Nitrate uptake by Karenia brevis. II. Behavior and uptake physiology in a nitrate-depleted mesocosm with a bottom nutrient source

摘要

ABSTRACT: Karenia brevis may optimize growth by alternately maximizing exposure to light, migrating up into an oligotrophic water column during the day, and to nutrients (nitrate), by migrating down to the sediment-water interface at night. Understanding how cell behavior contributes to the acquisition of light and nutrients that are separated in space is critical to understanding how K. brevis populations persist in oligotrophic environments. In response to previous modeling efforts that parameterized cell physiology and behavior in nitrate-replete conditions, we examined similar cellular characteristics in a stratified 1.5 m deep mesocosm. The upper 2/3 of the mesocosm, encompassing the surface and middle samples, was nitrate depleted (0.5 µM NO₃–) and simulated an oligotrophic water column. The lower 1/3 of the mesocosm contained 10 µM NO₃– corresponding to elevated nutrient levels near the sediment–water interface. We sampled uptake rates at 3 depths during the day at light levels of 350, 125 and 60 µmol quanta m–2 s–1 and again at night in the dark. Nocturnal uptake of nitrate in the mesocosm was significantly less than diurnal uptake. Nocturnal uptake rates in the mesocom were intermediate between cells exposed to prolonged nitrate-depleted and nitrate-replete conditions. Both migration, as indicated by diel aggregation patterns, and cell physiology indicate that descent to regions of higher nutrient concentrations were sufficient to maintain average growth rates of 0.3 div d–1. Thus, both the physiology and behavior of K. brevis may support populations near the sediment–water interface, where they may grow undetected in offshore oligotrophic water columns.