Seasonality/aseasonality of aquatic macrophytes in Southern Hemisphere inland waters

Abstract

The term aseasonality is used in this paper to describe environmental conditions which either lack annual seasonal change or have periodicities of change which are longer or shorter than the seasons. Environmental factors act on plants either as stresses or disturbances and changes in environment can signal the onset of conditions which are favourable or unfavourable to plant growth and reproduction. Plant life-histories are thus adapted to these environmental factors and respond to them with both seasonal and aseasonal periodicities, depending on their manner of occurrence and effect on the plants. A review of pertinent studies from the Southern Hemisphere shows that plants of the same life-form (submerged, floating, emergent) might differ in the types of adaptation and response to environmental conditions according to latitude but that the periodicity of response could be seasonal or aseasonal regardless of latitude. The concept of seasonality versus aseasonality is therefore misleading and an oversimplification of the variety of periodicities with which the environment acts on plant genotypes. Limnological principles of the Northern Hemisphere are applicable to aquatic macrophytes in the Southern Hemisphere but there is a particular need for research into the effects of biotic variables and water level fluctuations on aquatic plants and communities in the latter.

Effects of Water Level Fluctuation on the Structure and Function of the Littoral Zone

“…Macrophytes: reduction of vegetation beds following drawdown of lake level is used as a management tool in fish ponds, small lakes, and hydroelectric reservoirs (e.g., Quennerstedt, 1958;Bennet, 1962;Grimas, 1962;Lantz et al, 1964;Mathis, 1965;Beard, 1969;Hestand et al, 1973, Nichols, 1975Galdsby and Sanders, 1977;Hejny andHusak, 1978, Cooke, 1980). Changes in the composition and reduction in the productivity of vegetation as a result of WLFs have been described by Robel, 1962;Ager and Kerce, 1970;Clayton, 1982;Mitchell and Rogers, 1985. These effects have been explained by reduction in habitat availability, erosion, deterioration of light conditions, and desiccation.…”

“…However, a given change in water level may have different consequences in different situations. For example, drawdown associated with erosion and resuspension of bottom sediments reduces light penetration and limits plant development (Swanson, 1967;Geen, 1974;Walker and Tyler, 1984;Mitchell and Rogers, 1985;Osborne et al, 1987). The same effect can be caused by rapid flooding of a shallow lake (Hestand and Carter, 1974).…”

“…Water quality can be greatly affected by WLFs, as a result of the introduction of organic matter and nutrients from re-flooded terrestrial areas (Nees, 1964;Cooper, 1966;McLachlan, 1970;Hestand et al, 1973;Osborne et al, 1987), the increased concentration of dissolved materials due to water loss by evaporation (McLachlan et al, 1972), and the increased turbidity due to shoreline erosion and resuspension of bottom sediment (e.g., Grimas, 1962;Swanson, 1967;Geen, 1974;Walker and Tyler, 1984;Mitchell and Rogers, 1985;Osborne et al, 1987), or to rapid re-flooding of shallow lakes (Hestand and Carter, 1974).…”

“…Shoreline morphometry, especially in convoluted shorelines where changes of water level can significantly modify the area of lagoons and consequently change habitat availability and affect biomass production (Monro and Larkin, 1950;Pieczynska, 1972;Mitchell and Rogers, 1985;Osborne et al, 1987). 2.…”

Human impact on the ecological performance of Potamogeton species in northwestern Germany

“…Phenotypic plasticity, that is the possibility to produce opportunistically (Tomlinson 1982) shoots, internodes, leaves, or other plant parts of various sizes and shapes as a reaction to changes or forthcoming changes in the environment (see Mitchell & Rogers 1985). -Synchronous shoot polymorphism, which will eventually lead to differential shoot mortality in case of destructive events.…”