Cycles of macrophytes and phytoplankton in Pukepuke Lagoon following a severe drought.

A three year survey of the aquatic vegetation of a New Zealand dune lake is described. Pukepuke Lagoon is a shallow (max. depth 85cm.), slightly saline lagoon with 15ha of open water. Its inflows drain intensively farmed land and are rich in nutrients, especially phosphorus. Dense macrophyte crops characterised the lagoon each spring but the component species differed each year. After a severe drought which dried most of the lagoon bed in the 1969-70 summer, Veronica anagallis-aquatica and Ranunculus fluitans were the dominant species in 1970; then with normal water levels Chara globularis became dominant throughout 1971, followed by Potamogeton crispus in 1972. Factors that enabled the dominant species to gain an early competitive advantage arc discussed. Drying of the lagoon bed in 1970 is regarded as the major disrupting feature that initiated the sequence of plant species described. Phyto-plankton crops, estimated from chlorophyll a pigment, were low during macrophyte growth phases but reached peaks in the summer of 1971-72 and 1972-73; the former being responsible for the destruction of Chara. The competitive balance between phytoplankton and macrophytes is a delicate one but the shallow water of Pukepuke Lagoon tends to favour the macrophytes