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