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Early in the summer of 2018, algae seemed to proliferate in the Trinity River below Lewison Dam more than anyone could remember. Limited local scientific knowledge of algae in the Trinity led to this present study to ascertain general patterns of periphyton in the river. The study design is intended to provide an overview of spatial and temporal patterns of growth, and to document periphyton community composition. Thus, the investigation includes two elements: (1) photographic monitoring (photomonitoring) of periphyton growth (a score of abundance) on approximately 2-month intervals and (2) sampling of biodiversity (taxonomic richness) in winter and summer. Monitoring occurs at multiple mainstem sites and within major tributaries along the 64 km restoration focal reach for the TRRP, plus several locations above Trinity Lake. Multiple years of monitoring will be necessary to understand temporal patterns of periphyton growth in relation to varying river flows as disturbance events. This report covers only the first year of monitoring, yet is sufficient to establish baseline knowledge of periphyton presence, diversity, and to test two hypotheses: one on the prevalence of the diatom Didymosphenia geminata and another on the prevalence of cyanobacteria in general.

 

Photomonitoring results showed that periphyton cover scores within the mainstem river were generally high immediately below Lewiston Dam and that some sites had the greatest cover scores in the cooler months. Tributaries were spatially variable in cover scores and timing of peak periphyton cover score. A possible correspondence to tributary geology is noted for future study.

 

Biodiversity samples identified a total of 35 taxa across sites (gamma diversity; 1 bryophyte, 12 filamentous algae, 15 diatoms, and 5 cyanobacteria) and an additional 72 morphotypes. Cladophora was observed at many sites but was lacking immediately below Lewiston Dam where Spirogyra accounts for most of the filamentous algae. Didymosphenia geminata was found at up to 10 sites, primarily within the mainstem Trinity River. Cyanobacteria were also widespread, but generally noted as low in abundance.