Fish Communities in Small Aquatic Ecosystems
Total Page：16
File Type：pdf, Size：1020Kb
Fishcommunities in small aquatic ecosystems: caves, gueltas, crater and salt lakes The inland water fishes of Africa S mall aquatic ecosystems generally have several similar physical and biological characteristics. Except for their size, they are, most of the time, extreme ecosystems. For example, soda lakes have very high salinity and/or alkalinity; crater lakes are deep with a low level of oxygen in the deep layers of the water; in gueltas and chotts, the water is perennial in only a few and the temperature is high; in caves, organisms live in permanent darkness. In relation with these severe con ditions, fish have developed physiological and behavioural adaptations that allow them to survive and perpetuate. Thus, specific and usually endemic species inhabit these environments. In some cases, small species flocks exist, as can be observed in the Great lakes of East Africa. Finally, most of these environments are vulnerable to even minor distur bances. Continued vigilance, protective measures and conservation recommendations must be a high priority to protect them from actual or potential local threats. Strict ly speaking, the crate r lakes and the graben/horst lakes are diffe rent, whi le they both owe the ir formation to tecton ic events . A crater lake is a lake that formed in a volcanic crater or a caldera. In Afri ca, most of the crater lakes are located in dorman t or extinct volcanoes, and tend to be freshwater. The w ater clarity in such lakes can be exceptional due to the lack of inflowing streams and sediment. Crater lakes are well represented in tropi cal Africa, especially in the Guinean rainforest zone of Cameroon, wh ere th ere may be 36 or more (McGre gor Reid & Gibson, 2011). There are also crate r lakes in Eastern Afr ica, mainly high-m ountain crate r lakes in Ethiopia (Degefu et al., 2014), but the natural fish richness is generally very low or non existent. A graben is the result of a block of land being downthrown, producing a valley w ith a distinct scarp on each side . Grabens often occur side-by-side with horsts. Horst and graben struc tu res are indicative of tensional forc es and crustaI st retching. This kind of lake is very present in th e eastern arm (Gregory branch) of the African Rift Valley. This Gregory rift valley is most clearly defined in Kenya w here classical grabens penetrate the regional dom al upwarping. A string of lakes follow its course sout hwar d through Kenya and into Tan zania, including Turkana, Baringo, Bogoria, Nakuru, Elmenteita, Naivasha, Magadi, Natron. Eyasi and Many ara. Fishcommunitiesin small aquaticecosystems DIDIER PAUGY & CHRISTIAN L EVEOUE Lake level is a product of the balance of inputs (precipitation, inflowing streams, groundwater inflows) and outputs (evaporation, outf low s, ground w ater seepage). The relative importance of th ese factor s w ill govern the responsiveness of particular basins to changes in climate, and these w ill be examined in the following section (Salt and alkaline lakes), Salt and alka line lakes A salt lake is gener ally a landlocked body of w ater wh ose concentration of salts (typically sodium chloride) and other dissolved minerals is significantly higher than most lakes (often defined as at least three grams of salt per litre). In some cases, salt lakes have a higher concentration of salt than seawater, but such lakes would also be terme d hypersaline lakes. That is, for example, the case of Lake Retba in Senegal ("lac Rose" meaning "Pink lake" as it is know n by locals) where the salinity is between 80-350 g/I according to the seasons (Sow et a/., 2008). An alkaline lake that has a high carbonate content is sometimes called a soda lake. This type of lake is alkaline w ith a pH value above 7.5. In addition, many soda lakes also contain high concentrations of sodium chloride and ot her dissolved salts, making them saline lakes as well. High pH and salinity often coincide. Salt and soda lakes form w hen the w ater flowing into the lake, containing salt or minerals, cannot leave because the lake is endorheic (without outlet). The water then evaporates, leaving behind dissolved salts and thus increasing its salinity, making a salt lake an excellent place for salt production. The hypersaline and highly alkalic soda lakes are considered some of the most extreme aquatic env ironments on Earth. In spite of their apparent inhos pitablen ess, soda lakes are often highly product ive ecosystems, compa red to their (pH-neutral) freshw ater counterparts. An important reason for this high productivity is the virtually unlimited availability of dissolved carbon dioxide. Neverth eless, overly high salinity or alkalinity will also lead to a unique flora and fauna in the lake in question; sometimes, in fact, the result may be an absence or near absence of life near the salt lake. In fact it depend s on the tolerance of organisms to the mineral concentrations that exist in the lake. For African fish like cichlids , we generally agree that the highest salinity must not exceed 50-60 g/I (see also chapter Diversitv ofresponses to extreme environ mental conditions). Thus, some specialized species, like Alcolapia grahami, have been introduced w ith success in Lake Nakuru (Kenya), wh ere salinity can reach 20-25 g/I (Vare schi, 1982). However, authors (Gamier & Gaudant. 1984) have recorded the presence of a tilapia (Sarotherodon melan otheron) in Lake Retba (Senegal) where salinity reaches at least 80 g/I but can rise up to 350 g/I during the dry season. In their paper the authors say " In its North-Eastern part, the lake is extended by an intermittent creek that is supplied in water through inputs from groundwater. The inland water fishes of Africa It is in one of these wat er holes that we have observed many cichlid fish of w hich one specimen was captured in April 1978" . At present thes e holes no longer exist. but there are still small ponds of brackish water where a dwarf form of Sarotherodon still lives. In these ponds as well as the ancient holes, water is less concentrated than in the lake itself, which is really unsustainable for tilapines . Soda lakes occur naturally thr oughout the worl d (see Table 21.1 below fo r African ones), typically in arid and sem i-arid areas and in connection to tectonic rifts like the East African Rift Valley. The pH of most freshwater lakes is on the alkaline side of neutrality and many exhibit similar water chemistries to soda lakes, only less extreme . -l'AB LE.21.1. Chemical Name Country pH Salinity (gIl) characteristics (pH Wadi Natrun lakes Egypt 9.5 50 and salinity) for Malha Crarer Lake Sudan 9.5-10.3 rnaiorAfrican salt lakes. Lake Abbe Ethiopia 9.8- 1I 115 Lake Arenguadi (Green Lake) Ethiopia 9.5-9.9 2,5 Lake Basaka Ethiopia 9.6 Lake Shala Ethiopia 9.8 18 Lake Chiru Ethiopia 10.3 58 Lake Abijatta Ethiopia 9.9 34 Lake Magadi Kenya 10 >100 Lake Bogoria Kenya 10.5 350 Lake Turkana KenyafEthiopia 8.5-9.2 2,5 Lake Nakuru Kenya 10.5 5-90 Lake Logipi Kenya 9.5-10 .5 20-50 Lake Manyara Tanzania 9.5-10 40 Lake Natron Tanzania/Kenya 9- 10.5 100 Lake Rukwa Tanzania 8.5-9 .2 Lake Eyasi Tanzania 9.3 Generally speaking, tilapias 5./. have a w ide and varied diet and can occupy a large range of habitats, from freshwater to hypersaline conditions; therefore, its dist ribut ion is unlikely to be limited by many environ mental, physical or chem ical condit ions. How ever, the one factor that appears to affect tilapia is its vulnerability to cold temperatures. Tilapia is traditionally viewed as a trop ical to wa rm-temperate species. Salt concentration may constitute a limiting facto r for fish and for other many organisms. Thus, in Africa a marked decrease has been observed in the num ber of zooplankton and foramin ife ra specie s in increasingly confined water (Debenay et el., 1989). Likew ise, studies on the Casamance river have shown that fish taxa disappear with increasing salinity and that fish fauna becomes Fish communities in smallaquatic ecosystems DIDIERPAUGY & CHRISTIAN LEVEOUE monospecific (Sarotherodon melanoth eron) above 75%0 (Albaret, 1987). The capacity of S. m elanotheron to w ithstand salty environments suggests that this species has the capacity to modify its life-history traits, mainly reproduc tive and grow th ones. The most profound changes are only visible in hypersaline conditions wh ere the species is able to limit its growth, to reduce its size-at-maturity, and to change its fecundity (Panfili et el., 2004b). The main modification in the reproductive traits concerns the change in size at maturity, w hich dec reases in S. melanotheron as salinity increase s. Convers ely, the influence of intermediate or high salinities (35-90%0) don't show any clear significant relationship wi th fec undity or oocyte size of thi s species (Panfili et el., 2004b) Some authors mentioned several species of tilapias isolated in the alkaline lakes of the Great Rift Valley of Africa, living in extreme conditions of temperature, salinity and pH (Reiter et et., 1974). It is true for some of them like Oreochromis mo ssambicus or Sarotherodon melanotheron, which may survive for som e time in harsh conditions. But, it seems that only one com plex of fish species (Alcolapia) can actually be found in saline and alkaline lakes and is adapted to very harsh conditions.