New aspects of migratory behavior of phytoplankton in stratified waters: Effects of halocline strength and light on Tetraselmis sp. (Prasinophyceae) in an artificial water column

Abstract

There is very little information in the literature about how phytoplankton flagellates respond to rapid changes in salinity (e.g., haloclines of different strength). Here we present such data obtained from experiments with Tetraselmis sp. (Prasinophyceae) in a specially designed artificial water column. The experiments were performed with surface salinities varying from 27.4 to 33.4‰, whereas bottom salinities were essentially constant (34.0-34.6‰). Cells were introduced near the bottom. The first stage of the ascent was an accumulation of cells in the lower part of the halocline. The second was a transit to the upper part of the halocline after a variable time lag, and the third stage was a further ascent to the surface layer, again after a variable time lag. Our results reveal a strong positive phototaxis of the swimming cells under continuous surface irradiances in the range 17-144 mol quanta m Ϫ2 s Ϫ1 . Maximum swimming speeds were found to be 0.9 m h Ϫ1 . Increasing halocline strength (⌬S ϭ 0.7, 3.0, and 6.6‰), resulted in reduced swimming activity, but the cells managed to pass through after an adaptation period. Under a 14 : 10 light : dark (LD) cycle, the cell concentrations of the bottom layer decreased gradually for each cycle, and cells accumulated in the surface layer during the light period. During the dark period, cell concentrations were also increasing in the halocline for each cycle. A downward migration started about 2 h before the light period ended and was slower than the ascent of cells from the halocline. It therefore seems that positive phototaxis was stronger than the positive geotaxis as a driving force of cell motility. We conclude that strong haloclines can prevent phytoplankton flagellates in the surface layer from reaching the nutrient-rich deeper layer during the night and therefore play an important regulating role in the bloom dynamics of phytoplankton.

Resource Patch Formation and Exploitation throughout the Marine Microbial Food Web

“…Although swimming is widespread among phytoplankton (Horner et al 1997), its ecological importance has typically been considered primarily in relation to photic migration (Kamykowski et al 1998), physical stratification (Erga et al 2003), or sex (Govoronova and Sineshchekov 2005). Our observations indicate that phytoplankton motility and chemotaxis can significantly increase exposure to nutrient patches, imparting a considerable nutrient advantage to chemotactic cells.…”

Effects of salinity on diel vertical migration behavior in two red-tide algae, Chattonella antiqua and Karenia mikimotoi

“…Therefore, coastal flagellates should have the ability to quickly sense salinity changes and to control swimming patterns. In some flagellates, (Raphidophyceae) and Heterocapsa triquetra (Ehrenberg) F. Stein (Dinophyceae), cessation of upward swimming and a decrease in swimming speed have been observed when they transit a strong halocline in laboratory conditions (Erga et al 2003, Bearon et al 2006, Jephson et al 2011. Some flagellates such as Gyrodinium aureolum Hulburt form a thick bloom in a strong halocline (Bjørnsen & Nielsen 1991).…”

Behavioral response as a predictor of seasonal depth distribution and vertical niche separation in freshwater phytoplanktonic flagellates

“…Extensive investigations have, nevertheless, found that the movements of phytoplanktonic flagellates are often biochemically mediated (Miller and Diehn 1978;Iseki et al 2002) and are usually driven by sensory responses to environmental stimuli such as light (Kessler et al 1992), gravity (Hä der and Lebert 2001), temperature (Kamykowski 1981), chemical gradients (Lee et al 1999;Clegg et al 2004a), and even salinity (Erga et al 2003). Moreover, recent observations indicate that this behavior may be deceptively sophisticated (Kamykowski and Yamazaki 1997;Bearon et al 2004;Clegg et al 2004b).…”