Introduction
Aquatic ecologists or limnologists mostly concern themselves with an elemental view of the aquatic food web, such as nutrient cycles or ecological stoichiometry. Whereas aquaculture nutritionists concern themselves with certain essential biomolecules containing those elements. In temperate shallow-lake ecosystems including European fishponds, primary nutrients and plankton dynamics have a rhythm, closely following the PEG (plankton ecology group) model. There is no such model yet on a biochemical view of aquatic food web. What about the dynamics of biomolecules such as specific amino acids and fatty acids relevant for fish nutrition (?).
We present some analysis, how Earth’s physical forcing (temperature, photoperiod), primary ecosystem nutrients (carbon, nitrogen, phosphorus) and consumers’ grazing pressure (thermally corrected fish growth, plankton consortia relative trophic position) shape the key amino acids and fatty acids flows in planktonic food web to fish in standing freshwater bodies.
Materials and Methods
A set of six identical (0.2 ha area, 1 m depth), stocked (316 kg ha−1 Cyprinus carpio), fed (wheat 641 kg pond−1; June to September) experimental fishponds (n=6) in Vodnany, Czechia (49°09′24.9”N 14°09′54.2″E; 404 m a.s.l.) was investigated from May to October 2022. Monthly water quality, climate data, underwater pond temperature data loggers, fish body weight measurements (growing from 0.3 to 2 kg), ‘carp gill filterable’ net plankton consortia (>200-500 µm mesh) were collected. The net plankton consortia (from beginning of fish feeding in June to cessation of fish feeding in October) were lyophilized and analyzed for amino acids and fatty acids content. To understand grazing pressure (feeding demand), thermally corrected growth (TGC) of fish (n=12 month−1 pond−1) in ponds were calculated, with precisely calculated degree days in ponds. The relative trophic position (RTP) of monthly plankton consortia was estimated from the difference of [glutamic acid (−) phenylalanine]. As a forecasting of algal blooming tendencies, phytoplankton phosphorus resource use efficiency (RUEP) was estimated from [particulate organic carbon/ Total P content in water]. A simple visual trend analysis, followed by Pearson’s 2-tailed correlation and 2nd order polynomial regression analysis with a strict alpha-level (p=0.01) and minimum 25% variability explanation (R2 >0.25) criterion (using OpenAI data analyst 4o and R) was used.
Results
Amino acids: Most critical indispensable amino acid for fish ‘lysine’ in gill-filterable plankton consortia became deficient mid-season (August) in fishponds, but not ‘methionine’. A strong relationship could not be established with Earth’s forcing (temperature and photoperiod) or ecosystem nutrients (C, N, P). A relationship with grazing pressure (RTP) could be documented. Plankton consortia lacking higher trophic level organisms such as zooplankton (over-grazed by carp due to extraordinary TGC >2.5 units in preceding months) became deficient in lysine. Missing lysine and lowered RTP also coincided with peaking of algal blooming tendencies in fishponds (RUEP).
Fatty acids: saturated fatty acids (SFA) in planktonic food, that is a symbol of energy reserve in food web and easiest metabolic fuel for mitochondrial β-oxidation in fish, progressively declined from the beginning (June) till end of season (October). A strong relationship with decreasing photoperiod (<14 hours), air temperature (<20°C), carbon accumulation (>10 mg dissolved C l−1) and consumer grazing pressure (fish TGC >2.5 in preceding months) was documented for SFA deterioration. The long-chain (>C20) polyunsaturated fatty acids (LC-PUFA) is upregulated in food web due to a strong relationship to cold response (air temperature <19°C, photoperiod ≤12 hours), but require a nitrogen presence (>1.25 mg dissolved inorganic N l−1) and low consumer grazing pressure (fish TGC <1). Very long chain (>C22) polyunsaturated fatty acids (22:5n-3, 22:6n-3) within PUFAs are only observed during long photoperiod (>14 hours), but not related to temperature or nutrients or grazing pressure.
Discussions
Nutrition from natural food to fish in ponds is not flawless. In fishponds, fatty acids were majorly influenced by Earth’s forcing, ecosystem nutrients, followed by some effects of consumer’s grazing pressure. Whereas perturbations in amino acids are mostly sensitive to grazing pressure by consumers, and resultant trophic fractionation of amino acids in the pond food web. Perhaps switching-off lysine synthesis at lower trophic level (RTP). Knowing these trends, we could adapt to (complement) the flaws in natural food base. The nutritional quality of European fishponds could be enhanced by balanced pond feeding (nutrition) strategies, appropriate fish stocking densities and artificially creating zooplankton refuges.
Acknowledgment: The study was funded by Ministry of Agriculture, Czech Republic (project NAZV QK22010177). Martin Blaha, Jaroslav Vrba, Lenka Kajgrova, Petr Blabolil, Petr Dvorak, Tomas Kolarik, Vit Nahlik and Vitezslav Plicka are acknowledged for field sampling. A special thanks to Bela Urbanyi (MATE, Hungary) for some ideas.