplankton populations is normally not accounted for
In August, the water column was strongly stratified with warm (4–6◦C) and less saline surface waters (33–34.2) with transformed Atlantic waters or intermediate waters found below the surface layers in most location, except for BF and the deeper layers in RF where Arctic waters prevailed
The total abundance of copepod carcasses was not higher during the polar night compared to spring and summer, but due to the high proportion of relatively large dead Calanus specimens in January, the dead copepod biomass was overall higher during the polar night than during spring and summer when the abundance of dead Calanus spp. was low

Summary

Introduction

Not all zooplankton are alive in the natural environment, but the contribution of dead individuals to zooplankton populations is normally not accounted for. Neglecting the contribution of dead organisms can lead to erroneous estimations of abundances, biomass and productivity and may bias our understanding of population dynamics and energy fluxes in aquatic ecosystems (Elliott and Tang, 2011a; Frangoulis et al, 2011; Jónasdóttir et al, 2019). Zooplankton carcasses may play an important role in aquatic food webs and the carbon cycle They provide a substrate for bacteria, may promote nutrient retention within the water column (Tang et al, 2006b; Tang et al, 2009; Dubovskaya et al, 2015) and contribute to the formation of macroaggregates and to the detrital pool of aquatic systems (Simon et al, 2002; Isinibilir et al, 2011), and they may serve as nutritious food for benthic organisms (e.g. Søreide et al, 2013; Jónasdóttir et al, 2015). Marine snow, fecal material produced by zooplankton and fish (Turner, 2015; Saba et al, 2021), zooplankton carcasses are likely an important but rarely quantified source to the passive carbon flux (Sampei et al, 2009b; Sampei et al, 2012)

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