Herbivorous protist growth and grazing rates at in situ and artificially elevated temperatures during an Arctic phytoplankton spring bloom

Abstract

To assess protistan grazing impact and temperature sensitivity on plankton population dynamics, we measured bulk and species-specific phytoplankton growth and herbivorous protist grazing rates in Disko Bay, West Greenland in April-May 2011. Rate estimates were made at three different temperatures in situ (0 °C), +3 °C and +6 °C over ambient. In situ Chlorophyll a (Chl a) doubled during the observation period to ∼12 µg Chl a L−1, with 60–97% of Chl a in the >20 µm size-fraction dominated by the diatom genus Chaetoceros. Herbivorous dinoflagellates comprised 60–80% of microplankton grazer biomass. At in situ temperatures, phytoplankton growth or grazing by herbivorous predators <200 µm was not measurable until 11 days after observations commenced. Thereafter, phytoplankton growth was on average 0.25 d−1. Phytoplankton mortality due to herbivorous grazing was only measured on three occasions but the magnitude was substantial, up to 0.58 d−1. Grazing of this magnitude removed ∼100% of primary production. In short-term temperature-shift incubation experiments, phytoplankton growth rate increased significantly (20%) at elevated temperatures. In contrast, herbivorous protist grazing and species-specific growth rates decreased significantly (50%) at +6 °C. This differential response in phytoplankton and herbivores to temperature increases resulted in a decrease of primary production removed with increasing temperature. Phaeocystis spp. abundance was negatively correlated with bulk grazing rate. Growth and grazing rates were variable but showed no evidence of an inherent, low temperature limitation. Herbivorous protist growth rates in this study and in a literature review were comparable to rates from temperate waters. Thus, an inherent physiological inhibition of protistan growth or grazing rates in polar waters is not supported by the data. The large variability between lack of grazing and high rates of primary production removal observed here and confirmed in the literature for polar waters implies larger amplitude fluctuations in phytoplankton biomass than slower, steady grazing losses of primary production.

Microzooplankton Distribution and Dynamics in the Eastern Fram Strait and the Arctic Ocean in May and August 2014

“…The rest of the populations either declined or did not change significantly over 24 h. This asynchronisity in the growth of dominant species appears to reflect a general pattern of microzooplankton community dynamics, where multiple populations oscillate out of phase, whereas short-term incubations provide only a snapshot of these dynamics. The rapid species-specific growth rates of ciliates and dinoflagellates in our experiments support the idea that low temperatures do not constrain their growth more than that of their phytoplankton prey (Sherr et al, 2013;Menden-Deuer et al, 2018). Further, the ability of these protists to achieve their intrinsic maxima rapidly is likely an adaptation to the fluctuating and spatially heterogeneous environment (Franzè and Lavrentyev, 2014).…”

“…In two surface experiments (P4 and P7), microzooplankton consumed > 100% of primary production. This is not unusual (Calbet and Landry, 2004;Menden-Deuer et al, 2018) and likely reflects the dynamic equilibrium between phytoplankton growth and grazing (Irigoien et al, 2005) and, possibly, the effect of large predator removal. Herbivory, as measured in dilution experiments, is a community process.…”

“…Ciliates and dinoflagellates inhabiting polar seas are welladapted to their cold and icy environment (Sherr et al, 2013;Franzè and Lavrentyev, 2014;Menden-Deuer et al, 2018) and food availability is central among factors controlling their populations dynamics and composition (Caron and Hutchins, 2013). In addition to the distinct vertical patterns, microzooplankton biomass and composition displayed pronounced seasonality.…”

“…Microzooplankton can feed on P. pouchetii as evidenced by the outcome of dilution experiments at P3-40 m and P4 and published research (Verity et al, 2007;Grattepanche et al, 2011). However, the presence of colonial P. pouchetii is often reported in conjunction with low or insignificant herbivory rates in dilution experiments Stoecker et al, 2015;Menden-Deuer et al, 2018). In addition to forming large colonies, P. pouchetii can produce toxic polyunsaturated aldehydes (PUAs) such as 2-trans-4-trans-decadienal (Hansen et al, 2004).…”

Seasonal Variability and Drivers of Microzooplankton Grazing and Phytoplankton Growth in a Subtropical Estuary

“…For instance, the average grazing impact at temperatures <5 • C is often lower than the global average (i.e., <67%; Caron et al, 2000;Calbet et al, 2011), yet grazing rates of 0.5 day −1 have been recorded under these conditions. This suggests that microzooplankton can remove primary production even at near-freezing temperatures (Lawrence and Menden-Deuer, 2012;Sherr et al, 2013;Menden-Deuer et al, 2018). Likewise, temperatures >15 • C may not always translate in higher grazing rates, due in part to complex feeding interactions (e.g., grazer-prey selectivity) that may mask temperature effects (Kimmance et al, 2006;Chen et al, 2012).…”