Co-occurrence of functional groups in phytoplankton assemblages dominated by diatoms, chrysophytes and dinoflagellates

In four deep karstic lakes (Lake Vransko, Lake Visovacko, Kozjak and Prosce) several functionally well-adapted phytoplankton groups interchange in dominance during the seasonal cycle from April to September and likely tolerate the constraining conditions of nutrients deficiency more successfully as an aggregated group than as taxonomic units. This paper considers the functional approach by Reynolds et al. (2002), a method capturing much of the species ecology and which is successfully used for describing phytoplankton successions. Deep karstic lakes are attributed with the early spring (April) formation of stratification, low phytoplankton biomass, and with maximum biomass accumulating in July. The co-occurrence of functional groups in those lakes is evident but typically distinct according to different biomass proportions. The functional groups A, B, C, D, P, E and LO, represented by different Cyclotella, Ulnaria/Fragilaria, Dinobryon, Ceratium/Peridinium species, co-occurred in 50% and up to 90% of phytoplankton biomass in all investigated lakes. By observing the co- occurrence across the Croatian karstic region we: (i) demonstrate the regional distribution of phytoplankton assemblages in relation to low nutrient environment, and (ii) build a simple uniform pattern which successfully describes a variety of phytoplankton in deep karstic lakes and provides information of reliable indicators for those systems.