by Joanne Bell, Grow Lab Manager, The Aquaponic Source

We had an unfortunate invasion of planktonic algae (floating microscopic plants) in one of our grow lab systems. Unlike other forms of algae that cling to surfaces, this type is free-floating in the water. In lakes and ponds, some planktonic algae is considered beneficial. But too much can deplete oxygen and kill fish, according to Texas A&M’s Agrilife Extension.

Since, planktonic algae feeds on nitrates, it “bloomed” within days making it look as if pea soup were running through our system. A similar bloom in a tabletop aquaponic system appeared to cause the loss of a goldfish, so we didn’t want to take any chances with our tilapia and bass. Not to mention, depleted oxygen levels could alter future test results.

We discussed our options: bleach the system or run a UV sterilizer. Both would eliminate the algae (and the beneficial bacteria). UV sterilization would also deplete boron, manganese and iron. (Howard M. Resh, PhD, Hydroponic Food Production, 2001) But, bleaching the system is very labor-intensive. So, we settled on the lesser of the two evils: adding a UV sterilizer to our aquaponics system.

As expected, the water became crystal clear within 2 weeks, at the same time the plants began to show symptoms of nutrient deficiencies.

We thought you’d like to see the impact of UV sterilization-induced nutrient deficiencies on plants. It was most apparent in our tomato plants.

Tomato plants in system w/ UV sterilizer

UV sterilzation effects aquaponics

(Note growth habit: tall and stringy, discolored and curling leaves.)

uv sterilization effects aquaponics

Upper leaves. (Note: curling and discoloration.)

Tomatoes in system w/o UV sterilizer.

Healthy tomato plant

(Note: healthy green, compact, bushy plant.)

healthy tomato leaves

Lower leaves. (Note: less curling.)