Introduction
Fishponds, artificial ecosystems built exclusively for fish production, had been integral parts of the European landscape for centuries. The current fishponds have become less predictable regarding production processes, hence, also less cost-efficient, more stochastic, and greatly unstable systems. While human activities purposefully eutrophicated fishponds in the last century to increase their productivity, little is known about stoichiometric shifts of macronutrients available for planktonic food webs in these shallow ecosystems. Understanding the complex interactions between nutrient stoichiometry and ecosystem dynamics in fishponds is crucial for effective management and conservation efforts aimed at preserving these vital aquatic ecosystems while keeping them as protein-production units. We hypothesized that (a) the nutrient regime might vary both on regional and seasonal scales and (b) fishery management interventions might enforce stoichiometric imbalances. The long-term excessive nutrient loading may thus lead to stoichiometric imbalances and further worsen the bad nutrient state of fishponds yet affected by a region and a period of the vegetative season.
Material and methods
We surveyed 31 fishponds from lowland to highland regions of Czechia (altitudinal range from 170 to 730 m a.s.l.). All fishponds are polymictic, eutrophic to hypertrophic waterbodies (area 0.23–449 ha) with controlled fish stock. The fishponds under study were used for rearing ongrowing common carp exclusively. They were sampled monthly over a vegetative season from April to September (Apr–Sep). Data spanning the vegetative seasons were taken as observation units, herein referred to as pond-year, resulting in 150 pond-year cases. Dissolved inorganic N to total P ratio (DIN:TP) was used as an indicator for inferring in situ N versus P limitation of primary production from chemical data. DIN was calculated in-silico: DIN = NO3-N + NH4-N. We analyzed the data using generalized linear mixed models (GLMMs) with Gaussian distribution and log10 and log10+1 transformations to explore the drivers of chlorophyll-a and DIN:TP concentrations in water.
Results
We surveyed the seasonal trends of chlorophyll-a and the dynamic of available nutrients to the planktonic food web in 31 ponds used exclusively for fish production. Results of data compiled over various temporal and spatial scales suggested that fishponds might be stressed by seasonal and regional stochiometric imbalances of available macronutrients that could drive the concentration of chlorophyll-a (Fig. 1).
Discussion
Pond aquaculture is becoming progressively more marginal and challenged by many factors, such as prices, sustainability, environmental concerns, and resource utilization. Besides altitude, supplementary feeding (e.g., cereals and triticale; rich in phytate P) seems to be connected to the DIN:TP shift. The dynamics of available nutrients changed over the vegetative season with the cumulative input of supplemental feed. Most of the feed (65%) is fed in the second part of the growing season, at the time when the natural prey is over-grazed. We suggest that the current pond management practices deserve a revision, as warranted by changing nutrient status, stoichiometry, and climate.