Kevin Wood

How should we measure aquatic plants in shallow rivers?

Wednesday 16th July 2014

Shallow rivers in southern England, such as Dorset’s River Frome, have a rich flora of submerged aquatic plants. In particular, water crowfoot (Ranunculus penicillatus ssp. pseudofluitans), with its long trailing stems and white buttercup-like flowers, is a characteristic species of a healthy river. Aquatic plants have many key roles, not only in providing habitat, shelter and food for other organisms, but also in regulating hydrological, chemical and physical processes. As examples, plants slow water flow, increase dissolved oxygen concentrations, and trap sediments. The roles help support the community of invertebrates, fish, and other organisms for which shallow rivers such as the River Frome are famous.
Unfortunately, aquatic plants face a number of threats. A range of factors can reduce aquatic plant abundance and hence reduce their capacity to fulfil their key roles. Climate change, over-abstraction of water, nutrient loading and associated eutrophication from human sewage and agriculture, grazing by mute swans, and the siltation of river gravels, have all been identified as factors which can degrade aquatic plant communities. To monitor the health of the river flora throughout a catchment, we need to be able to make rapid, accurate measurements of plant abundance. Where alternative measures are used, we need to understand how these differ. The three most widely used measures of plant abundance are: (i) Cover – the percentage of the river bed area covered by plants; (ii) Volume – the percentage of a given volume of water occupied by plants; (iii) Biomass – the weight of plant material in a given area or volume of river. Traditionally, these measurements of plant abundance have been made from detailed in-stream. However, these methods are time-consuming and so it is difficult to carry out measurements at large numbers of sites within a short time period. In 2009 we carried out a project, funded through the Natural Environment Research Council, which examined:

(1) Whether estimating plant cover from the river bank could be as accurate as more detailed in-stream measurements.
(2) Whether 3 different measures of plant abundance (biomass, cover and volume) showed the same seasonal trends.

We repeatedly measured the plant community in March, May, July and September, at two locations on the River Frome: Maiden Newton and East Stoke. At each site we sampled the aquatic plant community within a 10 m long reach, in 10 bank-to-bank transects spaced one metre apart. Before any in-stream measurements were made, we estimated the plant cover from the bank, in order to compare our bank-side and in-stream estimates of plant cover.

We found that our in-stream estimates of plant cover, volume and biomass were all positively related to each other. However, these relationships were not 1:1 and also differed between the two sites and between months, suggesting that these relationships are local in space and time. We also found that plant volume showed a different seasonal trend compared with cover and biomass. We argue that plant morphology and growth form are likely to influence the relationships between cover, biomass and volume, and the seasonal trends that they show.

We found that our bank-side and in-stream estimates of plant cover were strongly, positively related, although bank-side estimates were around 27% higher, suggesting that bank-side surveyors tended to overestimate plant cover. Understanding the biases in bank-side estimates is vital as this will allow us to correct for them in future work.

This pilot study shows that estimating aquatic plant cover visually from the river bank could offer a rapid, simple alternative to detailed in-stream measurements and hence allow us to monitor changes in aquatic plants at more sites, more frequently than has previously been possible. We were subsequently able to use some of the methods tested here in a project which investigated the effects of multiple biotic and abiotic factors on the plant community of the River Frome.

For more information please see the published paper:
Wood, K.A., Stillman, R.A., Clarke, R.T., Daunt, F. & O’Hare, M.T. (2012). Measuring submerged macrophyte standing crop in shallow rivers: a test of methodology. Aquatic Botany, 102: 28-33.

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