368
M. Kejna et al.
not exceed 1800 m
3 per capita per year [42–44], showing significant seasonal and
year-to-year variability [41].
The agricultural development of the representative catchment of the Struga
Toru´ nska affects the quality of its water. In the upper stretch of the river, the water
is affected by the heavily eutrophicated Lake Mlewieckie, through which the Struga
flows. The water coming out of the lake has an increased organic matter content and
low concentrations of biogenic compounds. In the summer, hypoxia is observed. The
water monitored at Koniczynka, unlike that at Lipowiec, demonstrated a good level
of dissolved oxygen content and was low in organic compounds but the concentration of biogenic compounds, especially nitrate nitrogen, significantly increased
along with mineral substances and suspended matter.
Seasonal variations in the analysed chemical parameters of the water are evident.
The concentration of nitrate nitrogen was found to increase in winter and early spring
months and to fall to a minimum in the summer. That was due to increased demand
for the most easily absorbed form of nitrogen at the time of dynamic growing. On
the other hand, the minimum concentration of phosphates took place in April, at the
start of the growing season, and greatly increased as the season continued. Such a
pattern was observed earlier by Sullivan and Drever [45] and Clark et al. [46]. High
concentrations of phosphorus in the summer are related to reduced water supply
and thus, the development of anaerobic conditions, which favour the release of phosphorus compounds accumulated in river bed deposits [47]. In the spring and summer,
there are also enhanced processes of organic matter decomposition. In the water of
the Struga Toru´ nska, the values of BOD5 are highest—and aerobic conditions most
unfavourable—in the summer.
The increased biogenic content in the winter and spring may be related to intensive
flushing during river surges caused by melting snow and winter and early spring
rainfalls [48]. In agricultural catchment areas, surface runoff from nearby fields
fertilised using mineral and organic substances increases, especially in the spring.
As a result, ample quantities of dissolved substances are introduced [49, 50].
The drainage systems within the catchment of the Struga Toru´ nska are instrumental in conveying biogenic compounds, as they make so-called “preferred pathways” carrying large quantities of dissolved solids into the river [51]. This was
confirmed in research by Krasowska and Banaszuk [52] concerning agricultural
catchments with developed drainage systems.
Looking into the changes of primary concentrations of the water in the Struga
Toru´ nska in 1993–2017, a considerable year-on-year variability was found, which
was determined by hydro-meteorological factors. Aerobic conditions improved
and BOD5 decreased, but the content of dissolved substances expressed by electrolytic conductivity clearly increased due to soil erosion and washing out of
mineral compounds introduced during agricultural activities. Similar tendencies were
observed in other rivers of the region [53].
The impact of agriculture on the quality of surface water is so substantial that the
whole catchment of the Struga Toru´ nska was considered in 2012 to be an area of
particular exposure to pollution with nitrates from agricultural sources [54].
M. Kejna et al.
not exceed 1800 m
3 per capita per year [42–44], showing significant seasonal and
year-to-year variability [41].
The agricultural development of the representative catchment of the Struga
Toru´ nska affects the quality of its water. In the upper stretch of the river, the water
is affected by the heavily eutrophicated Lake Mlewieckie, through which the Struga
flows. The water coming out of the lake has an increased organic matter content and
low concentrations of biogenic compounds. In the summer, hypoxia is observed. The
water monitored at Koniczynka, unlike that at Lipowiec, demonstrated a good level
of dissolved oxygen content and was low in organic compounds but the concentration of biogenic compounds, especially nitrate nitrogen, significantly increased
along with mineral substances and suspended matter.
Seasonal variations in the analysed chemical parameters of the water are evident.
The concentration of nitrate nitrogen was found to increase in winter and early spring
months and to fall to a minimum in the summer. That was due to increased demand
for the most easily absorbed form of nitrogen at the time of dynamic growing. On
the other hand, the minimum concentration of phosphates took place in April, at the
start of the growing season, and greatly increased as the season continued. Such a
pattern was observed earlier by Sullivan and Drever [45] and Clark et al. [46]. High
concentrations of phosphorus in the summer are related to reduced water supply
and thus, the development of anaerobic conditions, which favour the release of phosphorus compounds accumulated in river bed deposits [47]. In the spring and summer,
there are also enhanced processes of organic matter decomposition. In the water of
the Struga Toru´ nska, the values of BOD5 are highest—and aerobic conditions most
unfavourable—in the summer.
The increased biogenic content in the winter and spring may be related to intensive
flushing during river surges caused by melting snow and winter and early spring
rainfalls [48]. In agricultural catchment areas, surface runoff from nearby fields
fertilised using mineral and organic substances increases, especially in the spring.
As a result, ample quantities of dissolved substances are introduced [49, 50].
The drainage systems within the catchment of the Struga Toru´ nska are instrumental in conveying biogenic compounds, as they make so-called “preferred pathways” carrying large quantities of dissolved solids into the river [51]. This was
confirmed in research by Krasowska and Banaszuk [52] concerning agricultural
catchments with developed drainage systems.
Looking into the changes of primary concentrations of the water in the Struga
Toru´ nska in 1993–2017, a considerable year-on-year variability was found, which
was determined by hydro-meteorological factors. Aerobic conditions improved
and BOD5 decreased, but the content of dissolved substances expressed by electrolytic conductivity clearly increased due to soil erosion and washing out of
mineral compounds introduced during agricultural activities. Similar tendencies were
observed in other rivers of the region [53].
The impact of agriculture on the quality of surface water is so substantial that the
whole catchment of the Struga Toru´ nska was considered in 2012 to be an area of
particular exposure to pollution with nitrates from agricultural sources [54].