Vertical Velocities


The analysis of the vertical velocities was limited to the top one-half of the water column (top one-third at site ADCP1) in order to avoid any influence of local bottom topography. This was done by vertically averaging the velocities over appropriate upper bins. 


Seasonally averaged vertical velocities were negative (indicating sinking) throughout the day at sites ADCP5, ADCP6, and ADCP7. Only site ADCP1 included a sustained period of positive (rising) velocity from about 12:00 to 24:00 h. Site ADCP3 displayed no clear trend (fig. 6). The diel cycles were most pronounced at sites ADCP1 and ADCP5 where the daily minimum vertical velocity was negative and occurred nearly simultaneously with the maximum surface current speed (site ADCP5), or 2–3 hours later (site ADCP1). Daily minimum vertical velocities occurred several hours prior to minimum ΔT°. At site ADCP5, the daily maximum vertical velocity was still negative, indicating sinking, and occurred just prior to minimum surface current speed and the maximum ΔT°. At site ADCP1, the daily maximum vertical velocity was positive, indicating rising, and followed minimum surface current speed and was coincident with the maximum ΔT°. At both sites, the maximum upward (site ADCP1) or minimum downward (site ADCP5) vertical velocity was coincident with the near-surface maximum in RB (fig. 3). Similar to diel patterns of RB, the diel patterns of vertical velocity also were out of phase with the theoretical model for light-driven vertical movement of buoyant cyanobacteria colonies.


Pearson correlation coefficients between filtered vertical velocities and horizontal current speeds (at subseasonal time scales) in the top one-half of the water column were significant and negative at sites ADCP1 and ADCP5 (table 1). These negative correlations were consistent with weaker currents resulting in disentrainment of positively buoyant particles, and with the correlation between RB and horizontal current speeds at site ADCP1 that showed more accumulation of mass near the surface with weaker currents. The correlation between near-surface vertical velocity and horizontal current speeds also was significant at site ADCP7, but the correlation was positive, suggesting that sinking particles were disentrained as horizontal current speeds decreased. 


The correlations between near-surface vertical velocities and air temperature generally were small with the exception of those at site ADCP1, where the correlation was positive (table 1). The sign of the correlation indicates that, when mixing was suppressed by high surface temperature, positively buoyant particles were preferentially disentrained. This is consistent with the results of the correlation of RB with air temperature (fig. 5), which indicated the potential for accumulation of mass near the surface at high air temperatures. Thus, at least at the deepest site (ADCP1), a tendency for mass to accumulate near surface with both weak horizontal currents and high temperatures was supported by the behavior of both RB and vertical velocities.


First posted March 16, 2011