Measuring light attenuation
Rationale
Light attenuates logarithmically (following Beer-Lambert law) from surface towards bottom due to absorption and scattering by optically active substances in water and water itself.
Light is measured in various units due to its wave-particle duality and addition of subjective units that consider human eye sensitivity. From a primary productivity point of view, units measuring number of photons are the most suitable. LI-COR sensors measure Photosynthetic Photon Flux Density (PPFD), which is number of photons in the PAR range incident per unit surface area per unit time (e.g., µmol/s/m2). HOBO sensors measure illuminance (lx), which is a subjective unit of light intensity, and on a wider spectrum than PAR. Secchi depth is a proxy for water transparency.
Light attenuation is measured at daytime. To get vertical light attenuation profiles, anchoring the boat is recommended to avoid horizontal drift away from the measurement device. However, it is important to avoid ropes getting tangled under the boat.
Secchi disc
Secchi disc is used to measure Secchi depth, which is a visual measure of water transparency.
Secchi depth is measured preferably at noon from the shaded side of the boat (to minimize reflection from water surface). Secchi discs are commonly 20–30 cm in diameter and they can be either white or black and white. At TZS, they are found in the wetlab of the boatshed. The ideal Secchi depth precision is 10 cm in less transparent and 20–50 cm in more clear waters.
Common practice:
- Lower the disc until it is no more visible and continue lowering by a meter or so
- Lift it up until it is visible again -> write down this depth
- Repeat 1–2 times and use the average of the depths as the final Secchi depth
Another way of measuring, following the HELCOM standard:
- Lower the disc until it is no more visible -> write down this depth
- Lower it by 0,5 m and lift up until it appears as a greenish-blueish (this might not be the case for other than pelagial measurements) spot -> write down this depth
- Repeat these two steps at least once and use the average of these depths as the final Secchi depth
In addition to the Secchi disc, one might consider having
- Extra weights to attach to the bottom of the disc in waters with currents, e.g., rivers. Some other white heavy items, such as canisters filled with sand, can also be used, but in that case the correlation between Secchi depths measured with Secchi disc and another white item must be tested in waters with weaker currents before the actual use.
- Polarized glasses or a bathyscope to reduce reflection from water surface
Light profile with LI-COR sensors
The LI-COR set includes a surface sensor, a rope and a frame with two underwater sensors, a spherical one that measures light from every angle, and a vertical light sensor (with the red protection cap) that measures only downwelling light.
At TZS, LI-COR can be found in the CTD-room in the basement. The rope and underwater sensors are in a black bucket; the surface sensor and the handheld device are in the cabinet in the corner of the room. No other equipment (except for note-taking) is needed.
Before use, create a new configuration for the project following the instructions of the manual (to measure relative light levels at different depths only with the vertical & surface sensors, the configuration JANINA can be used).
At the field:
Remove the red protection cap from the vertical underwater sensor, attach the wires to corresponding ports (numbered 1, 2, 3) and press power on. Choose the configuration you want to use, and the device is ready to measure.
Place the surface sensor to a horizontal surface under unobstructed sunlight and start lowering the rope from the sunlit side of the boat. The orange tapes (marked up to 10 meters) mark the depth for the spherical sensor when the lower edge of the tape is at water surface level. Add 15 cm to the number on the orange tape to get the depth of the vertical sensor.
Take readings at desired depths. It is recommended to have one person hold the rope at right level and another person to take notes. The readings wander a lot especially near the surface, and they won’t stabilize. For underwater sensors, try to determine an average of the wandering numbers and write it down. For the surface sensor, use the highest of the wandering numbers if the measurements are done in a rocking boat, because the wandering of the surface sensor readings results from the rocking of the boat and the sensor turning away from the sun, but that doesn’t happen for the water sensors.
Changes in cloudiness between measurements from different depths do not matter, because the underwater readings will be normalized to surface sensor readings in data analysis. However, if the cloudiness changes a lot between writing down the underwater readings and the surface sensor readings from the same depth, the measurement should be repeated so that the readings correspond to each other.
Turn the power off after measuring a profile. Rinse with fresh water (not the handheld device or the surface sensor) after use.
In case of some error, turning the device off and on again the usually helps. Generally, readings from the spherical sensor should be bigger than those from the vertical sensor. If the underwater sensors are in air, they won’t still show similar readings to the surface sensor because of different coefficients for air and water (unless configured to do so).
To make a light profile (e.g., in Excel), divide the underwater sensors’ readings with the surface sensor reading from the same depth to account for the changes in light intensity at the surface during making one light profile. Plot these relative light levels at the x-axis and depths at inverted y-axis, and fit a logarithmic curve to the profile.
Note that in wavy conditions, the near-surface readings are inaccurate, because the thickness of the water column above the sensors change a lot.
Light measurements with HOBO sensor array
In this setup, the HOBO sensors are attached to a moored rope reaching from bottom to surface. At TZS, there are some ready ones (without the loggers) in the boatshed in black boxes on the floor. If making the setup from the beginning, this equipment is needed:
Heavy weight, e.g., a canister filled with sand
Small weights to be attached to the rope below the sensors to keep the loggers at constant depths and to account for water level variations.
Rope, length = a few meters more than the measurement site depth. Ideally, have so much extra that you can pull all the loggers to surface without lifting the bottom weight when cleaning/checking the loggers. However, don’t allow excess rope to lie at the bottom and get tangled. Therefore, less excess rope can be used at shallow sites.
HOBO light loggers (with some kind of logger id marked to them e.g., with tape). Have several of them per rope, because some might stop working (battery dies, water leaks in...). Loggers in this setup were UA-002-64 pendant temperature/light 64K data loggers.
Nut for each logger, outer diameter ~2–3 cm
Cable ties
Buoy - choose the size according to the site. Small buoys are preferable to avoid shading the loggers, but if high waves are anticipated, use bigger buoys.
Contact info tag attached to the buoy
Measuring tape
To avoid biofouling, loggers (except the sensor eye) can be covered with copper tape
Marker (doesn’t need to be waterproof)
Scissors/knife to clip the cable ties shorter and to remove and reattach the weights when adjusting the weights
Before building the setup, start the loggers and set them to start logging at desired time. Attach the weight to the bottom, and the buoy to the other end. Secure every tie with cable ties.
Attaching the loggers to the rope: pull a cable tie through the small handle in the HOBO. Put a nut between the rope and the logger so that both ends of the cable tie go through the nut. Stuff the cable tie through the rope, close and tighten. The nut keeps the logger perpendicular to the rope.
Take the rope to the site at this stage and take the smaller weight, more cable ties, scissors/knife and measuring tape with you.
At the site, test how much weight needs to be added to the rope (below the loggers) to stretch the rope straight but not drag the buoy below surface. Attach the weight(s) with cable ties. Then, put the rope in the water and mark the water line to the buoy. Lift the rope back to the boat and measure exact depths of the loggers with measuring tape starting from the water line mark you just made.
The rope is now ready. When putting it to water (also later when checking/cleaning the sensors), make sure to turn all the HOBOs so that the sensors face up.
During the experiment, clean the sensors to remove biofouling. Test to find the right frequency. (in May 2021, twice a week was more than enough).