THE ALGAE OF WAIMANGU CAULDRON (NEW ZEALAND): DISTRIBUTION IN RELATION TO pH 1

The pH of the main body of thermal water and of the outlet of Waimangu Cauldron (Frying Pan Lake) is 3.8, whereas hot springs flowing into this lake are alkaline, pH 8.2–8.7. Where these waters meet there are pH gradients, and it is possible to find a series of habitats differing in pH but having approximately the same temperature (54–56 C). Where the pH is above 4.8 the blue‐green alga Mastigocladus laminosus is present, and where the pH is below this value the eucaryotic alga Cyanidium caldarium is the sole photosynthetic microorganism at temperatures less than 55 C. Rocks resting on the bottom of Waimangu Cauldron surrounded by water of pH 3.8 have Mastigocladus laminosus growing on them, but measurements of pH directly in these algal mats showed that the microenvironment of the alga was actually above pH 4.8. A simple experiment showed that it is probably the rock, per se, that initiates a tiny island of alkalinity in the acid lake, permitting M. laminosus to become established. The outlet of Waimangu Cauldron is a hot acid stream which becomes progressively alkaline as it descends through Waimangu Valley, due to inflow of alkaline water from springs and a cold stream. Blue‐green algae were found in this stream when the pH was greater than 5.0 and C. caldarium when the pH was less than 4.0. In one region where the pH was between 4.0 and 5.0, both blue‐green algae and C. caldarium were seen, although in moribund state. These data, provide further information on the lowest pH values at which blue‐green algae are able to grow.