Dissolved Substance Variability in Temporary Waters

The concentrations of dissolved substances in temporary waters vary more than in most permanent waters. This is due, largely, to three physical processes to which temporary waters are subjected: drying out, refilling, and freezing.

Taking Sunfish Pond as an example (Figure 3.2), from the time that it fills in early spring to just before it dries up the conductivity of the water typically increases four-fold. This concentration of chemical ions is due primarily to evaporation. Cole (1968) described the changes in water chemistry that occur as this proceeds—the normal trend is from carbonate to sulphatochloride to chloride water as calcium carbonate and, subsequently, calcium sulphate and possibly sodium sulphate are precipitated.

The loss of calcium ions is accompanied by relative increases in sodium and magnesium ions. Some calcium remains however, as calcium chloride—as the latter is extremely soluble. Even this, though, may precipitate out of solution in ponds in extreme climates. Cole cites the example of Don Juan Pond in Antarctica. This is a calcium chloride pond, only 10 cm deep, situated in a very dry, cold region. Needle-like crystals of calcium chloride hexahydrate form on the pond bottom and in the water. Species richness in such ponds is likely to be very low. Recent work on the ciliates of a salt pan in Spain has provided some information on how species composition is influenced by such strong chemical variables (Esteban et al. 2002).

At its normal concentration of 8.1% sodium chloride (~2.5 times that of sea water) the Spanish pan water contained only seven species. However, upon dilution with filtered fresh water in the laboratory, more species began to appear from cysts, ultimately producing a total of 34 species. Species appearance was clearly controlled by water chemistry rather than by habitat accessibility and, additionally, provided evidence to support the contemporary belief that most microorganisms are cosmopolitan in nature and, globally, are not represented by vast numbers of species.

When temporary waters first fill after the dry phase, the water is likely to have a chemical signature that changes soon thereafter—although this has not been well studied. McLachlan et al. (1972) recorded the events as Lake Chilwa, a shallow, saline lake in central Africa, filled after having been virtually dry for over a year. Refilling took 5 months during which time there was a rapid dilution of initially high levels of dissolved salts, and a high suspended sediment load resulting from erosion of the lake bottom; this turbidity gradually decreased over a further 2-year period. Increased turbidity is often evident also during the spring thaw of temporary ponds and streams in temperate regions. Initial nutrient release from the mineralization of dead tissues accumulated during the dry phase takes place largely at the sediment surface.

Thus, at first and where light levels are suitable, near-bottom-dwelling autotrophs are likely to be favoured, especially green and blue- green algae. In intermittent ponds, this may be followed, as nutrients circulate into the water column, by an increase in phytoplankton which may accelerate into blooms that, in eutrophic waters, may compromise subsequent periphyton production through reduced light penetration (Bronmark and Hanson 1998). Light penetration may also be affected by the release of tannins from leaves that have accumulated on the bed, especially in the autumn in temperate regions. In addition, Cameron and LaPoint (1978) showed that although not directly toxic, the tannins from Chinese tallow (Sapium sebiferum) inhibited feeding in the isopod Asellus militaris and the amphipod Crangonyx shoemackerii, causing high mortality in both.

In intermittent streams with permeable substrates, below-bed flow has been shown to be an important source of nutrients, particularly nitrogen. Grimm et al. (1981) proposed that biological production in a stream should be greatest at points on the bed where subsurface water rises to the surface. This model was subsequently validated by Valett et al. (1994) who showed that hyporheic upwelling could locally stimulate the productivity of surface algae, and probably bacteria (Dent et al. 2000), with benefits to grazing invertebrates. In other streams, however, hyporheic sediments have been shown to remove nutrients, for example, through denitrification (Duff and Triska 2000). Dieterich and Anderson (1998) have pointed to the potential, applied importance of some summer-dry streams for the efficient removal of nutrients and pollutants.

Concentration of chemicals in temporary waters may occur also as the result of freezing. Daborn and Clifford (1974), in a study of a shallow aestival pond in western Canada, found that, as the winter ice cover thickened and the volume of the pond decreased, there was a rapid rise in conductivity, alkalinity, water hardness (calcium and magnesium ions), sulphate, and orthophosphate. They found this cryogenic 'salting out' to be a consequence of the stable and selective nature of ice crystals—only a few chemical elements or compounds have an appropriate configuration that allows them to become incorporated into the crystal structure of ice. They further observed that the quantity of dissolved and particulate matter in the top layer of ice influenced the pattern of ice break up in the spring—primarily through control of the penetration of sunlight.

The chemical composition of temporary waters may also be influenced by rainwater. Paradise and Dunson (1998) monitored tree holes in three regions of Pennsylvania subject to high, but different inputs of hydrogen and sulphate ions. Although no regional trends were detected, tree hole insect densities and species richness were related to water volume, the concentrations of sulphate and sodium ions, and to dissolved organic carbon. More complex chemicals are known to influence the behaviour of temporary water inhabitants. For example, oviposition by the mosquitoes Aedes aegypti and Ae. albopictus has been shown to be significantly higher when females were exposed to water in which larvae had been previously reared (Allan and Kline 1998). Also, the percentage of eggs hatching of another mosquito, Anopheles diluvialis, was largest in infusions made from intermittently flooded swamp soil, and in hexane extracts of swamp water, suggesting that the hatching factor is a chemically stable organic compound (Jensen et al. 1999).

Inland, saline waters of course have significant chemical signatures that need to be considered. They are included in this book because not only do many of the small ones dry completely, annually, but larger, more permanent ones experience substantial changes in water level, thus producing intermittently wet habitats in their littoral zones. Salinity is thought to be an important ecological determinant, and has been highly correlated with species richness and composition. However, W.D. Williams et al. (1990) found that although this relationship held over the salinity range of 0.3-343 gl^-1 in Australian salt lakes, it broke down over intermediate ranges.

Many taxa found in the latter had very broad tolerances, suggesting that other environmental factors (both abiotic and biotic), together with chance (stochasticity), determined their distribution. These authors cited Herbst's (1988) study of the population dynamics of Ephydra hians, the brine fly, as a good example of how population dynamics are affected by different factors at different salinity levels. Larvae of this species in North American lakes are found over the range 20-200 gl^-1. At the lower end of this scale, larval densities seem largely to be limited by biotic influences, such as predation and competition. Near the upper end, however, they are constrained by physiology. Salinity per se did not seem to be an important determinant of this species' abundance.

Tibby et al. (2000) outlined the problem of determining whether whole community response in hydrologically closed systems, such as lakes and wetlands, is driven by the direct physiological effects of salinity stress, or by the restructuring of microhabitats that results from changes in water level and salinity. They extracted a core from the bed of a shallow, fluctuating lake in Kenya and analysed the fossil macroflora, diatoms, invertebrates, and sediment, representing the period 18701991. The data showed distinct species-specific responses to water level, salinity, and the development of papyrus swamp. Significantly, the latter two explained 51% of the observed historical variation in benthic community composition. The authors concluded that a significant proportion of the published correlation between salinity and invertebrate community structure along the full gradient of inland aquatic ecosystems may be an indirect effect of broad, but diffuse, relationships between salinity and the distribution of various types of benthic microhabitat. Thus local populations may well be regulated by the fluctuating availability of specific habitat and associated resources rather than by the limit of their osmoregulatory capacity. Details of the communities found in saline temporary waters will be given in Chapter 4.