The Distribution of Duckweed Lemna Perpusilla in a Small Southern California Lake: An Experimental Approach

The duckweed Lemna perpusilla in a small, alkaline lake was studied during 1970 and a map of its distribution was prepared. Duckweed, the only floating, unrooted higher aquatic plant present, was distributed around most of the edge of the lake but its density was extremely patchy. Experiments were made involving manipulation of the duckweed density, modification of its distribution, and measurement of the factors which experiments suggested were significant influences on its pattern of abundance. Attention was particularly focused on the importance of other plants in molding the duckweed pattern. Duckweed was absent from the central area of the lake primarily because there was nothing to prevent the plants from being transported by waves generated by the prevailing winds from the southwest. Growth on water taken from the center of the lake was poor compared to growth on water from near the shore. Duckweed can exist in the Potamogeton zone at a much higher density, and the low density does not result from grazing by animals. Growth of plants exchanged between the zone and a zone nearer the shore was depressed in the Potamogeton zone and stimulated in the other. Several experiments suggested that the lowered growth rate was attributable to the higher pH of the water, probably produced by the photosynthesis of other aquatic plants. Where it was dense enough, duckweed was able to overcome this effect by shading the plants beneath. It can be reasoned, from the structure of plants that retain their progeny inserted in the reproductive pockets, that the average ability per frond to produce further progeny is thereby impaired. Also, plants broken up into single fronds produced progeny at a faster rate than unbroken plants. Dense patches of duckweed occurred among the Scirpus plants and along the edge of the shore. Duckweed growth was more rapid on mud than on water but plants transplanted to selected places around the lakeshore did not reveal any differences in the ability of the mud to support their growth. Dense populationswere associated with breaks in the dense cover of Scirpus stems, which reduced the light intensity beneath them by 80%—90%. Comparison of growth of plants on mud in situ, in shaded and unshaded areas, indicated that these openings in the cover are responsible for allowing dense populations to develop. Development of dense patches along the unshaded shore of the lake was limited by their inability to grow fast enough to keep pace with the receding water and hence appeared only where the slope was steep. Most dense patches were eventually stranded on the mud and died, drastically reducing the density because very few new plants (growing from turions or seeds) could be recovered when water was added to dried mud. Therefore the patches must be built up again the following summer. Traditional methods of distribution study would not detect the two distinct causes of patchiness of duckweed in this lake.