Decomposition of vascular plants, including freshwater macrophytes, generally proceeds in three biological stages: initial rapid loss of the matter due to leaching, microorganism decomposition and macroinvertebrate processing, plus mechanical fragmentation of the litter from the outset (for review, see Webster & Benfield, 1986). Smock & Stoneburner (1980) offer an useful scheme to use visual cues to estimate four stages of leave decomposition: furled, green (without external signs of decomposition), partially decomposed yellowing leaves and decomposed brown ones.

In our context, the most important parameter is decomposition rates of freshwater macrophytes.

Generally, freshwater Hydrocharitaceae (Elodea, Hydrocharis and Valisneria), Nymphaeacea (Nuphar lutea, other pond-lilies) and Najadaceae (Najas flexis, numerous Potamogeton spp.) belong to the group with the highest decomposition rates. Decomposition rates for these species can 2-3 times (Webster & Benfield, 1986) exceed the corresponding values for tree leaves (for example, k = 0,035 day⁻¹ for conditioned leaves of alder, Alnus glutinosa, in Germany: Hieber & Gessner, 2002). In Podostemaceae (such as Podostenum ceratophyllum) and Typhaceae (Typha latifolia, other cattails), decomposition rates are less achieving the minimum (less than 0,002 day⁻¹) in Juncaceae (such as Juncus effusus).

Decomposition rates of freshwater macrophytes is strongly dependent on site, water temparature and other factors (e.g., Brock et al., 1982; Rodgers & Breen, 1982).

In particular, Hill & Webster (1992) have studied decomposition rates of hornleaf riverweed Podostemum ceratophyllum, Canadian waterweed Elodea canadensis, curly pondweed Potamogeton crispus, American water-willow Justicia americana (Acanthaceae) and broadleaf cattail Typha latifolia in the New River, Appalachia. Decomposition rates for these species are 0,037; 0,026; 0,021; 0,016 and 0,007 day⁻¹, respectively. For comparison, litter decay of submerged common rush, Juncus effusus, mentioned above is extremely slow (0.001 day⁻¹), with only the 23% weight loss after 268 days of natural decomposition (Kuehn at al., 2000; freshwater wetlands in Alabama).

Macroinvertebrates

Vegetable litter processing by microinvertebrates in freshwater ecosystems is considered in numerous papers (fer review, see Anderson & Sedell, 1979; Cummins & Klug, 1979).

For example, Smock & Stoneburner (1980) have studied the response of macroinvertebrates to the progressive decomposition of American lotus, Nelumbo lutea, using four stages of leave decomposition described above. It is shown that macroinvertebrate densities increase significantly with the onset and progressive senescence of leaves, as reflected by decreasing chlorophyll concentrations. In total, macroinvertebrates of 17 families are observed with the maximum density 12093 individuals m^-2 leaves surface, in the fourth stage of leave degradation. The chironomid Polypedilum nymphaeorum and three species of Naididae (Oligochaeta) (such as Pristina leidyi and other) exhibit positive responses to presumably increasing levels of food as leave decomposition is progressed. According to Smock & Stoneburner (1980), P. nymphaeorum larvae probably switch from feeding on periphyton to utilization of decomposing plant tissue and associated microbial decomposers once Nelumbo leaves began to decompose.

Attractiveness

Sterry et al. (1983) have studied in an olfactometer the behavioural responses of the freshwater pulmonate snail, Biomphalaria glabrata, to homogenates of various aquatic macrophytes. Among the eleven species studied, three are indifferent, two contain weak arrestants and three induce strong repellent effects. Only two species, European marshwort, Apium nodiflorum (Apiaceae), and lesser duckweed, Lemna paucicostata, induce significant attractant and arrestant effects comparable to those obtained with the terrestrial lettuce, Lactuca sativa, as controls. To the point, among species of the same family ivy leaved duckweed, L. trisulca, is less attractive for snails than L. paucicostata. It is shown also that homogenate of decaying L. paucicostata (beginning from the 15 % of decomposition) is much more strong attractant and arrestant than homogenate made of fresh plant. Sterry et al. (1983) believe that the attractiveness of decaying duckweed for B. glabrata is mainly determined by short chain carboxylic acids, in combination with some other compounds.

Nine categories of carboxylic and amino acids have been found to act as attractants and arrestants to B. glabrata (Thomas et al., 1983). B. glabrata respond more strongly and consistently to short chain unsubstituted monocarboxylic acids, propanoate and butan ... Read more »

Generally, Gammarus pulex and other amphipods are rather omnivorous than strictly detritivorous freshwater crustaceans (for rivew, see MacNeil et al., 1977).

In particular, G. pulex is most attracted to the aufwuchs on the conditioned discs made of poplar (Populus canadensis) leaves and less to the leaves themselves (De Lange et al., 2005).  According to data obtained by these authors, fungi and bacteria within the conditioned leaves are more important than green algae Scenedesmus obliquus.

Exudates of isopod Asellus aquaticus trigger active feeding behaviour in G. pulex as potential predators (Bengtsson, 1982). It is shown that 15 individuals of A. aquaticus placed in the 1000 ml bottle actively release in the water amino acid exudates. Within the first 2 hours of incubation, arginine (8022 ng per liter), lysine, tryptophan and histidine are most abundant. Wisenden et al. (1999) have also shown that G. minus display feeding responses to odor of squashed sympatric isopod Lirceus fontinalis.

Wudkevich et al. (1997) have exposed Gammarus lacustris to chemical stimuli from injured conspecifics and to chemical stimuli from two types of natural predators: dragonfly larvae, Aeshna eremita, and pike, Esox lucius. An exposure to these three stimuli causes G. lacustris to reduce significantly the level of their activity suggesting the presence of an alarm pheromone in the body tissues of G. lacustris. Similarly, chemical stimuli from sculpin, Cottus gobio, and brown trout, Salmo trutta, induce short decreasing locomotory activity of G. pulex, whereas an odor of freshwater signal crayfish, Pacifastacus leniusculus, is indifferent (Åbjörnsson et al., 2000). There are no significant differences in activity of G. pulex exposed to water scented by sculpin or trout, these responses are also independent of the previous diets (G. pulex or isopod A. aquaticus) of predatory fish.

Yet, Kullman et al. (2008) have examined the tendency to aggregate in G. pulex in the absence and presence of predatory fish odor. In conditioned with the threespined sticklebacks Gasterosteus aculeatus water, amphipods significantly prefer to stay close to conspecifics.

Basic References

Åbjörnsson K., Dahl J., Nyström P., Brönmark C. 2000. Influence of predator and dietary chemical cues on the behaviour and shredding efficiency of Gammarus pulex. Aquatic Ecology 34, 379-387

Bengtsson G. 1982. Energetic costs of amino acid exudation in the interaction between the predator Gammarus pulex L. and the prey Asellus aquaticus L. Journal of Chemical Ecology 8, 1271-1281

De Lange H.J., Lürling M., Van Den Borne B., Peeters E.T.H.M. 2005. Attraction of the amphipod Gammarus pulex to water-borne cues of food. Hydrobiologia 544, 19-25

Kullmann H., Thűnken T., Baldauf S.A., Bakker T.C.M., Frommen J.G. 2008. Fish odour triggers conspecific attraction behaviour in an aquatic invertebrate. Biology Letters 4, 458–460

MacNeil C., Dick J.T., Elwood R.W. 1977. The trophic ecology of freshwater Gammarus spp. (Crustacea: Amphipoda): problems and perspectives concerning the functional feeding group concept. Biological Reviews of the Cambridge Philosophical Society 72, 349-364

Wisenden B.D., Cline A., Sparkes T.C. 1999. Survival benefit to antipredator behavior in the ... Read more »