The importance of the singular roles of macrophytes in shallow aquatic ecosys‐ tems, described in Section 1.1 and Chapter 2, changes with plant species, composi‐ tion, and density, but also lake morphology, nutrient status and climate can play a role (e.g., Moss et al. 1997, Scheffer 1998, Jeppesen et al. 1999a). Dynamics and con‐ centrations of allelochemicals change with season or time and spatially (e.g., Blin‐ dow & Hootsmans 1991, Gross 2000). Stress factors may cause increased produc‐ tion of allelochemicals (e.g., Tang et al. 1995, Reigosa et al. 1999, Granéli & Johans‐ son 2003), but can also make target organisms more sensitive to allelochemicals (e.g., Einhellig 1995, Tang et al. 1995). In both cases the result is an enhanced allelo‐ pathic effect on the target species.
Light and nutrient limitation of target (Granéli & Johansson 2003) and donor (Ren‐ gefors & Legrand 2001) species are often mentioned as important factors influen‐ cing the extent of allelopathic effects (Ervin & Wetzel 2003). Fitzgerald (1969) stu‐ died the competition or antagonism among bacteria, algae and aquatic weeds and showed that nitrogen limitation and not phosphorus limitation of the donor spe‐ cies stimulates allelopathy. Gross (2003b) described that total bioactive hydrolysa‐ ble tannin levels in the submerged freshwater angiosperm Myriophyllum spicatum were highly influenced by light, while nitrogen availability had an effect on telli‐ magrandin II levels, but not on total bioactive hydrolysable tannin levels. Ray & Bagchi (2001) showed a negative relationship between the addition of phosphate or magnesium and the production of an algicide by the cyanobacterium Oscillatoria laetevirens. Earlier, Wu et al. (1991) studied the production of geosmin by Anabaena. Cells without any artificial supply of nitrogen (only gaseous nitrogen) produced more geosmin than cells that received excess nitrogen.
The allelopathic activity of the planktonic cyanobacterium Trichormus doliolum was shown to be affected by phosphorus (Von Elert & Jüttner 1997). The release of allelochemicals increased 30‐fold under P‐limited growth of the cyanobacterium. Wu & Jüttner (1988) demonstrated that, rather than nutrient depletion, the growth rate of Fischerella muscicola was important for the synthesis of allelochemicals. However, Rengefors & Legrand (2001) who cultured the dinoflagellate Peridinium, observed that it may sometimes be difficult to determine whether nutrient limita‐ tion or growth limitation enhances allelochemical production.
GENERAL INTRODUCTION 35 Contradictory to the previously described observations, Gross et al. (1994) sugges‐ ted that neither phosphorus nor nitrate, nor light limitation increased the produc‐ tion of allelochemicals in the cyanobacterium Fischerella. An energy shortage caus‐ ed by nitrate depletion or light limitation resulted in a dramatic decrease of inter‐ nal concentrations of allelochemicals. Von Elert & Jüttner (1996) suggested that target organisms, like cyanobacteria and chlorophytes, are more susceptible to allelochemicals from the cyanobacterium Tri‐ chormus doliolum under light limited conditions. Later, however, Von Elert & Jütt‐ ner (1997) demonstrated that phosphorus and not light was the controlling factor. Experiments of Mulderij et al. 2005b (Chapter 5) showed that the light intensity for culturing phytoplankton influenced the sensitivity of phytoplankton to allelopathic substances of the macrophyte S. aloides. As indicated by Gross (2003b) light and also processes like oxidation, polymerisa‐ tion, or cleavage may influence the stability of allelochemicals. The study by Twist & Codd (1997) is an example of light as a factor influencing the stability of allelo‐ pathic compounds. They investigated the stability of the cyanobacterial hepato‐ toxin, nodularin, and observed photodegradation of the allelochemical after expo‐ sure to ultraviolet radiation. The pH may also influence the production of allelo‐ chemicals. For Oscillatoria laetevirens, Ray & Bagchi (2001) showed that pH nega‐ tively affects the algicidal production. Monahan & Trainor (1970) found that the green alga Hormotila stimulated the growth of another green alga (Scenedesmus) at pH 6.3, but inhibited it at pH 7.7. Other examples of pH dependent allelopathic activity are from studies on marine phytoplankton species (Legrand et al. 2003). In some cases, the temperature also influences allelopathic activity, as revealed by studies with terrestrial crops (Reigosa et al. 1999) and investigations of marine en‐ vironments (Legrand et al. 2003).
1.8
Thesis outline
This thesis mainly focusses on the allelopathic effect of two structurally different macrophytes on phytoplankton growth. Chapter 2 gives an overview of current knowledge on the role of aquatic macrophytes (floating, submerged and emergent) on food webs in shallow aquatic ecosystems.
The experimental studies in this thesis describe the role of two typically rapid colonizers, the macrophytes Chara sp. and S. aloides. The project started with basic experiments to identify the inhibitory effect of Chara on phytoplankton growth. Previous studies showed that Chara (in particular C. globularis) can inhibit phyto‐ plankton growth. Most of those studies were, however, performed with extracts of aquatic macrophytes. The use of extracts indicates the presence/production of alle‐ lopathic substances in/by macrophytes, but it does not per se mean that they are in‐ deed excreted. To show that allelopathic substances are really active after produc‐ tion and excretion, we conducted experiments with macrophytes exudates or cell‐ free filtrates of the macrophyte culture water. Chapter 3, describes the response of three green algae to semi‐continuous addition of cell‐free filtrates from a mixed culture of Chara contraria and C. globularis. The exudates from Chara can influence phytoplankton growth in two ways: changes in the duration of the lag phase (i) and the growth rate (ii). With our experimental set‐ up, we were able to demonstrate allelopathic effects of Chara spp. on phytoplank‐ ton. Then several experiments with S. aloides, were performed. Chapter 4 reports on the allelopathic effect of the aquatic macrophyte, S. aloides, on the green alga Scenedes‐ mus obliquus. S. aloides not only influenced the length of the lag phase, but also changed the exponential growth rate and affected the morphology of Scenedesmus.
Natural phytoplankton communities do not only contain green algae. Therefore, we also tested the allelopathic activity of S. aloides exudates on different phyto‐ plankton species (green algae, toxic and non‐toxic cyanobacteria), as described in Chapter 5. Moreover, we tested the effect of light intensity on the sensitivity of phytoplankton to allelopathic substances from S. aloides. With this experimental set‐up we could also compare the responses of phytoplankton species and strains.