Individual study: Effects of ditch management on caddisfly, dragonfly and amphibian larvae in intensively farmed peat areas in Utrecht, Noord Holland and Zuid Holland, the Netherlands
Twisk W., Noordervliet M.A.W. & ter Keurs W.J. (2000) Effects of ditch management on caddisfly, dragonfly and amphibian larvae in intensively farmed peat areas. Aquatic Ecology, 34, 397-411
Farmers could contribute to enhancing the conservation value of water-filled ditches on agricultural land by undertaking more sympathetic management. However, knowledge as to what measures farmers can realistically undertake and methods of best practice are poorly known. Therefore, a study was set up in peat areas of the Netherlands to assess how dredging and ditch cleaning methods, and nutrient supply in the adjacent fields, affects the larvae of caddisflies, dragonflies and amphibians.
Study areas: The study was carried out between 1994 and 1996 in Utrecht, Noord Holland and Zuid Holland provinces. A total of 240 ditches on 84 dairy farms were selected. The fields were 30–60 m wide, separated by ditches 2–10 m wide, and 10–200 cm deep with water depths of 10-90 cm.
Management and environmental variables: The larvae of caddisflies (Trichoptera), dragon/damselflies (Odonata) and amphibians were selected for study; as well as their conservation value and indicators of ditch quality, it is commonly thought that these larvae are affected by ditch management regimes. The following variables were recorded:
Physical conditions: soil type (mesotrophic or eutrophic peat), ditch width, depth, ditch water table, hydrological isolation and Cl− concentration.
Ditch management :
Cleaning machine (ditch-scoop, mowing-basket, mowing-drum);
Cleaning period (July–September, September–October);
Dredging machine (pull-shovel, punched pull-shovel, suction pipe);
Dredging period (April–August, September–October, November–March);
Years since dredging;
Field management - i.e. field nitrogen (N) and phosperous (P) inputs.
Larvae sampling: Sampling was undertaken once between mid-May to the end of June, i.e., before farmers started cleaning the ditches. The larvae were sampled with a pond net (20 x 30 cm; mesh size 0.5 mm) over a 100 m section in each ditch; 20, 1-m samples per section. The number of larvae per species was counted.
Generalised linear modelling was used to identify the most relevant management/environmental variables influences relating to presence/absence of larvae.
In the ditches, nine caddisfly species (Triaenodes bicolor the commonest; present in 35% of ditches), four dragonfly species (blue-tailed damselfly Ischnura elegans most common; 52% of ditches) and five amphibian species (common frog Rana temporaria most common; 13% of ditches) were found.
Overall, in this rather complex study, the type of dredging machine, the dredging period, water depth and the frequency of dredging were all found to influence the presence of larvae of all study groups.
The presence of amphibian larvae was also markedly affected by the cleaning period; the chances of finding larvae were highest in ditches cleaned between July and mid-September. The presence of caddisfly larvae was influenced by both the cleaning machine and the cleaning period; highest presence was recorded for the ditch-scoop in combination with cleaning in September or October, and the mowing-basket with cleaning between July to mid-September. Why these combinations were best is unclear, but vegetation alterations may play a part; using a ditchscoop and mowing-basket or cleaning between July and mid-September gives the richest emergent vegetation (see: www.conservationevidence.com/ViewEntry.asp?ID=1415).
Water depth: Water depths of around 60-90 cm had the highest chance of caddisfly and dragonfly larvae being present (c.75-80%), declining to 25% for dragonflies and c. 5% for caddisflies at 10 cm depth. In deeper water more stable oxygen levels may be beneficial. In ditches with moderate to deep water, no effect of water depth on the presence of amphibian larvae was apparent.
Field management: The chance of finding caddisfly larvae diminished as the P supply increased. The amount of nutrients found do not affect the larvae directly; the effect is probably caused by simplification of vegetation structure as a ditch eutrophicates; in eutrophic ditches algae mats and duckweed Lemna dominate. The dragonfly and amphibian species found appeared less sensitive to eutrophication than the caddisflies.
Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://www.springerlink.com/content/h41275568281tk24/fulltext.pdf