Limnologists have been intrigued by the pronounced seasonal fluctuations in the abundance and composition of phytoplankton in aquatic ecosystems. Environmen‐ tal conditions (e.g., nutrient limitation) influence both donor and target species (Section 1.7), but also without apparent abiotic limitations we find differences in sensitivity to allelopathic substances, because different target species may show varying responses to the same allelochemical (see Chapters 3 & 5). Cyanobacteria are reported to be more sensitive to allelochemicals than chlorophy‐ tes (e.g., Planas et al. 1981, Körner & Nicklisch 2002). In a study with aqueous ex‐ tracts of the aquatic macrophyte Ceratophyllum demersum and natural phytoplank‐ ton, Jasser (1995) showed a decline of the biomass of cyanobacteria, but a simulta‐ neous increase in the biomass of chlorophytes. Gross et al. (1991) who showed that the allelochemical, fischerellin, produced by three species of the cyanobacterium Fischerella, inhibited other cyanobacteria (Anabaena, Synechococcus, Phormidium, Sy‐ nechocystis) and, to a lesser extent, chlorophytes (Ankistrodesmus, Nannochloris, Sce‐ nedesmus). Later, Gross et al. (1996) described that extracts of Myriophyllum spicatum inhibited various filamentous and coccoid cyanobacteria (Anabaena, Synechococcus and Trichormus) more strongly than chlorophytes (Scenedesmus, Stigeoclonium). Al‐ so, Usenko et al. (2002) observed that species of Cyanophyta were inhibited by exo‐ metabolites from macrophytes like Ceratophyllum, Potamogeton, Stratiotes, Trapa etc., but species of Chlorophyta and Bacillariophyta were stimulated. Leu et al. (2002), on the contrary, showed that both cyanobacteria (Anabaena and Synechococcus) and the chlorophyte Chlamydomonas were equally sensitive to allelochemicals from M. spicatum. Recently, Mulderij et al. (2005b, Chapter 5) studied the sensitivity of cya‐ nobacteria and chlorophytes to exudates from S. aloides.
So, differential sensitivity of phytoplankton was observed when species of diffe‐ rent functional phytoplankton groups (e.g., cyanobacteria, diatoms, green algae) were compared, but it also exists between species that belong to the same group. Kogan & Chinnova (1972) showed differential sensitivity between the three cyano‐ bacteria Anabaenopsis intermedia, Anabaena karakumica, and A. robusta to exudates from Ceratophyllum demersum. Also Gross et al. (2003b) showed that Anabaena sp. and Synechococcus elongatus were more sensitive to Ceratophyllum extracts than A. variabilis. Berger & Schagerl (2003) conducted experiments with nine species of cya‐ nobacteria and showed that Microcystis aeruginosa and Cylindrospermum sp. were
GENERAL INTRODUCTION
33
resistant to substances from C. aspera, while other cyanobacteria (Anabaena spp. and Microcystis flos‐aquae) were not. Later, they (Berger & Schagerl 2004) demonstrated that M. aeruginosa and Cylindrospermum sp. were not resistent to C. aspera. Nakai et al. (1999) showed that the cyanobacterium Microcystis was more sensitive to sub‐ stances from Myriophyllum spicatum and Ceratophyllum demersum than the cyano‐ bacterium Anabaena. Körner & Nicklisch (2002) showed that Microcystis, was more sensitive to M. spicatum than the cyanobacterium Aphanizomenon. The green algae Chlorella and Selenastrum, showed another response than Scenedesmus, when expo‐ sed to exudates from a mixture of Chara globularis and C. contraria (Mulderij et al. 2003, Chapter 3). Blindow & Hootsmans (1991) showed that Chara globularis exu‐ dates inhibited the growth of the green alga Scenedesmus, but apparently they can‐ not inhibit Ankistrodesmus.
Both Keating (1977, 1978) and Rojo et al. (2000) report allelopathic activity of ma‐ crophyte exudates which may influence the dynamics of phytoplankton popula‐ tions. Differential sensitivity of phytoplankton, within and between functional phytoplankton groups, may therefore favour other phytoplankton species and thereby influence species succession (Körner & Nicklisch 2002, Legrand et al. 2003). An example is the alternating dominance of the dinoflagellate Peridinium and the cyanobacterium Microcystis in the Sea of Galilee (Vardi et al. 2002).)
Allelopathy was also mentioned in management approaches for natural and agri‐ cultural ecosystems (e.g., Rizvi et al. 1992, Einhellig 1995, Anaya 1999). Pesticides could successfully be replaced by biological agents (allelochemicals). According to Legrand et al. (2003), little success has been achieved with such replacements in aquatic ecosystems, what is perhaps related to the instability of allelopathic com‐ pounds (Nakai et al. 1999). For controlling blooms of cyanobacteria, however, Ho‐ ward et al. (1996) suggested several management approaches including killing of cyanobacteria with chemical and biological agents. Although they mentioned chemical and biological agents, they specify only a chemical agent (e.g., copper sul‐ phate), that causes disintegration of cyanobacterial blooms. Other ways to control cyanobacterial blooms may be addition of leaf litter (Ridge et al. 1999) or decom‐ posing (barley) straw (Newman & Barrett 1993, Pillinger et al. 1994). Our present state of understanding of allelopathy, with special emphasis on the use of allelo‐ chemicals in biological control of nuisance species and its consequenses, is still too fragmentary.
1.7
Allelopathy and environmental influences
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.