Macrophytes in Drainage Ditches
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Macrophytes in drainage ditches Functioning and perspectives for recovery Jeroen P. van Zuidam Thesis committee Promotor Prof. dr. Marten Scheffer Professor, Aquatic Ecology and Water Quality Management Group Wageningen University Co-promotor Dr. Ir. Edwin T.H.M. Peeters Assistant professor, Aquatic Ecology and Water Quality Management Group Wageningen University Other members Prof. dr. Frank Berendse, Wageningen University Dr. Sarian Kosten, Radboud University Nijmegen Dr. Liesbeth Bakker, NIOO KNAW, Wageningen Dr. Theo Claassen, Wetterskip Fryslan, Leeuwarden This research was conducted under the auspices of the Graduate School for Socio-Economic and Natural Sciences of the Environment (SENSE). Macrophytes in drainage ditches Functioning and perspectives for recovery Jeroen P. van Zuidam Thesis submitted in fulfilment of the requirements for the degree of doctor at Wageningen University by the authority of the Rector Magnificus Prof. dr. M.J. Kropff, in the presence of the Thesis Committee appointed by the Academic Board to be defended in public on Wednesday September 11th 2013 at 1.30 p.m. in the Aula. Jeroen P. van Zuidam Macrophytes in drainage ditches: Functioning and perspectives for recovery 112 pages. Thesis Wageningen University, Wageningen, NL (2013) With references, with summaries in Dutch and English ISBN 978-94-6173-589-8 Contents 1. General Introduction ........................................................................................................................... 9 1.2 Macrophytes in drainage ditches ..........................................................................................10 1.3 Environmental conditions and human influence ...................................................................11 1.4 Research questions and approach .......................................................................................13 2. Macrophyte species composition and biomass in drainage ditches: the importance of both local and regional environmental factors .......................................................................................... 19 3. Occurrence of macrophyte monocultures in drainage ditches relates to phosphorus in both sediment and water .......................................................................................................................... 35 4. Cutting affects growth of Potamogeton lucens L. and Potamogeton compressus L. ...................... 53 5. The role of propagule banks from drainage ditches dominated by free-floating or submerged plants in vegetation restoration ........................................................................................................ 63 6. Differential response to climatic variation of free-floating and submerged macrophytes in ditches .............................................................................................................................................. 79 7. Synthesis .......................................................................................................................................... 89 7.1 Perspectives for management of drainage ditches ..............................................................89 7.2 A polder scale vision on water quality management: redistributing functions and ecological goals ....................................................................................................................93 Summary ............................................................................................................................................... 97 Samenvatting ......................................................................................................................................... 99 Acknowledgements ............................................................................................................................. 103 Curriculum Vitae .................................................................................................................................. 105 List of publications ............................................................................................................................... 107 Financial support ................................................................................................................................. 109 A typical line shaped drainage ditch with a well-developed vegetation. Photo taken in Dwarsgracht, Overijssel, the Netherlands. 8 1. General Introduction 1.1 The paradox of the drainage ditch As the largest part of the Netherlands is located below sea level, management of water has always been a challenge for its inhabitants. Besides building dikes to prevent sea and river water from flooding the land, drainage systems were constructed to drain for example peat areas to make them suitable for agriculture. As part of drainage systems, ditches can be defined as manmade permanent shallow waters usually less than 8 m in width and not deeper than 1.5 m in which continuous one-directional flow of water is mostly absent (de Lange, 1972). The construction of ditches in the Netherlands already started at the beginning of the common era in the Northern part of the country in the province of Friesland (Rooijendijk, 2009). At present around 250.000 km of drainage ditches can be found resulting in the Netherlands having the highest density of ditches in the world. The network of drainage ditches can be regarded as a typical element of the Dutch polder landscape (Figure 1) and part of the cultural heritage of the Netherlands. Figure 1. Aerial photo of the typical polder landscape near Kockengen in the Netherlands with a high density of drainage ditches (source: Hoogheemraadschap de Stichtse Rijnlanden) The main function of drainage ditches is transporting water but they also accommodate a wide variety of plant and animal species and therefore contribute largely to the biodiversity of the agricultural landscape (Painter, 1999; Armitage et al., 2003; Williams et al., 2004). The ecological characteristics of drainage ditches closely resemble those of cut-off channels along rivers. Both are small dimensioned linear water bodies, often with stagnant water and extensive vegetation development resulting in fast terrestrialization. Additionally both systems are influenced by a certain degree of disturbance. In drainage ditches vegetation is removed by maintenance activities while in cut-off channels vegetation is removed by periodic flooding events (Bornette et al., 1998; Roozen et al., 2008). In that sense drainage ditches can be seen as a surrogate for the channels that have disappeared in the Netherlands due to the construction of dikes and the agricultural use of floodplains along rivers. This creates the paradox of the drainage ditch: Although man made and artificial they are important for the ecological quality and biodiversity of the Dutch landscape. The ecological quality of drainage ditches has deteriorated in many countries the last decades (Herzon and Helenius, 2008). A commonly observed, drastic change in aquatic systems is that from a diverse, mainly submerged vegetation to a dominance of free-floating plants (STOWA, 1992; Janse and Van Puijenbroek, 1998; Kadono, 2004; Gettys et al., 2009). Dense mats of free-floating plants block the 9 1. General Introduction exchange of oxygen from air into water and lower light availability in the water column, the latter causing decreased photosynthesis, limiting oxygen production. With an oxygen consuming sediment at the ditch bottom these conditions often result in anoxia (Villamagna and Murphy, 2010). Anoxic conditions and decreased light availability result in a loss of submerged vegetation and associated macrofauna (Scheffer et al., 2003; Herzon and Helenius, 2008). As a consequence total biodiversity will decrease. From an ecological perspective drainage ditches have been overlooked for a long time with the consequence that research on the ecological functioning and the mechanisms behind deterioration of drainage ditches has lagged behind that of for instance lakes, rivers and streams. Therefore, knowledge on the relations between environmental conditions and vegetation functioning is rather limited which, consequently, makes it difficult to determine good management practices to maintain or restore diversity. 1.2 Macrophytes in drainage ditches Several studies show that macrophytes are essential for the ecological functioning of aquatic systems. Amongst others they offer habitat for fauna due to their architecture (Waters and San Giovanni, 2002; Christie et al., 2009; Bakker et al., 2010) but also regulate abiotic conditions by for instance removing nutrients from water and sediment (Bouldin et al., 2004; Cooper et al., 2004). Due to the small dimensions of ditches, vegetation development generally occurs throughout the whole width of a drainage ditch. Small ditches therefore can quickly develop from open water to land (Verdonschot et al., 2011). While helophytes usually develop in the ditch bank, the water column is mainly colonized by submerged and floating plants. In a well-developed aquatic vegetation several submerged and floating plant species are found with biomass production throughout the whole water column. This type of vegetation shows a biomass distribution with a vertical architecture (Figure 2, upper graph) due to the presence of several species occupying different parts of the water column. At elevated nutrient levels the vegetation contains more highly productive, canopy forming, floating leaved and free-floating