Spatial Changes in Invertebrate Structures as a Factor of Strong Human Activity in the Bed and Catchment Area of a Small Urban Stream

Abstract

The threats to small urban streams lead to a decrease in their water quality and dysregulate their ecological balance, thereby affecting the biodiversity and causing degradation of indicators that determine the ecological potential. The aim of our study was to determine the impact of abiotic conditions induced by intensive human activity on the community structures of invertebrates (zooplankton and macroinvertebrates) in the small urban stream Bukówka in the Szczecin agglomeration (NW Poland). This stream exhibits the same characteristics as a large river, in which the mass of live organic matter increases with their length. The composition of invertebrates (zooplankton and macroinvertebrates) was strongly influenced by the changes caused by humans in the stream bed. The construction of small reservoirs and bed regulation in this small urban streams had a similar effect on the quality of the water and ecological potential as in large rivers, but at a lower scale.

1. Introduction

Small urban watercourses are a common and desirable component of the landscape of agglomerations [1]. However, urban running waters are intensively affected by humans, most often due to the pollution caused by them and their constant interference in the shape and continuity of the riverbed [1]. These threats lead to a decrease in the water quality of streams and rivers and dysregulate the ecological balance, thereby affecting the biodiversity and causing a reduction in the value of indicators that determine the ecological potential [2]. In addition, these exert harmful effects on other basins. Urban streams flow into larger rivers or lakes, often affecting the abiotic and biotic conditions of the water bodies [3]. These recipients can constitute sites of permanent existence of resident fauna or act as a transitional shelter to native or migratory fauna [3,4]. They might also function as a refuge for fauna that colonize the whole bed from the headwaters to the mouth.

Micro(zoo)plankton and macroinvertebrates are good indicators of the physical and chemical influence on the running water bed [2,5,6,7]. Any spatial change in the invertebrate composition of running waters reflects the physical and chemical effects induced by dam impoundments and bed regulation. Regulated, dammed, and polluted streams or ditches are characterized by smaller numbers and lower diversity indexes of macroinvertebrate taxa compared to forest streams [2,8,9]. On the other hand, zooplankton almost always respond positively to dam impoundments and new conditions that are conducive to their development and reproduction [10]. Among the studies on the presence of micro- and macroinvertebrates in small streams, most have focused on a common topic—the impact of environmental factors (current velocity, water retention time, inorganic nutrients concentration, and other physico-chemical compounds) on their taxonomy and abundance [11,12,13,14,15]. Small urban streams undergo very rapid environmental changes, as these are under the strong influence of local factors that disrupt their functioning, such as human activity in the catchment and in the bed or rapid water runoff from the urban surface.

The urban impact on the spatial composition of invertebrates can be studied via small streams, such as Bukówka located in the Szczecin agglomeration (NW Poland). This watercourse is subject to intensive human activity over its entire length, such as regulation, bed damming, and pollution. The water of the stream is characterized by a high concentration of conductivity, total dissolved solids (TDS), and chlorides; these being very common parameters for indicating human activity. In addition, the lower section of the Bukówka stream is exposed to water inlet from the recipient, the Oder river, within the estuary.

The aim of our study was to determine the impact of abiotic conditions induced by intensive human activity on the structures of invertebrates in this small human-mediated urban stream.

2. Methods

2.1. Study Area

This study was performed in the Bukówka stream (and drainage of the lower stretch of the Oder river, NW Poland) (Figure 1). Bukówka is a regulated and non-natural stream flowing in the Szczecin agglomeration. It is the left tributary of the estuary of the Oder river. Bukówka is 9-km long, has a catchment area of 69 km², and is characterized by a mean slope of 0.075%. The stream provides water to the recreational, strongly eutrophicated reservoir “Słoneczne lake”, also called a pond; therefore, a dam and its impoundments are located along the stream. The total area of this reservoir is 7 ha, and its maximum depth is 1.5 m.

Each section of the stream was regulated and channelized. It means that the bed of the stream was extremely changed by humans. The bed of the upper section (Site 1—U1; Site 2—U2) was covered by sand and dead organic matter. The sediments of the reservoir—“Słoneczne Lake” (POND)—were marshy and also contained dead organic matter. The bottom of the reservoir was densely covered by emerging macrophytes (dominants: Glyceria maxima, Phragmites australis, and Sparganium erectum). Due to the abundance of phytoplankton in the reservoir and low visibility, there was no occurrence of submerged macrophytes. The dominant components of the sediments in the middle section (Site 3—M3; Site 4—M4) were sand and gravel, together with a small amount of organic matter. No macrophytes were observed in this section. The lower downstream sections (Site 5—D5; Site 6—D6) were characterized by a high amount of dead organic matter at the bottom, without the presence of minerals. The riparian zone of the bed in the lower section was covered by emerging macrophytes (dominants: P. communis, G. maxima, and S. erectum). Moreover, this section contained many small floodplains and slack water areas.

The percentage of land use in the catchment was calculated using the Corine Land Cover inventory. The land use in the catchment was separately estimated in the total catchment area and the buffer zone, which was located within 100 m of the stream shoreline. For our calculations, we used the QGIS Wien software [2,7,8] (QGIS Development Team). The land use of the Bukówka total catchment is occupied by agricultural areas (52%), artificial surfaces (44%), seminatural areas (3%), and water bodies (<1) (GIS data). In the 100 m buffer zone the catchment is mostly covered by artificial surfaces (94%) and definitely less by seminatural areas (5%) and water bodies (<1). Along the entire stream there are a numerous rainwater runoffs.

No field sampling permits with regard to the site locations were needed.

2.2. Sampling Methods

2.2.1. Environmental Factors

Environmental factors were measured monthly from March to October in 2016 (n = 8) in the same day when the invertebrate samples were collected. Temperature, dissolved oxygen, pH, and conductivity, as well as the content of chlorophyll a, total dissolved solids (TDS), and chloride were measured in situ using the Hydrolab DS5 multiparameter probe (USA). In addition, water velocity, width, and depth were measured at each site using an OTT electromagnetic water flow sensor (Germany). The values of the measured environmental parameters are reported in

Table 1. Mean ± SD of environmental variables at the examined sites in the Bukówka stream. Temp—temperature; Cond—conductivity.

Site	Temp	Dissolved Oxygen	Ph	Cond	Chlorophyll A	Chloride	Total Dissolved Solids	Width	Depth	Current Velocity
	(°C)	(mg L−1)		(µS cm−1)	(mg L−1)	(mg L−1)	(mg L−1)	(m)	(m)	(m s−1)
U1	14.0 ± 7.3	7.1 ± 2.8	8.5 ± 0.5	866 ± 118	3.4 ± 1.7	2141 ± 3860	0.5216 ± 0.1174	0.88 ± 0.48	0.10 ± 0.05	0.030 ± 0.020
U2	13.8 ± 7.2	5.8 ± 3.0	8.5 ± 0.3	792 ± 161	5.3 ± 3.2	2150 ± 4128	0.4932 ± 0.1147	0.74 ± 0.41	0.12 ± 0.07	0.053 ± 0.037
POND	17.4 ± 7.6	8.9 ± 3.5	8.8 ± 0.4	536 ± 306	193.0 ± 108.9	1769 ± 2933	0.4217 ± 0.1285	-	-	-
M3	16.7 ± 6.1	7.1 ± 3.2	8.6 ± 0.4	608 ± 312	222.9 ± 122.6	1530 ± 2328	0.4277 ± 0.1339	0.94 ± 0.41	0.10 ± 0.04	0.285 ± 0.145
M4	15.0 ± 6.1	8.3 ± 2.0	8.6 ± 0.3	727 ± 160	81.1 ± 42.8	1980 ± 3359	0.4623 ± 0.1033	1.63 ± 0.47	0.11 ± 0.02	0.392 ± 0.166
D5	14.2 ± 6.0	7.1 ± 1.9	8.5 ± 0.3	1026 ± 231	39.1 ± 17.8	1669 ± 2224	0.6504 ± 0.1373	3.08 ± 0.30	0.18 ± 0.05	0.265 ± 0.058
D6	15.4 ± 7.4	7.9 ± 3.2	8.5 ± 0.4	903 ± 246	42.6 ± 25.5	1951 ± 3034	0.6392 ± 0.0636	4.19 ± 1.04	0.73 ± 0.19	0.038 ± 0.033

.

2.2.2. Zooplankton

Zooplankton were collected monthly from March to October in 2016 (n = 8). At each site a 50 L water sample was collected from the surface of the river current using a 10-L bucket. The water was filtered through a plankton net having a mesh of 30 μm. The samples were concentrated to 150 mL and fixed in a solution of 4%–5% formalin. Then, they were divided into ten subsamples measuring 3 mL each, and the contents of each were counted using a Sedgewick–Rafter counting chamber. A Nikon Eclipse 50i microscope was used for the identification of organisms. Species were identified using the keys described by Radwan [16] and Rybak and Błędzki [17]. Rotifers were divided into two categories according to their habitat preference—pelagic species (plankton) and benthic, epiphytic, and epilithic species attached to the substratum or otherwise known altogether as benthic species [16].

The sites for zooplankton sampling were selected in the lotic, free-flowing sections of Bukówka. Two sites were located in the upper section (U1, U2) before the reservoir, two in the middle section (M3, M4) below the reservoir, and two in the lower-downstream section (D5, D6) of the stream. In addition, a control site was chosen in the open water zone of the reservoir (POND). In the lower section, there was the inlet of waters from the recipient, the Oder river.

The Lagrangian scheme was followed in the Bukówka stream, according to which the cross sections were sampled in a downstream sequence with the zooplankton sampling interval approximating the time of travel between the sites. The sampling sites of Bukówka were chosen taking into account the following: (1) influence of the reservoir, (2) influence of the water inlet from the Oder river, (3) differences in environmental conditions, and (4) ease of access.

2.2.3. Macrozoobenthos—Macroinvertebrates

Macroinvertebrates were collected twice in 2016—in April and October. Sampling was conducted using a hydrobiological net, by scraping a fragment sized 0.5 m² from the bottom. The material was preserved in 96% ethyl alcohol. In the laboratory, the organisms were separated and identified to the family level before they were counted, excluding Oligochaeta, which was counted at the same level. A specialist key was used for the identification of the organisms, and their abundance was expressed in ind⋅m².

The following indexes and metrics were used to estimate the quality of water: BMWP-PL (Biomonitoring Working Party for Polish Standards), Margalef biodiversity index, ASPT (Average Score Per Taxon), Shannon–Weaver biodiversity index, TBI (Trent Biotic Index), GOLD (% of Gastropoda, Oligochaeta, and Diptera in the total abundance), and %DIPTERA (% of Diptera in the total abundance) [18]. The Polish BMWP-PL (Biological Monitoring Working Party) index is based on the British BMWP, where the list of indicator taxa (families) has been adapted to Polish standards. Each family from the BMWP-PL list is assigned. Each family on the list is assigned a score, indicating the sensitivity of the taxon. The higher the score, the more sensitive the taxon is to pollution. The BMWP-PL score is obtained by summing the number of taxa points found in the sample. Value >100 = high water quality, 70–99 = good water quality, 40–69 = moderate water quality, 10–39 = poor water quality, and <10 = bad water quality. The Margalef biodiversity index is an index calculated from the formula: D = S/logN where S—the number of taxa in the family rank, and N—total number of individuals from all species. Values >5.5 indicated a high water quality, 4.0–5.49 = good water quality, 2.50–3.99 = moderate water quality, 1.0–2.49 = poor water quality, and <1.0 = bad water quality.

2.2.4. Statistical Analyses

Taxonomical similarity of zooplankton and macroinvertebrates between the sites was determined by calculating the Jaccard’s index. The Mann–Whitney U test (p < 0.05) was used for verifying the significance of differences in the number of species of total zooplankton and the abundance of each zooplankton group between the sites (p < 0.05). The same test was also used to identify significant differences in the taxa number and density of macroinvertebrates between the examined sections (p < 0.05). To illustrate the ordination of the zooplankton groups and macroinvertebrates groups in terms of their abundance with regard to environmental factors, a canonical correspondence analysis (CCA) was used.

It was expected that, as a basin of stagnant water, a reservoir has a large positive effect on the reproduction and development of zooplankton. Hence, when making the CCA between the zooplankton abundance and the environmental factors of the stream as running water, the environmental conditions of the reservoir were not taken into consideration, as they do not characterize the running but standing water and could affect the final results.

3. Results

3.1. Zooplankton

From the upper section of the stream to the lower downstream section, the species number of zooplankton was found to increase (Figure 2). A significantly lower species number was observed at the sites selected in the upper section (p < 0.05).

However, the Jaccard taxonomical similarity coefficient showed that the upper, middle, and lower downstream sections were clearly divided between themselves (Figure 3). This indicated that the taxonomical composition of each section was made up of different species, as described in

Table 2. Taxonomic composition of zooplankton observed at sites of the Bukówka stream.

	Site
Species	U1	U2	POND	M3	M4	D5	D6
Rotifera
Anuraeopsis fissa		+	+	+	+	+	+
Asplanchna sieboldi			+	+		+
Bdelloidea	+	+	+	+	+	+	+
Brachionus angularis			+	+	+	+	+
Brachionus calyciflorus		+	+	+	+	+	+
Brachionus quadridentatus			+	+	+	+	+
Brachionus sp.			+	+	+	+	+
Brachionus rubens			+	+	+	+	+
Brachionus urceolaris					+
Cephalodella gibba	+	+		+		+	+
Cephalodella sp.	+	+	+	+	+	+	+
Cephalodella ventripes	+	+	+	+	+	+	+
Collotheca mutabilis					+
Colurella adriatica	+	+	+	+	+	+	+
Colurella sp.	+			+		+	+
Colurella colurus	+	+		+	+	+	+
Colurella uncinata	+	+	+	+	+
Conochilus unicornis			+	+	+	+
Euchlanis dilatata	+	+	+		+	+	+
Filinia brachiata			+	+	+	+	+
Filinia longiseta			+	+	+	+	+
Filinia sp.				+
Filinia terminalis			+	+	+	+	+
Keratella cochlearis cochlearis	+	+	+	+	+	+	+
Keratella cochlearis hispida						+
Keratella cochlearis tecta	+	+	+	+	+	+	+
Keratella quadrata	+	+	+	+	+	+	+
Keratella testudo				-	+	+
Keratella testudo gossei						+
Keratella ticinensis				+	+		+
Keratella valga				+	+
Lecane bulla	+						+
Lecane closterocerca	+	+	+	+	+	+	+
Lecane flexilis						+	+
Lecane hamata			+	+	+
Lecane luna		+
Lecane sp.	+		+		+	+	+
Lepadella acuminata		+			+	+
Lepadella ovalis	+	+	+	+	+	+	+
Lepadella sp.	+					+
Monommata longiseta						+
Mytilina mucronata							+
Notholca acuminata					+	+	+
Notholca squamula	+					+
Polyarthra dolichoptera			+	+	+	+	+
Polyarthra remata			+	+
Polyarthra sp.			+	+	+	+	+
Polyarthra vulgaris		+	+	+	+	+	+
Proales sp.				-	+
Rotifera not identified	+	+	+	+	+	+	+
Synchaeta pectinata				+	+
Synchaeta sp.			+	+	+	+	+
Taphrocampa sp.	+
Trichocerca capucina		+
Trichocerca dixon-nutalli	+	+	+		+	+	+
Trichocerca elongata	+	+	+
Trichocerca pusilla		+	+	+	+		+
Trichocerca rousseleti						+
Trichocerca sp.	+		+	+	+	+	+
Trichocerca stylata			+
Trichocerca weberi		+			+
Cladocera
Acroperus harpae							+
Bosmina longirostris				+		+	+
Ceriodaphnia sp.							+
Chydorus sphaericus		+				+
Diaphanosoma brachyurum			+				+
Moina micrura							+
Pleuroxus aduncus	+
Polyphemus pediculus							+
Scaphaloberis mucronata			+	+			+
Cladocera juv	+
Copepoda
Eucyclops macruroides			+	+			+
Eucyclops macrurus							+
Eurytemora velox						+	+
Macrocyclops albidus		+
Thermocyclops oithonoides							+
Nauplii Cyclopoida	+	+	+	+	+	+	+
Copepodid Calanoida						+	+
Copepodid Cyclopoida	+	+	+	+		+	+

.

All the zooplankton groups contributed to the increase in the abundance of zooplankton in the stream below the reservoir and the slight decrease of abundance in the last site in the middle section (Figure 4). This influence was especially noticeable in the case of pelagic rotifers, the abundance of which was significantly lower in the upper section than the other sections (p < 0.05). Further from the reservoir, the abundance of pelagic rotifers was lower. Although not very clear, a similar pattern was observed in the abundance of benthic rotifers.

A different pattern of abundance was observed in the case of crustaceans (at adult and larval stages). Their stream abundance was found to be increased below the reservoir, and then decreased, while at the last site in the downstream section, the abundance increased again (Figure 4). Cladocerans (p < 0.05) and adult copepods were characterized by a much higher abundance in the lowest site compared to the other sites. At both sites in the upper section, benthic rotifers were found to be dominant (Figure 5). On the other hand, from the pond to the last site in the lower downstream section, pelagic rotifers were found to be dominant. However, their abundance decreased in the lower downstream sections. In the last site of the stream, the abundance of adult crustaceans, especially cladocerans, was found to increase.

The first CCA axis explained 41% of the total variability in zooplankton group abundance with regard to the environmental factors (Figure 6). The second axis explained 9% of that variability. The morphological factors of stream (depth and width) correlated best with the first axis. The best correlation with the second axis was found for chlorophyll a and current velocity. Cladocerans and copepods (that the most correlated with the firs axis) correlated positively with depth and width and negatively with current velocity.

3.2. Macrozoobenthos—Macroinvertebrates

During the entire study period, 24 taxa of macroinvertebrates were identified at all sites. No significant differences were found in the number of taxa between the three sections of the Bukówka stream.

According to Jaccard’s index, the most similar sites in terms of taxa were U1 and U2, which were in the upper section of the stream, and M3 and M4, which were in the middle section (

Table 3. Taxonomic similarity of macroinvertebrates between sites (Jaccard’s index).

Site	U1	U2	M3	M4	D5	D6
U2	76.5	X
M3	58.8	52.6	X
M4	50.0	44.4	64	X
D5	64.3	56.3	47	73	X
D6	44.4	47.4	32	47	50	X
POND	44.4	47.4	32	47	61.5	60

). In addition, sites M4 and D5 were similar in taxa composition. The largest differences in the composition of taxa were observed between the site M3 and the POND.

The Mann–Whitney U test used to test for differences in family abundances in different sections of the stream showed that the number of snails belonging to the Hydrobiidae family was significantly higher in the upper section of the stream than in the middle section (p < 0.05) (Figure 7). In the upper section, a higher number of Chironomidae larva and Oligochaeta were observed than the lower downstream section (p < 0.05). In the middle section, the Erpobdellidae family was more abundant than in the upper one (p < 0.05). No significant differences were found in the abundance of taxa between the middle and the lower downstream section (p > 0.05).

The first CCA axis explained 27% of the total variability in macroinvertebrates group abundance with regard to the environmental factors (Figure 8). The second axis explained 23% of that variability. The morphological factors of stream (depth and width) correlated best with the first axis. The best correlation with the second axis was found for temperature, chlorophyll a, and current velocity. Viviparidae, Platycnemidae, Notonectidae (that correlated the most with the first axis), and, to a lesser extent Coenagrionidae, correlated positively with depth and width and negatively with current velocity.

The values of the calculated indexes and metrics were characteristic of poor-quality water (

Table 4. The values of the calculated indexes and metrics at the examined sites. Orange color stands for poor quality of water (IV class), yellow for moderate water quality (III class), green for good quality of water (II class), and blue for very good quality of water (I class).

Site/Month	BMWP-PL	Margalef Index	ASPT	Shannon-Weaver	TBI	GOLD (%)	% Diptera
U1/April	35	3.33	3.89	0.990	3	91	42
U1/October	45	4.01	3.75	0.990	4	90	49
U2/April	37	4.16	3.36	1.648	4	85	35
U2/October	47	5.86	2.94	0.796	6	94	74
M3/April	46	3.84	3.54	0.964	6	94	36
M3/October	31	3.40	3.44	1.684	5	36	10
M4/April	26	3.35	3.25	1.160	4	93	17
M4/October	25	3.09	3.57	1.476	5	53	40
D5/April	17	2.49	3.40	0.399	3	100	11
D5/October	31	3.45	3.44	1.236	4	43	23
D6/April	16	3.49	3.20	0.907	3	59	6
D6/October	43	5.78	4.30	0.989	7	79	73

). Both the BMWP-PL index and TBI pointed to class IV water quality for most of the sites, the biodiversity indexes (Margalef and Shannon–Weaver) were relatively low, and the GOLD and %DIPTERA metrics indicated a relatively high number of ubiquitous species in the studied stream.

4. Discussion

4.1. Zooplankton

The spatial pattern of the richness and abundance of zooplankton in the Bukówka stream showed that the value of these parameters increased with an increase in the distance from the river source. It was identified that these parameters were dependent on many environmental conditions that favored the growth of the zooplankton population. Thus, the abundance of zooplankton and their species number spatially changed in a similar manner as other running waters, including large rivers that were found to be affected by human activity [19,20]. The greatest impact on the development of typical planktonic organisms (pelagic rotifers, cladocerans, and some copepods) in Bukówka was posed by the reservoir as a basin due to its low current velocity and long water retention time. Hence, even a small dam that created a very shallow reservoir caused a significant difference in the richness and abundance of zooplankton between the upstream and the downstream sections. A similar pattern of the distribution of the zooplankton community was reported in small forest streams and rivers impounded by human-made dams or beaver dams in some studies [7,10,14,21]. We observed that in the impoundment (POND)—the “Słoneczne lake” of Bukówka—the richness and abundance of zooplankton were high, which was comparable to or even exceeded those in the typical temperate limnetic basins (e.g., eutrophic lakes or reservoirs) [22,23]. Our earlier studies on the increase of the zooplankton abundance in streams or rivers caused by damming did not show such high densities as observed in the Bukówka reservoir. It can be concluded that the reservoir was characterized by desirable trophic conditions for filter-feeding plankters because of the high concentration of chlorophyll a. Phytoplankton and other primary producers can reach high densities in Bukówka due to the exposure of the stream and reservoir to pollution in the form of excessive nutrient enrichment. This issue is often encountered in urban lakes in which zooplankton are found at very high densities [24]. Our present study shows that the abundance of zooplankton can also attain high values in urban reservoirs and streams.

A very common observation is that the abundance of zooplankton decreases along the section stretching from the dam (reservoir) or lake outlet to the lotic sections downstream to the mouth [20,23]. Probably this pattern of decreasing abundance is an effect of fish predation on zooplankton, grazing by suspension-feeding, filter-feeding, or net-spinning of macrozoobenthos, and settling of sediments [6,25,26]. However, in the lower downstream section of Bukówka, the abundance of some zooplankton groups (cladocerans and copepods) was higher than the sites in the middle section. This spatial pattern is much different from that observed in running waters and could result from the following: (1) low current velocity in the most downstream site, (2) occurrence of slack waters densely covered by macrophytes in the last section, and (3) water influx from the recipient.

It is generally known that zooplankton disperse passively from the outlet of stagnant basins to downstream and can colonize new habitats [27]. Drifting zooplankton can colonize the river if its bed offer zones with low current velocity or with stagnant water (e.g., slack waters, impoundments, and floodplains) [7,10]. The main variables that contribute to the increase in the richness and abundance of zooplankton in the Bukówka stream at the more downstream sites are as follows: a decrease in the current velocity, a longer water retention time or greater areas of open-water zones, higher number of slack water areas, covering of floodplains by macrophytes, and adjacent water bodies [14,28,29]. The majority of microfauna, including freshwater zooplankton, are unable to persist if the current velocity goes beyond 0.1 m s⁻¹ [28,30]. The low values of current velocity noted at the last site of Bukówka (D6) were favorable for the occurrence of zooplankton and the increase in their populations. Moreover, crustaceans and other plankters could have migrated or been washed out from the riparian zones or slack waters connected with the stream water [12]. The highest width and depth of the stream bed at the last site might have also favored this phenomenon.

However, we hypothesize that the greatest impact on the higher densities of crustaceans at the most downstream site was posed by the influx of crustacean assemblages in the inlet waters from the recipient Oder river into the stream. This might have occurred during the high-water period. The reason for arriving at this conclusion is the occurrence of Ceriodaphnia sp., Moina micrura, Polyphemus pediculus, Eucyclops macrurus, Thermocyclops oithonoides, and Eurytemora velox, which were found only in the lower downstream sections of Bukówka. Most of these crustaceans are not typical of small-stream fauna. Crustaceans that could came from the Oder river could have migrated up the stream mainly in the last site (D6) because of low current velocities, which were too high at the D5 site for most of the crustaceans. Thus, it can be concluded that the highest impact on the zooplankton-shaping structures in the last section of the stream was posed by the inlet water from the Oder river; however, it is a hypothesis.

4.2. Macroinvertebrates

A relatively low number of taxa were found in the studied stream, regardless of the section considered. One of the reasons for this observation was the regulation of the stream. Sudduth and Meyer [31] reported a high number of pollution-tolerant taxa related to urban stream banks in their study. Such a pattern is caused by the regulation of stream banks, leading to the homogeneity of the environment [32,33]. Only an appropriate bioengineering mechanism could slightly increase the abundance of sensitive species [31,34]; however, a lot of other factors also decrease the diversity of urban streams. The stream examined in the present study flows through a part of the city that has no natural or forest area and does not have many trees adjacent to the stream. Couceiro et al. [35] showed that deforestation in the urban regions decreases the taxa richness of macroinvertebrates. In addition, the authors explained that water pollution (by nitrogen and phosphorus compounds) affects the composition of macroinvertebrates similar to deforestation. Similar pattern was observed by Gołdyn et al. [36] in small urban river in Poznań agglomeration (Poland), where stormwater runoff into this river was a source of significant contamination with heavy metals, organic matter and other pollutants, causing a strong transformation in the communities of benthic macroinvertebrates. Bukówka is a stream in the close neighborhood of allotments, from where a large amount of fertilizers is released. Moreover, the presence of a pond in the middle of the stream can be associated with the high values of TDS and conductivity and release of a large amount of total nitrogen and phosphorus, mostly from anthropogenic activities, such as the use of baits in fishing ground or feeding of aquatic birds. Many authors claim that biogenic compounds found in excess in urban streams negatively affect the macroinvertebrate composition [37,38,39]. Moreover, a channelization and regulation of Bukówka bed caused the low values of current velocity that affected the higher densities of taxa typical for ponds, lakes and large river, what was observed in lower section of stream. Therefore, low taxa richness and relatively high abundance in Bukówka are typical of a small, degraded stream, which is under the influence of high anthropogenic pressure.

All the biological indexes of macrozoobenthos are dependent on taxa compositions, and most of them take abundance into account. The values of TBI and BMWP-PL and its modification—ASPT index—were rather low for the examined stream, pointing at its poor water condition, and it is similar for many other Polish streams that are under anthropogenic pressure [40]. However, these indexes are basically calculated based on the taxa composition (the examined site has a lot of sensitive stenobiotic taxa). In addition, both Margalef and Shannon–Weaver biodiversity indexes were lower in Bukówka compared to other streams in Poland, and also correlated with abundance and taxa richness [41]. The last two metrics—GOLD and %DIPTERA—can shed some light on the whole pattern: these are based on ubiquitous taxa of macroinvertebrates, and thus, the higher their value, the poorer the conditions of the stream. In the case of Bukówka, the values of both metrics were high, which shows a high proportion of pollution-tolerant taxa in the abundance of macrozoobenthos. Taking all the indexes into account—and the fact that the statistical analysis of the basic environmental parameters did not reveal any significant biological correlations—we can state that streams like Bukówka represent a different type of running water, which are strongly affected by human activity. Therefore, urban streams should be given more attention, constantly monitored, and treated with special measures.

The macroinvertebrates of Bukówka were mainly represented by ubiquitous species, which are typical of streams flowing through urban agglomerations. Bukówka is a regulated stream flowing along the entire section through highly urbanized areas, carrying a large impact of the Oder River in its final section. Furthermore, the examined watercourse is not characterized by too many microhabitats for macroinvertebrates, and the riverbed is homogeneous what is non-natural for running waters. According to many authors, such a pattern leads to the degradation of the richness of bottom fauna, and consequently, to a decrease in the value of the biological indexes. So, it would be very important to pay attention to the need to rehabilitate the Bukówka stream to reach better values of ecological potential and biodiversity.

As Karr [42] claims, ecological integrity is the sum of the elements of biological diversity and processes occurring in an environment. Zooplankton and benthic macroinvertebrate taxonomic composition and abundances, macroinvertebrates metrics, and physicochemical parameters used to evaluate the state of the Bukówka stream can be considered together as biological and abiotic elements. It seems our results constitutes good ecosystem integrity in terms of the observed water quality and aquatic biology. Substantially, all these elements show that the worst conditions in the stream occurred in the last section in which pollution accumulated from the entire basin and where the riverbed was most transformed. However, all parameters react similarly on the changes that occurred in the stream.

5. Conclusions

In small urban streams the amount of live organic matter increases with their length. An additional and important factor that affects the increase of this matter is the intensive human activity in the catchment area and in the bed. The construction of reservoirs and bed regulation also have a similar effect on the ecological potential in small urban streams as in large rivers, but of course at a lower scale. The composition of invertebrates is also influenced by the influx of water from the recipient river (especially water with a different chemical composition). It seems that the structures of invertebrate assemblages in Bukówka are significantly governed by stream-scale variables, similar to the findings of Knott et al. [43].