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a Potomac River, Maryland b Savannah River, Georgia c White Clay Creek, Pennsylvania (Patrick 1961) d Clay and detritus bottom stream, southern Ontario (Moore 1972) e Epiphytes on Cladophora glomerata in the Skawa River, Poland (Chudyba 1965) f Flagellates present but not identified to species a Potomac River, Maryland b Savannah River, Georgia c White Clay Creek, Pennsylvania (Patrick 1961) d Clay and detritus bottom stream, southern Ontario (Moore 1972) e Epiphytes on Cladophora glomerata in the Skawa River, Poland (Chudyba 1965) f Flagellates present but not identified to species

River Ecology Structure
FIGURE 6.2 Factors controlling the biomass and physical structure of periphyton in streams. (Reproduced from Biggs 1996.)

substrates including clay tiles and glass slides can also be allowed to become colonized, and soft sediments can be collected by inserting the top of a petri dish into the sediment, and then sliding a glass plate underneath to sample a known area and depth. Cell counts usually are reported as numbers per square centimeter, or if cell volume also is estimated, as biovolume per square centimeter. To estimate the algal biomass it is usual to weigh a dried sample, then weigh a second time after combusting all organic matter at ~500°C. The loss of mass represents the organic material present (all of which was converted to CO2 by combustion), and is referred to as ash-free dry mass (AFDM). Because chlorophyll a is the most abundant pigment in plants, its ab-sorbance is measured most frequently. These are all static measures, often referred to as standing crop. Measurement of primary production, or the rate of formation of new biomass, also is important. Methods based on the release of O2, either within an enclosed chamber or for the entire stream, are described later in this chapter.

The simplest framework for discussing limiting factors is to take them one at a time, although in reality these factors interact in complex ways. This is because the importance of any one factor in limiting plant growth depends upon whether some other factor is in even shorter supply. Thus, light might be limiting if nutrients are plentiful, and their importance can alternate seasonally or from place to place. In addition, it is possible that two or more factors simultaneously limit growth. For these reasons it is difficult to address any one factor without specifying the context of other environmental variables. Before considering singly and in detail the factors that potentially limit benthic algae, it may help to present a brief overview of the role played by each.

Light can be a limiting factor in small streams under dense forest cover, where benthic algae populations tend to reach maxima just prior to canopy development and then decline through the summer. However, other factors often override light. Nutrients, particularly phosphorus (P) and nitrogen (N), might be expected to exert a critical influence over autotrophs in rivers just as they do in standing freshwaters, but this turns out to be rather complicated. Flow provides continual delivery of water, thus minimizing the depletion of nutrients, and thermal stratification does not occur to restrict mixing of nutrients throughout the water column, so nutrient exhaustion should be less likely to occur in rivers than in lakes, where its importance is well established. In some studies, enrichment of stream water with N and P has had no effect on benthic algal development, and in others it has. Phosphorus and N have been found to be limiting, sometimes in combination, depending on location. Other nutrients, trace metals, and bicarbonate probably are of some importance, temperature increases rates of metabolism, and grazing can limit algae populations, sometimes severely. Fast currents restrict the establishment of macrophytes and influence the distribution of benthic algae in terms of both taxa and growth forms. Where discharge is variable, flood events and scour by suspended sediments can cause major reductions in algal standing crops. As a consequence, in very rainy climates the growing season is restricted to the time between the last spring flood and the first autumn flood, and summer standing crops depend upon the number of consecutive flood-free days. However, in running waters where extremes of discharge are minimal, direct effects of current may be less important to the benthic algae. Overall, it appears that the effects of light, nutrients, and scour are of greatest importance, and each can be strongly limiting at a particular location or time. Now, what is the evidence for this scenario?

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