The eutrophication of natural waters is a common problem. Eutrophication describes the accumulation of nutrients (especially phosphorus and nitrogen compounds) which stimulates the growth of organisms (mainly phytoplankton). This results in a lack of oxygen and ultimately in the death of living beings in the water.
This study investigates the effect of stored plants next to natural waters on the eutrophication tendency. The results have shown that it is advisable to move the cut plants from the edge of the water quickly.
Keywords: eutrophication, biochemical oxygen demand, BOD5 test, urban water bodies, stored plants, plant juice
Cut plants are often left on the banks of urban water bodies. We suspect that the outward-oozing plant liquid may affect the quality of the water, which can increase the eutrophication tendency. ((Kalff, Jacob, Limnology. New Jersey: Prentice Hall, 2001.)) ((Martin, James, L. Hydro-Enviromental Analysis. Boca Raton: CRC Press, 2013)) ((Schönborn, Wilfried, Lehrbuch der Limnologie [Limnology]. Stuttgart: Schweizerbart, 2003.)) ((Schwoerbel, Jürgen and Brendelberger. Heinz, Einführung in die Limnologie [Limnology]. Heidelberg: Spektrum, 2005)) ((Thienemann, Alfred, Die begriffliche Unterscheidung zwischen See, Weiher, Teich. – Rheinische Heimatpflege, Jg. 12 (½), Düsseldorf 1940)) ((Wetzel, R.G. Limnology – Lake and River Ecosystems. London: Academic Press, 2001))
The plant juice running out from a pile of plants was studied with the help of the five-day biochemical oxygen demand (BOD5) test. ((Schönborn, Wilfried, Lehrbuch der Limnologie [Limnology]. Stuttgart: Schweizerbart, 2003.)) The BOD5 test determines the level of oxygen consumption within five days. The less oxygen consumed, the higher the water quality (or the less likely it is being burdened with organic substances). A high level of oxygen consumption is more likely to result in eutrophication and the death of the entire body of water.
The benefit of the BOD5 test is its indirect measure for the burden of water with bio-degradable organic substances. Decomposers which break down dead animals and plants at the bottom of a body of water consume a certain degree of oxygen, hence there is a correlation between organic matter build up and the oxygen consumed. ((Schönborn, Wilfried, Lehrbuch der Limnologie [Limnology]. Stuttgart: Schweizerbart, 2003.))The more the decomposers reduce organic substances, the more oxygen they will consume. ((Schönborn, Wilfried, Lehrbuch der Limnologie [Limnology]. Stuttgart: Schweizerbart, 2003.))((Schwoerbel, Jürgen and Brendelberger. Heinz, Einführung in die Limnologie [Limnology]. Heidelberg: Spektrum, 2005))
Results and Discussion
As Figure 3 shows, the plant juice oozing out from each pile of plants (namely, piles of leaves, grass and algae) has high BOD5 values. For comparison, a BOD5 value of 10ml/g is the worst grade a body of water can be assigned. 
From this, we conclude that the juice running out from the pile of plants contains large amounts of organic materials. During degradation, lots of oxygen is consumed by micro-organisms (decomposers). Therefore, the liquid of this plant could affect a neighbouring water body negatively and increase the risk of eutrophication.
Furthermore, we have found that the BOD5 values of plant juice decrease with an increasing storage duration (Figure 4).
We have been dealing with the question of whether the juice of plants will have a negative influence on a body of water. For this reason, we used fresh algae clusters which were piled up at the Lake of Elfrath as well as a pile of grass next to Sollbrüggenpark pond. 5 We measured and compared the following BOD5 values (Figure 5):
- BOD5 values of plant juice running out from a pile of plants (blue column);
- BOD5 values of the water next to the pile of plants where the plant juice had run into the water (orange column); and
- BOD5 values of the water five metres away from the entry point where the water quality would not be affected by the juice of the plants (yellow column).
As Figure 5 shows, the algal mounds at the Lake of Elfrath have seemingly no effect on the value of BOD5 in the body of water. Both of the values measured in two locations in the water (at the edge of the pile, where the plant juice runs into the water, and a few meters away from the plant-liquid entry point) are approximately equal. However, the grass pile in Sollbrüggenpark seems to have an influence on the value of BOD5 in the water: the value at the edge of the pile is higher than at the measuring point several metres away.
One possible reason for this difference is the distinct nature of each water body. The Sollbrüggenpark is a small and flat pond which is neither moved nor mixed by the wind. The Lake of Elfrath is a large water body. The water of the lake is more or less constantly in motion, swirled by the wind.
We probably noticed no effect in the Lake of Elfrath because our sample was constantly replenished by the fresh water currents.
Overall, we have found that a pile of vegetation right next to a water body could adversely affect its water quality and eutrophication tendency. However, a practical influence was only noted in a small pond (in Sollbrüggenpark) and not in a bigger lake (the Lake of Elfrath). Even if the expected impact is relatively small, it seems advisable not to store plant remains in close proximity to water bodies. The pile of plants should either be stored several meters away from the shore until its removal, or it should be removed immediately.