A two-part model linking multidimensional environmental gradients and seasonal succession of phytoplankton assemblages

Algal assemblages and their respective niches are analysed for a large riverine impoundment in semi-arid Australia. The extreme variability of the flow regime and the extensive period of low or no flow produced conditions including prolonged stratification akin to those of a lake during the dry season. Hierarchical agglomerative clustering of phytoplankton data and comparison with a principal component analysis of corresponding environmental data are used to demonstrate the linkage between steep environmental gradients and variations in the phytoplankton assemblages. The factor score plots separate the algal assemblages according to key environmental parameters including increasing clarity of the water column, temperature and modification of water chemistry. Assemblages normally contained cyanoprokaryotes (Oscillatoriales), euglenophytes or non-flagellated chlorophytes during flows followed by flagellated chlorophytes and cyanoprokaryotes (Nostocales) during the dry season. Species present during the period of reduced flow occupied conditions similar to those presented in the literature for temperate and/or tropical lakes. Common species included Pandorina morum, Eudorina elegans, Anabaena circinalis, Aphanizomenon issatschenkoi, Aphanizomenon aphanizomenioides and Cylindrospermopsis raciborskii. A two-part model is proposed which predicts the most likely genera of phytoplankton with respect to multidimensional environmental gradients.