Zooplankton Acoustic Profiler
The Fraser River freshet is starting (snow pack melt in the coastal mountains), and the suspended sediment loads coming out of the river mouth into the Strait of Georgia are also increasing. This is especially evident at our Delta Dynamics Laboratory site, located just south of the mouth of the Fraser River. In these data plots we can see that at low tide, when the river runs fastest, there is a spike in the near bottom turbidity at our SoG East DDL site, associated with the cascade of suspended sediment, as evidenced in the 200kHz ZAP echo-gram. During this first week of May (2012), the low tides have been occurring late in the day (16:00-17:00 PDT, 23:00-24:00 UTC), and we have seen a steady increase in the near bottom turbidity signal just after low tide. See the Strait of Georgia East DDL plots page
for the latest images.
After years of collecting, presenting, and delivering raw echo-sounder data, we have (finally) got around to confirming the calibration of our echo-sounder systems and can now produce calibrated backscatter volume values [Sv]. Shown here is the ZAP echo-sounder data from April 18, 2012 in Saanich Inlet. The raw counts have now been converted into proper volume backscatter values [dB re 1/m], taking into account system source and receive levels, beam pattern, range and spreading affects, and acoustic pulse settings. For reference, we expect smaller targets like zooplankton to produce values in the -80 to -55 [dB re 1/m] range, depending on the volume density of scatterers, while small fish will produce values in the -65 to -50 [dB re 1/m] range, also depending on target density and where in the beam they are. Calibrated Sv values should be available for download shortly.
The VENUS observatory measures the tide and large surface waves with our pressure sensors, but internal waves also exist and can sometimes been seen in the inverted echo-sounder data. Here is a one hour record of the 200 kHz ZAP inverted echo-sounder data from the Delta Dynamics Laboratory (DDL) on January 11, 2012 (20:00 UTC). The train of solitary internal waves shows up as an undulating layer of scattering material in the upper 20m.
Internal waves are generated when an “internal” density interface is disturbed. The ocean is stratified in density, with dense sea-water always underlying less dense (lighter) sea-water. Sea-water density varies as a function of both temperature and salinity, with colder water typically being more dense than warmer water, and more saline water having a higher density than fresher water. Throughout most of the Salish Sea, salinity dominates as the dictating parameter for determining sea-water density. The influence of the Fraser River and other coastal rivers is to produce a relatively thin (10-30m) layer of relatively fresher sea-water near the surface. The density difference at the boundary between the fresher surface layer and the deeper salty layers is ideal for supporting internal waves. In this image we can also see that the troughs are associated with surface convergence, and tend to accumulate near-surface flotsam and bubbles (elevated scattering material/back-scatter). From above, we can often see the surface signature of these internal wave troughs as long bands of flotsam and concentrated capillary waves (roughness), separated by smooth surface water over the divergent crests of the internal waves.
Dynamic sediments and sediments with active biogeochemistry often generate a variety of chemical compounds, some of which will be gaseous. This inverted echo-sounder image from October 30, 2011 at the Delta Dynamics Laboratory in the Strait of Georgia at 108m water depth near the mouth of the Fraser River has captured numerous clouds of rising bubbles. Such bubbles have several distinct characteristics. First, we can see the uniform rise velocity, suggesting nearly constant bubble size. Bubbles have been detected that dissolve with height (common in unsaturated water conditions, as the bubbles shrink they slow down and the traces arc to the right) or bubbles that grow as a result of reduced hydrostatic pressure (the traces curve upwards). This image coincides with low tide, another condition that is known to encourage gas release as the water pressure is at a minimum. We have also detected bubbles rising from schools of fish. One final observation: the ensonified volume of water is an 8 degree cone expanding upwards from the transducer head. The bubbles may appear in the acoustic back-scatter at any height as they enter the beam, and with the strong tidal currents in the Strait of Georgia, they can easily be advected horizontally both into and out of the beam, thus the random appearance of bubble clouds into and out of the ensonified volume.
The ASL Zooplankton Acoustic Profiler (ZAP) located at the base of the slope of the Fraser River Delta (DDL platform) recorded this hourly image of echo-sounder backscatter between 4:00 and 5:00 UTC September 5, 2011 (21:00 – 22:00 PDT September 4, 2011, just after dusk and at rising tide). The image reveals a dense school of large fish between 10 and 20m depth, and many individual fish between 20 and 80m depth.
Although we cannot know for sure what the species are, the Pacific Salmon Commission (PSC)’s troll test-fishing program indicated the presence in the vicinity for that week of both sockeye (Oncorhynchus nerka) and pink (Oncorhynchus gorbuscha) salmon with a very high proportion of pink salmon. Fish were gathering in the Strait and preparing to migrate up the Fraser River over these days. The PSC Mission Hydroacoustic Station recorded extremely high runs a few days later, with approximately 600,000 and 800,000 salmon passing the monitoring site on Sept 7 and 8, respectively, up from 86,000 on August 31 (personal communications with Yunbo Xie of PSC).
During rising and high tide, the sea pushes back against the river, reducing its flow at the mouth. As the tide falls, approaching low tide, the flow at the river mouth increases, and the sediment laden river water is “released/floods” into the Strait of Georgia. The river water, especially during periods of faster moving flow, is laden with silt and fine sands. As the flow spreads out into SoG, slows, and even mixes with the salt water, the larger/heavier sediments are released and sink, mostly likely by flocculation and differential settling. Shown here are data from three different instruments mounted on the Delta Dynamics Laboratory (DDL), capturing the cascade of sediment laden water as it sinks through the water column. The top panel is the 200 kHz echo-sounder record from the ZAP (S/N 1009). The second panel is the vertical velocity as measured by the co-located 300 kHz ADCP, capturing the downward movement of water and sediment (blue region). The bottom panel is the pressure record from the co-located CTD, showing that the sediment waterfall occurs during the low tide. This signal has been occurring for the last few weeks, and may continue throughout the spring freshet. Sediment accumulation during the freshet can be as much as 1 m in locations, and after tidal resuspension and redistribution throughout the year, has an average of 10-20 cm over many square kilometers.
Download High-Resolution Image (PDF): Sediment fall at the mouth of the Fraser River
The VENUS Zooplankton Acoustic Profiler (ZAP) instrumentation records an echo-profile every 2 seconds. The resulting series of profiles can be sorted by each day into a data cube (left), with hour of the day along the x-axis (centered at mid-night), day of the year in the y-axis (into the page), and vertical range above the bottom in the z-axis. This data cube can then be sliced in various planes to reveal daily and seasonal patterns. On the right is a one hour section of the data expanded to full resolution.
Reference: Borstad, G., L. Brown, M. Sato, D. Lemon, R. Kerr, and P. Willis. 2010. Long zooplankton time series with high temporal and spatial resolution. OCEANS 2010, pp.9. DOI:10.1109/OCEANS.2010.5664585.
The Zooplankton Acoustic Profiler (ZAP) data from Saanich Inlet reveals the diel migration of zooplankton, many individual fish, and even strange acoustic backscatter layers.
See the latest data from this instrument here.
Seasonal variations including the length of the day, the intensity of the sunlight, supply of nutrients, and the phytoplankton concentration result in dramatic variations in the zooplankton concentrations and patterns throughout the year.