The stoichiometric response of a freshwater plankton community to different rainfall scenarios

To investigate the impact of varying rainfall and run-off conditions on a freshwater plankton community, we conducted an in-situ mesocosm experiment at Lake Erken in Sweden. Three distinct sets of nutrient pulses, differing in frequency, intensity, and chronology, were selected as experimental scenarios. Lake Erken is a large mesotrophic to eutrophic lake and part of the Swedish SITES AquaNet mesocosm and monitoring network (Urrutia-Cordero et al. 2021). The mesocosm system at the Erken Laboratory consists of 16 polyethylene cylinders with a depth of 1.5 meters and a diameter of 0.8 meters attached to a floating platform. On the morning of the 6th of July 2022, the mesocosms were carefully filled with lake water from 1.5 m distance from the edge of the platform by using a gasoline pump (3HP, MecTools). The experimental manipulations started one day after filling the mesocosms. The experimental treatments differed in the frequency/intensity (from one extreme pulse to small daily pulses) and chronology (regular or irregular temporal pattern) of the nitrate, phosphate and dissolved organic carbon (DOC) pulses and encompassed the following treatments: (i) regular daily pulses, (ii) multiple irregular pulses with variable amounts, (iii) one extreme pulse and (vi) an unenriched control. The three nutrient pulse treatments received the same total amount of additions at the end of the simulated rainfall period (20 days) which was followed by a 17-day recovery period. Each treatment was run with a replication of 4. Homogenized water samples for size-fractionated stoichiometric analyses have been taken from each mesocosm using a Ruttner water sampler every fourth day which resulted in 10 data points (6 in the simulated rainfall period and 4 in the recovery period). The water sample from each mesocosm was separated into two size classes (<105 µm and >105 µm) and filtered onto one filter for particulate organic carbon and nitrogen (POC/PON) and one filter for particulate organic phosphorus (POP), respectively, using pre-combusted and acid-washed 0.45 µm glass microfiber filters (Whatman, GF/C). Additionally, filters for biogenic silicate (BSi) have been taken for the smaller size fraction (Whatman NC45 membrane filters, cellulose nitrate, 0.45 µm, 25 mm diameter; CAT no 10401106). To separate between the stoichiometric response of zooplankton and larger phytoplankton within the >105 µm size fraction, individual Cladocerans (as unselective filter-feeders) and copepods (as selective filter feeders) have been picked from an additional water sample at the start, mid and end of the experiment. Depending on the availability within the water samples, 5-10 Cladocerans and 10 copepods were placed in small tin capsules for the analysis of POC/PON and POP, respectively, for each mesocosm. The filters and tin capsules for the POC/PON analysis were dried at 60 °C and then measured using a CHN analyzer (Flash EA 1112, Thermo Scientific, Walthman, MA, USA). The filters and tin capsules for the POP analysis were pre-combusted and analyzed by molybdate reaction after potassium peroxydisulfate digestion (Wetzel and Likens 2000) and photometrically analyzed using a microplate reader (SYNERGY H1, BioTek®). The POP analysis of the picked zooplankton was unsuccessful, therefore the zooplankton POP data is missing from the data set. The filters for BSi were oxidized and autoclaved for 30 minutes using plastic tubes (Sarstedt). After cooling down to room temperature, the samples were swirled and reagents (molybdate reagent, oxalic acid and ascorbic acid) were added before photometrically measuring them in a plate reader (SYNERGY H1, BioTek®) at 810 nm using 48-well plates after 1.5 hours of reaction time (Grasshoff et al. 1999). The absorption of each sample was converted into silicate concentrations via a silicate standard curve (based on 1.09947.0001 Merck; 1000 mg Si/ampoule, SiCl₄ in 14% NaOH) which was treated similarly to the samples.