Freshwater Habitats Of Plankton

There is a wide variety of inland aquatic systems within Australia - ranging from rivers and streams to lakes and reservoirs, farm dams and ponds, billa-bongs and wetlands (Figure 2.7). Due to low rainfall and high evaporation in many parts of the country, there is often a scarcity of permanent water bodies. Rivers and streams are often ephemeral - containing flowing water only after rainfall. Natural lakes are rare - reservoirs built to conserve water for town water supply and for irrigation are more common.

Inland waters - as distinct from estuarine or marine environments - are often considered to be fresh, with low concentrations of dissolved salts.

droi shallow, ephemeral bee bee

shallow, ephemeral

Figure 2.7 Diagram of a stream network and pool formation as phytoplankton habitat. Upland streams provide an input of nutrients, but are poor phytoplankton habitat. The naturally ponded sections have a reduced flow rate, which allows water residence times to match cell doubling times. Riffle zones can provide habitat for benthic forms. The weir provides permanent water, but can become stratified and de-oxygenated.

Figure 2.7 Diagram of a stream network and pool formation as phytoplankton habitat. Upland streams provide an input of nutrients, but are poor phytoplankton habitat. The naturally ponded sections have a reduced flow rate, which allows water residence times to match cell doubling times. Riffle zones can provide habitat for benthic forms. The weir provides permanent water, but can become stratified and de-oxygenated.

However, salt lakes may have salinities greater than that of sea water. Williams (1980), in arbitrary terms, defined fresh water as that with a salinity of less than 3 grams per litre of dissolved salts. In lowland areas with low rainfall and high evaporation, the salts of inland waters are often dominated by sodium and chloride, rather like sea water. In upland headwater streams and reservoirs, the waters are much fresher and calcium and magnesium bicarbonates may be the predominant salts present.

Rivers and streams are the primary routes of catchment drainage. During flood events, rivers may break out of the confines of their river channel, with their waters then spreading out over the floodplain. On these occasions, they can also transport large quantities of sediment and nutrients downstream from the catchment. In contrast, during droughts stream flow in permanent rivers is sustained by drainage from adjacent groundwater systems, while many others cease flowing completely, with only isolated pools remaining. The characteristically shallow nature, steep gradients and high flow velocities of upland rivers and streams keeps their waters well mixed (Figure 2.7). Many of the larger Australian rivers are impounded behind dams as they emerge from highland areas. After exiting these areas many inland rivers, such as those within the Murray-Darling Basin, then traverse many hundreds of kilometres of flat, lowland country. Gradients are small and channels become broad and meandering, or split into anabranches and distributary channels - with many terminating in extensive wetland areas. Lowland rivers may be impounded in natural ponds or by constructed weirs where water depth will increase, flow velocities will decrease and the resultant ponds and weir pools then assume more lake-like characteristics, including stratification of the water column during summer in some if they are deeper than 3 metres. Fine sediment washed in from the catchment make many of these water bodies turbid. Nutrient and light availability, rate of flow, and stratification will all affect plankton community composition and abundance in these rivers (Mitrovic et al. 2003).

Flowing river systems are generally not good habitats for plankton, because the organisms entrained within the water column are continually displaced downstream. However, some of the larger lowland rivers may develop their own riverine phytoplankton communities - known as potamoplankton -which develop within parcels of water as these traverse the length of the river. Most algal growth in smaller, shallower, faster flowing streams, however, is confined to clumps of filamentous algae attached to a secure substrate to prevent themselves from being washed away, and to films of microscopic algae coating the surfaces of rocks, mud, sticks and aquatic macrophytes. These algae obtain the substances they require to sustain their growth as the water flows over them. The weir pools and ponded sections of lowland rivers and streams may, however, become suitable habitats for phytoplankton to form blooms. Some rivers also have small embayments, inlets, or backwater areas where water movement may be minimal. These areas - known as 'dead zones' - are areas where phytoplankton can develop (Mitrovic et al. 2001).

Lakes, reservoirs, farm dams, ponds, billabongs and wetlands are characterised by prolonged residence times of the water they contain, and the limited mixing of water within them - apart from that caused by wind-driven currents and internal-heat-transfer processes. Deeper lakes and reservoirs undergo strong thermal stratification during the warmer months of the year, caused by the preferential solar heating of the surface waters. Water density decreases as temperature increases, so warm water overlies colder water and creates horizontal density gradients that resist vertical mixing and enhance the stability of the water column. Chemical and biological demand for oxygen in deeper regions, accompanied by limited replenishment from the surface due to the lack of vertical mixing, can lead to very low oxygen levels in deep lake waters. Deoxygenation of the deeper waters has major effects on the chemistry of other substances, especially nutrients, which can be mobilised from the lake sediments under such conditions. The thermal stratification and mixing regimes of lakes and reservoirs influences water column stability, nutrient availability and light availability at different times of the year - and, consequently, the plankton community structure and abundance in these water bodies.


Lake Makoan in Victoria provides a good example of a reservoir that underwent a change of state: from a clear-water, macrophyte-dominated system, to a turbid, phytoplankton-dominated system. The lake dried out during droughts in the 1980s, the macrophytes died and the fine sediment on the lake bottom was exposed. This became suspended in the water column when the lake refilled. The water became very turbid, and light could not penetrate to the bottom for the macrophytes to re-establish. Instead, with high nutrient concentrations, cyanobacterial blooms took over.

The plankton of lakes has been termed limnoplankton, while that of ponds heleoplankton. While some species of phytoplankton may be characteristic of rivers, lakes or ponds, there are sufficient common species found in all three habitats that the classification of phytoplankton communities into these groupings has only very general application.

Farm dams are often very turbid environments, so lack of light within the water column may limit phytoplankton growth. These, and other small ponds, are often typified by high amounts of organic substances in the water, which is often thought to favour certain kinds of motile unicellular algae known as euglenoids (Chapter 5, Section 5.6). Wetlands and billabongs are generally shallow, and much of the submerged area may be occupied by aquatic macrophytes, especially angiosperms, but also by some large mac-roalgae, known as charophytes, that grow from the sediments. These mac-rophytes, and algae that grow attached to them (termed epiphytes) may compete with phytoplankton for light and nutrients, so that wetlands may not be good habitats for phytoplankton. Shallow water bodies may be clear water, macrophyte-dominated systems, or turbid, nutrient-enriched, phytoplankton-dominated systems (Scheffer 1998) (Box 2.2).

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