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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2009–5025

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Algal Growth

Chlorophyll a is used commonly as a surrogate for phytoplankton biomass. Other studies have shown that during the summer, Aphanizomenon flos‑aquae is the predominant phytoplankton species in Upper Klamath Lake (Kann, 1998; Wood and others, 2006; Hoilman and others, 2008;). The algal biomass in flooded Caledonia Marsh is assumed to have been composed primarily of Aphanizomenon flos‑aquae, because the waters that flooded Caledonia Marsh were from Upper Klamath Lake. This assumption is supported by field observations, although at the time of the Caledonia Marsh levee breach, chlorophyll a concentrations in Howard Bay were the lowest of the growing season (fig. 12), indicating that any algal growth in the flooded area occurred after the flooding event.

Chlorophyll a concentrations at sites FCM1 and FCM3 increased steadily from June 28 until the middle of the sampling period, becoming more variable by the end of the sampling period (fig. 6). In contrast, chlorophyll a concentrations measured at site FCM2 were significantly lower than those measured at the other FCM sites (table 2). The difference between sites indicates that one or more factors (such as growth inhibition or nutrient limitation) affected algal growth at site FCM2.

One factor that may have contributed to lower algal growth at site FCM2 was the presence of humic substances, which are products of the degradation of plant material and a major component of DOC. The difference in chlorophyll a concentrations at the FCM sites could be indicative of algal inhibition or reduction in availability of inorganic micronutrients by humic substances (Saunders, 1957; Wetzel, 1968; Kim and Wetzel, 1993; Klug, 2002). This hypothesis is supported by observations of the brownish color of the water at FCM2 in comparison to FCM1 and FCM3. Humic substances constitute a major portion of chromophoric (colored) dissolved organic matter, which often correlates with the concentration of DOC (Wetzel, 2001), and has been shown to impart color to the water in proportion to its concentration (Ishikawa and others, 2006). This hypothesis remains speculative, however, because the concentration of humic material was not measured directly, and while higher DOC concentrations were measured at FCM2 than at the other two sites, the differences were not statistically significant (table 2).

Another possible cause for differences in chlorophyll a concentrations among FCM sites is nutrient limitation. Total nitrogen to total phosphorus ratios are indicative of the potential for a nutrient to be limiting in a system. Ratios of total nitrogen to total phosphorus generally were less than 10 for all the FCM sites (fig. 13), which indicates a nitrogen limiting system (Forsberg and Ryding, 1980). Most of the ratios of lakewide sites lie in the range of nitrogen and phosphorus limitation (>10 and <17, respectively). The lower ratios at FCM sites than at the lakewide sites, particularly at site FCM2, reflect differing nutrient dynamics and might explain why chlorophyll a concentrations measured at site FCM2 were lower than those at the median lakewide sites (fig. 10) even though there was a surplus of bioavailable phosphorus at site FCM2.

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