Ó Springer 2005
Aquatic Ecology (2006) 40:69 –83 DOI 10.1007/s10452-005-9018-y
Macroinvertebrate communities of karst springs of two river catchments in the Southern Limestone Alps (the Julian Alps, NW Slovenia) Natasˇ a Mori* and Anton Brancelj National Institute of Biology, Vecˇna pot 111, SI-1000, Ljubljana, Slovenia; *Author for correspondence (e-mail:
[email protected]) Received 5 January 2005; accepted in revised form 14 September 2005
Key words: Community analysis, Ecology, Freshwater, Macrobenthos, Springs
Abstract The macroinvertebrate communities of 16 karst springs in the Southern Limestone Alps (Slovenia, SE Europe), were studied from May to September 1999, together with the major chemical parameters of the water and selected physical characteristics of the springs. The springs are located in an area of 800 km2, between 410 and 955 m a.s.l., and drain into two rivers whose catchments are separated by mountain ranges up to 2864 m. The Socˇa river drains into the Adriatic Sea and the Sava into the Black Sea. Springs showed relatively large fluctuations in discharge and small variations in temperature (normally about 1 °C) during the period of study. Seventy macroinvertebrate taxa were collected. Chironomidae (Diptera), Limnephilidae (Trichoptera), Enchytraeidae (Oligochaeta), Tricladida (Turbellaria) and Belgrandiella kuesteri (Gastropoda) were the most widespread taxa, collected from the majority of sampling sites. The environmental characteristics showed a great heterogeneity, and there was a large variation in the taxonomic composition and relative abundance of taxa between springs and during seasons. The coexistence of taxa with highly divergent ecological requirements is characteristic of the spring communities studied. They range from stygobionts and spring-restricted species to ubiquitous species. The differences observed in spring fauna in the two river catchments were probably dictated by differences in local environmental parameters and not by the geographical location in the different river catchments.
Introduction Groundwater-fed springs constitute a unique interface between surface and groundwater ecosystems (Webb et al. 1998) and are geographically and functionally relatively isolated from each other (Williams and Williams 1998). The uppermost part of the karst area (zone called epikarst) is densely fractured due to direct exposure of the rock to erosion processes, including mechanical fracturing and dissolution by carbonic acid produced in the soil layer (Williams 1983). Karst
springs, permanent or temporary, usually appear at the contact zone between fractured and less fractured limestone or dolomite rock, or in the valleys, where impermeable sediments or saturated zones appear (Gams 2003). Groundwater-dominated springs exhibit a degree of thermal stability throughout the year and, as a result, can provide an ecologically stable environment (Williams 1991). The area around the source of a spring within which the annual variation in water temperature does not exceed 2 °C, is defined as the eucrenal zone (Erman and Erman 1995). In the
70 karst areas the temperature of spring water is dependent on the mode of groundwater flow and its residence time within the aquifer (Gams 2003). Additional characteristics of karst springs are highly fluctuating discharge rates and hard water. High porosity and permeability of strata are the main reasons for the great fluctuations in discharge rate. The biological communities that inhabit springs are distinct from those in the downstream parts of streams fed by those springs (Vannote et al. 1980) and from those in other streams in the same geographic area (McCabe 1998). Springs are places where relicts of former geological times have survived, being protected from extremes of climate (cold winters or warm summers). Many species that are confined to springs (crenobionts) are species that occur far outside their normal geographical range (Hynes 1972). The fauna in and near the spring mouth often include specimens of groundwater dwelling animals (stygobionts) (Botosaneanu 1998). Species that are restricted to a spring environment frequently live alongside species that occur in a wide range of freshwater habitat types and are widely distributed (eurytopic organisms) (Williams and Williams 1998). Spring species also show considerable variation in vagility, which is relatively high in winged insects, while amphipods, triclads, water-mites and others are less vagile (Hoffsten and Malmqvist 2000). The composition of macroinvertebrate communities in springs is shaped by historical and geographical factors (Williams and Williams 1998), and by environmental characteristics, including geology (Bonettini and Cantonati 1996), flow permanence (Smith et al. 2003), pH (Glazier 1991), alkalinity and macrophyte cover (Glazier and Gooch 1987). The Southern Limestone Alps (the Julian Alps) are situated on the south eastern border of the Alps and are formed mainly by sedimentary Upper Triassic and Jurassic limestone (Jersabek et al. 2001). The region was directly influenced by glaciation during the Pleistocene (Fridl et al. 1998). In the Alpine area spring fauna have been rarely studied (Cantonati 1998; Gerecke et al. 1998) and in Slovenia no systematic regional study of spring communities has been made. The survey whose results are presented here was conducted with the aim of comparing the spring fauna of two river catchments, one draining into
the Black Sea and one into the Adriatic Sea, and to find out if the hydrological barrier between the two results in distinct spring macroinvertebrate communities.
Materials and methods Study area The springs are located within the catchments of the Rivers Socˇa and Sava, in the foothills of the Southern Limestone Alps (the Julian Alps, NW Slovenia) (Figure 1). The rivers drain waters from the karst massif of the Alps and flow, the former into the Adriatic Sea and the latter into the Black Sea. The climate of the region is mountainous, with a mean annual temperature of 8 °C and annual rainfall from 1600 to 3000 mm (Ogrin 1998). The springs are situated along deep, narrow Alpine valleys at altitudes from 410 to 955 m. A group of five springs come to the surface in the Trenta valley, in the Socˇa River basin. The Socˇa flows in a southerly direction and its valley is directly open to the Adriatic Sea and consequently to a warmer climate. Two other two groups of springs are part of the River Sava catchment. The group of six springs emerge in the Vrata and Radovna valleys and the group of five springs is located around Bohinj Lake and in the valley of the Sava Bohinjka River. The distance between the two most distant springs is approximately 40 km in the east –west direction and 20 km in the north – south direction. Most of the springs discharge directly into the channel of the spring stream and are described as rheocrenes. One limnocrene spring (Hynes 1972) called Kropa (9 – see Figure 1), discharges into a pond which overflows into the channel, and another, a hygropetric rheocrene spring (Schwoerbel 1959) called Tresli (13 – see Figure 1), are exceptions, and both emerge from the rock with a relatively moderate flow. Nine springs were perennial, two were intermittent and five springs were linear. Linear springs are characterised by erosion in a channel in which the upper end is usually dry for a certain period of the year, and by fluctuation of the area of groundwater emergence up or down in the channel, depending on local precipitation and groundwater level (Smith et al. 2003).
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Figure 1. Locations of sampling sites in the Julian Alps (NW Slovenia). 1=Vrata, 2=Krma, 3=Frcˇkov rovt, 4=Zatrep, 5=Lipnik, 6=Cˇrna recˇica, 7=Soteska, 8=Nomenj, 9=Kropa, 10=Voje, 11=at the lake, 12=Tolminka, 13=Tresli, 14=Krsˇ ovec, 15=Roja, 16=Krajcarica.
Sampling techniques Samples were collected in May, July and September 1999. Physical characteristics of the springs were noted on the spot (spring type, flow permanence, flow rate, substrate, periphyton, moss and organic matter cover). Substrate composition – percentage of boulders and cobbles (>6.4 cm), pebbles and gravel (6.4 –0.2 cm), sand and silt (
