DOI: 10.5683/sp3/home4c
title: Metadata for: Ecosystem and mixoplankton responses to pulsed allochthonous inputs to lake mesocosms
title: Métadonnées pour : Réponses des écosystèmes et des mixoplanctons aux apports allochtone pulsés dans les lacs

description: The research objective was to understand ecosystemic responses to different types and schedules of allochthonous inputs, with a focus on the autotrophic and mixotrophic (bacterivory) responses of nanophytoplankton to such events. We conducted a mesocosm experiment in the summer of 2022 in which we added organic matter as pulses, either through insect larvae, tree leaves or a combination of both to in-lake mesocosms (Lac Triton, Station de biologie des Laurentides). A range of response variables was sampled to capture ecosystem metabolism, community composition, and the relative contributions of autotrophic, heterotrophic and mixotrophic activities.
description: At two timepoints in the experiment : at the middle of the experimental period (day 25) and by the end (day 51) we measured: 


Dissolved CO2 (pCO2 in ppm) was measured using an environmental gas monitor (EGM-4; PP Systems, USA) using the headspace method (Hope et al., 1996). 


PhytoPAM II (Heinz Walz GmbH, Germany) to measure photosynthetic capacity of phytoplankton cells using: the maximal electron transport rate of photosystem II (ETRmax in μmol electrons/(m2 · s)) and the photon flux at which light-saturation of photosynthesis occurs (Ik in μmol quanta/(m2 · s)). 


Continuous monitoring of dissolved oxygen (DO in mg/L), temperature and pH (6-hour intervals) with datalogger probes that were submerged throughout the entire experiment.


Heterotrophic bacterial abundance (cells/ µL): Water column samples (3mL) preserved in 10% paraformaldehyde and glutaraldehyde mixture in cryotubes. Enumerated using a Becton, Dickinson and Company Flow Cytometer (BD Biosciences, USA) analyzed to a maximum of 10 000 events. 


Phytoplankton community samples: 125mL of water was preserved in glass bottles with Lugol’s Iodine solution. Later in the lab, 5ml or 10ml of sample was sedimented in chambers such that taxa could be identified and measured using an inverted Nikon microscope at 400x magnification. Volumetric conversions were then used to estimate biovolumes (µm3/µL) used as a proxy for biomass.


Mixotrophic taxa were identified according to literature to estimate the percentage of biomass represented by mixotrophic taxa in each community assessed (Le Noac’h, 2024, J. Plankton Res.).


To estimate the degree of mixotrophy by phagotrophy more directly, we used CARD-FISH (Catalysed reporter deposition - Fluorescence In Situ Hybridization) (Piwosz et al., 2021; Beisner et al., 2019; Kubota., 2013; Medina-Sánchez et al., 2005). From water collected in each mesocosm, 60mL was treated with 37% formaldehyde (1/10th dilution) and then filtered on polycarbonate filters (2um pore size, 25mm filter width, Whatman©) and then folded and frozen in cryotubes at – 20°C until later lab analysis. For CARD-FISH lab analysis, the Eub338 fluorescent probe (Amann et al., 1990) was used to mark bacteria and the Alexa 488 fluorescent probe (Invitrogen©) was used to mark eukaryotic DNA. We assessed the proportion of phagotrophy based on densities with CARD-FISh for Plagioselmis sp. based on observed frequency of bacterial consumption. Two replicates of each treatment were analysed for each timepoint in the experiment for this variable at 400X on an upright microscope using the DAPI fluorescence filter for DNA, GFP for bacteria and Cy5 for autofluorescence from chloroplasts.