Macrophytes

Macrophytes include vascular flowering plants, mosses and liverworts, some encrusting lichens, and a few large algal forms such as the Charales and the filamentous green alga Cladophora. Light and current are among the most important factors limiting the occurrence of macrophytes in running waters. Major plant nutrients, particularly phosphorus, can be limiting in nutrient-poor waters but are likely to be present in excess in eutrophic lowland rivers. Three ecological categories include those that are attached to the substrate, those that are rooted into the substrate, and freefloating plants. Attached plants include the mosses and liverworts, certain lichens, and some flowering plants of the tropics. These are all largely found in cool, headwater streams. The mosses are unusual in their requirement for free CO2, rather than bicarbonate, as their carbon source. In shaded, turbulent streams, their contribution to primary production may override that of the periphyton. Mosses also support very high densities of macroinvertebrates. Rooted plants include submerged (e.g., Hydrocharitaceae, Ceratophyllaceae, and Halorgidaceae) and emergent (e.g., Potamogetonaceae, Ranunculaceae, and Cruciferae) forms and require slow currents, moderate depth, low turbidity, and fine sediments for rooting. They are most common in mid-sized rivers and along the margins of larger rivers where they reduce current velocity, increase sedimentation, and provide substrate for epiphytic microflora. Tough, flexible stems and leaves, attachment by adventitious roots, rhizomes or stolons, and vegetative reproduction are important adaptations. Free-floating plants (e.g., Lemnaceae and Pontederiaceae) are of minor importance in running waters at temperate latitudes as they depend largely on lacustrine conditions. They may accumulate significant biomass in subtropical and tropical settings. Macrophytes in lotic ecosystems contribute to energy flow predominantly through decomposer food chains, as few macroinvertebrates feed on the living plants.

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Navicula x500

Synedra x250

Figure 8 Representative diatoms common in stream periphyton. From Hynes HBN (1970) The Ecology of Running Waters. Liverpool: Liverpool University Press.

Mfci.jttlll'JUlliĀ»

Navicula x500

Synedra x250

Figure 8 Representative diatoms common in stream periphyton. From Hynes HBN (1970) The Ecology of Running Waters. Liverpool: Liverpool University Press.

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