Jord Vand Luft Dyr og planter Grønland Samfund
  Udgivelser Overvågning Om DMU Viden for alle Nyheder
In English

Production, regulation and ecophysiology of periphyton in shallow freshwater lakes.

Lone Liboriussen

PhD thesis


Periphytic communities are important components of shallow freshwater lakes, not only because they can be very productive food web components, but also because they may serve as chemical modulators and restrict the distribution of aquatic vegetation. Knowledge of the regulation mechanisms controlling abundance, composition and production of the periphytic communities may hence be valuable tools in lake management and biomanipulation.

To stress the importance of production by attached microalgal communities and furthermore explore the effect of pelagic nutrients and season on the distribution between habitats, the production of pelagic and sediment-associated (epipelic) microalgae were measured over a year in a pre-defined area (depth 0.5 m) in each of two lakes: one nutrientrich and turbid and one less nutrient-rich and with clear water.

Total annual primary production was only 34 % higher in the nutrientrich than in the less nutrientrich lake. Epipelic microalgae were the dominant primary producers in the clear lake (77%), whereas phytoplankton accounted for most of the production (96%) in the turbid lake. Seasonality of the proportional distribution between habitat differed considerably between the lakes. In the turbid lake the epipelic algae contributed more in winter, when the water was relatively clear, than during summer, when the water was more turbid. By contrast, the epipelic contribution in the clear lake was slightly reduced by phytoplankton blooms in spring and autumn relative to the rest of the year. Production by the epiphytic algal communities within the reed stands was estimated to be insignificant compared to that of phytoplankton and epipelon.

Our results support the general theory of a redistribution of the microalgal production from the sediment to the water column with increasing pelagic nutrient concentrations rather than a proportional increase in primary production with eutrophication. A conceptual model of the seasonal variation in the relative contribution of epipelic primary production to total primary production in shallow lakes of contrasting nutrient state was proposed from the results. In nutrient-rich turbid lakes pelagic production is expected to dominate throughout the year, while the clear lakes with lower nutrient levels may be dominated by epipelic production. At some intermediate nutrient level a lake may be expected to shift from mainly epipelic production during the clear winter period to dominance of phytoplankton production in summer, because the self-promoting processes of epipelic production collapse in spring when a threshold phytoplankton density and temperature is exceeded.

The relationship between the pelagic nutrient concentration and the potential for periphytic growth and production on chemically inert substrata in the littoral zone (0-1 m) was studied in July and September along an inter-lake gradient in total phosphorus (TP) ranging from 11 to 536 µg l-1 (summer mean). Mean periphytic algal biomass related unimodally to the inter-lake TP gradient in both July and September, and lakes of intermediate TP (60 - 200 µg l-1) possessed the optimum combination of physical, chemical and biological conditions for periphytic growth. A shift from nutrient to light limitation seemed to occur along the inter-lake TP gradient in July, whereas nitrogen control may have been important in the most nutrient-rich lakes in September. Rates of community respiration, maximum light-saturated photosynthesis (Pmax) and production related better to the periphyton biomass than to the pelagic TP concentration. The periphytic communities generally became more heterotrophic from July to September, and mean net production (24 hours) was positive in most lakes in July and negative in all lakes in September. Net production was not related to the inter-lake TP gradient in July, but decreased with TP in September.

The results stressed that the nutrient concentration alone is a poor predictor of the standing biomass and production of periphyton. Periphyton may be regulated simultaneously by bottom-up (resources) and top-down (herbivory) forces, but the interactive effect of these forces is largely unknown. In long-term enclosure experiments conducted simultaneously in a clear and a turbid shallow lake, lightmediated bottom-up and fish-mediated top-down forces strongly controlled the biomass of periphyton and phytoplankton. By contrast, the light- and fish-mediated effects on the benthic macro-grazers were highly dependent on the structural composition of the community. Fish reduced the biomass and abundance of snails in the clear lake, and of ostracods in the turbid lake, but had no effect on chironomids in the turbid lake.

The results implied that the snails in the clear lake were, at least partly, reduced by indirect mechanisms, because both adult and juvenile snails were reduced, and only the latter were assumed to be within the size range vulnerable to predation. Light had no effect on the total biomass of benthic grazers in the clear lake, but strongly influenced life history traits of the snail community. The light-mediated bottom-up regulation of the primary producers was generally stronger in the turbid than in the clear lake, but the light regulation did not unambiguously cascade to the primary consumer level. Top-down regulation of the benthic grazer community was stronger in the clear lake than in the turbid lake, and in both lakes strong cascading effects on periphyton were seen.

In conclusion, both light-mediated bottom-up and fish-mediated top-down forces controlled the biomass of periphyton, while the reductive impact of the two factors on the benthic grazer community was highly dependent on the structural composition of the community. Although light and temperature are usually considered sub-optimal for algal growth in winter, periphyton growth and standing biomass may be higher in winter than in late spring. A study of monthly winter-spring growth and accrual rates of periphyton revealed that the periphyton accrual rates were higher from January to February than during any other months from February to May. The ratio of productivity to respiration and the chlorophyll-specific productivity of periphyton were negatively correlated with community biomass, and though low periphytic algae densities were developed under the ice, the community had a high productive potential. The study showed a high potential for periphytic growth and production in winter-spring.

Full report (939 KB)


Helle Thomsen


DMU  |  

Box 358 | Frederiksborgvej 399 | 4000 Roskilde | T: 4630 1200

 CVR: 10859387

 EAN: 5798000867000