The impact of </Didymosphenia geminata> on the community structures of invertebrates in streams around the Rocky Mountain Biological Lab

Abstract

The long-term relationships between animal and plant species and their relationships to their abiotic environment are closely tied to climate change. Stream macroinvertebrates are excellent indicators of freshwater conditions and respond measurably to environmental changes. Previous studies have observed that the Holarctic diatom Didymosphenia geminata (Didymo) exhibits nuisance growth patterns (putting out polysaccharide stalks, blanketing the bottoms of streams in thick algal mats), and proliferations vary with stream flow and nutrient composition (especially present in low-flow, low-phosphorus streams). As climate change accelerates weather extremes intensify and we see more low-flow years that exacerbate D. geminata blooms. D. geminata may affect the mobility and food acquisition of freshwater invertebrate indicator species. For 45 years a study of the streams around Rocky Mountain Biological Lab has recorded macroinvertebrate community composition as well as a number of abiotic factors, and more recently, D. geminata growth. This study looks at the relationships between macroinvertebrate community composition, specific families and D. geminata blooms during the 2020 season, and community composition and D. geminata data collected from summers 2015-2020 to explore relevant relationships. Questions addressed in this study include: 1) How does D. geminata affect community composition (focusing on community shifts); 2) What is the relationship between D. geminata growth and Chironomidae abundance; 3) What is the relationship between D. geminata growth and Heptageniidae abundance; and 4) What is the relationship between D. geminata growth and Baetidae abundance. Results show that proliferation of D. geminata causes significant shifts in the community composition of stream invertebrates; invertebrates in the family Chironomidae (Diptera) benefit from the Didymo mats as a refuge habitat within which they can forage on epiphytic diatoms and detritus; while Heptageniidae (Ephemeroptera), which require hard substrates to forage on diatoms in stream beds, are inhibited by it; free-swimming families like Baetidae (Ephemeroptera) are unaffected by the presence of these blooms. The impact of this diatom may reach across its native region and also into zones where it is considered invasive. D. geminata may be classified as an ecosystem engineer due to the physical alteration and invertebrate community shifts associated with it. These changes introduce ripple effects on other organisms like trout, which experience lower growth rates in streams dominated by low quality food (Chironomidae). As climate change accelerates and flow variability increases the drastic impacts of this nuisance diatom will in turn accelerate. Consequently, understanding the effects of this organism is key to understanding the impact of climate change on streams across the Holarctic zone.

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