The grazed samples had significantly more diatom species on them than the ungrazed samples. The most likely explanation for this observation is that grazing actively encouraged species overturn, reducing dominance by rapidly colonising diatoms such as
Achnanthes minutissima and the various Fragilaria taxa.
The grazed Myriophyllum samples had significantly more diatom species than the ungrazed samples, indicating that grazing encouraged the development of a richer diatom flora on this plant type. There were no significant differences noticed on the
Elodea plants. The increase in diatom species numbers on the grazed Myriophyllum
plants compared to the ungrazed plants may have been due to the relative architectural complexity of the Myriophyllum leaves. Robson and Barmuta (1998) found that invertebrate species numbers increased with increased complexity of substrate architecture. Sommer (2000) also considers spatial heterogeneity to be an important factor in the increase in algal species richness with increased grazing. If invertebrate diversity increased, then this would feasibly lead to an increase in diatom species, since some invertebrates would preferentially feed on certain diatom species, leading to other
diatoms coming in and replacing them. In the case of the grazed Myriophyllum plants, the increased leaf complexity (compared to Elodea) would lead to an increase in invertebrate diversity, and therefore the observed increase in numbers of diatom species.
There were few overall differences in diatom composition between the grazed and ungrazed samples, although more pronounced differences were noticed in Gunthorpe than in Green Plantation. In particular, Achnanthes minutissima was present in much higher abundance on the ungrazed than the grazed plants in Gunthorpe; the abundance of this diatom was similar between treatments in Green Plantation. This may have been due to differences in the invertebrate community present in Gunthorpe. Snails and chironomids can remove large quantities of adnate diatoms (Sumner and Mclntire, 1982; Wellnitz and Ward, 2000), which includes Achnanthes minutissima. The difference in diatom composition between the two plant types was also negligible.
The grazed plants had significantly more adnate than stalked diatoms present when compared with the ungrazed plants in both study lakes. This has been found in a number of similar studies (Karouna and Fuller, 1992; Tuchman and Stevenson, 1991; Steinman et a l, 1987). However, Dillon and Davis (1991) found that the diatoms ingested by various snail species were indistinguishable fi'om another, and other workers reported that grazing actually increased overstorey, stalked forms (Dudley, 1992; Samelle et a l, 1993). In these cases, experiments were carried out on the chlorophyte Cladophora, which has a different morphology entirely to that of the aquatic macrophytes used in this study. This may explain the difference in results. In the present study, there was no significant difference in adnate:stalked ratio between the two plant types used, indicating that plant architecture had no effect on the physiognomy of the benthic diatom community.
Chironomids were very abundant in the three study lakes, particularly in Felbrigg. The high abundance of these particular grazers could have led to an increase in the abundance of diatoms with a prostrate/motile physiognomy, as was seen in Wellnitz and Ward (1998). Chironomids preferentially feed on stalked diatoms, which would reduce competition for light and nutrients for the lower-storey species, such as Achnanthes minutissima and Cocconeis placentula. Cattaneo et al. (1998) found that submerged plants had higher abundances of chironomids and diatoms than fioating plants, which
of plants such as Ceratophyllum demersum gave a firmer attachment and protection against snails, which could explain why diatoms were so abundant on these plants. However, it was found that the snail Lymnaea peregra was particularly associated with
Elodea plants, although this was attributed to the snail’s preferred epiphytic composition of filamentous green algae, which was more commonly found on this plant (Lodge, 1986).
The caged and uncaged samples produced different indicator ratio values, but nevertheless ordered the two lakes correctly in terms of their water quality. Grazing would not, therefore, strongly affect a monitoring programme if these particular indicator diatoms were used. However, these data are based on only two sites and it would clearly be advisable to explore the effect of grazing on indicator taxa along a much wider water quality gradient at a greater number of lakes.
5.
Vertical distribution patterns
Slater (1991) and Blindow (1987) hypothesised that there may be differences in epiphyton composition between the parts of the submerged plant sampled. Slater (1991) showed that there was less similarity between the diatom assemblages on leaves from the tops of various macrophytes (mainly Potamogeton species) and more similarity between the assemblages from leaves collected lower down the plants. It was hypothesised that as the plant is growing upwards, the lower leaves are often older than the leaves nearer the top, as well as being nearer and hence influenced by the bottom sediment. It was suggested by Slater (1991) that as the diatom assemblages varied on different parts of the plant, then all parts should be sampled to gain a representative picture of the diatom flora. This suggestion agrees with the results presented here, since there was no appreciable difference between the top, middle or bottom of the plants sampled. A combined sample is, therefore, recommended for use in a monitoring programme.
However, the diatoms growing on the top sections of plants are probably subjected to a variety of different pressures, including intense wave disturbance, changes in light regime, and competition with phytoplankton/metaphyton that may temporarily become attached to the substrate. The bottom regions of the plants can be contaminated by
epipelon (including the motile Naviculas and the Fragilarias), and receive much less light than the upper regions. Although the results presented here suggest that a combined sample would be suitable for monitoring purposes, more research should be carried out on the effects of the sediment dwelling species on the community inhabiting the lower reaches of plant sections, particularly in lakes that possess a high abundance of Fragilaria taxa.
Summary
1. Plant architecture effects
• There were no overall differences between numbers of species found on the two plant types.
• Plants were generally dominated by Achnanthes minutissima, Cocconeis placentula,
and various Fragilaria species, with few differences between plant type.
• There was a negative correlation between indicator species ratio and decreasing water quality, and very little difference in indicator ratio between Elodea and
Myriophyllum. Therefore plant architecture has relatively little influence on diatom composition and hence need not be a major consideration when using epiphytic diatoms in a monitoring programme.
2. Grazing effects
• Grazing significantly increased diatom species numbers on both plants. • Grazed plants had significantly more adnate than stalked diatoms present.
• The ungrazed plants in Gunthorpe had a higher abundance of Achnanthes minutissima than the grazed plants. The diatom communities did not appear to be significantly affected by the grazing treatment in Green Plantation.
• Grazing resulted in a lower indicator ratio in Gunthorpe, and a higher ratio in Green Plantation, indicating that a noticeable, but unpredictable effect was occurring.