Species-specific growth rate estimates of sea-ice diatoms derived from PDMPO incubations under different irradiance levels in Tempelfjorden and Van Mijenfjorden (Svalbard) in April 2021

Dataset Overview: The dataset includes estimates of growth rates for sea ice diatom species exposed to different irradiance levels. The data set includes:

“Growth rate estimates.xlsx”: The data generated in this study. NS = non-stained cells, HS = half-stained cells, n = counted cells. “Dataset_uncertainty.txt” + “R script.txt” + “Read me.txt”: The R script used for the uncertainty estimations, together with the .txt data files used in R and a document providing clarifications about those data. Species-specific growth rates were obtained using the fluorochrome PDMPO (2-(4-pyridyl)-5-[[4-dimethylaminoethyl-aminocarbamoyl)-methoxy]phenyl]oxazole) staining method to measure biogenic silica deposition during cell division. This method allows growth rate estimates based on the number of stained diatom valves (the siliceous part of the diatom cell wall). The data provides insights into species-specific growth patterns, including responses to varying light intensities and the potential for growth in darkness. This approach highlights interspecific growth variability and contributes to understanding diatom ecology under arctic sea ice conditions.

Study Sites and Sampling: Samples were collected in April 2021 from two Svalbard fjords: Tempelfjorden (TF) and Van Mijenfjorden (VMF). These sites were selected for their contrasting environmental conditions. At each site, ice cores were taken and, immediately after coring, the bottom skeletal layer of the ice was scraped off with a knife to collect ice algae communities. Samples were incubated in filtered seawater (FSW) enriched with nutrients to avoid nutrient limitation, and the fluorescent dye PDMPO was added.

Experimental Setup: The stained samples were incubated under various irradiance levels (0.4 to 204 µmol photons m⁻² s⁻¹) and in complete darkness, using an LED light source. Incubation times varied between 24 to 48 hours depending on the site. After incubation samples were fixed with hexamine-buffered formalin solution.

Microscopical Analysis: Samples were analyzed using an inverted epifluorescence microscope equipped with specific filters for PDMPO fluorescence. Cells were classified into unstained (nt), half-stained (nt') and fully-stained (nt'') frustules, based on the presence/absence of a PDMPO signal in the two valves of each frustule.

Growth Rate Calculation: The relative proportion of half-stained and unstained frustules was used to calculate species-specific growth rates following Husmann and Klaas (2022). The equation used is detailed below:

                                                 μ[day^-1]=ln⁡((nt+nt')/(nt+nt'/2))∙(1/t)                                          

Where: nt: Number of non-stained cells. nt': Number of half-stained cells. t: Incubation time in days.

Considerations: Empty diatom frustules (considered dead) were excluded from the counts. Fully stained cells were rare and hence were often grouped with half-stained cells for calculation purposes. Only species with sufficient cell counts (at least 40 cells per irradiance level) were included to minimize errors.

Additional Notes: Uncertainty associated with growth rate estimates was assessed following the framework proposed by Husmann and Klaas 2022 (https://aslopubs.onlinelibrary.wiley.com/doi/full/10.1002/lom3.10505). For each species and irradiance treatment, Monte Carlo simulations were used to generate 10,000 binomial realizations based on the observed proportion of half-stained cells and the total number of counted cells. Simulated proportions were subsequently transformed into growth rates using the PDMPO-derived growth equation. The coefficient of variation (CV) and 90% uncertainty intervals were derived from the resulting simulated distributions using the 5th and 95th percentiles.