Ecological stoichiometry and nutrient reduction strategies for intensive yet sustainable hybrid pond systems – concepts for regenerative aquaculture

In this study, we explored some ecological stoichiometry theory (ES theory) and plankton ecology group model (PEG model) concepts that may assist regenerative mechanisms in hybrid pond aquaculture that are gaining popularity worldwide. Using fish-free pond-water mesocosms, we examined the response of algal growth to additions of fish feces and carbonaceous material under realistic or challenging dilution scenarios during both dynamic and steady states (Experiments I-II). Using pond living labs, we examined uncertainties in zooplankton body composition under variable bottom-up and top-down proxies, mediated by nutrient stoichiometry and fish stock index (FSI), respectively (Experiment-III). Experiment I, which explored graded levels of N and P at the expense of cellulose-lignin type organic C, showed P-enrichment explaining treatment differences, with elevated C-inputs partially suppressing algal proliferation. Experiment-II, which explored stoichiometric effects of molar C:P (at fixed N) and N:P (at fixed C) ratios, indicated that solids with lower C:P and N:P ratios may favor algal response, with N:P exerting influence likely over C:P under the experimental conditions tested. At high C but low P, brownification or dissolved oxygen risks were hinted. Experiment-III revealed that zooplankton (>90% cladocerans) may maintain their amino acids under fluctuating bottom-up/top-down pressures, except tyrosine (for storage proteins). The fatty acids 20:4n-6, 20:5n-3, and 22:6n-3 (for reproduction) in zooplankton exhibit greater variability under variable bottom-up/top-down pressures and irrespective of season. Such sensitives in cladoceran body nutrient reserves may be concomitant with shifts in population fitness (condition factor). Under the experimental conditions tested, our findings hint that by better linking the ES theory (C-amendment, P-reduction, but carefully avoiding brownification risk) and PEG model (fish−−scenario), regenerative aquaculture could be explored in future hybrid ponds.