The days are getting longer, the sun angle higher in the sky and sunlight will penetrate the water rather than deflect off the surface.

When the length of the day and light penetration is long enough, a photo compensation point is reached and algae will grow and reproduce. That is a good thing.

This base of the food chain will feed zooplankton, small aquatic insects, small fish and so forth up to larger fish and birds. The cycle of life will soon have the energy necessary to renew again. But sometimes things go awry.

Algae are technically not flowering plants but instead reproduce by other means. Both algae and plants are the primary producers of food through photosynthesis and have a notable surplus of oxygen needed by other life forms, certainly humans.

Algae are roughly divided into two groups, the primitive forms with a simple form of cell organization called prokaryotes such as the blue-green algae (cyanobacteria) and those with more complex cell organization called eukaryotes. The prokaryotes were probably the first life forms about three billion years ago.

Back in my early days as a biologist specializing in pollution and ecosystems I was tasked to investigate fish die-offs around the state and document causes from our base at the Water Pollution Control Laboratory in Sacramento.

Samples would be taken, analysis done and the system watched for another occurrence. Certain patterns became apparent. One of two situations were common to almost all events, either a toxic spill or high nutrients were the culprit.


The case for toxicity is usually easy to document but high nutrients were a little more complex.

Fish kills were most common in small lakes where fish were planted in the hopes of presenting fishing opportunities. Often, nutrients from lawn and flower over-fertilizing, highway organics such as oil and road kill and erosion ran off into the water column. Algae and aquatic rooted plants became abundant in the nutrient broth, zooplankton and insect life flourished as well as each tropic level of life. Green is good right?

But the large biomass of aquatic life steadily increased during the summer depending on the largess of nutrients. Both the aquatic animals and plant life respired and depended on the oxygen produced in the day. But, when the sunlight creating photosynthesis disappears for the night, respiration doesn't stop.

If the oxygen is not enough to last until the sun is high enough to begin the oxygen making process again, certain low tolerant fish and plants would die and decompose on the bottom.

The decomposing biomass also requires large amounts of oxygen and less would be available to last the next night. If enough plants, animals and decaying material were dependent on the daily production of oxygen, a sudden mass die off could occur, usually just before dawn.

Sometimes, residents would awake to notice fish floating and the smell of decomposing material in what would seem like a sudden event that actually took most of the summer to build.

The example in these small ponds would sometimes occur in larger systems much the same way.

While working on a stream monitoring study near the geysers power plants in the mid-seventies I was asked to join a crew investigating dead fish in Clear Lake.

We concluded that high nutrient loading from leach fields and land runoff created a similar potent low oxygen effect under the summer conditions. But, was that the complete story? Clear Lake also had high levels of blue-green algae at that time. What is known today about blue-greens adds a little to the story. Next up: Blue-green algae.

Jim Steele is a retired Cal Fish and Game scientist, registered professional forester, part-time consultant and full-time Lake County resident-volunteer.