The Diatom Assemblages

Before considering how the sampling of diatom assemblages can affect diatom-based water quality estimations, it is perhaps best first to identify the habitats in which they are found. Within most lotie systems the benthic algae - broadly defined as the periphyton - are the most important primary producers (Pryfogle & Lowe 1979), inhabiting almost all available surfaces. The majority of river diatoms, particularly in faster flowing systems, are benthic and can be divided into four main communities. These are: the epilithon - growing on stones or rock substrata, the epiphyton - growing on other algae and submerged aquatic macrophytes, the epipsammon - growing on sand grains and the epipelon - growing on or within fine silt (Round 1981). In addition there are also several less distinct associations which can be recognised e.g. epizoon, endolithon, endophyton and metaphyton. Although it is important to be aware of the existence of these lesser groups they are relatively rare and thus not considered here. There is inevitably some cross-over among the species found in each habitat (Cox 1990) but in general most taxa can be categorised into one of the major associations. A great deal of confusion can therefore arise if one is unaware exactly which habitat is being sampled (Round 1991). For example, the sampling of stones in slow-flowing rivers is often hindered by a covering of silt and/or other algae, thus giving a combined sample from three different habitats. Any ecological interpretation from such a sample should be treated with caution (Round 1991) particularly when comparisons are being made between different samples.

1.4.1.1 The Epipelon

The epipelon is the community living attached to, or moving within, the fine silt and mud of streams, rivers and lakes. Complex interactions within the silt could bias an ecological investigation where water quality is the focus. Phosphate ions are absorbed on to silt

particles and thus river sediments often have much higher P concentrations than the overlying water. Thus if epipelic taxa, which live within the surface of the sediments, can utilise this phosphorus source, they may be indicative of more nutrient-rich conditions than nearby species deriving their nutrients from the water column alone. Navicula gregaria ‘b ’, a species normally associated with more eutrophic waters, was found in the epipelon of a relatively nutrient-poor stream by Cox (1990b). This species is presumably able to utilise the nutrients bound within the sediment and therefore would not give a true representation of the ambient water quality. Perhaps because of the complex relationships between silt nutrient binding and diatoms this community should be avoided for water quality estimations.

1.4.1.2 The Epiphyton

Within most streams and rivers there is an abundance of aquatic macrophytes and almost all of these can be found to have a covering of diatoms (Round 1981, Morin 1986). There is however some contention as to exactly how diatoms react with the host plant. Some studies have shown that little or no difference occurs between epiphytic communities and those growing on artificial, inert, substrata (Cattaneo & Kalff 1979). In contrast, a strong host specificity was reported by Eminson and Moss (1980) for four host species of macrophyte. This host specificity was however only found under very nutrient-poor conditions and almost disappeared in more nutrient-rich waters.

From experimental results aquatic macrophytes have been shown to excrete organic compounds (Wetzel 1969) and these must have some influence on the epiphytic algae. The results of Eminson and Moss (1980) suggest that in more nutrient-rich waters the external influences are far more important. They go on to conclude that, from an evolutionary viewpoint, it would be disadvantageous for aquatic plants to release nutrients as the resultant epiphyton would reduce incident light significantly. The shading effect of epiphytic algae has been shown to restrict macrophyte growth within lakes. Sand-Jenson and Borum (1984) calculated that plants found growing in water of less than Im depth could have penetrated to 3.5m in the absence of attached epiphytes.

Another problem in the study of the epiphyton is that of colonisation and succession. Aquatic macrophytes form a dynamic substratum which is continuously producing new surfaces as it grows. Thus any one plant can support a number of epiphytic assemblages at different stages of succession (Jones & Meyer 1983).

When sampling with water quality estimation in mind it is imperative that representative diatom samples, which reflect the water quality, are taken. These samples should not be influenced by other factors such as host specificity and irregular exposure times. The epiphyton, due mainly to a lack of understanding of these complexities, has not been used for the purpose of water quality assessment. The majority of studies to date have used either artificial substrata or the epilithon (Round 1993).

1.4.1.3 The Epilithon

Due mainly to its almost universal availability the epilithon is the most widely and most often sampled of all the natural communities (Cattaneo & Amireault 1992). This habitat includes the diatoms growing on bed-rock, boulders, stones, gravel and small pebbles. It is usually possible to find some sort of solid substratum to sample at stream and river sites, although problems can occur in very slow flowing, lowland rivers (Round 1993). The structure of the epilithon is not however simple and has been compared to that of a forest (Round 1993) with a number of canopy levels. The different growth forms of diatom taxa form complex layers; some in close contact with the rock, e.g. Achnanthes spp., some growing on mucilage stalks, e.g. Gomphonema spp., while other species form chains, e.g.

Tabellaria flocculosa (Lock et a l 1984, Pringle 1990). Also associated with these different layers are motile species which move in-between trapped silt particles and mucilage stalks, e.g. Navicula spp. and Nitzschia spp. (Round 1981). The tendency for these layers to trap silt has led to some confusion in the literature as to exactly what comprises the epilithic diatom flora. Stevenson and Hashim (1989) suggested that the flora of most microhabitats was comparable. Round (1991) on the other hand considers the contamination of the epilithon by silt flora a serious source of error in sampling.

Even within the true epilithon considerable spatial and temporal variability has been reported (Jones 1978, Jones & Meyer 1983, Korte & Blinn 1983). The dynamic nature of

streams and rivers must in part be responsible for this but other factors too, such as rock type (Descy 1979, Allan 1995) and immigration and emigration of individuals (Stevenson & Peterson 1991) are also important. The geology of an area will inevitably influence the water chemistry and in turn the species composition. Within a river system, however, introduced foreign rock types have not been found to support significantly different floras to the local substratum (Gale et a l 1979). The reason for this is likely to be due to bacterial and fungal conditioning of the rock surface reducing the effects of the geology on the diatom community (Korte & Blinn 1983, Blinn 1986).

A more important effect of substratum is surface structure. Smooth substrata, such as glass microscope slides, have often been found to support a less diverse and less prolific community than rough substrata (Butcher 1940, Siver 1977, Antoine & Benson-Evans 1986). The reasons for this are given as: 1. a rough surface provides more opportunity for diatom colonisation (Antoine & Benson-Evans 1986), and 2. the availability of microhabitats enables the epilithon to overcome adverse conditions such as flood events or increased sediment load (Allan 1995). When sampling the epilithic diatom community it is therefore recommended that a number of stones be sampled, and the results pooled, to avoid the inherent heterogeneity in the epilithon (Round 1993).