Unimproved fords provide an easy and inexpensive method of stream crossing. However, vehicles compact the stream bottom and flatten the banks. One simple and relatively permanent solution to this problem is the use of concrete slabs, which can hinder the movement of fish and invertebrates, especially in the presence of low water levels. Although the impact of fords on the stream invertebrate fauna seems clear, there is little available data on this topic, as most reports instead focus on water quality. The impact of a concrete slab ford on the invertebrate fauna of a foothill stream was examined which led to the hypothesis that the benthic invertebrates inhabiting the ford and stream sections immediately upstream and downstream of the crossing would differ significantly from those at a non-transformed natural site. Results revealed that the invertebrate fauna in the vicinity of the ford was altered over relatively long stream sections. The ford disrupted the natural balance between deposition and erosion, with the former dominating above the ford and the latter dominating below the crossing. In small streams, where there are often several similar crossings, this effect would likely be multiplied. The ford's interference in the stream's natural balance between deposition and erosion caused a change in the grain size of the substrate and altered the available organic matter, which directly affected the qualitative and quantitative composition of the macrozoobenthos. As the habitat becomes more homogeneous, the community of invertebrates becomes less diverse. Our results can support management decisions for erosion control.
Stream regulation and soil erosion caused by forest exploitation are the most important anthropogenic changes in Carpathian streams (Kukuła, 2003). However, stream channels are also affected by bridges, fords and culverts constructed as part of the road infrastructure used for transporting timber, with the greatest impacts being caused by culverts and fords (Gibson et al., 2005).
At unimproved fords, which provide the easiest and least expensive method of stream crossing, vehicles compact the stream bottom and flatten its banks, requiring the banks to be strengthened. One simple solution is the use of concrete slabs, although their stability may be compromised over time by the continuous erosion of the streambed (Clarkin et al., 2006). In addition, such slabs hinder the movement of fish and invertebrates, especially when water levels are low (Vaughan, 2002; Bouska and Paukert, 2009; King and O' Hanley, 2016). Such structures also change the character of the streambed, with sediment deposition occurring upstream of the ford and increased erosion downstream (Clarkin et al., 2006).
Although the impact of fords on stream invertebrates seems obvious, there is little data supporting this notion, as relevant reports tend to focus on effects on water quality (Neal et al., 2007; Wear et al., 2013; Kidd et al., 2014). Therefore, the aim of this study was to assess the impact of a concrete slab ford on the invertebrate fauna in a foothill stream, to support management decisions. The hypothesis was that the benthic invertebrates inhabiting the ford and stream sections immediately upstream and downstream of the crossing would differ significantly from those at a non-transformed natural site.
Research was conducted in 2011 and 2013 in the Hołubelski stream (with springs 390 m amsl; length: 5.9 km; basin: 8.7 km2), a tributary of the San River. Benthic samples were collected at four sites with different degrees of anthropogenic changes. Morphometric parameters of each sampling site are indicated in Figure 1. Site 1 was located in a non-transformed section about 600 meters upstream. Site 2 was a section up to 15 meters upstream from the ford, where the water flow was slowed, and fine sediment accumulated. Site 3 was the ford itself, where samples were collected from the concrete slab surface. Site 4 was a section approximately 40 m below the ford, where erosion had created a 2.5 m-high waterfall, and gravel and pebbles had been removed by a strong current.
Benthic samples were collected due to seasonality, in the spring, summer and autumn each year. At the time of each collection physicochemical parameters (temperature, conductivity, dissolved oxygen, and the concentrations of NO3-, PO43-, and Cl- ion) were measured. This ford was in use during the time of sampling. Benthos were collected with a bottom scraper (330 μm mesh) (Merritt and Cummins, 1996). The collected organisms were fixed in 4% formalin, followed by 70% ethanol. Invertebrates were identified, counted, measured, divided into size classes, and weighed. Specimens of macroinvertebrate samples were identified to the lowest possible taxonomic level (mostly genus/family). Standard curves showing the relationship between the mass and length for each taxon were drawn. We assumed that we would try to find the main pattern of invertebrate reactions; hence we did not analyze the seasonal dependency. The numbers and biomass of invertebrates were calculated per 1 m2 of bottom surface. The mean density of the main taxa was compared between sampling sites through contingency table analysis (Zar, 1999). We used Margalef's diversity index (DMg = (S-1)/lnN, where: S – numbers of taxons, N – number of individuals) to assess invertebrates diversity (Magurran, 2004). The allocation of each taxon to the functional feeding group (FFG) depended on the mouthpart morphology and FFG allocations from literature (Merritt and Cummins, 1996).
The average conductivity of the water in the Hołubelski stream was above 400 µS; dissolved oxygen and oxygen saturation were high (∼11 mg O2 l-1 and ∼100%); the nitrate concentration was below 0.5 mg l-1; and the phosphate level was below 0.02 mg l-1. Physico-chemical parameters of water were similar in all sites (Table 1). In summer months, the water in the shallow un-shaded section of the stream (Site 3) was heated faster.
Undisturbed Site 1 displayed the highest density and biomass of invertebrates: about 3.8 thousand organisms per 1 m2, and almost 11 thousand mg m2. There were numerous Mayflies (Heptageniidae and Baetidae), Stoneflies (Perlidae, Nemouridae and Leuctridae), and Caddisflies (Hydropsychidae, Limnephilidae and Sericostomatidae). Beetles were represented mainly by Elmidae, and Crustaceans by the Gammaridae family (Table 2).
Compared with Site 1, the density and biomass of invertebrates at Site 2 was lower, and the structure of the community was significantly different (Figure 2). The macrozoobenthos at Site 2 included Chironomids and Mussels. The Mayflies at this site were dominated by larvae of family Ephemeridae. The Limnephilidae family prevailed in trichopterafauna in the Site 2. Site 2 also exhibited relatively numerous Gammaridae, Baetidae Mayflies and Nemouridae Stoneflies.
In the ford section, at Site 3, Baetidae and Heptageniidae Mayflies, Gammarids, Chironomids and numerous Simuliidae were found (Table 2). Total density in the ford section (Site 3) was much less than at other sites (Table 2), while the invertebrate composition also differed (Figure 3).
The invertebrate fauna at Sites 1 and 4 differed significantly (Figure 2). Heptageniidae Mayflies and Elmidae Beetles occurred at lower densities at Site 4 (Table 2). Site 4 also exhibited many Gammaridae Crustaceans, relatively numerous Baetidae and Heptageniidae Mayflies, Hydropsychidae Caddisflies and Tricladida (Table 2, Figure 3). Margalef's diversity index was lowest at site 3 (3.05), compared to the other sites (9.22 – site 1, 8.32 – site 2, 9.10 – site 4).
Six major FFGs were recognized in this study, these include; collector filterers, collector filterers / predators (Hydropsychidae: net-spinning Caddisflies), collector gatherers, predators, scrapers, and shredders. Analysing the functional composition of the assemblage, showed that shredders were the most abundant FFG with a maximum density of 2646.3 ind. m-2 at site 1. The highest density of collector filterers was found in site 2, whereas its density at site 4 was low. Collector filterers/predators showed their highest density at site 4. At site 3 this group, as well as predators, and scrapers were absent (Figure 4).
The physico-chemical parameters of the water in the Hołubelski stream were typical of clean water. These parameters indicated that the waters at each of the study stations were typical of naturally polluted mountain streams comparing to reference data from Bieszczady National Park (Kukuła and Bylak, 2010). The streambed differed between the sites. Only Site 1 was undisturbed, exhibiting a mosaic of habitats typical of foothill streams, including a bottom covered with gravel and pebbles and zones of sand and silt near the stream banks. Site 1 displayed the highest density and biomass of invertebrates. There were numerous Mayflies, Stoneflies, Caddisflies, Elmidae Beetles, and Gammarids. This community was typical of natural foothill streams, in which the taxonomic composition depends on habitat diversity (Habdija and Primc, 1987).
The most radical change in the bottom substrate was observed in ford section, at Site 3, where the natural bed was replaced by smooth concrete slabs. Fast-flowing shallow water, which is easily heated in summer, and a homogenous bottom created an environment to which only a few groups of aquatic invertebrates are adapted (Ward, 1993). These groups included Simuliidae flies, which are resistant to changes in water parameters and are well adapted to life in a fast current. In mountain streams, these flies are often found on rock surfaces (Habdija and Primc, 1987), which are similar to the concrete slabs in the ford. Baetidae Mayflies found at Site 3 remained on the slabs in areas covered with filamentous algae. These larvae often occur on the surface of stones covered with algae or moss (Brittain, 1982). They have also been observed in perforated concrete slabs, where they can find suitable shelter in holes in the concrete (Bylak et al., 2009). In this study, invertebrate fauna was less diverse, and some Gammarids, Heptageniidae Mayflies, and Chironomids were found on the surface of the slabs as a result of drift. These species cannot persist long under such conditions, as they must find hiding places (Ward, 1993; Tachet et al., 2002).
Upstream of concrete slabs, the water current is slowed, and particulate material accumulates (Clarkin et al., 2006). In the Hołubelski stream, such changes in the habitat conditions were evident over a relatively long distance. When a uniform layer of fine sediments covers stones and gravel, it results in the formation of sections with a homogeneous bottom, where the invertebrate fauna is less diverse (Wood and Armitage, 1997; Extence et al., 2013). Compared with Site 1, the density and biomass of invertebrates at Site 2 was lower, and the structure of the community was significantly different. The macrozoobenthos at Site 2 included Chironomids and Mussels, which prefer a fine-grained substrate (Longing et al., 2010). The Mayflies at this site were dominated by larvae of family Ephemeridae, found buried in the stream bottom (Nilsson, 1996). The fine particles of the muddy substrate obstruct the flow of oxygenated water, such that animals burying into the sediments must be adapted to adverse aerobic conditions (Wood and Armitage, 1997; Larsen et al., 2009).
Near the stream banks at Site 2, the bottom surface was characterized by leaf litter and shallower, well-oxygenated water and contained relatively numerous Gammaridae, Limnephilidae Caddisflies, and Nemouridae Stoneflies. These organisms require well-oxygenated water and coarse particulate organic matter (CPOM) as food (Merritt and Cummins, 1996). Site 2 also exhibited Baetidae Mayflies, which perform well under conditions of slow water flows (Brittain, 1982). This family, whose members feed on detritus and swim actively, has been observed in silted sections of small forest streams (Bylak and Wójcik, 2016).
Downstream of the ford, finer bottom material is removed by faster water, and the stream cuts deeper into the channel. At Site 4, sand, gravel, pebbles and organic particles were removed. Such transformation can be compared with the changes that occur after regulation (Clarkin et al., 2006). The accelerated flow of water and lack of finer material is unfavorable for organisms inhabiting this part of the stream (Bylak et al., 2009). The invertebrate fauna at Sites 1 and 4 differed significantly. The immediate effect of increased erosion is habitat homogenization (Bylak et al., 2009). Living in strong water currents is energetically expensive, and even rheophilic Heptageniidae Mayflies and Stoneflies require shelter under such conditions. High water flow conditions pose little difficulty to predatory Rhyacophilidae Caddisflies, which use silk strands to anchor themselves to stones (Merritt and Cummins, 1996). In the Hołubelski stream, this group appeared only at Site 4.
Site 4 also exhibited many Gammaridae Crustaceans, which are relatively resistant to anthropogenic changes and can often be found in regulated sections of streams at relatively high densities (Bylak et al., 2009). They usually prefer to live in calmer habitats closer to the shore or between stones (Tachet et al., 2002). At Site 4, despite the strong current, there were a few quiet places near the banks, where CPOM accumulated.
The transformation of the Site 4 habitat also increased the number of Hydropsychidae Caddisflies and Tricladida. Hydropsychidae larvae can survive between stones, spinning nets that serve as shelters, while Tricladida are able to live in stronger water currents by hiding under stones (Tachet et al., 2002; Cardinale et al., 2004).
Problems of soil erosion in the vicinity of stream crossings can be mitigated by improving the ford – replacing natural substrate with concrete slabs (Clarkin et al., 2006). Although this solution is effective at reducing surface erosion, and eliminating sediment inputs to the stream, it is challenging because of other forms of erosion such as channel erosion below the ford (Wear et al., 2013). It has long been known that the drop in stream elevation below the ford impacts migrating invertebrates, as well as upstream migrating fish (Soderstrom, 1987).
The presence of the concrete slab ford resulted in transformation in the invertebrate fauna over relatively long sections of the stream. This effect may be multiplied by the presence of several such crossings. The ford's interference in the stream's natural balance between deposition and erosion caused a change in the grain size of the substrate and altered the available organic matter, which directly affected the qualitative and quantitative composition of the macrozoobenthos (see Appendix 1 in the online supplementary files). As the habitat becomes more homogeneous, the invertebrate community becomes less diverse. Preserving stream health is crucial for the survival and vitality of aquatic fauna and for the protection of aquatic life. Our results demonstrate also the importance of proper separation of existing road infrastructure in other types of research, such as monitoring, to avoid distortion of the results. Our findings can also support some management decisions, especially for erosion control.
We would like to thank the anonymous reviewers for their careful reading of our manuscript and their insightful comments and suggestions.
Supplemental data for this article can be accessed on the publisher's website.