Introduction


Upper Klamath Lake, the largest freshwater lake in the State, is located east of the Cascade Range in south-central Oregon. The lake has been historically eutrophic (Eilers and others, 2004) and has experienced annual occurrences of large cyanobacterial blooms (primarily Aphanizomenon flos‑aquae, or AFA) in recent decades. Growth and decomposition of dense AFA blooms frequently cause extreme water quality conditions characterized by high pH (9–10.2) and ammonia concentrations (> 0.5 mg/L unionized) and widely variable dissolved oxygen (anoxic to supersaturated) (Wood and others, 2006; Hoilman and others, 2008). These conditions are detrimental to the survival of two endangered fish species endemic to the lake, the Lost River and shortnose suckers. In order to complement water quality samples with better spatial and temporal resolution, it may be possible to use measured acoustic backscatter as a surrogate for suspended solids. In addition, if properly calibrated, acoustic Doppler current profilers (ADCPs) can provide non-intrusive measurements to compute time series of suspended solids concentration (SSC) profiles concurrent with measurements of three-dimensional velocity profiles (Gray and Gartner, 2009).


Attempts to characterize suspended material from acoustic backscatter, a byproduct of ADCP velocity measurements, have increased as the use of ADCPs has become more widespread. Early studies used ADCP velocity and backscatter measurements to provide qualitative estimates of zooplankton biomass and diurnal patterns of vertical migration (Schott and Johns, 1987; Flagg and Smith, 1989; Heywood and others, 1991; Zhou and others, 1994), and even defined multiple diel migration layers (Plueddemann and Pinkel, 1989; Heywood, 1996; Pinot and Jansa, 2001; Ashjian and others, 2002; Sutor and others, 2005). Whereas most of these studies used relatively low frequency ADCPs (most often 150 kHz) in deep waters, Lorke and others (2004) applied acoustic backscatter from a 600 kHz ADCP to quantify distribution and movement of zooplankton, primarily Chaoborus flavicans, populations in lakes (< 50 m depth). Lorke and others (2004) found very good correlation between backscatter estimates and measured concentrations and a strong correlation between up and down migration and times of sunset and sunrise.


Others have attempted to quantify the concentration of suspended solids that primarily are inorganic material through laboratory or field calibrations. Laboratory experiments designed to calibrate backscatter to SSC were conducted by Thorne and others (1991) and Lohrmann and Huhta (1994). Thevenot and others (1992) developed calibration parameters for reliable estimates of SSC as part of a study to monitor dredged material using Broadband-ADCPs (2.4 MHz and 600 kHz). Thevenot and others (1992), Thevenot and Kraus (1993), and Holdaway and others (1999) found concentration estimates from calibrated backscatter were comparable or superior to estimates from transmissometers. Gartner (2004) computed SSC using 1.2- and 2.4-MHz ADCPs. Results compared favorably to those computed from optical backscatter sensors (OBS) (within about 10 percent of the range of OBS estimates and within about 40 percent of the mean of OBS estimates over a 7-day time series). Wall and others (2006) used ADCP backscatter to compute suspended sediment discharge. Regression of measured to computed SSC values resulted in an R² of 0.86 and a standard deviation of 7.9 mg/L over a range of about 5–75 mg/L. Finally, Topping and others (2007) applied a multi-instrument, multi-frequency approach; they related acoustic attenuation to concentration of suspended silt and clay and acoustic backscatter to concentration of suspended sand. Computed values were within 5 percent of those measured by conventional methods.


Examples of the application of ADCP backscatter measurements to the study of phytoplankton are scarce in the literature and typically provide descriptions of the spatial distribution of phytoplankton rather than predictions of phytoplankton biomass. The paucity of studies relating ADCP backscatter to phytoplankton may be a result of a limitation of the acoustic backscatter technique—the relation between particle size and acoustic frequency. Errors in estimates occur when the size of suspended material is too small or too large for the acoustic frequency being used.


Individual AFA cells typically are small (about 100–130 µm³) (Konopka and others, 1987) and are often grouped in filaments ranging from 8 to 50 cells (Huisman and others, 1999). Thus filament volumes might range from about 800 to 6,500 µm³ (equivalent radii about 5–12 µm). However, the AFA is found in flakes consisting of thousands of filaments and the number of flakes may exceed thousands per liter at the height of the bloom (Geiger and others, 2004). Thus, AFA flakes have a range of equivalent radii from about 50 to 250 µm. These particles far outnumber other scatterers in the lake and are well within the size range suitable for the frequency of ADCPs used in Upper Klamath Lake that are described here.


This paper describes a study to determine if relative backscatter, RB (acoustic backscatter corrected for transmission losses from acoustic beam spreading and attenuation in units of dB) and vertical velocity measurements by ADCPs can be used to better understand seasonal, subseasonal, and diel dynamics of cyanobacterial colonies (AFA) in a shallow freshwater lake. Here, the term seasonal refers to variations or trends that occur over time scales consistent with the ADCP deployment length (about 85–125 days). The term subseasonal is used to define variations or trends that have time scales greater than diel but less than seasonal, that is, time scales of days to weeks corresponding to weather patterns. The RB and vertical velocity data measured by ADCPs in Upper Klamath Lake were used to establish the typical diel cycles of mass distribution and movement through the water column, and how these cycles relate to the typical diel cycles in thermal stratification, mixing, and light availability. The theoretical model of light-driven vertical migration of buoyant cyanobacteria is based on the fact that density of cells changes during the day in response to availability of light and nutrients. When light is available in excess of growth capacity, cells become negatively buoyant by limiting the production of gas vesicles or by increasing carbohydrate production, thus gaining ballast. If nutrients are plentiful and cells are light‑limited, cells become positively buoyant by consuming carbohydrate ballast or increasing the synthesis of gas vesicles. As a result, theoretically, if movement is light‑driven then the cells of buoyant cyanobacteria generally rise in the water column during evening and early morning (during darkness) and sink in the water column during morning and afternoon (during daylight) (Konopka and others, 1987; Porat and others, 2001). Like the AFA dynamics, the external forcings that determine stratification and entrainment, that is, air temperature and wind speeds, also vary on subseasonal time scales. Therefore, the relation of RB and vertical velocity to wind-driven currents and air temperature on subseasonal time scales was investigated as well. Finally, SSC values (estimated from RB) were used to make inter-comparisons among sites, to investigate the relation to other water quality measurements (for example, chlorophyll a, dissolved oxygen, and water temperature), and to determine the movement of mass vertically through the water column. 


First posted March 16, 2011