Macrophytes in drainage ditches : functioning and perspectives for recovery
toon extra info.
Jeroen P. van Zuidam
|Auteur(s)||Zuidam, J.P. van|
|[S.l. : s.n.]|
|112 p fig., graf., tab|
|Proefschrift Wageningen toon alle annotatie(s)
Met lit. opg. - Met samenvatting in het Engels en Nederlands
|Scheffer, Prof. dr. M. ; Peeters, Dr. ir. E.T.H.M.|
|Samenvatting door auteur||
Drainage ditches can be found in many countries across the world and are important elements of the Dutch landscape. Although manmade they accommodate a wide variety of plant and animal species and, therefore, contribute largely to the biodiversity of the agricultural landscape. Due to their comparable ecological characteristics they might even serve as a surrogate for disappeared cut-off channels of river floodplains. However, due to disturbances such as eutrophication and frequent vegetation removal many ditches now show a monoculture of free-floating plants or of fast-growing submerged species like Elodea sp. rather than a diverse submerged vegetation. These monocultures deteriorate conditions for aquatic fauna, resulting in an overall loss of biodiversity. To maintain or restore a diverse vegetation, knowledge on driving forces behind the occurrence of different vegetation compositions is needed. Therefore this thesis focuses on identifying conditions and mechanisms that determine macrophyte community composition and functioning in drainage ditches in the Netherlands, with focus on effects of nutrient state, maintenance and seed bank properties.
To determine which environmental factors, operating at different spatial scales, relate to differences in macrophyte community composition, a field dataset from 67 ditches was analysed containing data on species and biomass composition and the environmental setting. In Chapter 2 the contribution was quantified of local and regional environmental factors in explaining patterns in species and biomass composition. Multiple factors at different spatial scales were related to the macrophyte communities, the most important being nutrient availability, occurrence of seepage, dimensions of the ditch and of the surrounding hydrological unit and soil type. The rather low explained variance per variable likely indicates a typical characteristic of macrophyte communities in drainage ditches: multiple factors affect their functioning without one being dominant. Local and regional factors contributed approximately two third and one third respectively to the total explained variance for both biomass and species composition data. The relations found with regional factors are likely explained by the strong connectivity between drainage ditches and the surroundings, causing intensive exchange of substances and biota.
Nutrients were regarded a dominant factor for the occurrence of different vegetation types in drainage ditches. Therefore, in Chapter 3 the most important nutrient fractions and levels were determined at which three vegetation types (monocultures of Elodea sp. or free-floating plants and diverse submerged vegetation) were observed in Dutch drainage ditches. The three vegetation types occurred in rather distinctive ranges of nutrient concentrations with total P in water and sediment being most important. Higher levels of sediment P distinguished the occurrence of monocultures (by Elodea sp. and free-floating plants) from the diverse submerged vegetation, while higher P levels in water distinguished free-floating plant dominated systems from ditches with a submerged vegetation (Elodea monoculture or other submerged plants). Remarkably, although nutrient levels clearly differed, no differences in total biomass were found between the three vegetation types. This was mainly caused by the, compared to other aquatic systems, relatively low total biomass of Elodea sp. dominated ditches, possibly resulting from increased P storage in Elodea sp. plant tissue, rather than increased productivity. Additionally, the small dimensions of most drainage ditches may have limited space for increased biomass development of free-floating plants (only growing at the water surface) and Elodea sp. despite higher nutrient availability.
Maintenance activities like mowing may endanger completion of the life-cycle or survival of macrophytes. In Chapter 4 the effect of cutting on survival and growth was determined for two Pondweed species by applying different cutting treatments under indoor experimental conditions. For both species short term negative effects of cutting on biomass production and survival were observed. Potamogeton lucens seemed to be a more tolerant species as strong effects were only visible when biomass was cut at the sediment–water interface. On the other hand, Potamogeton compressus seemed to be more vulnerable because every treatment (cutting halfway, at three quarters down the water column and at the sediment water interface) resulted in significantly lower below-ground and green shoot biomass production. Long term effects of cutting on P. lucens might occur through decreased development of the rhizome network while for P. compressus long term effects on reproduction might be expected as flowering decreased when cut at the sediment–water interface and turion formation only occurred in the uncut controls. The decreased biomass production by P. compressus may lead to a competitive disadvantage in the field as fast-growing, disturbance tolerant species such as Elodea nuttallii (Planch.) St. John may outcompete the species. Creating patchiness in mowing height and frequency or applying a mowing regime that leads to reduced biomass development while species still survive might create opportunities to both maintain the water transporting function of the drainage system while preserving the species in the ditches.
Many ditches in the Netherlands nowadays contain a monoculture of free-floating plants and restoring suitable site conditions for submerged plants might create opportunities for re-establishment from propagules. The potential of the sediment propagule bank to initiate recovery under suitable conditions was studied in Chapter 5 by performing a germination experiment with sediments from ditches with a submerged vegetation and a monoculture of free-floating plants. Compared to sediments from ditches with submerged vegetation, sediments from free-floating plant–dominated ditches produced significantly lower numbers of individuals and species of submerged and emergent plants, while numbers of individuals and species of free-floating plants were higher. These results suggest that sediments from free-floating plant dominated ditches have lower potential to recover a diverse plant community. This probably results from positive feedback mechanisms caused by the vegetation present, maintaining the free-floating plant–dominated state. Sediment disturbance seems to strongly favour the germination of free-floating plant propagules, especially from free-floating plant dominated ditch sediments. Ditch maintenance activities such as mowing and dredging, therefore, will likely favour persistence of the free-floating plant dominated state. To shift from dominance by free-floating plants to a more diverse plant community, alternative maintenance methods should be considered that cause less sediment disturbance together with measures that promote (re-)colonization such as temporary drawdown or re-introduction of species.
At the highest spatial scale considered in this thesis climatic conditions influence many of the regional and local conditions. In Chapter 6 the effect of climatic variation on the cover of free-floating and submerged macrophytes was studied by relating the North Atlantic Oscillation (NAO) winter index to macrophyte cover in ditches in the Netherlands over the past 26 years. Mild winters were related to higher coverage of free-floating and evergreen submerged plants (like Elodea nuttallii) while cold winters coincided with higher coverage of annual submerged plants in the next growing season. On organic soils this effect of milder winters on free-floating plant development was significantly stronger, likely due to increased nutrient availability. These results suggest that milder winters may cause free-floating and evergreen submerged plants to outcompete annual submerged plants which may lead to a loss of diverse submerged vegetation. Lowering nutrient availability is likely one of the measures that will limit the negative effects of climatic warming.
In Chapter 7 (Synthesis) it is discussed that, similar to cut-off channels, heterogeneity in environmental conditions is an important characteristic in drainage ditches to maintain plant diversity. The environmental factors discussed in this thesis may all add to creating heterogeneity in conditions and consequently differences in vegetation composition. A schematic overview of the found relations is presented. To restore and maintain a diverse vegetation a combination of measures is likely needed, that influences multiple factors related to vegetation functioning, thereby also counteracting the negative effects of climatic warming on the development of a diverse vegetation. Finally, the use of a polder scale vision on water quality management is discussed arguing that future management of drainage ditches can be more effective when based on an integrated view on the optimal design and management of the total polder. This may then result in spatial redistribution of functions and ecological targets within and across polders/hydrological units to exploit the ecological potential. Subsequently, habitat heterogeneity can be achieved by creating differentiation in design and maintenance of drainage ditches.
|Trefwoorden (cab)||sloten / aquatische ecologie / waterplanten / soortendiversiteit / milieufactoren / nutriëntenuitspoeling / polders / ecohydrologie / ecologisch herstel / drainage / baggeren|
|Toelichting||Sloten zijn overal ter wereld te vinden en vormen een belangrijk onderdeel van het Nederlandse landschap. Ondanks dat het door de mens gemaakte, kunstmatige wateren zijn, kunnen ze een grote diversiteit aan flora en fauna bevatten. Hierdoor dragen ze in belangrijke mate bij aan de biodiversiteit van het agrarisch landschap. Doordat sloten in ecologisch opzicht sterk lijken op afgesloten nevengeulen in rivieruiterwaarden zouden sloten zelfs een vervanger kunnen zijn voor de nevengeulen die op veel plaatsen zijn verdwenen. Verstoringen als eutrofiering en frequent vegetatie verwijderen hebben er echter voor gezorgd dat veel sloten momenteel een monocultuur bevatten van bijvoorbeeld kroos of Smalle waterpest (Elodea nuttallii). Deze monoculturen zorgen voor verslechterde leefomstandigheden voor waterdieren (vissen, amfibieën en ongewervelde dieren) waardoor de totale biodiversiteit van de sloot achteruit gaat. Om achteruitgang van de ecologische kwaliteit te voorkomen of terug te dringen is kennis nodig over de factoren die verantwoordelijk zijn voor het voorkomen van verschillende typen vegetatie. Het onderzoek in dit proefschrift richt zich daarom op het identificeren van condities en mechanismen die de samenstelling en het functioneren van watervegetaties in Nederlandse sloten bepalen, waarbij meer specifiek gekeken is naar de effecten van voedselrijkdom, onderhoud en de aanwezige overlevingsorganen.|
WUR, Leerstoelgroep Aquatische Ecologie en Waterkwaliteitsbeheer
Stichting Toegepast Onderzoek Waterbeheer (STOWA)