algae/notes/peg.html
The PEG Model
Sommer, U., Z Maciej Gliwicz, Winfried Lampert, and Annie Duncan. 1986. The PEG*-model of seasonal succession of planktonic events in fresh waters. Arch. Hydrobiol. 106 (4) 433-471.
The Plankton Ecology Group, which is a working group of SIL and INTECOL, created a descriptive model for the succession of plankton in lakes. The simple steps are listed below. The article analyzes each step and which lakes fit, which lakes don't fit, and why. This article demonstrates the utility of a rich text model for processes.
The following steps are quoted, transcribed, from the article.
1. Towards the end of winter, nutrient availability and increased light permit unlimited growth of the phytoplankton. A spring crop of small, fast-growing algae such as Cryptophyceae and small centric diatoms develops.
2. This crop of small algae is grazed upon by herviorous zooplanktonic species which become abundant due both to hatching from resting stages and to high fecundity induced by the high levels of edible algae.
3. Planktonic hervivores with short genration duration time increase their populations first and are followed by slower growing species.
4. The herbivore populations increase exponentially up to the point at which their density is high enough to produce a community filtration rate, and so cropping rate that exceeds the reproduction rate of the phytoplankton.
5.As a consequence of herbivore grazing, the phytoplankton biomass decreases rapidly to very low levels. There then follows a "clear-water" equilibrium phase which persists until inedible algal species develop in sifnificant numbers. Nutrients are re-cycled by the grazing process and may accumulate durin gthe "clear-water" phase.
6. Herbivorous zooplanktonic species become food-limited and both their body weight per unit length and their fecundity declines. This reults in a decrease in their population densities and biomasses.
7. Fish predation accelerates the decline of hervivorous planktonic populations to very low levels and this trend is accompanied by a shift towards a smaller average body size amongst the surviving crustaceans.
8. Under the conditions of reduced grazing pressure and susteained non-limiting concentrations of nutrients, the phytoplankton summer crops start to build up. The composition of the phytoplankton becomes complex both due to the increase in species richness and to the functional diversitfic ation into small "undergrowth" species (which are availble as food for filter-feeders) and large "canopy" species (which are only consumed by specialist feeders such as raptors or parasites).
9. At first, the edible Cryptophyceae and inedible colonial green algae become predominant. They deplete the soluble reactive phosphorus to nearly undetectable levels.
10. From this time onwards, the algal growth becomes nutrient-limited and this prevents an explosive growth of "edible" algae. Grazing by predator-controlled herbivores balances the nutrient-limited growth rate of edible algal species.
11. Competition for phosphate leads to a replacement of green algae by large diatoms, which are only partly available to zooplankton as food.
12. Silica-depletion leads to a replacement of the large diatoms by large dinoflagellates and/or Cyanophyta.
13. Nitrogen depletion favours a shift to nitrogen-fixing specis of filamentous blue-green algae.
14. Large species of crustaceans hervivores are rffplaced by smaller species of rotifers. These small species are less vulnerable to fish predation and are less affected by interference wiht their food collecting apparatus shich can be caused by some forms of inedible algae. Accordingly, their population mortality is lower ante their fecundity is higher than that of the larger species.
15. The small species of herviores coexist under a persistent fish predation pressure and the increased possibility of food partitioning which is associated with the greater speices complexity of the phytoplankton.
16. The population densities and species composition of the zooplankton fluctuate throughout the summer, the later being also influenced by termperature.
17. The period of autogenic succession is terminated by factors related to physical changes which includes increased mixing depth resultiong in nutrient replenishment and a deterioration of the effective underwater light climate.
18. After a minor reduction of algal biomass, an algal community develops which is adapted to being mixed. Large unicellular or filamentous algal forms appear. Among them diatoms become increasingly important with the progress of autumn.
19. This association of poorly-ingestible algae is accompanied by a variable biomass of small, edible algae.
20. This algal composition together with some reduction in fish predation pressure leads to an autumnal maximum of zooplankton which includes larger forms and species.
21. A reduction of light energy input results in a low or negative net primary production and an imbalance with algal losses which causes a decline of algal biomass to the winter minimum.
22. Herbivore biomass decreases as a result of reduced fecundty due bot to lower food concentrations and to decreasing temperature.
23. Some species in the zooplankton produce resting stages at this time, whereas other species produced their resting stages earlier.
24. At this period in the year, some cyclopoid species "awake" from their diapause and contribute to the over-wintering populations in the zooplankton.