Effects of C/N ratio on biofloc development, water quality, and performance of Litopenaeus vannamei juveniles in a biofloc-based, high-density, zero-exchange, outdoor tank system

A six-week trial was conducted to study the effects of different levels of organic carbon supplementation on biofloc color, structure and density, inorganic nitrogen and phosphorous dynamics, feed utilization and growth performance of Litopenaeus vannamei juveniles in an outdoor tank system under high-density and zero-exchange conditions. Organic carbon in the form of molasses was added daily to provide calculated carbon to nitrogen (C/N) ratios of 12:1 (CN-12), 15:1 (CN-15), and 18:1 (CN-18) based on the carbon–nitrogen content of the feed and the carbon content of the molasses during the entire experiment period No molasses was added in the control group which had a C/N ratio of 9 (CN-9). One additional treatment in which molasses supplementation was based on the actual level of total ammonia nitrogen (TAN) in the culture water (6 g of carbon was added for each 1 g of TAN as measured weekly), was also included and referred to as treatment CN-adjust. Juvenile shrimp (2.21 ± 0.11 g) were stocked into 20 experimental tanks filled with 500 L of biofloc-rich water. Each treatment had four randomly assigned replicate tanks stocked at a density of 300 shrimp m− 3. Shrimp were fed a formulated feed with 36% crude protein and cultured for 6 weeks. One week after the trial initiation, the color of suspended bioflocs began to differentiate, ranging from green to brown corresponding with the increase in C/N ratios. The higher C/N ratios shifted the predominately autotrophic biofloc to heterotrophic, simultaneously displaying higher levels of volatile suspended solids. The concentrations of TAN, nitrite-nitrogen, nitrate‐nitrogen, total inorganic nitrogen, and reactive phosphorus of all treatments fluctuated, showing a decline in total inorganic nitrogen concentration and an increase in reactive phosphorus concentration with the increase in C/N ratios. All five treatments exhibited survival rates above 95.5% with no significant differences between treatments. Better shrimp final weights and growth rates, with significantly higher yields and lower feed conversion ratio values were found for treatments CN-9, CN-adjust, and CN-12 than the CN-15 and CN-18 treatments. Three significant conclusions can be drawn from the results of this study: (1) VSS and turbidity values were superior quantifying parameters for quantitative determination of biofloc than SS or TSS in a biofloc-based system; (2) once a mature biofloc community is established, TAN and NO2−–N concentrations can be effectively controlled by either heterotrophic assimilation or autotrophic nitrification that helps maintain their concentrations at acceptable ranges for shrimp culture even at high stocking densities; and (3) the mixed type of biofloc dominated by both microalgae and autotrophic bacteria is more beneficial for shrimp performance in high density zero-exchange culture systems than systems dominated by heterotrophic bacteria, and also reduces production costs by reducing organic carbon input and oxygen use. Statement of relevance This paper provides useful info concerning the role of organic carbon supplementations in biofloc-dominated tank system operated with no water exchange and the changes in selected water quality indicators and performance of the Pacific white shrimp, Litopenaeus vannamei under these conditions.