VENUS has been operating in Saanich Inlet now for 2 years. Installed in February 2006, a suite of standard oceanographic instruments has been collecting data from a depth of 100 meters. Apart from a few gaps during the first year associated with a number of high voltage power outages, the record clearly shows both annual and inter-annual variations. The first set of images reveals variations in the hydrographic properties of temperature, salinity, density, and dissolved oxygen. These sensors are all co-located on the VENUS Instrument Platform (VIP) at 96 m depth, connected to the VENUS Node in Saanich Inlet. The final image is a composite of 9 daily echo-grams from the bottom mounted 200 kHz inverted echo-sounder, which tracks both fish and plankton populations.
The entire two year record of temperature, salinity, density (in the form of sigmat) and dissolved oxygen, starting in late February 2006 through early February 2008 is shown in Figure 1. The CTD instrument samples every minute, resulting in over 500,000 samples per year. To reduce the data density on these plots, we have plotted both the minimum and maximum values for every 24 hour period. This selection reveals both short and long term variability, while ensuring that extreme values are captured. Key signals include the winter cooling, which at this depth progresses into May, followed by summer warming that extends through to the end of October. Temperatures plateau through November and December, after which they cool again through spring. Salinity values at this location are correlated with temperature. There is a slight inversion in the vertical temperature profile near the bottom, with density stratification supported by a strong salinity gradient. At 96m, the salinity trend over winter (Nov-May) is a gradual freshening, followed by increased salinity during the up-welling season from May through September.
Oxygen, too, is tied to both temperature and salinity, with fresher/cooler water having higher dissolved oxygen concentrations [ml/l] than saltier/warmer water. Deep water renewal events are evident at one month intervals, in agreement with Masson (2002). During these density driven renewal events, salinity, temperature, and oxygen all increase. However, the displacement of anoxic water from deeper in the Saanich Inlet basin upwards passed the VENUS site, forces a longer term trend of decreasing oxygen during the renewal period, which is July through October. These trends and events are further evident in the plots of individual years, Figure 2 (2006) and Figure 3 (2007). Another remarkable signal is the high daily variability in many of the signals over short periods, with near full scale transitions occurring over time scales as short as minutes to hours. A case in point is late May 2006 (Figure 2), when oxygen values varied between hypoxic conditions of 0.2 ml/l to 3 ml/l during several oscillations over a number of days.
A composite of echo-grams from early 2007 to early 2008 is shown in Figure 4. Each panel represents one day of the inverted 200 kHz echo-sounder record, scaled in relative [db], from top right to lower left: Feb 8, 2007, April 6, 2007, June 6, 2007, July 19, 2007, Sept. 9, 2007, Oct. 30, 2007, Dec. 5, 2007, Dec. 31, 2007, and Jan. 31, 2008. This instrument pings once every second, recording over 800 echo-return intensities per profile. It is particularly well suited for revealing zooplankton and fish. Key features in this image include the diurnal migration of the zooplankton, most likely dominated by Euphausia pacifica, from depth to the surface at dusk, retreating at dawn back to the bottom and depths below the VENUS Instrument Platform (VIP). Long nights and short days during the winter give way to short nights and longer days in the summer. During the spring bloom (April, second panel), phytoplankton densities are sufficiently high to enhance the near surface echo-intensities throughout the surface layer. Starting in late January, we see the return of significant zooplankton populations, as well as dense schools of small fish, possibly herring. VENUS is working on summary statistics that will show the trends and seasonal timings of these characteristics.
by Richard Dewey, VENUS, a division of ONC, University of Victoria
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