Algae are an important component of a natural river ecosystem.  They photosynthesize, converting light into sugar for usable biological energy that forms the foundation of most foodwebs.  Cold, oligotrophic (nutrient-poor) rivers such as the Trinity are generally not perceived as having a lot of algae.  Back in 2018 a bloom of algae in the ‘restoration reach’ of the Trinity (roughly Lewiston to Helena) raised concerns; the bloom brought greater visibility of algae than we had seen in the Trinity in recent memory. 

For TRRP scientists, those concerns translated to numerous questions.  But questions that should be overlaid on the large body of literature from countless scientific studies done elsewhere.  That literature shows (repeatedly) a number of fundamental patterns: 

• A broad diversity of algae is natural in cold rivers like the Trinity.
• Young salmonids feed on small invertebrates that come both from the terrestrial environment and from within the river where algae is a primary food source. 
• In the river, algae grow primarily on the river bottom (it is “benthic”) while algae suspended in the water column (“planktonic”) is prevented from becoming thick by the swift outflow of the water. 
• Algae can be scoured from the river bottom during large floods, forming a reset for the benthic environment (what ecologists call a ‘disturbance’). 

Studies by Dr. Mary Power (U.C. Berkeley) and her associates, primarily in the nearby Eel River, had gone further with the studies of disturbance.  They found that while floods scour algae, they also scour some of the larger benthic invertebrates, particularly caddis fly larvae.  Caddis fly larvae consume large amounts of algae and build cases around themselves from wood and sand to prevent fish from eating them. Their studies found that floods are necessary for controlling caddis fly populations so that algae may rebuild, that chironomid larvae (nonbiting midges) then become more abundant, and then young salmon have more food available.

Prior to 2018, TRRP scientists had their hands full with issues higher up the food chain and thus more directly related to salmon production; algae had not been specifically studied within the Trinity restoration reach from an ecological point-of-view.  Questions arose… Was the bloom inappropriate for the Trinity? How much algae ‘ought’ the Trinity have?  Are the types of algae in the Trinity appropriate for producing chironomid larvae and thus fish food?  How does the abundance of algae in the Trinity correspond to natural flows or scheduled dam releases? 

In 2020 I began monitoring algae with a simple method to address these questions while not detracting from the ongoing focus on salmon.  I have been regularly measuring an index of algal abundance based on their visibility in underwater photography.  I have also been sampling the algae in winter and summer to identify the types of algae present.  A report from the first year of sampling is available at https://www.trrp.net/library/document/?id=2549. A second, multi-year report should be completed in 2024.

The algae in our focal reach of the Trinity River can be roughly placed into 3 groups: cyanobacteria (once known as ‘blue-green algae’), filamentous algae, and diatoms. 

Diatoms

Diatoms are amazing single-celled algae…  They form cell walls of silica (glass) with intricate patterns that look almost like spirograph art under a microscope, and many are actively mobile!  Some also are capable of capturing nitrogen, making them a particularly valuable food source for invertebrates, which then become a food source for young salmon.  These diatoms tend to have an orange color, so when aging Cladophora looks orange in the river, it actually is becoming covered with nutritious diatoms.  Diatoms also live directly on rocks, where they can build up to form a slippery surface.  When this “scum” of diatoms is removed from a river rock and placed under a microscope, not only is the beauty of those glass cell walls revealed, but also bazillions (OK – not a scientific term) of Chironomid larvae… fish food! 

While I lack specific data on the effect on invertebrates (and thus fish food), the pattern corresponded well with the studies by Dr. Power as well as observations from an ongoing invertebrate study being done in the Trinity. Indications are that Trinity and Lewiston dams, which prevent floods in winter, are preventing the ecological disturbance that builds algae communities to feed chironomid larvae for an optimal food supply as young salmon hatch and emerge from gravels.  Traditional ROD flows that don’t flood the river until April leaves an overly mature algae community (and invertebrates) in place while salmon fry emerge, then scours away the algae (the base of the food chain) when young salmon populations are peaking.  A shift in flow management to provide those scouring flows in winter should set the stage for more natural algal growth as well as optimal food supplies for emerging young salmon.