Zróżnicowanie przestrzenne wybranych składników biogennych w śródpolnych oczkach w Polsce Północnej = Spatial differentiation characterising selected biogenic components in mid-field kettle ponds of northern Poland

Abstract

Closed basins are important and fixed elements of a post-glacial landscape, in which they may occupy rather a large percentage of the total area. Sometimes these fill to become bodies of water known as kettle ponds. Each such basin has its own closed catchment at the surface and, owing to the limited circulation of matter in these types of depressions, biogenic components often accumulate to excess. In that context, the work detailed here had as its main objective the identification of sources of biogenic substances like nitrates, ammonium-nitrogen and phosphates, with a view to determining the range of variability characterising their presence in small mid-field ponds present in the catchments of two of northern Poland’s rivers, i.e. the Parsęta and the Borucinka. Most of the kettle ponds under study could not be said to have particularly high contents of nitrate, given an average for the bodies of water studied within the Parsęta basin equal to 2.98 mg·dm-3. There was nevertheless considerable variability in concentrations of the ion – ranging from 0 to as much as 51.30 mg·dm-3. It was the “Sadkowo” pond that might be singled out here for its extremely high concentration (the aforesaid maximum value of 51.30 mg·dm-3; along with a mean value of 13.41 mg·dm-3). In this case a local factor is likely to have been operating – i.e. areal runoff pollution from fields involving nitrogenous compounds. Ponds in the Borucinka catchment had only low concentrations of nitrate, with a mean value for all kettle ponds studied there of just 0.24 mg·dm-3 (with values in the overall range 0 to 2.17 mg·dm-3). Concentrations of the analysed component were thus lower in the Borucinka catchment than in the Parsęta basin, with a key influencing factor likely to have been the sizes of the bodies of water studied. The Parsęta-basin examples were in fact smaller kettle ponds whose catchment features and morphometric parameters are such as to ensure higher concentrations of biogenic (especially nitrogenous) compounds in any standing waters. Mean concentrations of ammonium ions obtained for the Parsęta basin were of 0‑2.41 mg·dm-3, the value averaged for the six ponds being 0.95 mg·dm-3. However, four other Parsęta-basin ponds excluded from the study in fact reported very high values for NH4 + – of up to 25.55 mg·dm-3. The Borucinka catchment again contrasted with the Parsęta basin, with noted concentrations of ammonium-nitrogen both low and of limited variability (in the 0‑1.88 mg·dm-3 range). The average figures for all the depressions studied there was 0.09 mg·dm-3. The situation as regards the two forms of nitrogen was thus similar, with concentrations lower in the catchment of the Borucinka and higher in the basin of the Parsęta. Sizes of bodies of water would seem to be a factor influencing spatial differentiation of NH4 + concentrations. Where phosphate was concerned, kettle ponds within the Parsęta basin had a mean concentration of 0.57 mg·dm-3, with reported values from one pond to another ranging from 0 to 4.46 mg·dm-3. The Borucinka p ds again had lower concentrations of this biogenic substance across a narrower range of values (0 to 3.69 mg·dm-3, mean 0.19 mg·dm-3).