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Newsletter and Technical Publications
Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- Phytotechnology and Ecohydrology -


Mechanisms of algal succession in reservoirs

Succession is a widely-accepted, biological concept implying a sequence in which species or groups of species dominate a plant community.

In reservoirs, where the process of eutrophication is often observed, phytoplankton succession shifts from dominant, quick-growing, colonising, opportunistic species like diatoms and green algae to the slower-growing, resource-stress tolerant species like cyanobacteria during the annual cycle (Table 8.2). Many factors are responsible for such a sequence of succession. The most important one is stability of the water column and resource limitations. The limiting resources may be a nutrient, light, temperature, turbulence, water retention time, or biotic factor such as selective predation by a predator or parasite population. Knowledge about the hierarchy of factors which are responsible for phytoplankton succession and species domination in a reservoir is the best tool for effective management of these water resources.

Table 8.2. Characterisation of survival strategies of different phytoplankton groups
Characteristic Diatoms and green algae Cyanobacteria
Growth rate quick slow
Investment of cell energy higher reproductive effort higher non-reproductive activities (i.e., vertical migration to maximise the exploitation of available resources, toxin production, etc.)
Uptake and retention of nutrients by cells very low high nutrient affinity
Cell size small high (often cells form a colony)
Resistance to zooplankton grazing low high (by large colony size and toxin production)
Sedimentation rate high low (buoyancy regulation by presence of gas vacuoles)
Periods of domination periods of habitat colonisation during perturbation and water mixing conditions resources depleted during stagnant water conditions
Light requirments high (better adaptation to changes of light intensity) low
Examples of algae Chlorella, Scenedesmus (green-algae), Cyclotella, Melosira, Asterionella, Fragillaria, (diatoms) Microcystis, Oscillatoria, Anabaena, Aphanizomenon

Depending upon the trophic status of a water body, the specific composition of the phytoplankton community may differ, but the sequence of succession will be similar. During clear water conditions, with adequate concentrations of dissolved nutrients, the rapid increase of communities dominated by faster-growing species will be observed. High productivity (low biomass, but high rates of production) may continue progressively for some time, but, because of reduced light penetration and nutrient depletion, the competitive advantage is likely to move in favour of slower-growing, conservative species that are better adapted to nutrient deficits or lower levels of insolation. As long as the physical stability of the water column remains relatively constant, domination of the aquatic plant community by phytoplankton species with specialised adaptive strategies usually occurs. Depending upon which specific nutrient or other factor is most critical within the population, a specific species composition will be observed (Figure 8.4).

Fig. 8.4. Dominant algal assemblages determined in terms of the relative availability of (unspecified) limiting nutrients and column mixing (Reynolds, 1980, changed): (D) diatoms; (G) green-algae; (C) cyanobacteria; and (F) dinoflagellates.

These successional trends may be interrupted by perturbations, which significantly alter the physical structure of the environment. The most important are wind-induced mixing episodes or mixing of the water column connected with changes in the water retention time within reservoirs.

Temporal (seasonal) succession

The seasonal succession of phytoplankton in reservoirs is regulated by a number of abiotic and biotic factors, such as water temperature, the concentrations and ratios of nutrients (especially phosphorus and nitrogen) available for phytoplankton growth, and the composition and biomass of zooplankton (Figure 8.5). However, in reservoirs, the biological and physico-chemical processes are closely related to hydrology, and, especially, to water retention time (Figure 8.6). There is usually a positive correlation between water retention time and water temperature, for example. Increased water retention time is the main factor responsible for water temperature increases.

Concentrations of soluble reactive phosphorus (SRP), which is one of the main limiting nutrients in freshwater aquatic systems, decrease with longer water retention times. This process is explained, in part, by the high precipitation and sedimentation rates associated with allochthonous and autochthonous seston, and, in part, by the incorporation of SRP into biota. However, when SRP concentrations decrease below 30 µmg/dm3, the phosphorus deficit may be compensated for by higher levels of acid and alkaline phosphatase activity. These elevated phosphatase activity levels accelerate the release of mineral phosphorus from organic matter and amplify the recirculation of phosphorus during periods of deficiency (Box 8.1).

Fig. 8.5. Sequence of abiotic and biotic factors that regulate the occurence and biomass of phyto- plankton in the Sulejów Reservoir (Zalewski et al. 2000) (lager image)

Fig. 8.6. Factors responsible for the regulation of phytoplankton succession in a reservoir

The conditions in throughflow reservoirs with low water retention times - turbulent mixing, lower temperatures, changes in light intensity, and high nutrient and silica concentrations - are favourable for diatom growth (e.g., for Asterionella, Stephanodiscus, Melosira, Tabellaria, Fragillaria, and Cyclotella). Growth in these species is especially limited by:

  • low silica and phosphorus concentrations,
  • low light intensities and penetration,
  • thermal stability,
  • parasitism and zooplankton grazing.

Notwithstanding, some diatom species can be more or less affected by nutrient availability (Figure 8.4).

In reservoirs with long water retention times, in which thermal stratification is observed during stable conditions, the succession of phytoplankton leads to the formation of cyanobacterial blooms in nutrient-rich, eutrophic waters. The main cyanobacterial genera responsible for the formation of blooms are Microcystis, Aphanizomenon, Anabaena, and Planktotrix (Oscillatoria). These species usually form blooms with only one cyanobacterial species dominating. Factors controlling cyanbacterial growth include:

  • water temperature,
  • nutrient availability,
  • light penetration and intensity,
  • parasitism and protozoan grazing,
  • destratification and stability of the epilimnion.

In waters with lower nutrient availability, chlorophytes (green-algae such as Spherocystis), chrysophytes (such as Cryptomonas), and dinoflagellates (such as Ceratium) may occur (Figure 8.4).

Water retention times in reservoirs can influence not only the phytoplankton community composition but also its biomass. The average cyanobacterial biomass observed during periods of short (less than 60 days) retention time was two times lower than that observed during periods with long (greater than 60 days) water retention times (Figure 8.7).

Fig. 8.7. The effect of water retention time on the abundance of cyanobacterial biomass in a lowland reservoir (Tarczynska et al. 2001)

 

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