Periphyton removal by freshwater micrograzers

Abstract

In various experiments, the freshwater snail species Lymnaea peregra (Müll.), Physa fontinalis (L.), Valvata piscinalis (Müll.) and Bithynia tentaculata (L.) significantly removed periphyton from glass slides, while the two tested crustacean species, Asellus aquaticus (L.) and Gammarus pulex (L.), did not. B. tentaculata removed similar amounts of periphyton accumulated in the field as it did of laboratory-cultured periphyton.

Removal rates ranged from 0.1–2.2 mg animal⁻¹ day⁻¹ ash-free dry weight (afdw) for different species, different temperatures and differing amounts of initially available periphyton. Differences in removal rate per individual snail between and within species could largely be accounted for by differences in snail biomass and activity. A significant effect of temperature on periphyton removal and activity was observed in only one out of four experiments. Apparently, an efficient temperature compensation mechanism is present in the tested species. The four species differed in grazing trail width, linear velocity, and also in the density and irradiance transmittance of the periphyton that is left in the grazing trail. Thus, differences in periphyton removal capacities of field populations of the different species are to be expected.

With similar periphyton density and taxonomic composition, velocity nor activity of L. peregra was influenced by the type of substratum (glass slides versus plant surface of Potamogeton pectinatus L.). Taxonomic composition of the periphyton did influence velocity and activity. Higher periphyton density resulted in increased periphyton removal in L. peregra (up to a plateau at 0.2 mg cm⁻² afdw) and P. fontinalis but not in B. tentaculata and V. piscinalis.

In a growth experiment lasting 9 weeks, the density of periphyton on P. pectinatus was reduced in the presence of L. peregra. Taxonomic composition of periphyton on grazed plants differed from that on control plants: dominance by tightly adhering unicellular green algae in the presence of snails versus filamentous Cyanobacteria in their absence. Ungrazed control plants with a higher periphyton cover produced more leaf material than plants with snails, at the expense of the tubers. In a second growth experiment with P. pectinatus in the presence of the grazers B. tentaculata, V. piscinalis and juvenile L. peregra, all snail species significantly reduced periphyton density. Contrary to the previous experiment, however, no differences in plant growth were apparent. This was due to a lower periphyton and phytoplankton density in the ungrazed controls as compared to the previous experiment. Irradiance reaching the plants in the first growth experiment was close to the compensation point for P. pectinatus of this age and acclimized to this irradiance, while in the second experiment net photosynthetic rates were estimated to be about twice as high. This resulted in a fivefold difference in newly formed biomass after 9 weeks.

The three tested species clearly showed different activity patterns. L. peregra remained active throughout the experiment and was present on the plants and aquarium walls in considerable numbers. V. piscinalis was present on the macrophytes for about one month, during which oviposition took place. Subsequent post-breeding mortality rapidly removed all adults of this semelparous species. B. tentaculata showed a somewhat intermediate pattern. In the first part of the experiment, densities on plants were similar to those of L. peregra, but after oviposition a large proportion of the animals moved to the sediment and burrowed themselves, whilst large-scale mortality did not take place.

Two methods were developed to estimate irradiance transmittance improvement due to periphyton removal. These methods were evaluated for different species and conditions. On glass slides, transmittance of the remaining periphyton after a period of exposure to grazing animals can be estimated without significant error from its biomass and a hyperbola relating irradiance attenuance (1-transmittance) to biomass (method A), provided that at least 60% of the biomass is removed by the grazers. Else, more detailed measurements of trail width, linear velocity and attenuance in grazing trails of individual snails are to be preferred (method B). When the two methods were evaluated on the basis of periphyton growth and removal rate on P. pectinatus in the first growth experiment, method B appeared to estimate the removed amounts more accurately.