A 21-year record of vertically migrating subepilimnetic populations of Cryptomonas spp.

Abstract

The vertical distribution and diel migration of Cryptomonas spp. were monitored continuously for 21 years in mesotrophic Cross Reservoir, northeast Kansas, USA. The movements of these motile algae were tracked on multiple dates during July-October of each year using in situ fluorometry and optical microscopy of Lugol's iodine-preserved samples. Episodes of subepilimnetic diel vertical migration by Cryptomonas were detected and recorded on 221 different days between 1994 and 2014, with just 2 of these years (1998 and 2013) lacking any sampling events with deep peaks sufficiently large enough to track. Whenever a subepilimnetic layer of Cryptomonas was detectable, it was generally observed to ascend toward the bottom of the epilimnion beginning approximately at sunrise; to descend toward the lake bottom during the late afternoon and evening; and to remain as a deep-dwelling population until dawn of the following day. Moreover, there was high day-to-day consistency in the absolute water column depths at which the migrating algal cells would cease their ascending or descending movement. We believe this unique and remarkable dataset comprises the most detailed record of diel migratory behavior for any planktonic freshwater alga reported for a single freshwater lake.

Relative importance of top‐down vs. bottom‐up control of lake phytoplankton vertical distributions varies among fluorescence‐based spectral groups

“…It is possible that this result is driven by the dominance of light-sensitive taxa in the cryptophyte and green algae spectral groups in our study lakes, such as Cryptomonas, Chroococcus, and Chlamydomonas ( Fig. 6; Clegg et al 2004;Yang et al 2015;deNoyelles et al 2016). However, as we do not have depth profiles of microscopically identified taxa, we cannot definitively state which taxa are driving the sensitivity of peak width to light attenuation.…”

“…For example, the vertical distribution of diatoms, chrysophytes, and cryptophytes, which are highly nutritious and preferentially grazed by zooplankton, may be more driven by top‐down control than inedible cyanobacteria (Brett et al 2000, 2006), although cyanobacteria may also experience indirect top‐down effects as their abundance increases due to zooplankton grazing down more edible taxa (Lynch and Shapiro 1981; Holm et al 1983). In contrast, the vertical distribution of taxa that can control their motility or buoyancy in the water column, such as cyanobacteria with gas vesicles and green algae, dinoflagellates, cryptophytes, chrysophytes, and euglenoids with flagella, may be more driven by pH or bottom‐up resource gradients of light, nutrients, or CO 2 (Shapiro 1973; Oliver 1994; Clegg et al 2004, 2007; deNoyelles Jr et al 2016). Thermal stratification can act as a physical control affecting migration strategy or settling location of phytoplankton in the water column, potentially allowing motile taxa to take advantage of well‐stratified water columns by migrating to or maintaining buoyancy at a layer with optimal resources (Alldredge et al 2002; Sengupta et al 2017).…”

Temperature × light interaction and tolerance of high water temperature in the planktonic freshwater flagellates Cryptomonas (Cryptophyceae) and Dinobryon (Chrysophyceae)

“…, deNoyelles et al. ), i.e., they peak under conditions of reduced light and temperature. Our results demonstrate that if water temperature exceeds 20°C, the niche of Cryptomonas species is constrained to light levels >50 μmol photon · m −2 · s −1 .…”

“…, deNoyelles et al. ). This behavior may also serve to avoid temperatures above the species‐specific T opt in situ (Bergkemper et al.…”

“…However, flagellated algae might escape adverse conditions by migration to benign environments. Cryptomonas species perform phototactic responses to varying light conditions, conducting diurnal vertical migrations over several meters (Sommer 1985, Arvola et al 1991, deNoyelles et al 2016. This behavior may also serve to avoid temperatures above the speciesspecific T opt in situ photon Á m À2 Á s À1 .…”