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Scientific Investigations Report 2008–5201

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5201

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Nutrient and Chlorophyll a Water Samples

Aphanizomenon flos-aquae Bloom Dynamics

Photosynthetic pigments, like chlorophyll a, are measured as a surrogate for algal biomass (Wetzel, 2001) because the cost and time required to collect and analyze chlorophyll a is less than that required to make measurements of algal biomass using gravimetric methods. In this report, algal biomass is represented by chlorophyll a concentration. Furthermore, because 90–100 percent of the total phytoplankton biomass in Upper Klamath Lake consists of AFA between May and November (Kann, 1997), chlorophyll a represents primarily the biomass of AFA. The weekly water samples, although collected at only a subset of the sites in the study area, provided valuable context for understanding the data from the continuous water quality monitors. Trends and fluctuations in water quality parameters were commonly associated with trends and fluctuations in algal (AFA) biomass, as reflected in the chlorophyll a data. Maxima in concentrations of chlorophyll a coincided with supersaturated concentrations of dissolved oxygen and the highest pH values, indicating an algal bloom, whereas minima in concentrations of chlorophyll a coincided with undersaturated dissolved oxygen concentrations and lower pH values, indicating a bloom decline. A bloom decline is characterized by a sharp reduction in oxygen production through photosynthesis and is manifested as a decrease in dissolved oxygen concentration. These decreases may reach levels potentially harmful to fish when oxygen demand continues in the water column or sediments through the decomposition of organic material. In contrast, periods of growth in the bloom are generally manifested as supersaturated dissolved oxygen concentrations and high pH values, as photosynthesizing algae consume bicarbonate and produce oxygen.

Week-to-week variation in chlorophyll a concentrations can indicate either temporal variability (because the population is blooming and declining through time) or spatial variability (because the algal growth is inherently patchy and the patches move around the lake). These two types of variability can be distinguished largely by considering the concurrent dissolved oxygen and nutrient concentrations. Indicators of bloom decline and associated cell senescence are low dissolved oxygen concentration and increased dissolved nutrient concentration; the magnitude of the increase in dissolved nutrients and decrease in dissolved oxygen concentration often is proportional to the chlorophyll a concentration before the event. Variation in chlorophyll a concentration caused by patchiness, however, is not associated with increased nutrient or decreased dissolved oxygen concentration. Patchiness is an expected characteristic of AFA blooms. AFA cells contain gas vesicles that allow colonies to become positively buoyant; cells also create and consume carbohydrate ballast to regulate their position in the water column. The sampling protocols used in this study may be insufficient to detect chlorophyll a near the lake bottom. Additionally, colonies floating to the surface can form mats that are moved around by the wind, resulting in a low chlorophyll a concentration at the site of measurement when in fact a bloom is in progress elsewhere in the lake.

At site MDN, chlorophyll a data were collected from 2002 through 2006, allowing a direct interyear comparison. In each year, the algal bloom expanded rapidly to peak sometime between mid-June and mid-July, as measured by a maximum in chlorophyll a concentration (fig. 9). Between late July and mid-August in each year, chlorophyll a concentrations decreased concurrently with increased dissolved nutrient concentrations and decreased dissolved oxygen concentrations, indicating an algal bloom decline and the associated large-scale cell senescence. (For the purposes of this analysis, dissolved nutrients collected at site EPT are shown for 2006, because they were not measured at site MDN.) Dissolved oxygen concentrations of less than 4 mg/L (lasting between several hours and several days) were measured at site MDN in association with this type of bloom decline from late July to mid-October during 2002–06 (fig. 9). The most severe (longest duration) low dissolved oxygen events (LDOE) occurred during the last week in July and first week in August 2003 and 2005, and were associated with the largest declines in chlorophyll a and increases in levels of dissolved nutrients, particularly ammonia. The occurrence of a low dissolved oxygen event in October 2003, however, demonstrates that this type of event is not limited to midseason, and can occur more than once in a single season.

Concentrations of chlorophyll a at most sites reached two or more peaks during 2006 (fig. 10). The highest chlorophyll a concentrations occurred after the midseason bloom decline at sites HDB and MDT (table 4). Data collected during previous years also indicated late-summer blooms of AFA (Kann, 1997; Wood and others, 2006; Hoilman and others, 2008). Throughout the sampling season, chlorophyll a concentrations varied between sites, indicating patchiness in the bloom, which was corroborated by the observations of field crews. As the season progressed, the variability between sites increased, indicating that the spatial scale of the patchiness increased. This trend through the season has been observed previously (Hoilman and others, 2008). The chlorophyll a concentrations at site HDB had multiple substantial peaks throughout the season, which differed from patterns at the other sites. The bloom in Howard Bay was somewhat isolated from the main body of the lake, and probably developed its own seasonal dynamics in response to localized nutrients, wind, and circulation patterns within the bay (Wood and others, 2008).

Decreases in chlorophyll a concentrations that indicate a large decline in the bloom are of interest because previous studies indicate that this phenomenon sometimes coincides with water quality that is harmful to endangered suckers in the lake. For example, a bloom decline has been shown to be the cause of LDOEs and increased un-ionized ammonia concentrations (Wood and others, 2006). In 2006, high chlorophyll a concentrations in late June and early to mid-July were followed by low concentrations in late July and early August. Because chlorophyll a concentrations decreased simultaneously at most sites indicates that net algal production was reduced by factors extending over a large area; temperature, decreased sunlight, and nutrient limitation are factors that could lead to such rapid lakewide chlorophyll a decreases. The lowest chlorophyll a concentrations measured after the bloom were between sample collection dates July 25, 2006, and August 1, 2006. During this time, continuous monitors measured dissolved oxygen concentrations less than 4 mg/L at all sites except sites RPT and WMR, less than 2 mg/L at site MDN, and less than 1 mg/L at sites HDB, MDT, and EPT. Dissolved oxygen concentrations at sites RPT and WMR did not decrease to less than 4 mg/L, indicating that the spatial extent of the LDOE did not reach these two sites. Consistent with previous years (Wood and others, 2006, Hoilman and others, 2008), the multiple decreases in chlorophyll a concentrations after mid-August in 2006 were not associated with widespread low dissolved oxygen concentrations. Such late-season variability more likely is a result of increased patchiness (decreased spatial scale) of the bloom rather than cell senescence.

Total phosphorus and total nitrogen concentrations were correlated positively with chlorophyll a concentrations (fig. 11). Using the Spearman’s rank correlation coefficient, the relation between total nitrogen or total phosphorus and chlorophyll a samples with data from all sites combined was 0.741 (p < 0.05, n = 99) and 0.802 (p < 0.05, n = 93), respectively. Maxima in chlorophyll a concentrations in 2006 corresponded with maxima in total phosphorus and total nitrogen concentrations (fig. 11), and total nitrogen and total phosphorus concentrations increased concurrently with chlorophyll a concentration at the onset of the initial bloom (fig. 10). Previous studies have shown that total phosphorus and nitrogen concentrations, and chlorophyll a concentration in Upper Klamath Lake tend to increase simultaneously in spring, as determined by weekly or biweekly sampling (Kann, 1997, Wood and others, 2006). The increase in total nitrogen concentration has been preceded by an increase in heterocyst formation indicating nitrogen fixation by AFA leading up to the initial bloom (Kann, 1997).

Unlike deep lakes in which summertime chlorophyll a often can be predicted based on the phosphorus available at spring turnover (Dillon and Rigler, 1974), the seasonal dynamics of total phosphorus concentration in shallow lakes can be much more complicated, largely because of exchange between the water column and sediments (Havens and others, 2001). Nutrient mass balance studies have confirmed that the source of phosphorus to Upper Klamath Lake in spring and summer is largely internal loading from lake sediments (Kann and Walker, 2001), although the precise mechanism for phosphorus loading is unknown (Jacoby and others, 1982; Barbiero and Kann, 1994; Laenen and LeTourneau, 1996; Fisher and Wood, 2004). Initial results using pore-water profilers suggest that diffusive flux in combination with bioturbation is a possible mechanism for internal phosphorus loading in the lake (Kuwabara and others, 2007). Some phosphorus that is stored in sediments and contributes to current internal loads is the result of increased external loading over the last several decades (Boyd and others, 2002), which is evidenced by a decrease in nitrogen to phosphorus ratios in the upper sediments (Eilers and others, 2004).

Through the combination of internal loading of phosphorus, nitrogen-fixation capability, and buoyancy regulation, blue-green algae such as AFA can dominate other phytoplankton in systems with low nitrogen to phosphorus ratios (less than 29:1 by weight; Smith, 1983). Ratios of total nitrogen to total phosphorus of all samples collected in 2006 were less than 29, which indicate that the appropriate conditions existed to establish AFA dominance over other phytoplankton (fig. 12). Analysis of sediment cores collected in Upper Klamath Lake show increased abundance of AFA and decreased diatoms and green algae in recent decades (Eilers and others, 2004). The changes in abundance of AFA coincide with the decrease in ratios of nitrogen to phosphorus in the upper lake bottom sediments, which is attributable to increases in external loading of phosphorus. The lowest ratios (2.9–10) of total nitrogen to total phosphorus were measured at site WMR (near the mouth of the Williamson River), indicating that total nitrogen to total phosphorus ratios from riverine inflows were lower than the lake water sampled at the other sites.

Potentially phosphorus-limited samples, characterized by chlorophyll a to phosphorus ratios greater than 1 (White, 1989; Graham and others, 2004) and total nitrogen to total phosphorus ratios greater than 17 (Forsberg and Ryding, 1980), occurred during the initial bloom in late June and early July (fig. 13). Ratios of total nitrogen to total phosphorus were lower between mid-August and mid-October than the ratios measured in the beginning of the season. Ratios of chlorophyll a to total phosphorus also peaked at lower values in September and October than in July (fig. 13). Based on both ratios, late-season blooms were not limited by phosphorus to the same extent as the initial algal bloom in late June and early July. Higher orthophosphate concentrations from mid-August through mid-September than concentrations prior to mid-July also indicate that more phosphorus was available to support bloom growth after the first bloom decline. Understanding the role of phosphorus in eutrophication and excessive algal growth is important from a management perspective (Havens and others, 2001; Schindler, 2006), and reductions in external loads likely will lead to reductions in internal phosphorus loads in Upper Klamath Lake, which in turn will reduce the peaks in the bloom and the severity of bloom declines (Walker, 2001; Boyd and others, 2002). These results indicate that such a strategy would most effectively control the early-season blooms.

Peaks of orthophosphate and ammonia occurred simultaneously with the low chlorophyll a concentrations at the end of July (fig. 10). Low dissolved oxygen concentrations coincided with the decline of chlorophyll a concentrations and the increase in dissolved nutrient concentrations, suggesting that the nutrient peaks resulted from decomposition of dead algae cells, which converts organic nutrients to inorganic form while consuming dissolved oxygen. Nitrite-plus-nitrate concentrations were low compared with ammonia, which indicates that nitrification is not a rapid or effective process for removal of ammonia from the water column during post-bloom periods of low dissolved oxygen concentrations.

Spatial Variability in Nutrient and Chlorophyll a Concentrations

Lake bathymetry appears to influence dissolved nutrient concentrations. Median concentrations of ammonia and orthophosphate were highest at the deep trench sites MDT and EPT (fig. 14), particularly during the period of bloom decline, when dissolved nutrient concentrations were the highest of the season. Median concentrations of ammonia and orthophosphate were lowest at the shallow site, RPT. The same variation among the sites was seen in 2005 data, indicating a greater degree of mineralization (conversion of organic nitrogen to ammonia) at the deep sites. The oxygen consumption rates obtained from dark bottles (discussed in section, “Dissolved Oxygen Production and Consumption Experiments”) do not indicate a large oxygen demand from the water column during the period of bloom decline. Therefore, oxygen consumption and mineralization processes likely occurred primarily at the sediment–water interface. This idea is supported by the downward vertical velocities measured near-bottom in the trench with ADCPs in conjunction with sediments containing a large amount of organic matter, which together indicate settling of organic matter at the deep trench sites (Gartner and others, 2007).

Chlorophyll a, total nitrogen, and total phosphorus, concentrations were higher at sites HDB and MDT than at the other sites (fig. 14, table 4). The high concentration of chlorophyll a and total phosphorus at site HDB may be explained by the relation of the site to the circulation patterns in the open waters of the lake, in which Howard Bay is isolated from the main flow regime.

The greater depth of site MDT provides buoyant colonies the opportunity to concentrate at high density near the surface of the water because the water column is more resistant to mixing than at shallow sites. Vertical velocities measured by ADCPs indicated that the movement of the colonies was upward near the surface at MDT (Gartner and others, 2007). The high chlorophyll a concentrations in samples collected at site MDT were consistent with observations made by the field crews of thick mats of algae close to the water surface. Additionally, the primary location for companies collecting AFA for commercial production of algal food supplements is near site MDT, indicating that the location provides easy access to the high concentrations of algae near the surface. Furthermore, circulation modeling indicates a clockwise circulation that can capture large algal mats between Eagle Point and Howard Bay (Wood and others, 2008). Particle-tracking studies with the model have shown that water can be trapped in the cell for days.

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