DEFINICIÓN DE VARIABLES Y SU OPERACIONALIZACION

There was an indication of a negative correlation between TP and diversity on the submerged plants, and a positive correlation between these two parameters on the emergent plant substrates. The dominance index increased with increasing TP on the sediment substrates, and slightly increased on the submerged plants. In contrast, dominance decreased with increasing TP on the emergent plant samples. Diversity generally decreases vrith increasing eutrophication in aquatic ecosystems (Mason, 1991), and there is a tendency towards dominance of nutrient-tolerant algal species with enrichment. Whilst this conclusion would explain the correlations between increasing TP and decreasing diversity/increasing dominance in the sediments and on submerged plants, it does not explain why diversity increased with increasing TP on the emergent plants. Perhaps emergent plant diatom diversity was affected by factors other than TP (such as invertebrate grazing), whilst the submerged plant diatom communities were subject to greater influence by TP. It is also possible that the growth of the emergent plants themselves had an effect on diatom diversity, since it has been found that age of substrate has a profound effect on diatom species composition and density (Slater, 1991). Slater (1991) found that diatom assemblages on the bottom (therefore older) leaves of Potamogeton sp. tended to be more similar to each other than the assemblages from the top (younger) leaves. Further research is therefore required to establish why diversity increases with increasing TP on the emergent plants.

There was a negative correlation between Secchi depth and diversity on the submerged plants (NB: the SD gradient used was plotted from high to low SD). When Blickling was removed from the dataset, there was a very high positive correlation between diversity and Secchi depth on the emergent plants. This result was surprising, because one would expect diversity to decrease with decreasing SD, i.e. as light conditions became less favourable for diatom growth. There was some indication that light availability might be affecting diatom community structure in a study of two freshwater Antarctic lakes, but this was not investigated in full (Oppenheim and Greenwood, 1990). The effects of light availability on the development of benthic diatom community structure in different habitats has not been studied at all, and represents a significant gap in our knowledge. Most effort has been made to investigate the effects of light on epiphyton biomass, presumably because this is easier to measure than the effects on community structure. Lalonde and Downing (1991) found that epiphyton biomass decreased with depth, but did not answer the question of how light availability affects community structure in such situations. Although not concerned with diversity. Marker and Collett’s (1997) study of epiphytic algae on Phragmites in the River Great Ouse revealed that algal biomass peaked when the light climate was improved, since higher concentrations were found on the outer fringes of reedbeds, just below the water surface (where light is more readily available).

Blickling was an outlier in two of the analyses carried out on the full dataset. This lake is significantly larger in surface area than the other four, and was dominated by a cyanobacterial bloom for the whole sampling period. These two factors probably had a significant effect on the stability and functioning of the benthic diatom communities, since it was likely that they had been subjected to varying levels of disturbance by wind (due to the higher surface area), and lower light levels than the other lakes. pH was also very high during the summer in Blickling, which would have led to dissolution of some of the diatom frustules (and probably was the main factor contributing to problems with harvesting diatoms from the summer samples). The presence of this outlier in such a small set of lakes indicates the complexity of shallow lake ecosystems, and is important to consider in future studies of benthic diatom community structure.

There was no correlation between diversity in the sediment samples and TP/Secchi depth. This is probably because the epipelon is more responsive to nutrients available

the sediments of shallow lakes are likely to be better adapted to changing light conditions, since this parameter can change rapidly over a short period of time. It is therefore likely that a community response to light would not be seen.

The variable results seen in the diversity data contradict results seen in a study of Nepalese streams, where various substrates were sampled for diatoms with the aim to develop a water quality biomonitoring programme (Rothfritz et a l, 1997). It was found that diatom samples from both the epiphytic and the epilithic habitats reflected the same chemical gradient (in this case acid-base status, sulphate concentration and conductivity), but H’ diversity and evenness did not vary between habitats, suggesting no differences in diatom community structure. It was noted that the epiphytic samples held a significantly larger proportion of the total species pool of diatoms, which agrees with the results given here which indicated that the submerged and emergent plants captured the largest amount of floristic variation. The authors recommended comprehensive sampling in the assessment of diatom biodiversity, but single habitat sampling for water quality monitoring.

The submerged and emergent plants had a higher diatom species dominance than the sediments. Dominance in the sediment samples increased with increasing TP. However, there was a negative correlation between TP and dominance in the emergent plant samples. Species that were most dominant on the plants included Achnanthes minutissima and Cocconeis placentula, which were also reported as being dominant on various plant species in the eutrophic Lake Tâkem in Sweden (Blindow, 1987). Rothfiitz et a l (1997) found that Cocconeis placentula, Achnanthes lanceolata and

Achnanthes pediculus were significantly more abundant on bryophytes than in the epilithon in Nepalese hill streams.

Biodiversity is an important characteristic to consider when investigating ecosystems, since it has been argued that a large proportion of native species richness is required to maximise ecosystem stability and sustain function (Schwartz et a l, 2000). However, the hnks between biodiversity and ecological functioning are poorly understood, and there are several arguments indicating that biodiversity and functioning are linked (Chapin et a l, 1997; Tilman, 1997; Edwards and Abivardi, 1998), and others that there are no correlations between the two (Hooper, 1998; Hooper and Vitousek, 1997). The majority of diversity/ecological functioning studies have been carried out on terrestrial

plant and animal communities, with relatively few in aquatic environments (e.g. Finlay and Esteban, 1998; Marques et al., 1997). The links between biodiversity of benthic diatoms and the functioning of lake ecosystems in particular has not been studied, although attempts are now being made to rectify this (Sayer et a l, 1999). Stevenson (1996) states that interpretation of diversity scores in benthic algal communities is unclear.