Avital Gasith

Acute toxicity of ammonia to juvenile gilthead seabream Sparus aurata under reduced oxygen levels

Abstract The effect of dissolved oxygen concentration on the toxicity of ammonia to juvenile (0.4–3.2 g) gilthead seabream ( Sparus aurata ) was studied. The mean 96-h LC50 value of ammonia was 23.7 mg/l total ammonia-N (19.3–28.7, 95% C.L.; equivalent to 1.27 mg/l NH 3 -N). By comparison, S. aurata is somewhat less sensitive to ammonia than salmonids and similar in sensitivity to nonsalmonids. Increased toxicity of ammonia was observed with decreasing oxygen level. Our data suggest that below a threshold of approximately 40% D.O. saturation, the response of S. aurata to ammonia is no longer linear. Under reduced oxygen level most of the mortality occurs within a few hours, apparently as a consequence of an additive toxic effect. No evidence of pathological changes was found in the gills, liver and kidneys of fish with lost equilibrium in the presence of added ammonia. Acutely toxic conditions for S. aurata may prevail in medium-flow systems (2–6 kg fish/m 2 and 50% total daily water exchange) where oxygen and ammonia levels are influenced by microbial and algal activities. In such systems, ammonia toxicity may increase as a result of increasing ammonia levels and decreasing oxygen concentration following crashes of phytoplankton populations. In high-flow systems typical of most hatchery and nursery tanks as well as some grow-out ponds, it is likely that the chronic effects of ammonia toxicity will be more important than the acute ones. The data suggest that increasing oxygen level by aeration is sufficient to reduce acute ammonia toxicity under certain situations.

Prolonged lag in population outbreak of an invasive mussel: a shifting-habitat model

Biological invasions pose a great threat to the integrity of natural communities. Some invasive species demonstrate a population explosion shortly after arrival while in other cases a prolonged lag between arrival and population outbreak is evident. This paper describes a case of a prolonged lag and explores the possible mechanism for this lag. The Red Sea mussel Brachidontes pharaonis, a Lessepsian migrant, was first recorded in the Mediterranean seven years after the opening of the Suez Canal in 1869. Since then it spread along the Israeli coast and as far northwest as Sicily. Studies conducted in the late 1970s, when B. pharaonis was still rare, predicted that it would not establish dense populations along the Israeli coast and would not outcompete the indigenous mussel Mytilaster minimus, although it has strong negative effects on survival and growth of the native species. It was attributed to the invaders low intrinsic rate of increase relative to that of the native species, and to strong density-independent mortality generated by exposure to high wave action and sedimentation. In contrast to these predictions, we found massive formations of B. pharaonis beds after lag of about 120 years. We looked for distributional patterns that may explain this lag and found no south–north gradient but a strong habitat-dependent colonization. Most apparent are dense B. pharaonis mussel beds (density >300 per 100 cm2) on rocky platforms where mussel beds were absent in the past. These platforms lack the vermetid rim that is typical to this formation. In platforms protected by a biogenic rim, sediment accumulation is high and perennial algae flourish. None of the mussel species form beds in such habitats. We suggest that the delayed formation of B. pharaonis beds along the Israeli coast is a consequence of a recent shift in habitat conditions on some platforms. It is possible that receding of the biogenic rim at the edge of these platforms allowed more effective washing, reduced sediment accumulation, and reduced perennial algae cover making platforms more suitable for the mussels. Lower density-independent mortality allowed B. pharaonis to dominate on such platforms over the indigenous species. On beachrock, a habitat previously dominated by M. minimus, we recorded a rapid shift in numerical domination to B. pharaonis (from 1 : 7 to 1.4 : 1 Brachidontes/Mytilaster individuals) over a period of 4 years (1995–1999). This is probably a result of saturation of the habitat by B. pharaonis recruits originating from the established populations on platforms. Salinity changes and a potential genetic shift may also have contributed to the invasive mussel outbreak.

Resistance, resilience, and community dynamics in mediterranean-climate streams

Streams and rivers in mediterranean-climate regions (med-rivers) are subjected to sequential, yet contrasting hydrologic disturbances of drying and flooding. Although seasonally predictable, these disturbances can vary in intensity and duration within and among mediterranean-climate regions (med-regions). Consequently, med-rivers differ in the permanence of their aquatic habitats. To persist, species have acquired matched resistance and resilience adaptations. They gain resistance either by enduring the stress or avoiding it. Community recovery (or resilience) is achieved with cessation of hydrologic stress that permits maximization of re-colonization and reproduction. Endurance strategies are usually disturbance-specific, but avoidance enables organisms to cope with both drying and flooding, and is the prevalent resistance strategy. Correspondingly, community persistence depends to a large extent on the integrity of refuges, an aspect that has so far been little explored. Existing information suggests that seasonal community succession becomes more pronounced with increasing aridity and declining water permanence. The invertebrate community in semi-arid med-rivers can therefore undergo succession through three to four identifiable assemblages, whereas in perennial streams the difference between wet and dry period assemblages is smaller. Community turnover is influenced by the intensity of the hydrologic disturbances and varies between wet and drought years.

Effects of Water Level Fluctuation on the Structure and Function of the Littoral Zone

Water level fluctuations (WLFs) are most pronounced in impoundments, but are also observed in natural lakes over a wide size range, both on a seasonal and on an interannual time scale. In small and/or shallow lakes, WLFs can affect the metabolism and structure of the entire lake system. In most cases of large deep lakes, only the littoral region is affected. The effects on the biota, either beneficial or adverse, depend on local conditions.

Global scale variation in the salinity sensitivity of riverine macroinvertebrates: Eastern Australia, France, Israel and South Africa

Salinity is a key abiotic property of inland waters; it has a major influence on biotic communities and is affected by many natural and anthropogenic processes. Salinity of inland waters tends to increase with aridity, and biota of inland waters may have evolved greater salt tolerance in more arid regions. Here we compare the sensitivity of stream macroinvertebrate species to salinity from a relatively wet region in France (Lorraine and Brittany) to that in three relatively arid regions eastern Australia (Victoria, Queensland and Tasmania), South Africa (south-east of the Eastern Cape Province) and Israel using the identical experimental method in all locations. The species whose salinity tolerance was tested, were somewhat more salt tolerant in eastern Australia and South Africa than France, with those in Israel being intermediate. However, by far the greatest source of variation in species sensitivity was between taxonomic groups (Order and Class) and not between the regions. We used a Bayesian statistical model to estimate the species sensitivity distributions (SSDs) for salinity in eastern Australia and France adjusting for the assemblages of species in these regions. The assemblage in France was slightly more salinity sensitive than that in eastern Australia. We therefore suggest that regional salinity sensitivity is therefore likely to depend most on the taxonomic composition of respective macroinvertebrate assemblages. On this basis it would be possible to screen rivers globally for risk from salinisation.

Effect of an exotic prey on the feeding pattern of a predatory snail

The mussel Brachidontes pharaonis, which invaded the Mediterranean from the Red Sea about 120 years ago, has recently become abundant in many midlittoral and some infralittoral rocky habitats along the Israeli rocky shore. We investigated the influence of B. pharaonis, as a novel prey, on the foraging patterns of the large whelk Stramonita haemastoma in the field, and examined food preferences in the laboratory. S. haemastoma has shifted from indigenous species to feeding on the novel mussel when abundant. The whelk prefers to prey upon the invasive mussel over all indigenous species offered (e.g. barnacles and mussels), probably due to its larger size. In the midlittoral zone, the foraging activity of S. haemastoma is considerably low even where refuges are readily available (incisioned-rocks) and food density is high (mainly B. pharaonis). Higher proportions of whelks are actively foraging in the infralittoral zone but usually on smaller prey, mostly barnacles. We suggest that this differential foraging activity in the two zones is related to the degree of exposure to wave action. The midlittoral is inherently more exposed to wave action than the infralittoral, where sea conditions are more benign and the whelks may enjoy longer activity periods.

Structuring Role of Macrophytes in Lakes: Changing Influence Along Lake Size and Depth Gradients

Emergent, floating-leaved, and submergent macrophytes grow in the littoral region of most lakes. These aquatic macrophytes are influenced by geomorphology, environmental conditions, and biotic interactions (Sculthorpe, 1967; Hutchinson, 1975), while exerting their own influence on the lake environment and biota (Carpenter and Lodge, 1986; Engel, 1988). The capacity of macrophytes to provide a substrate for colonization of algae and invertebrates (Sozska, 1975; Cat-taneo and Kalff, 1980; Dvorak and Best, 1982; Cattaneo, 1983; Morin, 1986; Schram et al., 1987; Miller et al., 1989), to affect water and sediment chemistry as well as other limnological conditions (Carpenter and Gasith, 1978; Prentki et al., 1979; Jaynes and Carpenter, 1986), and to influence biogeochemical cycles and productivity (Wetzel and Hough, 1973; Godshalk and Wetzel, 1978; Wetzel, 1979; Carpenter, 1980; Cattaneo and Kalff, 1980; Carpenter, 1983; Wetzel, 1990) and biotic interactions (Crowder and Cooper, 1982; Heck and Crowder, 1991; Schriver et al., 1995; see also this volume) is well recognized. The understanding of the role of macrophytes in lacustrine systems is based mostly on process studies, small-scale investigations (ponds, test plots), observations in small lakes, and modeling (Carpenter and Lodge, 1986). It is intuitively obvious that the influence of macrophytes in most small or shallow aquatic systems is proportional to their abundance (density, biomass, or extent of cover) and productivity. Little is known about the role of macrophytes in situations in which they are less conspicuous, as in large

Microbial Population and Activity in Wetland Microcosms Constructed for Improving Treated Municipal Wastewater

The idea of using constructed wetlands for the treatment and improving of wastewater emerged in the second half of the last century. Despite relatively wide use of this environmentally friendly technology, relatively little is known about the microbial populations involved in biotransformation and removal of contaminants in this system. The aim of the current study was to investigate the assembly and function of microbial populations in vertical-flow constructed wetland microcosms designed to improve the quality of wastewater after activated sludge treatment. Also, the performance of 3-year-old wetland ponds was investigated. Even though the quality of the influent water was relatively high, improvement in water parameters such as coliform level, ammonia concentration, BOD, and TSS was observed. The performance of the wetland ponds was comparable to that of the microcosms. The microbial community composition of the biofilm formed on the surface of gravel particles in vegetated and plant-free microcosms was studied by denaturing gradient gel electrophoresis (DGGE) and sequencing of 16S rRNA gene fragments. Highly complex bacterial diversity was observed in the biofilm. Cluster analysis of DGGE patterns demonstrated that depth within the wetland microcosm has a stronger effect on microbial community composition of the biofilm formed on wetland matrix than vegetation. Measurements of fluorescein diacetate hydrolysis activity and nitrification potential revealed that hydrolytic activity was affected by both microcosm depth and vegetation presence, whereas nitrification potential was mostly influenced by depth. Resolving the bacterial assemblage of wetland biofilm, which often is considered a black box, will help to understand the interactions involved in the development of diverse and mature biofilm and its function.

Habitat condition and fish assemblage structure in a coastal mediterranean stream (Yarqon, Israel) receiving domestic effluent

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 I-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.

Comparative evaluation of the larvivorous fish Gambusia affinis and Aphanius dispar as mosquito control agents

Under laboratory conditions Aphanius was more successful than Gambusia in preying upon the 3rd, 4th and pupal stages of mosquitoes. The reverse was found for the first two instars. However. Aphanius consumed more 2nd instar larvae under the cover of vegetation when larger fish were able to penetrate shallow water and feed on the mosquito larvae.The two species showed a similar prey-size selection except for Aphanius of the medium size (31–35 mm) which ate larger larvae than Gambusia of the same size range.When provided access to the surface, neither fish species showed any adverse effect at oxygen levels as low as 0.5 mg l-1 (6% saturation). When denied access to the surface, both species behaved ‘normally’ at oxygen levels as low as 1.3 mg l-1 (15% saturation).This study suggests that Gambusia affinis and Aphanius dispar can complement each other as mosquito control agents in different habitat conditions. We suggest that in mosquito infested situation which are characterized by high organic matter and low oxygen levels biological control could best be achieved by introduction of a range of sizes of both fish species. Repeated introductions of the fish, in large enough numbers, may be required for ad-hoc alleviation of a mosquito problem. Best results are thus to be expected in relatively small water bodies such as oxidation ponds.