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Appropriate Season and Hydrologic Conditions for Sampling

Periphyton samples collected during NAWQA ecological surveys are generally collected concurrently with benthic invertebrate samples. Although not implicitly stated as an objective of NAWQA ecological surveys, the relation of benthic algal biomass and community composition with the abundance and distribution of herbivorous invertebrates and fish likely will benefit water-quality interpretations of the floral and faunal components of aquatic biological communities. The invertebrate protocol recommends sampling to be conducted during normal, low- or stable-flow periods and delayed approximately 4 weeks following a flood with a recurrence interval greater than 5 years (Cuffney and others, 1993). This recommendation is based partly on logistical consider ations because it is physically easier to sample a stream at a low, steady stage. Although algae are generally in all aquatic habitats, algal community structure and biomass can vary seasonally and in relation to antecedent hydrologic conditions. Light, temperature, current velocity, and nutrients are primary factors that influence the development, structure, and growth rates of periphyton and phytoplankton communities. Seasonal differences in these factors account for successional changes in the relat ive dominance of algal taxa in streams and rivers.

The intensity, duration, and quality of light influence the dominance of algal species, the structure of algal communities, and the relative amount of photosynthetic pigments produced by algal cells (Antoine and Benson-Evans, 1983; Shortreed and Stockner , 1983). For example, algal cells subjected to shaded conditions synthesize larger amounts of chlorophyll a, relative to algal biomass, than those exposed to high light intensity (Darley, 1982; Rosen and Lowe, 1984). Thus, the ratio of chlorophyl l a to ash-free dry mass (CHL/AFDM) of algal communities can be relatively larger during certain seasons of the year (for example, following leaf-out of deciduous, riparian trees during spring) and in shaded stream reaches. The effects of light quantity and quality should be con sidered in the selection of an appropriate sampling time and location.

Water temperature influences the metabolic and reproductive rates of algae, benthic invertebrates, fish, and other aquatic organisms. Although algal growth rates can be relatively lower during periods of cold water temperature, the standing crop or bioma ss of periphyton communities can be comparatively large because of the absence or inactivity of grazing organisms. Temperature also can influence periphyton species composition and community structure to a greater extent than it affects the total biomass or photosynthetic pigment concentration of the algal community.

Discharge and velocity conditions at the time of sampling, as well as conditions prior to the date of sampling, must be considered when determining the appropriate time to collect algal samples. Access to designated sampling reaches can be limited during seasonal high-flow conditions. Periodic high flows, or spates, result in scouring of benthic microhabitats and washout of periphyton and phytoplankton communities. However, modest increases of current velocity following storms may actually enhance rates of algal accumulation (Stevenson, 1990; Humphrey and Stevenson, 1992). Because rates of nutrient uptake and boundary-layer diffusion between the water and benthic algal mats increase with current velocity (Whitford, 1960; Horner and Welch, 1981), moder ate increases in velocity can result in increased algal biomass. Differences or changes in current velocity also are likely to affect the ratio of chlorophyll a to ash-free dry mass and the composition and structure of algal communities (McIntire, 1968; Belanger and others, 1985).