Unusual allometry between in situ growth of freshwater phytoplankton under static and fluctuating light environments: possible implications for dominance

Abstract

The effects of fluctuating light fields on the growth of phytoplankton are not well understood and conclusions in the literature have been equivocal. Most studies have examined responses such as productivity and chlorophyll a content (laboratory culture and field tests) or growth rates (laboratory culture tests). In this study we examined the in situ growth rates of different types of phytoplankton within two natural populations. Comparisons were made between populations grown in a static environment (suspended in a fixed position in the water column) and an equivalent population moving through the water column simulating the mixing of entrained phytoplankton. Growth under fluctuating light fields in this experiment only significantly (P < 0.05) increased the growth of the diatom Skeletonema and decreased the growth of Anabaena circinalis, Microcystis aeruginosa and Scenedesmus sp. All other phytoplankton, including the genera Nitzschia, Fragilaria and Dactylococcopsis, did not have growth rates that were significantly different between static and fluctuating light treatments. A general pattern where diatoms grew best, followed by chlorophytes with the toxicogenic cyanophytes M. aeruginosa and A. circinalis growing least well, was distinguished under fluctuating irradiance. This seems consistent with the common occurrence of these groups of phytoplankton in the natural environment. The cyanophytes Dactylococcopsis and Aphanothece did not follow this pattern, with the former growing better under-fluctuating light and the latter exhibiting an unusual growth pattern where growth was higher under lower light intensities.