Dataset: Temperature and nutrient dependent phytoplankton growth and herbivorous protist grazing rates

This dataset represents phytoplankton growth and herbivorous protists grazing rates measured under three different temperatures (in situ, Δ+3, Δ-3) and two nutrient regimes (nutrient-repleted and nutrient depleted) with seawater collected from the Narragansett Bay (NB) Long-term Plankton Time Series site (41.57 ºN, 71.39 ºW).

The experimental set-up consisted of a nested design: source water with plankton communities (< 200 µm) was used to set up long-term (10-day) microcosm incubations at three temperatures and two nutrient concentrations to monitor the community response to temperature and nutrient manipulations in terms of species composition and abundance over the incubation period. Seawater was filtered through a 200 micrometer (µm) mesh to eliminate macrozooplankton grazers. The 20-liter (L) acid-washed carboys were immediately transported to the laboratory. Each microcosm was used to assess phytoplankton growth and microzooplankton herbivory rates following the two-point modification of the dilution method (Landry and Hassett 1982) with 100 percent and 10 percent SW dilution levels. The initial dilution experiment conducted on day 0 (D0) was used to assess metabolic rates under in situ temperature and nutrient load. Then, on day 3 (D3), day 6 (D6), and day 10 (D10) using water from each microcosm, 6 dilution experiments per day (one per each temperature and nutrient level) were conducted for a total of 19 dilution experiments in 10 days.

Experimental bottles were incubated for 24 hours at -0.5 degrees C, 2.6 degrees C, and 6 degrees C under a 12:12 light: dark cycle of cool white fluorescent lights at 115 µmol photons m^-2 s^-1. Triplicate subsamples were taken from the 100 percent SW stocks after 24 hours from each incubation bottle for chlorophyll a and microscopy analysis. Chlorophyll a extraction and determination followed Graff and Rynearson (2011) and measurements were performed on a Turner Designs AU10 fluorometer. Plankton community enumeration and composition was performed on samples preserved in 2 percent acid Lugol’s iodine final concentration (Menden-Deuer et al., 2001). Phytoplankton cells were enumerated using a Sedgewick-Rafter slide (1-milliliter volume) and a Nikon Eclipse E800 light microscope while herbivorous protists were enumerated following the Utermöhl (1958) method settling between 2.5 and 15 milliliters. The entire surface area of the settling chamber was examined at 200x with a Nikon Diaphot 300 inverted microscope. Ciliates and dinoflagellates were identified and classified to the lowest possible taxonomic level by consulting several taxonomic guides (Kofoid and Campbell 1929; Tomas 1997; Strüder-Kypke et al. 2002).

Phytoplankton growth and herbivorous grazing rates were estimated from changes in total chlorophyll a concentration over the 24-hour incubation. The instantaneous phytoplankton growth rate (μ) depends on the assumption of unlimited, exponential growth and was calculated following the equation: μ = 1/t ln (N_t/N₀), where t is the incubation time in days and N_t and N₀ are the chlorophyll a concentration at the beginning and at the end of the experiment. Herbivory rates due to microzooplankton grazing were estimated as the difference between μ measured in the diluted (μ_10%) and whole (μ_{100 percent}) seawater sample g = μ_{10 percent} - μ_{100 percent}.