Soutenance mémoire
Allochthony in zooplankton biomass
The role of t-OM assimilation in aquatic food webs has been studied for a long time, particularly by stream ecologists who studied the role of allochthonous food sources for invertebrates and fish (Jones 1949, 1950, Teal 1957, Fisher and Likens 1972, Petersen and Cummins 1974). However, the notion of “allochthony” defined as the terrestrial contribution in aquatic biomass, has been used in literature only very recently. Researching the keyword “allochthony” in the peer reviewed literature of the Scopus® database, within “Agricultural and Biological Sciences” and “Environmental Sciences” disciplines, 140 articles were referenced from 1983 to 2016. The number of articles using “allochthony” rose very recently in 2007 and peaked in 2009 (16 references) reflecting a recent and close interest of scientific community in the last years.Allochthony in the biomass of aquatic organisms is almost exclusively calculated from stable-isotope (SI) ratios. The natural occurrence of SI is now widely used in ecology in particular for tracing the fluxes of organic matter such as allochthonous inputs in aquatic biomass within lake ecosystems (del Giorgio and France 1996, Jones et al. 1998, Cole et al. 2002, Carpenter et al. 2005, Rautio and Vincent 2006, Pace et al. 2007, Taipale et al. 2009, Berggren 2010, Berggren et al. 2015). The most widely used SI ratio in trophic studies is the relative abundance of 13C over 12C due to the C-based compounds that characterize organic matter molecules and the relatively high abundance of 13C compared to 12C in ecosystems. This 13C/12C ratio in animal biomass assimilated into their tissues closely reflects the ratio absorbed from their diet when applying a known fractionation (Post 2002, Fry 2006).
Zooplankton reliance on terrestrial organic matter at the ecosystem scale
Allochthony is often presented in zooplankton because of the strategical position that these organisms occupy in the aquatic food web. The ultimate goal is to understand the interactions between terrestrial and aquatic ecosystems or to quantify the aquatic reliance on terrestrial ecosystems and, as such, ecosystem-scale conclusions are often drawn from allochthony in zooplankton. However, the measured allochthony in zooplankton is the result of multiple processes occurring at different levels of the food web that reflects only a part of the extent of the aquatic reliance on the surrounding terrestrial ecosystems which is practically never quantified for the whole lake ecosystem. This reliance on terrestrial ecosystems depends on: allochthonous and autochthonous fluxes coming into the lake,differential metabolism allocation of t-OM by individuals and the importance of biomass and production at the ecosystem level.
From allochthonous and autochthonous sources to zooplankton
Terrestrial organic matter can overwhelm the environment in aquatic ecosystems and represent more than 90% of the DOC as well as more than half of the POC in boreal and temperate lakes (Meili 1992, Wilkinson et al. 2013b). DOC represents much larger inputs with a usual DOC:POC ratio between 6:1 and 10:1 (Wetzel 1995). This is particularly true when an aquatic ecosystem is surrounded by a coniferous forest producing massive humic inputs from the catchment basin where terrestrial dissolved organic carbon (t-DOC) is dominated by fulvic acids of relatively low molecular weight that can heavily subsidize bacterial production (Hessen 1992, Berggren et al.2010b). Bacterial biomass can then be composed of a large proportion of t-OC (Guillemette et al. 2016) and be consumed by heterotrophic or mixotrophic protists such as ciliates and flagellates (Martin-Creuzburg et al. 2005). Zooplankton predation of these organisms may cause them to inherit of the same degree of allochthony. This is currently believed to be the major flux of terrestrial organic matter in pelagic aquatic food web.
Differential terrestrial organic matter allocation by zooplankton
Once the food is ingested by zooplankton, including t-OM, it is differentially allocated by the individual in growth, in respiration, in lipid reserve accumulation, in reproduction or in excretion. However, due to technical issues, many studies focusing on allochthony can be considered as presenting the summary of these differential t-OM allocations. For example, a specific degree of allochthony for zooplankton may be the result of the amount of autochthonous versus allochthonous material consumed, the allochthony values of its prey, the allochthonous molecules stored in the lipid reserves, the amount of t-OM used for biomass synthesis and the t-OM respired and excreted by the individual. Zooplankton net production is characterized by biomass synthesis, i.e. individual growth and reproduction (Runge and Roff 2000).
Allochthony estimates indicate what is present in consumer biomass in fine and do not inform on the t-OM that has been respired by the organisms or transferred to the eggs. These differential allocations have never been tested in zooplankton but have begun to be explored for bacterial communities with a recent study discovering that bacteria preferentially retained t-OM in biomass (Guillemette et al. 2016), while algal C was used for bacterial metabolism and respiration. It is very likely that, as with bacteria, zooplankton utilize molecules from different origins preferentially for growth (biomass synthesis) or respiration (metabolism).
Zooplankton production
Counting and identification: Each individual of the zooplankton community sample was identified using Utermöhl chambers with an inverted microscope (Zeiss Axio Observer A1, x100), according to taxonomy guides from Edmondson (1959) and Czaika (1982). Every zooplankton samples (Ntot=71) were entirely identified except when the density was too high. In such samples, half or quarter of the abundance was counted after dividing the sample with a Folsom’s sample divider. A mean of 392 zooplankton individuals were identified per sample (0.95 confidence limits = 14.8% ; Postel et al. (2000)) assuring a minimum of 100 individuals counted and identified, except for seven samples with very low densities where about 60 individuals have been counted. Nauplii were pooled into two groups according to stages 1 to 3 (NI-III) and stages 4-6 (NIV-VI), whereas copepodites were identified to six stages from C1 to C6. Cladocerans were identified to genus and newly hatched individuals were classified as juveniles. Eggs from all species were counted as well.
Length – dry weight regression: Mean dry weights (DW) for all species and stages were estimated with length-DW regressions. Individuals were measured with an optical camera (AxioCam ERC 5S) and microscope software (AxioVision). Individual nauplii, copepodites and adult biomasses from copepod and cladoceran species were calculated from length-DW regressions and were compared to direct adult weight measurements. Weighing every stage of every species was impossible, but we verified the length-weight relationships with measurements for the adult stages of the four most abundant species of the community once a month. Adults were picked, counted (about 200 individuals), freeze-dried and directly weighed using a Mettler Toledo microbalance (XP26 DeltaRange).
Environmental and food web variables
Lake water inflow was measured from the main and secondary lake inflows (m3 s-1). Allochthonous C inputs in the lake were estimated from the lake water inflow multiplied with the water concentration of DOC from these two main inflows assuming that riverine DOC was entirely allochthonous. Phytoplankton biomass and production were estimated from chlorophyll-a and O2 concentration, respectively. Chlorophyll-a concentration was measured by fluorescence following Yentsch and Menzel (1963b). Gross primary production (GPP) was calculated using continuous (hourly) measurements and diurnal changes of dissolved oxygen (O2) concentrations in surface water as in Vachon and del Giorgio (2014). This method includes GPP measurement from phytoplankton, benthic algae and macrophytes. Bacterial production (BP) was measured following the [3H]-leucine method incorporation of Kirchman (1993). Triplicate aliquots of 1.5 mL water samples were exposed to 40 nM [3H]-leucine during 1 h. Average blank-corrected rates of leucine uptake were converted to rates of C production assuming the standard conversion factor of 1.55 kg C mol leu-1 multiplied with an isotopic dilution factor of 2. Bacteria were incubated at the constant 20°C to exclude the effect of temperature to BP (Adams et al. 2010)
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Table des matières
INTRODUCTION
Statement of the problem
State of the science
Allochthony in zooplankton biomass
Zooplankton reliance on terrestrial organic matter at the ecosystem scale
Objectives and hypotheses
Methodological approach and study site
Thesis structure
CHAPTER I :SEASONAL VARIABILITY OF ZOOPLANKTON PRODUCTION SUPPORTED BY TERRESTRIAL ORGANIC MATTER AND DRIVING FACTORS
Abstract
1.1 Introduction
1.2 Methods
1.2.1 Study site
1.2.2 Sampling and continuous measurements
1.2.3 Zooplankton production
1.2.4 Stable-isotope analyses and allochthony
1.2.5 Zooplankton production based on terrestrial source i.e. allotrophy
1.2.6 Environmental and food web variables
1.2.7 Statistical analyses
1.3 Results
1.3.1 Total zooplankton production
1.3.2 Stable isotopes and allochthony
1.3.3 Allotrophy
1.3.4 Environmental and food web variables
1.3.5 Multiple linear regressions
1.4 Discussion
1.4.1 Estimation of zooplankton production and influencing factors
1.4.2 Seasonal pattern of zooplankton allochthony
1.4.3 Zooplankton allotrophy and terrestrial C inputs
1.5 Acknowledgements
1.6 References
1.7 Supporting information
CHAPTER II :SEASONAL PATTERN OF ZOOPLANKTON LIPID RESERVES AND WINTER LIFE STRATEGIES
Abstract
2.1 Introduction
2.2 Methods
2.2.1 Study site and zooplankton community
2.2.2 Survival experiment
2.2.3 Water and zooplankton sampling
2.2.4 Fatty acid analyses
2.2.5 Statistical analyses
2.3 Results
2.3.1 Seasonal abundances in zooplankton community
2.3.2 Starvation experiment
2.3.3 Seasonality in water chemistry and putative food sources
2.3.4 Total lipids and FA composition in zooplankton
2.4 Discussion
2.5 Acknowledgements
2.6 References
2.7 Supporting information
CHAPTER III :SPATIAL DISTRIBUTION OF ZOOPLANKTON ALLOCHTHONY WITHIN A LAKE
Abstract
3.1 Introduction
3.2 Material and methods
3.2.1 Study lake and sampling
3.2.2 Characterization of resource heterogeneity
3.2.3 Stable-isotope analyses
3.2.4 Isotope mixing model
3.2.5 Statistical analysis
3.3 Results
3.3.1 Contribution of autochthonous and allochthonous sources to lake resource pool
3.3.2 Spatial heterogeneity in the putative zooplankton resource pool
3.3.3 Spatial distribution of allochthony in L. minutus
3.4 Discussion
3.4.1 Spatial heterogeneity of C resources
3.4.2 Spatial variability in putative allochthony
3.5 Acknowledgements
3.6 References
3.7 Supporting information
GENERAL DISCUSSION
Seasonal variability of allochthony
Allocation of terrestrial organic carbon in lipids
Combining stable isotopes and fatty acids
Driving factors of allochthony
Terrestrial organic matter
Autochthonous primary production
Zooplankton life strategy
Upscaling allochthony at the ecosystem level
CONCLUSIONS
REFERENCES
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