turbed sites (Angermeier & Karr, 1994). Comparing ..... and streams in different parts of the world (e.g. Karr, ..... Paller, M. H., M. J. M. Reichert & J. M. Dean, 1996.
Hydrobiologia 422/423: 319–330, 2000. M. Jungwirth, S. Muhar & S. Schmutz (eds), Assessing the Ecological Integrity of Running Waters. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.
319
Habitat condition and fish assemblage structure in a coastal mediterranean stream (Yarqon, Israel) receiving domestic effluent Sarig Gafny1 , Menachem Goren2 & Avital Gasith1 1 Institute
for Nature Conservation Research and 2 Department of Zoology; George S. Wise Faculty of Life Sciences, Tel-Aviv University, Ramat-Aviv 68878, Israel Key words: urban stream, Multi Dimensional Scaling, stream pollution, BOD
Abstract The Yarqon stream is the largest urban stream in Israel. In the past 40 y it was severely impacted by human action. Fish kills occur along the polluted sections of the stream almost regularly following winter floods and when poor-quality effluent is discharged into the stream. These events attest to the serious ecological state of the Yarqon stream. We studied the interaction between habitat conditions and fish assemblage structure for four consecutive years. Comparison of certain water quality variables among selected sites along the upper and central stream sections of the Yarqon indicated that the two stream sections differ significantly in electric conductivity and organic load (BOD) but not in temperature and dissolved oxygen. The average species richness and fish abundance in the upper, relatively undisturbed section is significantly higher than in the downstream, upper reach of the central section where domestic effluents enter the stream. In contrast, fish biomass is usually higher in the central stream section. Organic load was the only factor among the four water quality variables measured that significantly correlates with variables of the fish assemblage. A curve fit analysis between organic load and fish species richness and abundance suggests a threshold at ca. 10 mg l−1 BOD, above which most fish species totally avoid polluted habitats. Separate Multi Dimensional Scaling (MDS) analyses of water quality variables and fish assemblages clearly separated between the sites in the upper and the central stream sections. When the result of the fish MDS analysis was graphically superimposed on the result of the water quality MDS analysis, we found a close overlap between the two, suggesting a strong association between water quality and the fish assemblage structure in the Yarqon stream.
Introduction Streams and rivers are unique aquatic systems characterized by an unidirectional flow of water. Water and materials from the drainage basin cascade down along the geomorphologic gradient and determine water quality and habitat features. The riparian vegetation that grows along the banks limits the light that enters the stream, provides coarse particulate organic matter and habitats for wildlife and acts as a buffer zone against anthropogenic influences (Naiman & Bilby, 1998). In mediterranean-type streams, flow presents a high seasonal variability, with high discharge during flood events in winter and minimum or no flow in summer. These seasonal changes in the hydrological regime and its associated water quality conditions provide a structural framework for mediterranean-
stream ecosystems. In turn, these factors may be strongly influenced by human activity yielding detrimental ecological effects (Gasith & Resh, 1999). During the 20th century, river and stream pollution and misuse have decimated species richness (Gore, 1985; Haslam, 1990). The study of restoration and rehabilitation of streams and rivers is growing in importance (for a review of case studies see: Sparks, 1995; Kauffman, 1997), and the understanding of the complex processes involved in the maintenance of healthy, functioning river and stream ecosystems is improving (Herricks & Osborne, 1985; Allen, 1995). In water-stressed regions the competition for water between human needs for water and that of nature may result in severely diminished stream flow. This human impact is often exacerbated by water pollution (e.g. Gasith, 1992; Gasith & Resh, 1999). The degraded
320 status of the coastal streams of Israel is an example of the failure to recognize and control the compounded effect of water diversion and pollution (Gasith, 1992). Despite the continued deterioration of water quality and the intensification of water diversion from streams in Israel, since the early 1970s (Goren, 1974) no new information on the status of the fish populations in the coastal streams of Israel has been published until the present study on the Yarqon stream was initiated (Gasith et al., 1998). The Yarqon stream is the largest urban stream in Israel. In the past 40 years the stream has been severely impacted by human action. Fish kills occur along the polluted sections of the stream almost regularly following winter floods and when poor quality effluent is discharged into the stream (e.g. when the operation of waste-water treatment fails). These events attest to the serious ecological state of the Yarqon stream. Presently, efforts are being made to rehabilitate the Yarqon and other coastal streams in Israel. The aim of the present study is to assess the ecological health of the Yarqon stream and to identify the rehabilitation measures needed to mitigate the adverse human impact. For that purpose we used fish assemblage structure as an indicator of habitat condition. The working hypothesis is that habitat conditions influence fish species richness, abundance and size distribution. Ecosystem perturbation such as enrichment with organic matter is expected to reduce species richness, enhance or inhibit fish growth relative to fish in unpreturbed sites (Angermeier & Karr, 1994). Comparing of the structure of the fish assemblages in less perturbed sites with those in polluted sites is therefore expected to reveal the extent of the pollution impact along the Yarqon stream.
Methods The sites studied The Yarqon stream is the southernmost perennial stream in the Coastal Plain of Israel. This lowland stream (gradient 0.06%), meanders along 29 km through the Tel-Aviv metropolitan region and drains into the Mediterranean. The Yarqon drainage basin covers an area of approximately 1800 km2 , with an average annual rainfall of about 600 mm. The stream’s complex environment is composed of at least three sub-systems: (a) the channel and its aquatic life; (b) the riparian zone (mostly modified by human activity);
and (c) the built area, the center of human activities. Prior to 1950, the Yarqon stream carried over 200 million cubic meters (MCM) of freshwater annually discharged from its main springs (Rosh Ha’Ayin); the discharge resulting from run-off varied between less than one MCM in drought years and up to 300 MCM in rainy years (Ben-Zvi et al., 1995). Diversion of the Yarqon’s main springs in the mid-1950s for domestic uses has reduced the base flow to less than 1% of the average annual discharge (about 1.5 MCM). If not for the discharge of effluents (about 4.5 MCM yearly), this former ‘Yarqon River’, second in discharge after the Jordan River, would have become an intermittent stream in drought years (Gasith, 1992). Based on the degree of perturbation and water quality the Yarqon stream can be divided into three segments: (a) an upper 7.5 km relatively unperturbed section of relatively high water quality (Gasith et al 1998). This unpolluted section ends at the confluence of the Qane tributary and the Yarqon stream, where municipal effluents from a waste-water treatment plant (Kefar-Sava–Hod-Hasharon) enter the Yarqon stream; (b) the central section of about 17.5 km that is severely impacted by pollution, mostly of municipal effluents (entering via the Qane tributary and from the Ramat-Hasharon oxidation ponds); (c) the lowermost 4 km section which is under tidal influence and therefore is variably saline. Our study was limited to the freshwater, upper and central sections of the Yarqon stream.
Sampling and analyses Analysis of habitat conditions and fish assemblage structure was conducted on a monthly basis from June 1995 to August 1997. We included in the analysis data obtained in a preliminary study (Oct. 1994–June 1995; Gasith et al., 1998). The sampling sites along the upper and central sections of the Yarqon stream are shown in Figure 1. Samples were taken monthly in three or four sites in the upper section (Sts. 2a, 2b, 3a and 3b), two sites in the upper reach of the central section (Sts. 3bc and 3c) and one in the lowermost reach (St. 6b) of the central section. Stream morphometry (width ca. 8 m) was similar at all sites. Sites 2a and 3a are above small rocky dams and represent lentic (pool) habitats (depth=1–2 m), whereas sites 2b, 3b, 3bc, 3c and 6b are below rocky dams and represent lotic (riffle or shallow run) habitats (depth=0.5–1.5 m). The
321 small dams allow upstream migration of some species all year round (except in extremely dry summers) and winter upstream migration of the others. Temperature, dissolved oxygen and conductivity were measured with a YSI model 85 OxygenTemperature-Conductivity meter, turbidity was measured with a Hack turbidity meter (model 2100A) and organic load (BOD) was measured following the standard procedure (APHA, 1995). Fish samples were obtained by a non-selective electrofishing technique using a shore-operated electroshocker (EFKO model FEG 6000) producing 350 Volts (17 Amp), non pulse DC. Each site was sampled for 10 min. The fish were sampled without block nets but attempts were made to collect all fish encountered in the sampled segment. The fish were sorted, identified, measured (total length to the nearest 1 mm) and weighed (wet weight to the nearest 0.1 g). Analysis of variance using Systat for Windows (1992) and Statistica (Statsoft Inc., 1995) was applied to test the difference in water quality and fish assemblage variables among sites (statistical significance of p0.1), whereas NH3 –N was significantly higher at the central section (t=6.8 p0.1), where it ranged from a minimum of 800 µmhos cm−1 at 20 ◦ C in winter to a maximum of 1520 in summer (annual mean ca. 1060). The conductivity in the central section (Sts. 3c, 6b) was slightly higher than in the upper section (annual mean 1170; Figure 2b). The highest range and average values of conductivity were recorded in the lowermost reach of the central section (St. 6b; annual mean 1315). The daytime dissolved oxygen concentration varied among the sites studied, although the difference was not statistically significant (F=1.33; df=118; P>>0.1). It was usually 30–40% below saturation in the upper section and varied from a minimum of 2 to a maximum of 11 mg l−1 (20 and 110% saturation, respectively) in the central section (Figure 2c). The lowest oxygen concentrations were recorded below the inflow of the Qane tributary (St. 3c) during a 3-month period (Feb.–April) after a flood event in 1996. Below cascades formed by dams the dissolved oxygen concentration was slightly but significantly elevated (t=3.26 P0.001). Until August 1996 the average species richness in the upper section (Sts. 2a, 2b, 3a) was significantly higher than in the upper reach (St. 3bc, St. 3c) of the central section (paired t=5.58, p>0.0001). Most remarkable was the abrupt decline in fish abundance between two adjacent sites (< 100 m apart) representing the lowermost reach of the upper stream section (St. 3a) and the uppermost reach of the central section (Sts. 3bc, 3c; Figure 5b). This also suggests a threshold at ca. 10 mg l−1 BOD, above which the fish avoid the polluted habitats. Improved water quality in
324
Figure 2. Range (min–max), median and mean (±SD) of (a) water temperature; (b) conductivity; (c) dissolved oxygen; (d) organic load – BOD in selected sites along the Yarqon stream during 1994–1997.
the upper reach of the central section after the wastewater plant went into operation (May 1996) increased the fish abundance here (starting from July 1996). However, the fish disappeared from this stream reach in February 1997 after winter flood events (Figure 6). Fish biomass was highest but also varied the most in the central section (Figure 5a). The largest range of biomass was recorded in the upper reach of the central
stream section (St. 3c), where fish richness and abundance were the lowest (Figure 5a,b,c). It corresponded to the greater frequency of occurrence of larger fish here (Figure 7). The difference in size distribution of the fishes in the upper and central stream sections was best demonstrated by the size distribution of the cichlid T. zillii, which is currently the most common fish in the Yarqon (Table 2). Relatively small T. zillii (31–40
325 Table 2. Total number (n) and relative frequency of occurrence (%, in italics) of the fishes in different sites and habitat types (len= lentic; lot= lotic) of the upper and central sections of the Yarqon stream (1994–1997) Site
n 2a len
Habitat type
Upper section (%) 2b 3a lot len
3b lot
Central section (%) 3bc 3c 6b lot lot lot
Taxon CICHLIDAE T. zillii S. galilaeus
8896 51
8.9 29.2
12.2 62.5
16.2 8.3
12.5 0
4.3 0
3.5 0
42.4 0
CYPRINIDAE C. carpio A. telavivensis
671 1045
6.3 13.2
0.3 26.7
1.8 25.8
1.3 29.4
5.2 4.7
4.6 0.2
80.4 0
12
8.3
0
ANGUILLIDAE A. anguilla
199
5.5
4.0
7.5
16.6
7.5
0
POECILLIDAE G. affinis Xiphophorus sp.
3920 354
18.4 13.8
29.4 8.8
25.3 37.3
18.1 23.4
2.3 9.3
3.4 7.3
893
0
0
0
0
0
0
100
1 769
2 589
2 874
2 253
604
505
5 447
CLARIIDAE C. gariepinus
MUGILIDAE M. cephalus Total n
16 041
mm TL) prevailed in the upper section and their size distribution was skewed toward small individuals (Figure 7). Small numbers of relatively large T. zillii (>150 mm) were recorded in the upper reach of the central section and smaller T. zillii (71–80 mm) prevailed in the lowermost, partially recovered site which was the only location where the fish size was normally distributed (Figure 7). Likewise, the carps collected in the upper reach of the central section (120
