Influence of relative water motion on the growth, ammonium uptake and carbon and nitrogen composition of Ulva lactuca (Chlorophyta)

Abstract

In a series of multifactorial laboratory experiments, Ulva lactuca discs were grown in an apparatus in which they were exposed simultaneously to 3 simulated current speeds (7.5, 15, 22.5 cm s-1) and a still control, and either 3 ammonium concentrations (0–10, 35–45 and 115–145 μM) under ample uniform light (ca 200 μE m-2 s-1) or 3 light intensities (approximately 35, 90 and 270 μE m-2 s-1) with uniform surplus, ammonium. Disc growth rates were determined in each experiment as well as tissue nitrogen and carbon composition and fluxes of NH4, NO3/NO2 and PO4 in media. In a supplementary series of field experiments, U. lactuca discs were simultaneously exposed to 2 different water motion regimes in adjacent chambers at several sites characterized by widely different ammonium concentrations. In field experiments, growth rates were calculated and analyzed as a function of water motion at the various sites. The application of simulated current consistently enhanced disc growth rates in the laboratory, except at the lowest light intensity. In most cases this enhancement was fully realized at the lowest applied simulated current (7.5 cm s-1). Simulated current slightly enhanced ammonium uptake rates by U. lactuca discs, relative to rates in still water, except at the highest ammonium concentration. C:N ratios of discs generally declined with increases in simulated current, except at the highest ammonium concentration. This decline was primarily attributable to increases in per cent N and was, again, mainly realized at 7.5 cm s-1. The results suggested that simulated current compensated for N limitation, except when light was sufficiently low to become the overriding limiting factor, but that the enhancement of growth by simulated current could not be explained in terms of N metabolism alone. Field experiments showed that the higher level of water motion consistently enhanced growth at sites with comparatively low ammonium concentrations, but not at sites with moderate or high ammonium concentrations.