DOI: 10.1594/pangaea.974697
title: The stoichiometric response of a freshwater plankton community to different nutrient pulse scenarios: A comparison across sites and seasons

description: Three identical in-situ mesocosm experiments were conducted in the Swedish SITES AquaNet mesocosm and monitoring network (Urrutia-Cordero et al., 2021) to test the effects of nutrient pulses varying in intensity, frequency, and chronology on a freshwater plankton community. The three experiments were set up to simulate different rainfall scenarios with resulting run-off patterns and compare the effects across sites (two experiments conducted simultaneously at different lakes) and seasons (identical experiments conducted at the same lake in spring and summer). The first experimental run was conducted between the 6th of July and the 12th of August 2022, during the summer season, at Lake Erken and Lake Bolmen. The second experimental run was conducted between the 2nd of May and the 8th of June 2023 (spring) at Lake Erken. The lakes exhibit contrasting characteristics. Lake Erken is a large mesotrophic to eutrophic lake, whereas Lake Bolmen is a smaller oligotrophic lake (Urrutia-Cordero et al. 2021). The mesocosm system at Lake Erken consists of 16 polyethylene cylinders with a depth of 1.5 meters and a diameter of 0.8 meters attached to a floating jetty, whereas the system at Lake Bolmen is identical in equipment but placed out on the lake. Further details about the set-up of the systems, the filling of the mesocosms, installed sensor systems and additional measurements can be found in the experimental protocols: Further details about the set-up of the systems (Langenheder et al. 2024a), the conception of nutrient additions (Langenheder et al. 2024b), and installed sensor systems (Berger et al. 2024). The following experimental treatments consisting of simultaneous nitrogen, phosphorus, and dissolved organic carbon (DOC) additions were applied: (i) daily pulses of the same low intensity, (ii) multiple irregular pulses with variable intensity, (iii) one extreme pulse, and (iv) an unenriched control. The nutrient additions to each of the three nutrient pulse treatments summed up to the same total amount at the end of the simulated rainfall period (20 days) which was followed by a recovery period (17 days). Each of the four treatments was applied with a replication of four, resulting in a total of 16 mesocosms per experiment. Every fourth day, homogenized water samples were taken using a Ruttner water sampler from each mesocosm. The water sample from each mesocosm was then separated by a 105µm mesh into two size classes: seston smaller than 105µm and seston larger than 105µm. The size-fractioned water samples were then 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). Additional filters for biogenic silicate (BSi) have been taken for seston smaller than 105µm (Whatman NC45 membrane filters, cellulose nitrate, 0.45µm, 25mm diameter; CAT no 10401106). In subsequent laboratory analyses, the filters for POC/PON were dried at 60 °C and measured using a CHN analyzer (Flash EA 1112, Thermo Scientific, Walthman, MA, USA). The POP filters 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 BSi filters were oxidized, the samples were swirled and reagents (molybdate reagent, oxalic acid, and ascorbic acid) were added before photometrically measuring them using a microplate reader (same as above) following Grasshoff et al. (1999). The data for seston smaller than 105µm at Lake Erken in summer can be found in Happe et al. (2024).