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 butanoate, than to other related chemicals.

Samples of two species of aquatic macrophytes, lesser duckweed L. paucicostata mentioned above and coon’s tail Ceratophyllum demersum, have been homogenized, together with their epiphytic flora, and allowed to decompose in closed systems for up to 42 days (Sterry et al., 1985). Despite the differences in the morphologies, habitats and epiphytic flora of these species, it is found that short chain carboxylic acids, acetate, propanoate, butanoate and hydrogen are the major end products of microbial decomposition in both cases.

Basic References

Anderson N.H., Sedell J. R. 1979. Detritus processing by microinvertebrates in stream ecosystems. Annal Review of Entomology 24, 351-377

Brock T.C.M., Huijbregts C.A.M., Van de Steeg-Huberts M.J.H.A., Vlassak M.A. 1982. In situ studies on the breakdown of Nymphoides peltata (Gmel.) O. Kuntze (Menyanthaceae); Some methodological aspects of the litter bag technique. Hydrobiological Bulletin 16, 35-49

Cummins K. W., Klug M. J. 1979. Feeding ecology of stream invertebrates. Annual Review of Ecology, Evolution, and Systematics 10, 147-172

Hieber M., Gessner M.O. 2002. Contribution of stream detrivores, fungi, and bacteria to leaf breakdown based on biomass estimates. Ecology 83, 1026-1038

Hill B.H., Webster J.R. 1982. Aquatic macrophyte breakdown in an Appalachian river. Hydrobiologia 89, 53-59

Kuehn K.A., Lemke M.J., Suberkropp K., Wetzel R.G. 2000. Microbial biomass and production associated with decaying leaf litter of the emergent macrophyte Juncus effusus. Limnology and Oceanography 45, 862-870

Rodgers K. H., Breen C. M. 1982. Decomposition of Potamogeton crispus L.: The effects of drying on the patterns of mass and nutrient loss. Aquatic Botany 12,1-12

Smock L.A., Stoneburner D.L. 1980. The response of macroinvertebrates to aquatic macrophyte decomposition. Oikos 35, 397-403

Sterry P.R., Thomas J.D., Patience R.L. 1983. Behavioural responses of Biomphalaria glabrata (Say) to chemical factors from aquatic macrophytes including decaying Lemna paucicostata (Hegelm ex Engelm). Freshwater Biology 13, 465-476

Sterry P. R., Thomas J. D., Patience R. L. 1985. Changes in the concentrations of short-chain carboxylic acids and gases during decomposition of the aquatic macrophytes, Lemna paucicostata and Ceratophyllum demersum. Freshwater Biology 15, 139-153

Thomas J.D., Ofosu-Barko J., Patience R.L. 1983. Behavioural responses to carboxylic and amino acids by Biomphalaria glabrata (Say), the snail host of Schistosoma mansoni (Sambon), and other freshwater molluscs. Comparative Biochemistry and Physiology Part C: Comparative Pharmacology 75, 57-76

Webster J.R., Benfield E.F. 1986. Vascular plant breackdown in freshwater ecosystems. Annual Review of Ecology, Evolution, and Systematics 17, 567-594