48 DECEMBER 2013 • WORLD AQUACULTURE • WWW.WAS.ORG As an additional consequence of water retention in ponds and their management intensity, a significant decrease in bacterial loading was measured (Fig. 6). As a result of filtering by pond zooplankton (cladocerans in particular), the counts of intestinal bacteria declined from tens of CFU/mL in inlet water to almost zero in outlet water. Most evident was the decline in enterococci, the counts of which dropped, on average, from 32.0-34.0 to 0.75-0.78 CFU/mL in October, representing 97.7 and 98.9 percent reduction, respectively. Counts of E. coli also declined at pond outlets, with the most noticeable contrast seen in May, when their counts were significantly reduced, in one case from 22,000 CFU/ mL to only 0.2 CFU/ mL (<0.001 percent) at the outlet. However, no significant changes were recorded in the counts of mesophilic bacteria (22° C), which are important decomposers and thrive at temperatures between 20 and 45° C during warm summer months. Their decrease was most obvious only in October, with a 92.9 percent reduction (181,000 to 12,850 CFU/mL). However, such a decline is undoubtedly associated with the continuous drop of surface waters temperature below 20° C after early September (Fig. 6). An extraordinarily high self-purification capacity of carp ponds has been documented by Mlejnková and Horáková (2009), who showed that approximately three months after a semiliquid manure application, the level of faecal and organic indicators in treated ponds was not significantly increased and did not significantly differ from non-manured ponds. In a pond supplied with municipal wastewater, there was a highly significant decrease in average numbers of faecal indicators (E. coli) at TOP, FIGURE 4. Basic physico-chemical aspects of water quality in the inlet and outlet of the SIB pond during the growing season. BOTTOM, FIGURE 5. Nutrient concentrations in the inlet and outlet of the SIB pond during the growing season. the pond outflow. Similar results were reported by Kuczynski et al. (2003), who registered a thousandfold reduction in bacteria numbers and coliform titre values in waters discharged from ponds supplied with biologically pre-treated municipal sewage. In recent years, pond manuring has been strictly limited by law in the Czech Republic and is now almost never used because of the high trophic levels (occasionally to hypertrophy) reached. The main determinants of water quality in Czech carp ponds, therefore, are related to food input and hydraulic retention times. Saprobiological evaluation indicated that the inflow/outflow discharge ratio in semi-intensive carp on-growing and marketing ponds in South Moravia is the driving force affecting the pattern and extent of changes in water quality between pond inflow and outflow. Outflow water quality corresponded to the alpha-betamesosaprobity interface (SI 2.38-2.82) in all ponds under study, regardless of inflow water quality (Fig. 7). Outlet SI declined (water quality improved) from 2.89 to 2.38 in ponds supplied with poor quality water but increased (water quality worsened) from 2.32 to 2.82 when supplied with good quality water or remained unchanged when supplied with “medium” quality water (SI 2.59). A similar tendency toward water quality improvement was recorded in diversity of the macrozoobenthos community. Properly managed Czech carp fish ponds are safe for the environment during the growing season, serving not only as
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