WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2014 13 sunlight intensity and duration, transmission of underwater light, nutrient availability and average algal cell age. Water temperature and sunlight depend on geographic location, climate, and season— factors that cannot be controlled by the culturist except through site selection. Nutrient availability seldom limits algal productivity in intensive pond aquaculture because large amounts of waste nutrients are generated as a byproduct of feeding. Underwater light conditions and cell loss can, however, be manipulated in the PAS to increase phytoplankton productivity. The PAS algal basin is shallow (~0.5 m) so that phytoplankton cannot sink below the well-lit photic zone. A slow rate of mixing throughout the algal basin allows algal cells to avoid light limitation of growth, a common feature of static, hypereutrophic fish ponds. Mixing also enhances nutrient dispersion throughout the basin, thereby insuring that the pond volume can be fully utilized. Although algal growth generates oxygen as a byproduct of photosynthesis, subsequent aerobic decomposition of algal biomass consumes an approximately equivalent amount of oxygen. Aerobic decomposition also releases the same amount of carbon dioxide and ammonia initially assimilated by algae during growth. To provide net oxygen input and net nitrogen and carbon removal, algal biomass must be either removed or decomposed anaerobically. Algal biomass can be removed directly or indirectly by incorporating it into other life forms that are eventually harvested from the pond. Biomass can be decomposed anaerobically in pond sediments or isolated tanks or reactors. Controlled removal rates can increase algal productivity by decreasing average algal cell age, insuring a fast-growing algal culture that rapidly assimilates nutrients. Algal cell age can be manipulated by providing some method to continually crop or harvest algal biomass from the pond. Controlled algal harvest also prevents excessive algal abundance, thereby decreasing water column respiration and increasing light availability. Phytoplankton communities in the PAS are continually “cropped” using co-culture of filterfeeding fish such as tilapia (Turker et al. 2003a), or by using physical processes such as coagulation and precipitation of algal biomass. The primary objective of growing tilapia in the Arkansas ponds described earlier was to produce a secondary fish crop on otherwise unused food base in catfish ponds. In contrast, the primary objective of growing tilapia in the PAS was to provide a grazer to harvest phytoplankton and zooplankton. Tilapia must be confined in raceways or net pens, isolating them from other feed sources such as uneaten feed, fecal waste, or pond sediments; otherwise, planktongrazing efficiency was reduced. We referred to this technique as “co-culture” to distinguish the practice and intended effect from polyculture. Field trials demonstrated that tilapia co-culture could be used to maintain algal standing crops equivalent to a water transparency of 12 to 15 cm while ensuring high rates of algal productivity and substantially reducing the occurrence of undesirable cyanobacteria (Turker et al. 2003b). The combined use of shallow basins, pond-wide mixing, and continuous cropping of phytoplankton and zooplankton biomass provided a 3- to 4-fold increase in phytoplankton production compared to traditional ponds, allowing a proportional increase in the upper limits of fish carrying capacity and production. Continuous water flow throughout the PAS is critical. Water is directed through the culture raceways to maintain good water quality and ensure optimal fish growth. Uniform flow in the algal basin assures well-mixed conditions, resulting in more predictable and consistent phytoplankton productivity and community structure. To insure uniform continuous water movement, a device was needed that could move large water volumes at low energy input with minimal capital investment and low maintenance cost. Slowly rotating (1-3 rpm) paddlewheels moving very large water volumes at hydraulic heads of only 2-5 cm were found to be best suited for this application (Fig. 5). LEFT, FIGURE 5. Diagram of the 0.8-ha PAS unit at Clemson University. RIGHT, FIGURE 6. The 0.8-ha PAS unit installed in 2000 at Clemson University. (CONTINUED ON PAGE 14)
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