In the 32 rural ponds surveyed the pH ranged between 7 and 8.8 and the total alkalinity between 52 and 244 mg dm-3 (Table 14). Among the inorganic plant nutrients the NH4-N concentration was generally below 20 μg dm-3. However, higher concentrations were found in ponds 2, 11, 16 and 32. The concentration of the NO3-N was even lower. In most of the ponds the nitrate level was found to be less than 5 μg dm-3. The PO4-P concentration was about 1 μg dm-3 in 8 ponds and around 4–8 μg dm-3 in other 12 ponds. However, the other ponds showed as high as 52 μg dm-3. As evidenced in table 14, the nitrogen limitation is more pronounced than the phosphorous one. Though the survey was not for an entire season or for a longer period, even this single reading clearly indicates that the ponds are highly nutrient-deficient and particularly nitrogen-deficient. However, to have a more detailed picture on the nutrient status a year-long sampling is required using the more sensitive indofenal blue method for ammonia and ascorbic acid variant of the molibdane blue method for phosphate instead of the methods formerly used.
The nutrient status of the sediment, at least in the old rural ponds with thick anaerobic sediment, differs completely from that found in the overlaying water column (Table 15). The sediment is rich in organic and inorganic nutrients. The survey of natural fish food resources in all sediments of the 32 ponds has clearly shown the high content of organic matter in the old ponds. In the sediment, fish food compartments, organic carbon, detritus, benthic animals and zoobiotecton (Table 7) were investigated. The sediment organic-C ranged between 3.2 and 47.7 mg g-1 dry sediment. The highest amounts were determined in the old ponds with thick sediment layers. In pond 31 the organic carbon content was near to the value of 5 percent of the sediment weight. In the newly constructed or sediment excavated ponds the organic carbon content was very low. The organic matter accumulated in the sediment was highest in those ponds which have the highest surrounding human and animal livestock population. This relationship is obvious in the case of ponds 7, 14 and 16.
In pond 1, when the dissolved nutrient compartments in the different sediment layers (using the sediment squeezing procedure to collect the pore or interstitial water out of the sediment) were investigated, results showed that there was high organic content with a very large quantity of ammonia absorbed to the inorganic sediment particles and also freely dissolved in the sediment water. The dissolved phosphate phosphorous concentration was also very high in the sediment water. All the basic nutrients in the sediment water were about a thousand times higher than in the above-laying water column. This large nutrient store is, however, almost locked in the sediment and not utilized in the rapid nutrient cycling of the water column. Due to the small size as well as the steep and high embankment, only the weak convection current and the molecular diffusion bring up some nutrient from the sediment into the water column. The low population density of the common carp (a very effective bottom-stirrer) is not sufficient to maintain a proper condition at the sediment water surface. Further, the benthic animal populations were very scarce due to the anaerobic nature of the sediment and so their stirring activity is also lacking. After measuring the ammonia and phosphate concentrations in the different layers of the sediment pore water with the squeezing procedure, the daily ammonia and phosphate release by the sediment through molecular diffusion was determined. This was based on the values of the measured nutrient gradients and the molecular diffusion coefficients of the particular nutrient molecules and the Fick equation applied for calculation. The daily release of ammonia was around 20 mg m-2 and that of the phosphate was less than 2 mg m-2. This slow rate of nutrient release by molecular diffusion is not sufficient to cover the large nutrient uptake potential in the overlaying water column. On the same day, the ammonia uptake potential, Vmax measured with the Michaelis-Menten procedure was as high as 261 μg dm-3 h-1. This means that several grammes of ammonia would be required instead of 20 mg to cover the nutrient requirement in the water column.
Thus, the only mechanism which brings up the sediment nutrient into the water column is the Microcystis bloom. This particular blue-green algal group is able to utilize the sediment nutrient during the early developmental stage as well as during their diel vertical movement. This ability also explains the common occurrence of Microcystis bloom in ponds with nutrient-deficient water column but thick nutrient-rich and anaerobic sediment. The nutrients brought by Microcystis bloom into the water column at times become available for other algal species when the large Microcystis biomass starts to collapse and decay. This phenomenon was apparent in pond 1 which was investigated in more detail. The ammonia nitrate and phosphate concentrations were very low in this pond during January and February but in mid-March, after a very rapid collapse of the Microcystis population, the nutrient level increased sharply (Table 16). The 5–10 μg dm-3 ammonia concentration reached a value of 1 000 μg dm-3, nitrate concentration from 10 μg dm-3 to 700 μg dm-3, and phosphate concentration reached the value of 2 000 μg dm-3.
A detailed diel investigation of the oxygen and temperature condition in pond 1 is characterized by the anaerobic sediment and the limited oxygen and nutrient transport in this small and sheltered pond. During 2 diel investigations in February there was a very distinct oxygen stratification in the water column (Table 17). In the lower water layer near to the sediment the oxygen content was very low falling to a value of 0.1 mg dm-3. During the same days there was a distinct but very weak convection current in the water column brought about by cooling and sinking of the surface water during the second half of the dark period. However, the temperature difference was too small for a strong convection current supplying more oxygen to the bottom water (Table 18). During the diel investigation in March, accompanied by strong wind action, the vertical distribution of the oxygen was more homogenous and the oxygen level in the bottom water reached the value of 8 mg dm-3 in the afternoon. Pond 1 has a larger surface area of 0.75 ha and a lower and less steep embankment and is thus more exposed to the wind action. This unusually windy research day demonstrated the importance of any action promoting the transport processes within the water column and sediment water interface.
