Factors affecting Secchi readings

• Factors we cannot control– Secchi values vary due to

solar angle and earth’s tilt over the summer

– Waves – sometimes we have to make due with mother nature.

– Surface reflections off the water

– Observer vision characteristics

• Factors we can control– Time of day– Monitoring day conditions

(monitor on sunny calm days)

– Follow protocols– Increase number of

readings (duplicate or triplicate readings)

Factors we cannot control

• Time of day (10am-4pm): – Secchi values vary by about 6% due to solar altitude

changes in mid summer (over May-Sept. this increases to 15%).

• Waves – sometimes we have to make due with mother nature. 5” waves can decrease reading by 10%.

• Surface reflections off the water. • Observer vision characteristics.

Ways to minimize variations• Time of day (10am-4pm) – If readings taken outside this

6-hour time frame, we increase the variability.• Waves – Monitor on calm days.• Surface reflections – use shady side of the boat.• Clearness of the sky – Monitor on sunny to partially

sunny days.

• Remove your sunglasses.• Position your body as close to the water as is safe.

• Observer vision characteristics – make sure to match the observer’s name with his/her readings

Ways to minimize variations (cont)• Following descending/ascending (2 cloth pins)

protocols. – Variability in readings is less when averages are taken

as compared to just taking a descending or ascending readings. (Worst case was 1-meter between either ascending or descending reading and average of the two. Luckily, only 4.5% of samples were more than 0.5 meters different than the average).

– Secchi depth can change up to 17% of the max Secchi depth if averages are not used. The % of difference can be important for assessment purposes and will be more important for lakes with low Secchi depths.

• Feel free to repeat measurements – We gain accuracy with repetition.

Lake Observation notes

• Near Shore Observations– Lake level

• Deep hole Observations– Cloudy or murky (disc 1 foot below surface)– Water color (disc 1 foot below surface)– Perception of the water quality

• Any time– Weather and other observations