Migratory behaviour of Skeletonema grethae (Bacillariophyceae) in stratified waters

Abstract

Even though Skeletonema spp. are among the most important primary producers of the world oceans, little is known about their migratory behaviour and buoyancy properties. Here, we present results obtained from experiments on S. grethae in an artificial, salinity-stratified water column. Beforehand, growth experiments were conducted to find out how S. grethae responded to the set of environmental parameters being used in the water column. The highest growth rates were obtained for a salinity range of 25–35 psu, and an irradiance range of 60–330 μ mol quanta m⁻² s⁻¹ (1.3–1.9 div d⁻¹). Inoculum populations were introduced in the upper brackish layer in Experiment 1 (E1) and in the bottom layer in Experiment 2 (E2). During the experiments, the cell/colonies assumed different forms; in E1 they became long and thin with short and weakly silicified connecting processes between them, whereas in E2 there were only minor changes in form and cells looked more like the ‘normal type’, shorter with longer, more heavily silicified connecting processes. In addition, with respect to the buoyancy, subgroups developed, part of the population became ‘floaters’ and part became ‘descenders’, whereas others did not move at all. Such flexibility in migratory behaviour could be an advantage for survival and successful expansion into new areas. It also indicates that there is a phenotypic plasticity in Skeletonema morphology that seems to influence the floating properties of the cells. Long cells remained buoyant for ten days and short cells for more than 17 days. The presence of a strong halocline has a great impact on the vertical displacement of cells/colonies, as revealed by a halt in the ascent/descent of cells within or just below it, resulting in biomass accumulation at these sites. Based on our data nitrate availability could also be involved in the buoyancy control of S. grethae.

Keywords:

• Skeletonema
• vertical migration
• buoyancy
• halocline
• colony-form
• plankton

Acknowledgements

We are greatly indebted to technical staff at the Department of Physics and Technology, University of Bergen, for help with technical solutions and improvements of the experimental water column, and to Svein Nordland at the Department of Biology, University of Bergen, for assistance with computerizing the data sampling. We thank research scientist Svein Sundby at the Institute of Marine Research, Bergen, for providing data on seawater viscosity.