- Top of page
- Materials and methods
- Supporting Information
Global temperatures are rising and predictions for the twenty-first century range from an increase of 1·1–6·4 °C (IPCC 2007) and 2–4 °C increase for the temperate climate in the Netherlands (Van den Hurk et al. 2006). An important indicator of climate warming is changes in phenology (Bradley et al. 1999; Menzel 2002; Sparks & Menzel 2002). Higher temperatures, for example, affect the timing of events like leaf bud burst (Linkosalo et al. 2009) and flowering of terrestrial plants (Tooke & Battey 2010) and may lengthen the growing season of terrestrial (Menzel & Fabian 1999) and aquatic primary producers (Thackeray, Jones & Maberly 2008; Meis, Thackeray & Jones 2009). Warming showed to result in altitudinal and pole-ward shifts in terrestrial vegetation (e.g. Parmesan & Yohe 2003; Walther 2003) as well as the pole-ward shift of emergent aquatic macrophytes in Finland (Alahuhta, Heino & Luoto 2011).
Vegetation data from drainage ditches in the Netherlands revealed that cover of free-floating and evergreen overwintering submerged macrophytes in summer was positively related to mild winters, whereas higher cover of submerged macrophytes that die back in winter occurred after cold winters (Netten et al. 2011). Another study in shallow lakes showed that fewer frost days during winter may lead to lower submerged macrophyte cover in favour of higher phytoplankton biomass (Kosten et al. 2009). Other studies indicate that warming may affect aquatic ecosystems in a similar way as eutrophication (Moss et al. 2011).
Floating plants like duckweeds are in close contact with the atmosphere, and therefore, any effects of climate warming on the phenology of aquatic vegetation will be particularly visible in this group of plants. Phenological changes due to warming are most obvious in events that occur relatively early in the year (Aerts, Cornelissen & Dorrepaal 2006) because observed changes in temperature so far have been more pronounced in winter and early spring (Sparks & Menzel 2002). The onset of duckweed dominance might be such an event that can be related to warming.
Duckweed species occur world-wide, ranging from tropical to boreal regions as long as it is not too cold, too dry or too wet (Landolt 1986). They inhabit relatively small and shallow waters (Landolt 1986) like ponds, pools, small lakes, ditches and wetlands. These ecosystems are found world-wide (e.g. Downing et al. 2006) and can be remarkably rich in biodiversity (Scheffer et al. 2006). Nutrient availability in the water column is essential for free-floating plants as they have no direct access to the sediment nutrient pool, unlike submerged and emergent macrophytes (Hutchinson 1975). Small and shallow drainage ditches in agricultural areas often receive excessive loads of nutrients from the surrounding terrestrial environment, leading to the development of dense mats of duckweed (Landolt 1986; Kočić, Hengl & Horvatić 2008). The mats interfere with the exchange of oxygen between atmosphere and water (Pokorný & Rejmánková 1983) and limit photosynthesis below the mat (Morris et al. 2004) resulting in very low oxygen content (Janes, Eaton & Hardwick 1996; Villamagna & Murphy 2010). Those anoxic conditions strongly reduce the survival of macroinvertebrates and fishes (Davis 1975). Duckweed mats can thus be considered as good indicators of low water quality. In larger aquatic ecosystems in the tropics, such dense mats are often formed by water hyacinth, whereas species of the Lemnaceae family may dominate in smaller and sheltered ecosystems in all climatic regions.
Regional waterboards in the Netherlands have a long tradition of collecting data on aquatic ecosystems. As many drainage ditches are covered by free-floating plants, these systems offer the opportunity to explore effects of weather conditions on the timing of phenological events of these floating plants and whether effects of higher temperature can be compensated by lower nutrient levels.
The first objective of this study is to investigate whether changes in weather conditions over the period 1980–2005 resulted in an earlier start of dominance of floating plants in ditches in the Netherlands by analysing a large data set with field observations. The second objective is to evaluate the effect of different climate scenarios on (timing of) duckweed development and to evaluate the effect of lowering nutrients under the different climate scenarios by means of a duckweed biomass model.
- Top of page
- Materials and methods
- Supporting Information
Field observations on duckweed cover indicated that due to warmer weather conditions, dominance of duckweed (cover >75%) occurred earlier in the year. This study clearly showed that the start of duckweed dominance is well related to winter temperature and number of frost days. This finding supports the already recognized importance of winter conditions for terrestrial (Kreyling 2010) and aquatic (Kosten et al. 2009; Netten et al. 2011) plants. The model applied in the present study revealed a similar pattern: earlier start together with a longer duration of duckweed dominance. Although weather conditions in winter were significantly correlated with these results, spring conditions showed better correlations. This seems to support the conclusion of Alahuhta, Heino and Luoto (2011) that conditions during growing season were more important for the distribution of boreal helophytes than winter conditions. The observed earlier start of duckweed dominance is comparable with the earlier onset of algal blooms in Lake Washington, USA, which was also attributed to changes in climate conditions (Winder & Schindler 2004). Lengthening of the growing season as an effect of warming seems to be due to the earlier start of the growing season.
Winter weather conditions are important for duckweed development. Duckweed can overwinter as free-floating plant on the water surface or as frond resting on the sediment (Hillman 1961). Fronds on the sediment usually start to develop later in the season than plants on the water surface because they need a higher temperature (Jacobs 1947). In severe winters, frost may kill floating plants (Whiteman & Room 1991) and duckweed development mainly depends on fronds resting on the sediment delaying the start of duckweed dominance. Prognoses are, however, that severe winters will occur less frequently (IPCC 2007) potentially leading to an increased survival of duckweed floating on the water surface, resulting in a higher initial biomass at the beginning of the growing season leading to an earlier dominance of duckweed. Increasing spring temperatures may even further accelerate the start of duckweed dominance.
Floating plants have a primacy for light due to their position on top of the water column and prevent warming of the water column below the mats while enhancing temperature in their own habitat (Netten et al. 2010). An earlier start of duckweed dominance, therefore, may impede germination and development of submerged macrophytes due to lack of light or lower temperatures. Dense duckweed mats impede the exchange of oxygen between atmosphere and water, and under those mats, oxygen is no longer produced through photosynthesis. This leads to a situation with strong anoxic conditions offering no opportunities for most macroinvertebrate and fish species to survive (Davis 1975). Dominance of floating plants thus decreases diversity of macrophytes (Janes, Eaton & Hardwick 1996) as well as biodiversity of macroinvertebrates (Cremona, Planas & Lucotte 2008). Global warming and the associated earlier dominance of floating plants will probably lead to a further reduction in species diversity in drainage ditches, and this is different from the observations in mountain ponds by Rosset, Lehmann and Oertli (2010), who predict an increase in species diversity due to warming. These contrasting results may potentially be explained by the difference in climate regimes.
The model presented describes growth of duckweed over different seasons and years in semi-natural ecosystems based on a few, simple equations. Other duckweed models are either complex (e.g. Janse 1998), developed for only the growing season (e.g. Driever, Van Nes & Roijackers 2005), or applicable only under controlled situations with high nutrient load (e.g. Monette et al. 2006; Lasfar et al. 2007). The present model performed best when nutrient limitation was incorporated. Nutrient concentrations are constant in the model, although in the field they follow a seasonal pattern with usually higher concentrations of total nitrogen and, to a lesser extent, total phosphorus in winter and lower concentrations of these nutrients in summer (De Klein & Koelmans 2011). For ditches covered with a dense duckweed layer, this nutrient pattern might be different as additional phosphorus may be released from the sediment under anoxic conditions because binding by iron will no longer occur (Geurts et al. 2008). Furthermore, changes in climate conditions may also affect ecosystem nutrient-loading through increased run-off (Jeppesen et al. 2009) potentially leading to stronger effects.
The most important factors regulating biomass development of floating plants were included in the model, but some other factors like dispersal and competition were not. Although dispersal can be a relevant factor, it is not likely to be a limiting factor in drainage ditches as distances between ditches are very short within a drainage area and seeds can be transported by wind up to 250 m per day (Soomers et al. 2010). Competition between floating and submerged plants is assumed to be asymmetric with submerged plant depleting water nutrient concentrations (Scheffer et al. 2003). Effects of competition due to submerged plants might therefore be mimicked with lower nutrient concentrations in the model. The model was tested and validated for the temperate climate region. Blooms of duckweed are, however, a serious problem in most eutrophicated, small and shallow water systems around the world (Landolt 1986). Although the model can easily be modified to other regions by adapting the duckweed parameters to those species present in that region and by running the model with local weather conditions, it remains questionable whether it will work in all situations. In contrast to the temperate region is, for example, seasonal variation in temperature in the tropics weak. Seasonal dormancy induced by, for example, light might need to be included in the model, and this requires more fundamental changes to the model. Furthermore, changes in duckweed phenology due to warming might be undetectable in the tropics with the present model because the increase in temperature is relatively small in comparison with, for example, temperate and boreal regions.
The present results indicated that effects of warming seem to be similar to those of eutrophication suggested in previous studies (Jeppesen et al. 2009; Moss et al. 2011). Even under lower nutrient concentrations, duckweed dominance may probably occur more frequently under future climate scenarios. A similar pattern has been suggested for shallow lakes where cyanobacteria, another group of problematic primary producers, are expected to bloom more frequently even at relatively low nutrient concentrations under future climate scenarios (Wagner & Adrian 2009). Reducing nutrient concentrations, therefore, might mitigate the unwanted effects of warming on duckweed dominance. The modelling results demonstrate that this is possible and that the effort to reduce nutrients increases with warming but also depends on the current orthophosphate concentrations. The latter is in line with observations in Lake Geneva by Tadonléké (2010) where eutrophication status determined the response of phytoplankton productivity to warming. Simulations in the present study showed that at concentrations around 0·10 mg PO4 L−1, a reduction of 10–20% seems to be required, and this increases to up to 40–70% for orthophosphate concentrations of 0·50 mg PO4 L−1. However, the Netherlands Environmental Assessment Agency (PBL 2008) predicted that as a result of measures already taken to reduce nutrient-loading to surface waters in the Netherlands, phosphorus concentrations in surface waters will decrease by only 3% in 2025. This reduction is much less than needed according to the simulations and will probably not be sufficient to counteract warming effects. To prevent earlier onset of duckweed dominance and thus longer duration of duckweed dominance as a result of warming, management strategies have to be adapted.