Using macrophytes in urban stream rehabilitation: a cautionary tale

Published source details Suren A.M. (2009) Using macrophytes in urban stream rehabilitation: a cautionary tale. Restoration Ecology, 17, 873-883.
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Summary

Action: Transplant or replace blocks of vegetation

A replicated, randomized, paired, controlled, before-and-after study in 2005–2006 in five rivers near Christchurch, New Zealand (Suren 2009) found that three-leaved watermilfoil Myriophyllum triphyllum persisted over winter in only two of five sites where it was planted, and that planting accordingly had no significant effect on cover of other macrophyte species. After three months, watermilfoil had been washed away or outcompeted in three of five sites where it had been planted. Average cover across the five sites was <1% before planting, 30% after one week, 6% after three months and 2% after 10 months. Cover was <1% in unplanted sites. Cover of five other common aquatic macrophyte species was typically statistically similar in planted and unplanted sites. This was true both before intervention (9 of 10 comparisons; planted 5–29%; unplanted: 2–34%) and after (19 of 20 comparisons, for which planted: <1–9%; unplanted: 1–14%). Methods: Ten sites were used in this study: two 5–6 m long reaches in each of five rivers (12–70 cm deep; <0.1–0.9 m/s flow). Macrophytes were manually removed from all sites in May 2005. Whole plastic trays (24 x 32 cm) of three-leaved watermilfoil were then planted into five sites (one random site/river, 15–18 trays/site). The trays had been reared in plastic pools for 12 weeks before planting out. Macrophytes were surveyed along transects before weeding/planting (March and April 2005) and for 10 months after (May 2005–March 2006).