Rudi Roijackers

Floating plant dominance as a stable state

Invasion by mats of free-floating plants is among the most important threats to the functioning and biodiversity of freshwater ecosystems ranging from temperate ponds and ditches to tropical lakes. Dark, anoxic conditions under thick floating-plant cover leave little opportunity for animal or plant life, and they can have large negative impacts on fisheries and navigation in tropical lakes. Here, we demonstrate that floating-plant dominance can be a self-stabilizing ecosystem state, which may explain its notorious persistence in many situations. Our results, based on experiments, field data, and models, represent evidence for alternative domains of attraction in ecosystems. An implication of our findings is that nutrient enrichment reduces the resilience of freshwater systems against a shift to floating-plant dominance. On the other hand, our results also suggest that a single drastic harvest of floating plants can induce a permanent shift to an alternative state dominated by rooted, submerged growth forms.

Water-level fluctuations affect macrophyte richness in floodplain lakes

The characteristic ecology of floodplain lakes is in part due to their relatively strong water-level fluctuations. We analyzed the factors determining water-level fluctuations in 100 floodplain lakes (during non-flooded conditions) in the active floodplains of the Lower Rhine in the Netherlands. Furthermore, we explored the relationship between water-level fluctuations and macrophyte species richness, and analyzed the suitability of artificially created lakes for macrophyte vegetation. During non-flooded conditions along the Rhine, lake water-level fluctuations are largely driven by groundwater connection to the river. Hence, water-level fluctuations are largest in lakes close to the main channel in strongly fluctuating sectors of the river and smallest in isolated lakes. Additionally, water-level fluctuations are usually small in old lakes, mainly due to reduced groundwater hydraulic conductivity resulting from accumulated clay and silt on the bottom. Species richness of floating-leaved and emergent macrophytes was reduced at both small and large water-level fluctuations, whereas species richness of submerged macrophytes was reduced at small water-level fluctuations only. In addition, species richness of submerged macrophytes was higher in lakes that experienced drawdown, whereas no similar pattern was detected for floating-leaved and emergent macrophytes. The decline in amplitude of lake water-level with lake age implies that the number of hydrologically dynamic lakes will decrease over time. Therefore, we suggest that excavation of new lakes is essential to conserve the successional sequence of floodplain water bodies including conditions of high biodiversity. Shallow, moderately isolated, lakes with occasional bottom exposure have the highest potential for creating macrophyte-rich floodplain lakes along large lowland rivers. The water-level regime of such lakes can in part be designed, through choice of the location along the river, the distance away from the river and the depth profile of the lake.

Effect of temperature and nutrients on the competition between free-floating Salvinia natans and submerged Elodea nuttallii in mesocosms

In many aquatic ecosystems, free-floating plants compete with submerged plants for nutrients and light. Being on top of the water surface free-floating plants are superior competitors for light. Submerged plants can take up nutrients from the sediment and the water column, hereby reducing these levels for free-floating plants. Global warming may change chances of successful species invasion and can alter species dominance. We studied the combined effects of nutrient loading and increased temperature on the competition between the potentially invasive free-floating Salvinia natans (L.) All. and the naturalized submerged Elodea nuttallii Planch. St. John by an outdoor mesocosm experiment under temperate climate conditions (The Netherlands) over a period of 71 days. The free-floating S. natans benefited from increased temperature and increased nutrient loading and limited the chances for the submerged E. nuttallii to take advantage of these changed conditions. S. natans substantially increased temperature in the top layer, while limiting the temperature increase below the mat. Our results suggest that with global warming, invasive free-floating plants might become more successful at the expense of submerged plants.

External Nutrient Sources for Lake Tanganyika

This study assessed the external nutrient sources for Lake Tanganyika from August 1994 to August 1995. The physico-chemical characteristics of the three largest inflowing rivers (Rusizi, Malagarasi, and Lufubu) and the wet atmospheric deposition in Bujumbura (Burundi), Kigoma (Tanzania), and Mpulungu (Zambia) were analyzed. The magnitude of external loading of P and N were evaluated to determine their potential for supporting biological activity. Seasonal changes in the physico-chemical composition of riverine input were detected and were linked to the altitude and morphology of the river system. A flushing effect was noted. Higher discharge rates corresponded with increased concentrations of most constituents. Rusizi provided the most and Lufubu the least of total riverine nutrient input. Rusizis load equalled or almost doubled, depending on the nutrient, the combined loads of the other rivers. Seasonal changes in the atmospheric deposition chemistry were detected and were linked to biomass burning and atmospheric transport processes. Highest atmospheric deposition rates were encountered in more populated and industrialized Bujumbura. Long- and short-term atmospheric washouts were observed. Concentrations of rainwater components were on most occasions higher at the beginning than at the end of the rainy season. A similar trend was observed during separate rain events. Wet atmospheric deposition provided approximately 83% of dissolved inorganic nitrogen (DIN), 37% of total phosphorus (TP), 63% of total dissolved phosphorus (TDP), 65% of soluble reactive phosphorus (SRP), but only 1% of soluble reactive silicate (SRSi) of external source loading. The remaining load was derived from riverine sources. DIN, TP, TDP, SRP, and SRSi yearly net fluxes through wet deposition and riverine input were calculated as 58, 5.4, 2.5, 2, and 99 mmol per m2 lake surface, respectively. Annual external loading of N and P potentially induced a new production of 45 to 60 g C/m2/yr (ca. 7 to 14 % of annual primary production). External loading forms the main pathway for nutrients to enter the productive layers of Lake Tanganyika during stratified and oligotrophic periods (February to May).

Changing weather conditions and floating plants in temperate drainage ditches

Dominance of free-floating plants such as duckweed is undesirable as it indicates eutrophication. The objectives of this study are to investigate whether the onset of duckweed dominance is related to weather conditions by analysing field observations, to evaluate the effect of different climate scenarios on the timing of duckweed dominance using a model and to evaluate to what extent nutrient levels should be lowered to counteract effects of global warming. To analyse the onset of duckweed dominance in relation to weather conditions, duckweed cover in Dutch ditches was correlated with weather conditions for the period 1980-2005. Furthermore, a model was developed that describes biomass development over time as a function of temperature, light and nutrient availability, crowding and mortality. This model was used to evaluate the effects of climate change scenarios and the effects of lowering nutrients. The onset of duckweed dominance in the field advanced by approximately 14 days with an increase of 1 °C in the average maximum daily winter temperature. The modelled biomass development correlated well with the field observations. Scenarios showed that expected climate change will affect onset and duration of duckweed dominance in temperate ditches. Reducing nutrient levels may counteract the effect of warming. Synthesis and applications. Global warming may lead to an increase in the dominance of free-floating plants in drainage ditches in the Netherlands. The expected reductions in nutrient-loading to surface waters as a result of different measures taken so far are likely not sufficient to counteract these effects of warming. Therefore, additional measures should be taken to avoid a further deterioration of the ecological water quality in ditches.

Experimental analysis of the competition between algae and duckweed

We performed indoor competition experiments between algae and Lemna gibba L. in order to unravel mechanisms of competition. To separate effects of shading and physical interference from nutrient competition we grew the two groups physically separated while sharing the same water. A multifactorial design was used with five levels of initial nitrogen concentration (0.1- 50 mg N l-1) and four shade levels mimicking 0 -100 % duckweed shade on the algal compartment We performed indoor competition experiments between algae and Lemna gibba L. in order to unravel mechanisms of competition. To separate effects of shading and physical interference from nutrient competition we grew the two groups physically separated while sharing the same water. A multifactorial design was used with five levels of initial nitrogen concentration (0.1-50mg N l(-1)) and four shade levels mimicking 0-100% duckweed shade on the algal compartment. In the experiments in which algae were not shaded, the growth rate of Lemna was reduced strongly (60-62%) at moderate initial nitrogen concentrations (0.1-1 mg N l(-1)). The impact of algae was less at high N-loading and if algae were shaded. The reduction in the chlorophyll content of the fronds was even more dramatic (72-80%). Analyses of nutrients and pH indicated that algae inhibited the growth of Lemna by the removal of N, P and Fe, but also by their photosynthetic effect on pH. When algae and duckweeds were grown together, floating algae occurring at high nutrient levels partly covered the duckweeds and reduced the growth further than in the absence of physical contact. Since under those conditions Lemna growth was still marked, it seems likely that on the long run Lemna will always expand sufficiently to outcompete the algae at high nutrient levels.

A simple method for analysing the effects of algae on the growth of Lemna and preventing algal growth in duckweed bioassays

A simple novel method for indoor culture experiments with small floating water plants, such as Lemnaceae, is described. Experiments demonstrate that the method allows for longer lasting culture experiments with Lemna, avoiding algal growth and self-shading of fronds by overcrowding. This is achieved by enclosing the Lemna fronds in tubes, which are replaced during growth by tubes of increasing diameter. The RGR of Lemna reached optimal values (RGR > 0.3 day-1) when not hampered by algal growth. In competition experiments with algae growing outside the tubes and Lemna inside the tubes, the growth of the fronds decreased dramatically; chlorophyll content of the fronds was 91-97% less and RGR 29-55 % less. This method allows for multifactorial experiments, handling up to 100 experimental units per researcher. This paves the road to competition experiments among and between water plants for light, nutrients and space

The strength of limiting factors for duckweed during algal competition

Duckweed (Lemna gibba) growth was found to be strongly reduced by unicellular green algae (Scenedesmus conspicua, Chlorella sp., Chlamydomonas sp.) in indoor experiments. These algae reduced N, P, Fe and Mn concentrations of the medium drastically, moreover they increased the pH beyond 10. Subsequent additions of nutrients and pH neutralisation removed the growth inhibition of duckweed. This growth inhibition is, therefore, concluded to be due to pH increase and N, P and trace element (Fe, Mn) removal. Of the five factors significantly inhibiting duckweed growth, depletion of N was strongest, increase in pH was second, followed by reduction of P > Fe > Mn.