A survey involving repeated visits to 56 river sites with a variety of Batrachian Ranunculus species was carried out. Analysis of the data using CANOCO suggested that the following measured environmental variables were the most important in determining the plant community composition: pH, sediment nitrogen, phosphate and potassium concentration, shade, water velocity, water phosphate concentration, and elevation. In addition, management by cutting as well as grazing and poaching by cattle were important variables. Chemical analysis of plant and sediment samples revealed significant positive correlations between the concentrations of Ranunculus shoot tissue, phosphate and water phosphate, Ranunculus phosphate and sediment phosphate, and Ranunculus nitrogen and sediment nitrate. A series of transplant trials were carried out to determine to what extent it was possible for Ranunculus species to survive outside of the conditions in which they were found in the survey.
Three of the stresses identified in the survey were further investigated in experimental work: shade, low water velocity and eutrophication. A field experiment was carried out on a branch of the River Frome (a chalk stream dominated by Ranunculus penicillatus subsp. pseudofluitans). The change in community composition in shaded plots compared with unshaded plots was measured during the growing season. The following season the experiment was repeated, but with the additional stress of reduced water velocity. During the first season the Ranunculus cover was much less in the shaded plots, and the cover of other members of the plant community was also significantly less in the shaded plots. During the season with reduced water velocity, Elodea canadensis rather than R. penicillatus subsp. pseudofluitans became the dominant species, and the community reacted in a similar way to the shade stress. There was less total plant growth (more visible substrate) in the low velocity (increased stress) season, and a higher species diversity.
The effects of elevated phosphate concentration were investigated in an experiment in a pair of artificial recirculating rivers. One of the rivers had the phosphate input raised from 40 µgP l-1 to 200 µgP l-1, which resulted in prolific filamentous algal growth. R. penicillatus subsp. pseudofluitans and Potamogeton pectinatus plants showed a reduction in growth in the elevated phosphate (presumably due to shading from the algae), and where Ranunculus and Potamogeton plants were grown next to each other, the Ranunculus root biomass was significantly reduced.
Several experiments were carried out on the effects of disturbance on Ranunculus penicillatus subsp. pseudofluitans. Ranunculus clumps were cut repeatedly in the River Rye. It was discovered that after the first and second cuts, the shoots showed increased growth, but after the third cut they showed decreased growth compared with uncut clumps. By contrast, in two experiments carried out in polluted sites (Gogar Burn and the headwaters of the River Hull), there was no evidence of an increase in growth after a single cut.
Ranunculus species are associated with particular current velocities and particular sediment particle sizes. Does this correlation with sediment imply that the sediment causes the distribution, or is sediment texture itself simply a reflection of current velocity? An experiment in the Mouse Water found no correlation between Ranunculus growth and sediment particle size, suggesting that current velocity is the primary determinant.
Throughout the thesis, the data were interpreted in terms of C-S-R strategy theory. A greenhouse shading experiment ranked four Ranunculus taxa in terms of their stress tolerance (R. hederaeus > R. circinatus > R. penicillatus subsp. pseudofluitans > R. fluitans). The data from the survey were used to devise integrated indices of stress and disturbance for each site surveyed, and this was then used to determine the importance of stress and disturbance in the habitats occupied by the various Ranunculus species, and by implication the importance of stress and disturbance in the strategy in those species, i.e. the C-S-R strategy of each species. A further survey of 57 river sites with Ranunculus species present was carried out,a and various morphological attributes were measured on the plants. These attributes were then regressed against the strategy of the plant in order to determine which traits area associated with a particular strategy. Stress-tolerant species tend to have floating leaves, lack divided submerged leaves, and tend to have small, weak shoots. Disturbance-tolerant species lack floating leaves, and have large strong shoots. Competitive taxa tend to have long submerged leaves, and lack floating leaves.
The implication of these data for the conservation and management of rivers is discussed.
Introduction (in full)