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An experiment was carried out by separation of a shallow man-made lake into two basins with an earth dam. Unintended leaks allowed extremely fertile river water to feed both basins so that nutrients were unlikely ever to have been limiting to phytoplankton populations. The two basins had greatly differing fish populations. In the basin in which these were low, a chain of events took place in which zooplankton grazing reduced phytoplankton growth sufficiently t o allow establishment of aquatic macrophyte populations and an associated diverse benthic invertebrate community. I n the basin to which fish had free access from the river, phytoplankton populations remained high, and no aquatic macrophyte community developed. The implieations of thrse observations for shallow lake management are considered.

Events in the plankton community of a 120 ha, shallow (1.2 m), brackish lake have been studied with the help of two watertight enclosures, each of which isolated about 300 m3 of water and its underlying sediment. Analyses of soluble reactive phosphorus, total phosphorus, ammonia-N, nitrate-N, silicate-Si, chloride, photosynthetic pigments, phytoplankton and zooplankton counts are reported and are related to inputs of nitrogen and phosphorus into the lake from adjoining basins and the catchment, and to the hydrologic regime. The inputs of phosphorus are budgeted and a very large roost of black-headed gulls appears to provide most of the phosphorus supply. The results are discussed with reference to the eutrophication of the lake and decline of aquatic macrophytes which have occurred in the past ten years.

K e y words: Cross-linked polyacrylamide, evapotranspiration ratio, water use efficiency, water storage, crop establishment.Cross-linked polyacrylamide expands upon contact with water to form a pliable gel that in a growing medium constitutes a reservoir of moisture available for uptake by plants. Trials with seedlings of three crop species (Lactuca sativa L , Raphanus sativus L , Triticum aestivum L) using measurements of yield and evapotranspiration ratio show that gel-stored moisture is utilised with a greater ef3ciency than conventional forms of water. Moreover, gel storage of water provides a buffer against temporary drought stress and the potential for reducing the risk of failure of certain crops at establishment.The development of new techniques and materials for enhancing the efficiency of use of water supplies where they are scarce or where irrigation costs are high is a priority for the survival of rural communities in arid and semi-arid regions. Measures that permit improved use of water resources, and better survival and growth of agricultural and forestry crops, must be able to assist in buffering the periodic failure of seasonal rainfall, and be both economic and practical.Among recent initiatives to achieve better use of water supplies is the introduction of synthetic, cross-linked polymer additives for growing media. Polyacrylamide, polyvinylalcohol and starchacrylate co-polymer superabsorbents have the capacity to absorb substantial quantities of water and subsequently release this moisture from the gel matrix that forms upon contact between the original crystalline polymer and free water. Formulations in the polyacrylamide category can absorb up to 500 times their own weight of distilled water (Johnson 1984a), though this performance is notably reduced in the presence of soluble salts (Johnson 1984b).Addition of superabsorbent gel to growing media has been shown to increase water holding capacity by up to 400% (Johnson 1984a) and to decrease water stress 43 1 J Sci Food Agric 0022-5142/90/$03.50

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