Blue-green algae in the Waikato River: ecological and physiological studies of planktonic blue-green algae and other phytoplankton and their impact on water quality in the Waikato River

Abstract

The distribution of the wide spectrum of blue-green algal species which is present in the catchment of the Waikato River is delineated. The seasonal periodicity of these algae is described and related to environmental factors in the lower Waikato River. In the lowland lakes, Lake Rotongaio and the thermal streams, blue-green algae are dominant. Species of interest are the bloom-forming species: Microcystis aeruginosa, Microcystis flos-aquae, Anabaena spiroides and Coelosphaerium kutzingianum, and a common alga throughout the whole catchment, Anabaena oscillarioides. In the Waikato River itself blue-green algae are relatively low in numbers and the river is dominated by diatoms with the Melosira species (M. granulata var. angustissima, M. distans) predominating below Lake Ohakuri. In this lake an increase in algal density and change of species composition occur as a consequence of nutrient enrichment from thermal discharges and tributaries and the effect of the shift from riverine to lake environment. The series of hydrolakes serve as breeding grounds for the river phytoplankton, which also continue to multiply in the river below the hydrolakes, indicating reproduction in the running water. The use of Shannon diversity index confirms the general trend of deterioriation of water quality downstream. The phosphate and nitrate requirements of blue-green and green algae were shown to be higher than those of diatoms by nutrient enrichment experiments. The higher temperature and light requirements of blue-greens and greens were also demonstrated. The Waikato River with its high silica content, moderate range of temperature and running water habitat was therefore demonstrated to be a more favourable environment for the diatoms than for green and blue-green algae. Anabaena oscillarioides (pure culture isolated from the river) was shown to be phosphate deficient with low internal cellular phosphorus content and high induced alkaline phosphatase activity when grown in water from the Waikato. The calculated relatively high Ks (half-saturation substrate constant) for inorganic phosphate of 67μg 1-1 is greater than the usual river concentration and probably explains the low density of this alga in the river. The in situ doubling time for A. oscillarioides was calculated to be two days and the deficiency symptoms disappeared when river water was supplemented with 100μg 1-1 phosphate-P, with the doubling time reduced to 0.5 days. Growth rate of the alga is proportional to phosphate concentration within the range of 0-100μg 1-1 beyond which increases in phosphate concentration only affects final yield. Both Microcystis aeruginosa and Anabeana oscillarioides were shown to exert inhibitory effects on Chlorella and natural phytoplankton. Microcystis showed true inhibition which depended on the size of inoculum indicating that the production of extracellular products is probably responsible. On the other hand, the inhibitory effect of Anabaena is the consequence of nutrient competition with Anabaena having a selective advantage over the other algae due to its having a higher initial uptake rate of phosphate. Microcystis aeruginosa, a well known toxic alga, is dominant in the lowland lakes and common in the river. Different strains were isolated from various localities, including the Waikato River, Lake Wahi, a farm pond and an oxidation pond. Three of these strains were shown to be toxic to mice with the most toxic one killing mice in two hours. The possibilities of blue-green algal blooms in the river and in the off-river reservoirs are discussed. With the increase of nutrients and the change from river to lake environment, blue-green algal blooms must be considered a likely event. The design and management of off-river reservoirs to prevent blue-green algal blooms are therefore of great importance.