From bacteria to piscivorous fish: estimates of whole-lake and component-specific metabolism with an ecosystem approach.

Fabien Cremona,Alo Laas,Toomas Kõiv,Ain Järvalt,Veljo Kisand,Peeter Nõges,Tiina Nõges,Priit Zingel,Tõnu Feldmann,Helen Agasild

doi:10.1371/journal.pone.0101845

Fabien Cremona, Alo Laas + Show 8 more

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https://doi.org/10.1371/journal.pone.0101845

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Abstract

The influence of functional group specific production and respiration patterns on a lake's metabolic balance remains poorly investigated to date compared to whole-system estimates of metabolism. We employed a summed component ecosystem approach for assessing lake-wide and functional group-specific metabolism (gross primary production (GPP) and respiration (R)) in shallow and eutrophic Lake Võrtsjärv in central Estonia during three years. Eleven functional groups were considered: piscivorous and benthivorous fish; phyto-, bacterio-, proto- and metazooplankton; benthic macroinvertebrates, bacteria and ciliates; macrophytes and their associated epiphytes. Metabolism of these groups was assessed by allometric equations coupled with daily records of temperature and hydrology of the lake and measurements of food web functional groups biomass. Results revealed that heterotrophy dominated most of the year, with a short autotrophic period observed in late spring. Most of the metabolism of the lake could be attributed to planktonic functional groups, with phytoplankton contributing the highest share (90% of GPP and 43% of R). A surge of protozooplankton and bacterioplankton populations forming the microbial loop caused the shift from auto- to heterotrophy in midsummer. Conversely, the benthic functional groups had overall a very small contribution to lake metabolism. We validated our ecosystem approach by comparing the GPP and R with those calculated from O2 measurements in the lake. Our findings are also in line with earlier productivity studies made with 14C or chlorophyll a (chl-a) based equations. Ideally, the ecosystem approach should be combined with diel O2 approach for investigating critical periods of metabolism shifts caused by dynamics in food-web processes.

Highlights

The main processes governing aquatic ecosystem metabolism are primary production and respiration
For assessing the metabolism of lakes, the most popular approach in the last decades has been calculating Gross primary production (GPP) and R from measured integrated CO2 and/or O2 fluxes in the surface waters ([4], [5], [6]). These fluxes are supposed to represent the contribution of primary production and respiration of photoautotrophic functional groups and respiration of heterotrophic functional groups to the whole net ecosystem production (NEP)
The ecosystem approach pointed out the high share of phytoplankton in the whole-lake GPP and R, the importance of microbial planktonic consumers in the metabolism, and the low importance of benthic consumers in C emissions in shallow and turbid lakes

Summary

Introduction

The main processes governing aquatic ecosystem metabolism are primary production and respiration. For assessing the metabolism of lakes, the most popular approach in the last decades has been calculating GPP and R from measured integrated CO2 and/or O2 fluxes in the surface waters ([4], [5], [6]). These fluxes are supposed to represent the contribution of primary production and respiration of photoautotrophic functional groups (phytoplankton, phototrophic bacteria, periphyton and macrophytes) and respiration of heterotrophic functional groups (zooplankton, zoobenthos, heterotrophic bacteria, fish) to the whole NEP

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