Suspended Solids Concentration


Although using RB to discern seasonal, subseasonal, and diel variations of suspended material is enlightening, RB does not allow direct comparisons among measurements by different instruments or locations. However, use of measured or computed SSC (in units of mass/volume rather than dB) does allow quantitative comparisons between datasets, meaningful comparisons to other water quality variables, and investigation of the movement of mass through the water column. For this reason, profiles of SSC were computed using acoustic backscatter measurements at the five ADCP sites in 2005.


The time series of SSC calculated from RB show distinct differences between the sites (fig. 7). Overall through the season, the depth-averaged concentrations were highest at the deepest site, ADCP1. Concentrations were second highest at the northernmost site, ADCP3, but were lowest at ADCP5, another site in the northern part of the lake. These relative concentrations were consistent with the chlorophyll a samples collected at four sites during the 2005 season. The median chlorophyll a concentrations of 12 samples collected between June and September at site MDT (near site ADCP1), site MDN (near site ADCP3), site EPT (near site ADCP5), and site MDL (near site ADCP6) were 122.5, 99.0, 36.5, and 59.7 µg/L, respectively. The correspondence between the two variables was further tested with linear regression between the depth-averaged chlorophyll a and the median value of the computed depth-averaged SSC during the 2 hours surrounding the time of sample collection and at the most closely situated ADCP site (table 2). At the two sites that were coincident with a water quality site, sites ADCP1 and ADCP6, the correlation coefficients were 0.46 and 0.63, respectively. At the other two sites, which were each located about 1.5 km from the nearest water quality site, the correlation coefficients were 0.47 and 0.43 at sites ADCP3 and ADCP5, respectively. All correlation coefficients were at least weakly significant (p < 0.16).


The computed SSC also shows some correspondence with continuous measurements of dissolved oxygen (DO) from monitors located at the water quality sites, two of which are shown in figure 7. The seasonal minimum in DO at site MDN was an indicator of a severe decline in the AFA bloom between the last week in July and the first week in August (Hoilman and others, 2008). The lowest DO values during this time coincided with low SSC at sites ADCP1, ADCP3, and ADCP5. A common characteristic of a major bloom decline in the lake is that it is most severe in the deep trench and in the northern part of the lake (Wood and others, 2006; Hoilman and others, 2008; Lindenberg and others, 2008), which is consistent with the fact that the late July/early August minimum in SSC was most prominent at the sites in the northern part of the lake (sites ADCP3 and ADCP5) and in the trench (site ADCP1), and was least prominent at site ADCP6 (fig. 7). In order to quantify the correspondence between SSC and DO, the correlation coefficient between the daily median value of depth-averaged SSC and the daily median value of DO, measured 1 m from the bottom of the lake at the closest water quality site, was calculated (table 2). At all sites, the correlations with DO were significant with p<0.02. 


Computed SSC can provide insight into how mass moves vertically through the water column at diel time scales. The computed SSC at each bin in the vertical water column was used to calculate the fraction of total water column mass in the upper and lower one-half of the water column as a function of time (upper one-third and lower two-thirds of the water column at site ADCP1). Surface intensification is indicated when the fraction of water column mass in the upper one-half of the water column (upper one-third at site ADCP1) is greater than 50 percent (33 percent at site ADCP1); bottom intensification is indicated when the fraction of water column mass in the lower portion of the water column is greater than 50 percent (67 percent at site ADCP1). A typical 10-day period of record shows that mass was concentrated in the upper portion of the water column during those hours of the day when there was some thermal stability in the water column. As the surface started to cool and winds increased, the water column became well-mixed (fig. 8) from decreased stratification and increased turbulence. This short period of record is typical of the longer record, in which surface intensification as a daily phenomenon was common and bottom intensification was uncommon, but occurred occasionally at site ADCP1. 


When computed SSC was averaged over the season to produce the seasonally averaged diel cycle, a clear pattern of afternoon surface intensification emerged at each site (fig. 9). The mass in the top one-half of the water column (top one-third at site ADCP1) reached a maximum near the time corresponding to the maximum ΔT°. The mass in the bottom one-half (two-thirds at site ADCP1) reached a minimum a few hours ahead of the maximum in mass in the top one-half of the water column at all sites except site ADCP1, where it instead reached a minimum a few hours later than the maximum in mass in the top one-half. The depth-integrated mass was greatest at site ADCP1; however, the average daily change in mass was the smallest percentage of the total mass (about 19 percent, table 3), whereas the average daily change in mass was the largest percentage of the total mass at site ADCP6 (about 92 percent) and site ADCP3 (about 71 percent), the two most shallow sites. 


Pearson correlation coefficients between depth-integrated computed SSCand horizontal current speeds at subseasonal time scales were positive and significant at sites ADCP1, ADCP5, and ADCP6 (table 4). The Pearson correlation coefficients between depth-integrated computed SSC and air temperature at subseasonal time scales were negative at the two deepest sites, ADCP1 and ADCP5, and were not significant at the other sites. These correlation results indicate that there was more total suspended mass in the water column at the deepest sites when the air temperature was cooler and more total mass in the water column at three of the five sites when the current speeds were stronger. These relations are consistent with both increased resuspension as current speeds increase and increased sedimentation as air temperatures increase and are consistent with the correlations between vertical profiles of RB and current speeds, and between vertical profiles of RB and air temperatures. 


First posted March 16, 2011