What are temporary waters? Biological importance

Temporary waters, in general, are to be found throughout the world. Some types, such as the reservoirs of bromeliad leaf axils and turloughs (seasonal limestone lakes), are restricted by factors such as climate or geology. Others, such as temporary ponds and streams, and rain pools are ubiquitous—although these, too, may exhibit some regional differences (e.g. rain pools characterize the high, open grasslands of South Africa). However, all are, for the most part, natural bodies of water which experience a recurrent dry phase. Often, the latter is predictable both in its time of onset and duration. The defining element is the cyclical nature of the drought, as some permanent waterbodies may dry up in exceptional years. In the latter case, because most of the biota is not adapted to survive such conditions there will be significant mortality.

Temporary water species, on the other hand, are generally well adapted to dealing with water loss. Indeed, many species have spread beyond the boundaries of natural waterbodies to colonize those temporary waters that have been created by human culture, such as bird baths, and rainwater-filled tin-cans and tyre tracks. Although the contents of this book are devoted largely to temporary fresh waters, it is important to remember that a great number of inland saline waters also experience drought. Such habitats are widespread and they have received considerable attention elsewhere (e.g. W.D. Williams 1981; Hammer 1986; International Society for Salt Lake Research 2001). Coastal marine habitats, for example salt marsh ponds and supralittoral rock- pools, are also subject to drying, and will be touched on in this volume.

The physicochemical features of temporary waters strongly influence the biotas present, but biological factors may be important also especially with increased duration of the aquatic phase. Insects and crustaceans tend to dominate the fauna, but temporary water communities, as a whole, may comprise bacteria, protoctists, vertebrates, fungi, and an abundance of higher and lower plants. Many species exhibit opportunistic and pioneering traits, and also a range of drought- survival mechanisms, such as diapause and seed formation.

Whereas wetlands, in general, comprise a very important subset of temporary waters, and data from their study will be drawn on throughout this book, the latter is not intended as a comprehensive synthesis of wetland biology per se. For such information the reader is directed to, for example: Williams (1993), Finlayson and Van der Valk (1995), Mitsch and Gosselink (2000), Spray and McGlothlin (2004), and the websites of organizations such as the Ramsar Convention on Wetlands (http://www.ramsar.org/), and Wetlands International (http://www.wetlands.org/).

Biological importance. Despite perhaps being regarded as the cinderellas of aquatic science, temporary waters represent significant components of the global landscape. From a cultural perspective, cyclically fluctuating water levels often have determined the sustainability and evolution of riparian societies. A prime example is the annual flooding of the River Nile upon which the agricultural activities of both ancient and modern civilizations have depended. Further, from a resource perspective, wetlands, for example, represent a significant store of our planet's freshwaters.

However, temporary waters also have considerable significance to Biology per se. Blaustein and Schwartz (2001) outlined four reasons for studying temporary pools, specifically, but which well encompass other temporary water types: (1) temporary waters can contribute to our general understanding of 'ephemerality', especially as it relates to life histories, population dynamics, and community organization; (2) these habitats represent convenient systems in which to study ecological concepts, particularly as they are amenable to manipulation experiments, and their abundance allows easy replication. Further, those habitats with simple communities can be mimicked in semi-natural or even artificial set-ups; (3) temporary waters frequently harbour the vectors of disease-causing organisms that afflict mankind; and (4) temporary waters contain many species important to global biodiversity.

To these may be added: (5) that, in a biogeo- graphical context, there is evidence to suggest that temporary ponds may have acted as postglacial dispersal routes for taxa possessing dormant stages capable of 'island-hopping' (e.g. copepods) from glacial refugia (Stemberger 1995); (6) in an evolutionary context, there has been exploration of the idea that life may have evolved on earth more than once, and that an alternative environment-of- origin to the oceans may have been ponds that dry out periodically. In such ponds, chemicals in solution would have been progressively concentrated to a state that enabled the maintenance of protoplasmic systems (Hinton 1968).

Further, W.D. Williams (1988) has suggested that there is an alternative explanation to the hypothesis that the biota of permanent, standing freshwaters came from marine ancestors via either the terrestrial environment, rivers or estuaries, and which relegates the biota of temporary freshwaters to a subset of the 'permanent' biota that developed adaptations to resist desiccation and good powers of dispersal (Wiggins et al. 1980). The alternative viewpoint considers most permanent lakes to be geologically ephemeral (few are older than 20,000 years) and regards temporary water bodies as being very ancient, not as individuals but as a habitat type. Williams cited Lake George, a temporary freshwater lake near Canberra, Australia, as being 10 million years old. He hypothesized that from rivers and the terrestrial environment, a large contingent of the biota first colonized temporary freshwaters (perhaps temporary floodplain pools), and that a subset of this flora and fauna developed, or regained, the ability to withstand permanent inundation and hence were subsequently able to establish themselves in permanent lakes.

Under such a scenario, Williams stated that one would expect to see some of the following properties: the more evolutionary ancient groups of the biota should occur in temporary waters; much of the biota living in permanent waters should retain effective dispersal mechanisms—to counter the geological ephemerality of their habitats and as a reflection of their lineage; many of the 'active migrants' of the permanent lentic biota, should persist in lotic habitats, or have close relatives which do; and overall species richness in temporary waters should be greater than in permanent freshwaters. Evidence from the literature provides some support for each of these properties (Tasch 1969; Elgmork 1980; Fernando 1980; Schram 1986; Fernando and Holcik 1989; Lake et al. 1988); (7) with increasing interest in land-water ecotones, the margins of temporary ponds and streams have the potential to be important sites for modelling hydrological processes, nutrient transport and transformation, and the role played by the biota (Bradley and Brown 1997; Giudicelli and Bournard 1997); and (8) there is now evidence to indicate that variations in the physical environment of inland waters impact both molecular and morphological evolution by changing mutation rates and by exposing (through genotype-environment interactions) otherwise cryptic variation. Extreme environments tend to accelerate morphological change, promoting diversification (Hebert 1999). Temporary waters may be important sites of such altered rates of molecular evolution and, therefore, worth further study.