44 MARCH 2014 • WORLD AQUACULTURE • WWW.WAS.ORG grass, or hay have been applied to ponds to encourage sedimentation of soil particles. This treatment gives unpredictable results, and because application rates of 1,000 to 2,000 kg/ha of organic matter are necessary, low dissolved oxygen concentration can result. Soil particles remain suspended longer in weakly-mineralized water than in waters with greater concentrations of ions – especially divalent ions like calcium and magnesium. Therefore, liming sometimes will cause suspended soil particles to settle out in ponds with low alkalinity and hardness. Gypsum (calcium sulfate) applied at 100 to 200 mg/L to increase calcium concentration is the most common means of clearing turbidity. Gypsum treatment provides a residual effect because calcium concentration will remain elevated for several months to several years depending upon hydraulic flushing rate. Alum (aluminum sulfate) is sometimes applied to ponds to effect rapid flocculation and sedimentation of suspended soil particles (Boyd 1979). The treatment rate usually is around 20 to 30 mg/L but this chemical provides no residual effect. It is best to test the water to determine the alum rate because, if not enough alum is added to effect flocculation of suspended solids, the alum used in the initial treatment will count towards the amount needed for retreatment. Alum is highly acidic, making it potentially dangerous to workers, and doses greater than the total alkalinity can lead to low pH and fish stress or mortality. Calcium Concentration. High calcium concentration favors rapid precipitation of phosphate as calcium phosphate. For example, to achieve the same level of fish production, ponds in Israel with highly-mineralized water needed three times more phosphate fertilizer than required in ponds in Alabama with weaklymineralized water (Boyd 1990). The use of liquid fertilizers is particularly desirable in water with high calcium concentration because phosphate will remain in solution long enough for some of it to be absorbed by phytoplankton. Some coastal areas in the southeastern United States are underlain by aquifers containing naturally-softened groundwater than has high alkalinity (100 to 500 mg/L) but low total hardness (often < 20 mg/L). In a few places, such waters are used to supply sportfish ponds (Boyd et al. 1978) and fertilization leads to extremely high pH. When pH rises above 8.3, phytoplankton uses inorganic carbon from bicarbonate, causing carbonate concentration to increase. Carbonate concentration may become very high in water of low total hardness because the calcium concentration is insufficient to precipitate enough carbonate to prevent its continuing hydrolysis from increasing pH (Boyd 1990). Calcium sulfate added to ponds to increase total hardness to roughly the same concentration as total alkalinity can ameliorate high pH (Mandel and Boyd 1980). A suitable treatment rate can be estimated as follows: Gypsum rate (mg/L) = [total alkalinity (mg/L as CaCO3) – total hardness (mg/L as CaCO3/L)] x 2. In semi-arid and arid regions, total hardness often exceeds total alkalinity. This condition does not lead to water quality aberrations other than rapid precipitation of phosphate added in fertilizers. • Watershed Characteristics. Nutrient concentrations in runoff from watersheds are greater where soils are fertile than where soils are infertile. Watershed practices also may influence background nutrient concentrations in ponds. For example, in east-central Alabama, unfertilized ponds with woodland watersheds invariably had low phytoplankton abundance, while most unfertilized ponds in pasture watersheds in which cattle grazed had phytoplankton abundance similar to that of fertilized ponds. Manure that washed into ponds following storm events was a continuing source of nutrients for phytoplankton (Boyd 1976). Because of differences in the natural fertility of waters resulting from characteristics of watershed soils and human activities on watersheds, amounts of fertilizer nitrogen and phosphorus needed to develop a phytoplankton bloom will vary among ponds. Moreover, in some situations it is conceivable that potassium, other major nutrients or even trace nutrients should be included in fertilizers. Those providing advice on pond fertilization should be prepared to modify general fertilizer recommendations to assure effectiveness in specific ponds. Ponds with excessively large watersheds may receive so much inflow that fertilizer nutrients are flushed out, preventing development of a plankton bloom. In some cases, fertilization can be effective if delayed until summer when rainfall and runoff are less but ponds that continue to be flushed throughout the summer may not respond to fertilization. Deep water intakes for overflow water (Fig. 3) have been suggested as a means of preventing the loss of nutrients and plankton following heavy rainfall (Boyd 1990). The extent of adoption and the effectiveness of this procedure have not been evaluated. Aquatic Macrophytes. Aquatic macrophytes can be a major problem in sportfish ponds. The best way to prevent growth of rooted aquatic macrophytes is to deepen edges of ponds so that all areas are about 45 to 60 cm deep, establish a phytoplankton bloom early in the spring, and maintain the bloom throughout the growing season. Fertilization of ponds in which dense stands of macrophytes are already present often leads to greater macrophyte growth rather than increasing phytoplankton productivity. Fertilization will not control floating plants and mat-forming macrophytic alage. Discussion of herbivorous fish, herbicides, and other means of macrophyte control is beyond the scope of this article. Initiating and Maintaining Blooms. Plankton blooms may be difficult to initiate in the spring – especially in small, shallow ponds. Fertilizers can be applied at 1- or 2-week intervals in such ponds to stimulate the onset of phytoplankton growth. Once a bloom is established, care should be taken to prevent it from declining until the water clears or from becoming too dense and leading to low FIGURE 3. A deepwater intake for overflow water can prevent the loss of nutrients and plankton following heavy rainfall and watershed runoff.
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