Advection, growth and nutrient status of phytoplankton populations in the lower River Murray, South Australia

To investigate the link between river flow, nutrient availability and devel opment of algal blooms, growth rates of the major phytoplankton species wer e examined in situ in the lower River Murray, South Australia over the 1994 /1995 summer. Eight sites were selected over a 54 km reach between Lock 1 a nd Nildottie and growth rates estimated by monitoring mean cell density in time-aligned 'parcels' of water as they travelled downstream. Discharge at Lock 1 during the period of study (3000-5000 ML day(-1)) typified summer en titlement flows to South Australia. A large, shallow floodplain lake (lagoo n), with an hydraulic connection to the river, supported a large population of cyanobacteria in summer, but inputs to the main channel did not substan tially affect the abundance and composition of river phytoplankton. Mean ne t growth rates of Anabaena circinalis and A. flos-aquae f. flos-aquae were 0.132 and 0.176 day(-1), respectively, although individual rates varied fro m positive to negative. In contrast, the mean growth rate of the filamentou s diatom Aulacoseira granulata was -0.15 day(-1), reflecting a decrease in population size with advection downstream. Mean cell densities of the three species did not exceed 5000 cells mL(-1) throughout the study. Growth bioa ssays conducted in the laboratory indicated that nitrogen was often the nut rient limiting algal growth, although it was not established whether nitrog en was limiting in situ. A conceptual model is presented, linking these fin dings with those of other work on the lower River Murray, to summarize the physical and chemical environmental factors governing the abundance of cyan obacteria in this reach of the river. Copyright (C) 2000 John Wiley & Sons, Ltd.