Vessel-based Observations

Sampling in the field All values from instrumented buoys need to be calibrated and validated on a regular basis. By making weekly measurements with carefully maintained instruments we can correct the values from the buoy for any drift associated with changes in the moored instruments' calibration. A group from Dalhousie goes out weekly with the science team from BIO aboard the DFO research vessel R/V Sigma T. We deploy instruments to measure various water-column parameters and collect water for further analysis in the lab in order to estimate chlorophyll concentration and light absorption by particulate and dissolved matter. One of the instruments that we deploy is a Satlantic HyperPro profiler which measures vertical profiles of conductivity, temperature, pressure, downwelling irradiance, and upwelling radiance. The profiler also has a WET Labs ECO Puck triplet meter measuring chlorophyll and CDOM fluorescence and backscattering by green light. We also deploy a custom-built bio-optical package consisting of:

We hope to add a Satlantic SUNA nitrate sensor to the mix soon.

BBOMB Field Measurements
This figure shows the evolution of water temperature over time and depth. Seasonal changes are evident in the first 40 meters. At greater depth water temperature is nearly constant all year round. This same pattern is evident in water temperature climatology.
Phytoplankton are phototrophs, which means that they absorb light and turn it into chemical energy. Not all of the sunlight energy absorbed by a plankton cell is utilized by photosynthesis. Some of the energy is reemitted as photons by a process called fluorescence. The quantity of phytoplankton in the water column can be estimated by measuring fluorescence, although fluorescence intensity is affected by things other than biomass. This figure shows the evolution of fluorescence with time and depth as measured by weekly profiles. The spring bloom, a brief period of exceptionally high phytoplankton concentration that typically occurs sometime in March or April, is evident by high values of fluorescence.
This figure shows the evolution of salinity over time and depth. Notice that salinity near the surface is highly variable compared to salinity at depth. Salinity near the surface is strongly influenced by local precipitation and river runoff. The periods when salinity is exceptionally low on the surface correspond to periods of heavy precipitation.
As sunlight penetrates downward through the water column it is reduced, or attenuated, due to absorption and scattering by the various constituents in the water. The average change in intensity over depth can be represented by a single value called the downwelling diffuse attenuation coefficient (Kd). The unit for Kd is 1/m since it is a measure of loss over distance. This value is often calculated for monochromatic light since different wavelengths of light travel different distances in the water. In this figure we show the attenuation coefficient for 490nm, or blue-green light, computed from weekly profiles. Phytoplankton absorbs blue light with greater efficiency than green light. This is why water looks green when there is algae growing in it. The degree to which the blue light is absorbed, measured by the average attenuation, is related to the quantity of phytoplankton in the water column. The plot shows the average attenuation computed from weekly profiles. Note the indication of relatively high concentration of phytoplankton around the middle of April. This peak signals the annual spring bloom. A relatively minor bloom also occurs every fall.