Plankton community structure during an ecosystem disruptive algal bloom of Prymnesium parvum

Abstract

The purpose of this paper was to study the plankton food web structure in Lake Koronia during an extremely dense bloom of Prymnesium parvum. The Prymnesium bloom fulfilled the main criteria of an ecosystem disruptive algal bloom, since it was a massive persistent, monospecific bloom which disrupted the dynamics of the plankton community. It appeared after an N-fixing cyanobacterial bloom, when a decrease in temperature and low nitrogen conditions may have caused a positive feedback, which allowed the replacement of cyanobacteria by the invasive Prymnesium. The initiation of the bloom was further facilitated by the disruption of zooplankton grazing, with the biomass of the primary grazers, rotifers and cladocerans, dropping to undetectable levels. Furthermore, the rest of the phytoplankton responded to the onset of the bloom by an 82% decrease. Cyanobacteria and diatoms exhibited the highest decrease in biomass (95 and 99%, respectively) followed by chlorophytes (55% decrease). This enabled the uncontrolled increase of the P. parvum population to numbers close to the highest abundance ever recorded. At that point, the grazing community was reinstated and the system became replete with nutrients with an N:P ratio of 10:1. This coincided with the beginning of the bloom termination, which was followed by an increase in the biomass of competing phytoplankton.