Phytoplankton nutrient addition and grazer experiment

Dataset belonging to: Vonnahme et al., Contrasting roles of grazing and nutrient availability in shaping phytoplankton functional groups across temporal scales in Arctic fjords, submitted to Journal of Phycology Three sets of experiments were performed 1) Seasonal nutrient addition experiments at two stations in Nuup Kangerlua, 2) Spring nutrients addition and grazing experiments in Isfjorden, 3) Spring nutrient addition and grazing experiments in Nuup Kangerlua. Natural water samples from all sites were enriched with phosphate, silicate, and nitrate representing maximum winter concentrations and incubated in situ (Svalbard experiments) or in the laboratory under in situ temperature and constant light (Greenland experiments) for 1-2 weeks. In addition, the effect of Cirripedia larvae grazing was tested. In Nuup Kangerlua, a grazing treatment was created by adding a fixed number of Cirripedia larvae to a pre-filtered water sample, while in Isfjorden, the effect of grazing was tested by comparing natural water (with very abundant grazers) to a prefiltered sample (without grazers). In Nuup Kangerlua, water samples were taken from 5 m depth at NK-o and NK-i using a Niskin bottle in May (28th), June (13th and 16th), and July (6th) 2022, representing the spring bloom and summer period, respectively. Samples were incubated in triplicates in 50 mL cultivation flasks at 200 µE m-2 s-1 at 3°C for one to two weeks. The samples were gently swirled twice a day to keep the cells in suspension. Besides an untreated control (prefiltered water sample), the samples were enriched with 10 µmol L-1 NO3, 6 µmol L-1 PO4, or 45 µmol L-1 SiOH4 final concentration, representing typical winter values of the monitoring time series. To study the effect of grazing, a grazer addition treatment was added during the spring bloom 2024 (22nd of April) using water sampled from 5 m depth at NK-k. Triplicate water samples were pre-filtered through a 200 µm mesh and either incubated as control, or as grazer addition treatment after adding 5 individual Cirripedia larvae, previously collected with a WP2 net (45 µm mesh size). During the pre-filtration also large phytoplankton cells and colonies >200 µm may be removed, which introduces a potential bias of the experiment being limited to smaller phytoplankton colonies. Cirripedia larvae represented the most abundant zooplankton group at that time as per observation and previous study (Arendt et al., 2013). After the incubation, the samples were transferred to brown borosilicate glass bottles and fixed in neutral Lugol at final concentration 2% before and after the incubations. Samples were counted for 2 broader functional groups, due to their different ecological roles within the food web and for carbon export: diatoms and Phaeocystis. This grouping aligns with the key functional contrast in the system between silicifying diatoms and the non-silicifying, colony-forming Phaeocystis, which is highly relevant for nutrient (Si) and grazer responses. During the experiment, most phytoplankton cells could be assigned to one of the two functional groups. The samples were concentrated in 2 mL settling chambers and counted under an inverted light microscope, by examining transects or random fields of view until a minimum of 100 cells of both diatoms and Phaeocystis were counted. In Isfjorden, water samples were taken from 15 m depth at the Isfjorden Adventfjorden (IsA) monitoring site on the 19th of May 2022. The samples were incubated in triplicates in 500 mL cultivation flasks, which were large enough to allow for sampling of multiple variables. They were deployed in situ at 5 m depth attached to a mooring. The mooring included a planar and spherical PAR logger and a temperature logger (RBR Ltd., Ottawa, Ontario, Canada). Besides an untreated control, the samples were enriched by the addition of nutrients until final concentrations of 10 µmol L-1 nitrate, 6 µmol L-1 phosphate, or 45 µmol L-1 silicate, consistent with the treatments in Nuup Kangerlua. Due to high abundances of Cirripedia larvae in the water sample, the experiment was done with untreated and with prefiltered (180 µm sieve) water, representing a sample with and without grazers. After 10 days, the samples were transferred to brown borosilicate glass bottles and fixed in 2% neutral Lugol. The samples were concentrated in 2 mL settling chambers and counted under an inverted light microscope, counting a minimum of 100 cells of both diatoms and Phaeocystis.