Competition between free-floating and submerged macrophytes in a future of climate change

This research was about the asymmetric competition between free-floating and submerged macrophytes in shallow freshwater ecosystems. I studied the effect of climate change on the dominance of free-floating macrophytes in temperate regions. The research approach was a combination of outdoor mesocosm experiments, a laboratory experiment, a database analysis and a literature review. In Chapter 2a I explored the possibility to use inexpensive open-top chambers (OTCs) as passive artificial warming devices in experimental aquatic studies. Existing experimental set-ups for artificial warming are very expensive and do usually not maintain a natural vertical temperature profile in the water column, which is generally observed in small-sized water bodies. Effects of different sizes of OTCs on the daily water temperature course in outdoor mesocosms were evaluated. Results showed that OTCs cause a significant temperature increase corresponding with predicted climatic warming whereas the vertical temperature profile in the water column was maintained. Therefore, OTCs are well suited for studying the effects of climate change in small-sized aquatic mesocosm experiments. These OTCs were subsequently used in an outdoor mesocosm experiment where I studied the effect of temperature and nutrient concentration on the competition between free-floating and submerged macrophytes (Chapter 2b). Free-floating macrophytes benefited from increased temperature and increased nutrient loading and limited the chances for the submerged macrophyte to benefit from these conditions. Only with high initial cover (90%) and a low nutrient concentration submerged macrophytes were able to limit free-floating macrophyte growth (with 10% initial cover). Only in this situation submerged macrophytes could benefit from increased temperatures. At a higher initial cover of free-floating macrophytes (50% or 90%) submerged macrophytes did not grow better with the increased temperatures. I further showed that 30% cover of free-floating macrophytes resulted in 74% light reduction below the mat. In mesocosms with OTCs and fully covered with free-floating macrophytes, the temperature increased only above and within the floating mat, while below the mat the temperature remained unchanged. Under a dense mat of free-floating macrophytes oxygen is often scarce and nitrification may be hampered, leading to higher reduced nitrogen (NHx) concentrations. In a laboratory experiment the interactive effects of a high concentration of NHx with temperature, pH and light on submerged macrophytes were examined (Chapter 3). Results demonstrated that a high NHx concentration and higher temperature together with a low pH and low light conditions caused the strongest toxic effects on relative growth rate and leaf tissue fitness of Elodea canadensis. The negative effect of low pH seemed counterintuitive. However, the specialized bicarbonate-concentrating pathway at high pH of the this plant species resulted in higher carbon availability for detoxification. This is because this pathway lacks photorespiration and minimizes carbon loss at high temperatures. At low pH, higher temperatures also lead to increased photorespiration rates and thus lower detoxification capacity. Also the process of cyclosis may optimize detoxification under pH 8, while this process is not very active under lower pH (as it is HCO3 dependent). In general rising temperatures and increased reduced nitrogen levels will decrease the fitness of submerged aquatic macrophytes. The results described in the previous chapters suggested that in freshwater ecosystems a shift in dominance from submerged to free-floating macrophytes may occur with climate change due to warming and eutrophication. Analysis of an existing database provided field evidence on the impact of climate change on macrophyte cover in Dutch drainage ditches (Chapter 4). Ditches are often overlooked ecosystems with potential dominance for both free-floating and submerged macrophytes. In this study 26 years of field data on macrophyte cover were related to local weather variables and the North Atlantic Oscillation (NAO) winter index. Mild winters (positive NAO winter index) were positively related to coverage of free-floating macrophytes and evergreen overwintering submerged macrophytes. Cold winters (negative NAO winter index) were positively related to cover of submerged macrophytes that die back in winter. The effect of climate seemed to depend on the over-wintering strategy of the macrophyte species present. It also appeared that ditches on organic soils enhanced the positive relation of free-floating macrophytes with the NAO winter index. In these ditches an increased decomposition of organic matter and increased run-off resulted in increased nutrient loading, promoting free-floating macrophyte growth (as seen in previous chapters). Previous chapters gave insight in the competition between free-floating and submerged macrophytes in shallow freshwater ecosystem and how climate change might affect this. In Chapter 5 an ecosystem management approach was applied on the illustrative example of socio-economic and ecological problems caused by free-floating macrophytes. Adequate ecosystem management must be based on a dynamic balance of scientific knowledge development, societal acceptance and feedback, and practical implementation. In the synthesis (Chapter 6) the findings from this research on the competition with and without climate change have been integrated. Also a model is presented that describes the local warming effect that free-floating macrophytes can have. The synthesis provides insight into the competition between free-floating and submerged macrophytes and how one could deal with expected problems.