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

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Summary

The drained and natural wetlands surrounding Upper Klamath Lake may influence nutrient cycling in the lake. Natural wetlands, such as swamps and marshes, can sequester nutrients from tributary inflows; whereas, nutrients are released when oxidized peat soils that formed in previously drained wetlands are flooded. After a previously drained wetland is flooded, many years can pass before natural wetland functions are restored. In the short term, nutrients left over from agricultural practices or from the oxidation of peat soils may be transferred to the overlying water column. Flooded Caledonia Marsh, a drained wetland used for agriculture for many years, became of interest when the marsh was flooded accidentally due to a levee breach in early June 2006. The study focuses on what affect a newly flooded drained wetland has on nutrient concentrations in the lake.

Water quality characteristics at one marsh site, FCM2, were somewhat different relative to the other two marsh sites (FCM1 and FCM3), for reasons that might include a greater distance from the levee breach, proximity to a restored wetland, and possibly the presence of vegetation. A brown color in the water column at FCM2 was consistent with higher dissolved organic carbon (DOC) concentrations at that site, where the water column likely was colored by humic acids, a major component of DOC. Both DOC concentrations and specific conductance values, were higher at FCM2 than at the other two sites. The same factors were likely responsible for the higher values of both constituents. A difference in soil type and land use cannot be ruled out, but it seems likely that there was less mixing at FCM2 with water from Upper Klamath Lake, which was characterized by lower DOC concentrations and lower specific conductance values than any of the flooded marsh sites. Chlorophyll a concentrations, a surrogate for algal biomass (primarily the blue-green alga Aphanizomenon flos-aquae), were lower at site FCM2 than at the other two sites, which indicates that algal growth was limited by some mechanism at site FCM2. Some studies have shown algal growth inhibition in the presence of humic compounds, as measured by DOC.

Site FCM2 also differed with regard to trends in total nutrient species relative to chlorophyll a concentration. Concentrations of bioavailable nutrients—orthophosphate, ammonia, nitrite-plus-nitrate—were greater at site FCM2 than at the other marsh sites, indicating that nutrient limitation was not responsible for the lack of algal growth at site FCM2. Ratios of total nitrogen to total phosphorus were indicative of a nitrogen limitation at all sites, but to a greater extent at site FCM2. Ratios at the marsh sites were different from those at lakewide sites and indicate different nutrient dynamics in the lake relative to the flooded marsh.

Total phosphorus and orthophosphate concentrations were greater in the marsh than in the open waters of the lake. The difference in phosphorus concentrations between the marsh and the lake could have been due to the release of phosphorus from the inundated soils after flooding. Although total phosphorus concentrations in the hydraulically connected Howard Bay increased from 2005 to 2006, no significant increase in algal growth occurred during the same period. Therefore, short-term increases in bioavailable phosphorus that can be expected when the previously drained wetlands around the lake are reflooded will not necessarily lead to a proportional increase in algal growth in the lake. This also underscores that it should not be assumed that phosphorus is the primary factor limiting algal growth in the lake at all times and all areas of the lake.

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