Introduction
Phosphorus (P) is directly linked to eutrophication in shallow-lake systems, either alone (absolutely) or in tandem (relatively) with nitrogen (N). P in aquatic ecosystem can occur bound to sediment (legacy P), seston and nekton (organically bound P), and in free algal-reactive state (orthophosphate-P). The last one is of concern for standing freshwaters, which trigger an overshooting of primary productivity (algal blooms); under equally strong dissolved inorganic nitrogen fluxes (DIN) [1]. Already, the primary excretory product of fish is DIN (ammoniacal-N). In this context, the branchial-urinary losses of digested P by fish as dissolved reactive phosphorus (DRP) is likely the worst situation. Fish can act as pumps in fishponds converting organically (food) bound P to algal-reactive orthophosphate-P [2]. Present study aimed to provide a mechanistic understanding when the fish are source or sink of P, i.e., drive or mitigate dissolved reactive phosphorus (DRP) in fishponds. By doing so, regulating the eutrophication.
Materials and Methods
We hypothesize the more suppressed DRP excretion could be, the more efficient ecosystem P use would be. More P would stay locked in fish biomass (secondary consumers) or zooplankton (primary consumers) and less available to algal biomass (primary consumers). Fish biomass can act as source or sink of P; sometimes related to food’s digestible N: P stoichiometry and many times not. For example, at a constant N or P intake, DRP excretion might depend on supply of one or two indispensable amino acids, and digestible non-protein energy per unit of food intake. Other biological factors, such as presence or absence of scales or switching to basal metabolism might impart control on “phosphorus use efficiency” (PUE) and DRP excretion. Through isogenous but objectively formulated (WinFeedÒ) semi-purified research diets (fed 4% of body weight; split into 2 doses daily), PUE and DRP losses in a cyprinid model (Cyprinus carpio; age 1+; 22 g to 50.4 g) were investigated.
In a series of five experiments (twelve 120 L tank Guelph-RAS; 4 group x 3 replicate design; ~30 days each; ~250 g biomass tank−1), following objectives were investigated: (a) differences in PUE and DRP losses between scaleless and scaly fish under a P-deficient diet (0.21% P); (b) effect of graded levels of bioavailable P (0.21%, 0.83%, 1.39%, 1.92%) on PUE and DRP losses, at isonitrogenous (38% digestible protein, DP) and isoenergetic diets (185 kcal 100 g−1 digestible non-protein energy, NPE); (c) effect of graded levels of bioavailable lysine (1.43%, 1.71%, 2.05%, 2.39%) and methionine (0.21%, 0.49%, 0.69%, 0.89%) on PUE and DRP losses, at isonitrogenous (26% DP), iso-phosphorus (0.6%) and isoenergetic diets (185 kcal NPE 100 g−1); (d) effect of graded levels of dietary non-protein energy (168 kcal, 181 kcal, 183 kcal NPE 100 g−1) on PUE and DRP losses, at isonitrogenous (37% DP), iso-phosphorus (0.9%); (e) lastly, PUE and DRP losses at basal metabolism conditions (7°C, feed deprivation, 30 days). The entire series of laboratory experiments lasted 40 weeks. The analysis had a mass balance or nutritional bioenergetics approach between fed nutrients (with known and standard digestibility) and carcass/ biomass gain of nutrients (growth trial and carcass analysis) to compute N, P retentions and non-faecal losses [2].
Results
At P-deficient diet, PUE was higher in scaly carp (77±5.9% of intake) than scaleless carp (46.9±6.1%). DRP losses (51.1±5.9% of intake) can equal or exceed DIN losses at excess bioavailable P (>0.83 g 100 g−1 fed) or when digestible N: P ratio (≤4.4:1) approaches or breaches organismal/ carp body N: P ratio limit (~4:1).
DRP losses (57.8±4.5% of intake) equaled DIN losses when indispensable amino acid requirements are not fulfilled (Lys ≤1.43%, Met ≤0.39%), even at a comfortable digestible N: P ratio (~7:1). At bioavailable methionine (Met) ≥0.49% and lysine (Lys) ≥1.71%, DRP losses cease (≈0%). At high Lys-Met, P from water maybe absorbed.
When there is insufficient NPE (≤168 kcal 100 g−1), PUE deteriorate and aggravate DRP losses (81.6±4.3% of intake) – even if food N: P ratio is ≥6:1 (above organismal limit). But as a minimum NPE requirement is met (>181 kcal 100 g−1), further increasing of energy has no effect on PUE and DRP losses.
At state of basal metabolism, all pre-stored P remain conserved (PUE 98±1.6% of stored P). There are almost no DRP losses (1.6±1.6% of stored P). Carcass N: P ratio remain tightly conserved at 4.6±0.4: 1 – likely the “organismal storage median”.
Discussions
P release (re-cycling) from fish population may contribute the same or double order of magnitude as the total phosphorus loading from the watershed [3]. The DRP excretion rate of a feeding fish biomass can range from 11 to 375 mmol kg−1 h−1. Not only that, the molar ratio of N:P excretion (i.e., DIN: DRP) could range from 0.8 (bad or eutrophic) to more than 280 (or only N; non-eutrophic) [4].
Results suggest P storage is closely related to organismal body N: P stoichiometric (storage) limit (4:1) and food’s bioavailable N: P ratio. When food digestible N: P ratio come close to 4:1, PUE deteriorate and DRP losses aggravate. To the extent that molar excretion of DRP becomes same or exceed DIN (DIN: DRP= ≤1: 1). Then, algae are neither limited by N nor P; an overshooting of primary productivity happens [1].
However, there are two exceptions. Despite a good digestible N: P ratio in food (above 6: 1 or 7: 1), which is above organismal body threshold (4:1), a maximum P storage in fish biomass does not happen. It occurs when food base is diluted or missing digestible lysine, methionine, and NPE (i.e., “biologically insufficient levels” Lys <1.43 g, Met <0.39 g, NPE <168 kcal per 100 g food intake). The resultant DRP losses aggravate (51-82% of intake P) to the extent that molar excretion of DIN: DRP become ~1: 1 or worse.
We conclude there is negative organismal plasticity in P storage by carp, under insufficient NPE in pond (carcass N: P ratio can go up to 6:1). There is positive organismal plasticity in P storage by carp, under bountiful lysine and methionine in pond (carcass N: P ratio can go down to 3:1). We provide a basis of manipulating food base in carp ponds, to reverse DRP excretion or poor PUE by fish biomass towards a “net P-excavation effect”. Additionally, culture of scaly carp in P-deficient ponds and not putting feed during periods below 10°C surface water temperature is suggested.
Acknowledgment: The study was funded by GAČR 22-18597S and GAJU 047/2021/Z.
References
[1] Elser, J.J., et al., 2007. Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems. Ecol Lett 10(12), 1135-1142.
[2] Roy, K., et al., 2022b. The concept of balanced fish nutrition in temperate European fishponds to tackle eutrophication. Journal of Cleaner Production 364.
[3] Brabrand, A., et al., 1990. Relative Importance of Phosphorus Supply to Phytoplankton Production - Fish Excretion Versus External Loading. Canadian Journal of Fisheries and Aquatic Sciences 47(2), 364-372.
[4] Villeger, S., et al., 2012. Intra‐and interspecific differences in nutrient recycling by European freshwater fish. Freshwater Biology 57(11), 2330-2341.