No abstract

The seasonal succession of the major phytoplankton species is described for Lake Rotongaio, a small crater lake which flows into Lake Taupo via a short channel. The dynamics of only the 4 most abundant species are described. Expressed as cell numbers, Anabaena oscillarioides dominated in the warmer and stratified periods from November 1979 to April 1980 and again from September 1980 to the end of the observation period in January 1981. During destratification the diatoms Synedra ulna and Cyclotella menenghiniana attained maximum numbers. The green alga Chlorella vulgaris peaked immediately before the September 1980 stratification. When expressed as cell volumes, however, the diatoms were always greater than Anabaena. It is proposed that these seasonal changes can largely be explained as being long-term versions of short-term population fluctuations seen during thermal stratification. These are interpreted to have been responses to vertical mixing with resulting changes in the availability of light for photosynthesis in the epilimnion. During thermal stratification, Anabaena is buoyed upwards, whereas the diatoms sink to the hypolimnion. To compensate for such losses the growth rates of the diatoms must have been of an approximately similar magnitude to the growth rates leading to their winter maxima. The Synedra population decline may have been influenced by low temperature in mid-winter. Nutrient recirculation did not appear to be an important factor for phytoplankton biomass. There is little evidence for any important direct competition between the species for Received 3 March 1983; accepted 2 August 1983 resources; the phytoplankton dynamics and biomass appear to be mediated directly by external events and not, in any important way, by internally generated conditions.

Since 1977 the Sepik River backwaters between Pagwi and Angoram in Papua New Guinea have become increasingly covered by the exotic water-fern Salvinia molesta. By May 1979, Salvinia covered about 80 km2. Water chemical analyses for dissolved substances give no evidence that nutrients in the Sepik River and ‘lagoons’ might become limiting enough to affect the Salvinia population. The physical impact of the plant is reflected especially in the decline of the fisheries for Saltfish Tilapia (Sarotherodon mossambicus), crocodile hunting, collection of sago-palm, and in the slowing down or complete elimination of water transport. As a consequence, people in a number of villages are unable to reach markets and children cannot go to schools.A programme of management has been formulated which will involve the United Nations, central and provincial governments, and local inhabitants. However, it should be understood that there is little chance of complete elimination of the weed from Sepik River backwaters.

Hydrological observations made in January 1984 in the region near Cape Farewell, New Zealand, are described and previously published observations reviewed. It is shown that upwelling depends on the existence of the intermittent Westland Current, and is intensified by an onshore wind. Such a wind induces a fall in sea level near Cape Farewell, and the resulting favourable sea surface slope accelerates deep water over the bathymetric rise inshore of Kahurangi Shoals. The hydraulic response of the thermocline, coupled with a coastal convergence of the bottom Ekman flow, produce a strong upwelling source near Kahurangi Point. M90020

The surface distribution and metabolic uptake of nitrate and ammonium were investigated as they related to the movements of water in the upwelling zone off the north-west coast of South Island, New Zealand. Analyses were also made of chlorophyll a and paniculate nitrogen as indicators of phytoplankton biomass.The movement of water and resulting dispersion from the upwelling focus were shown to differ greatly from day to day; this could confound simple correlations between nutrient nitrate and phytoplankton. A suitable model to investigate metabolic processes due to influx of fertile upwelled water which could avoid this problem can be based upon assays of NO 3 and NH 4 uptake by the plankton. These assays, done with 15 NH 4 and I5 NO 3 , could provide an operational index for identifying the zone of nett production due to nutrient upwelling.