Samples from all sites except The Trees were dominated by the free-floating and mat- 

 forming filamentous green alga Rhizoclonium. This alga has been reported to cause problems in 

 standing and slowly flowing waters across the western United States (Wehr and Sheath 2003). 

 Algae interfere with water uses — e.g., fishing, swimming, boating, and irrigation — only when 

 standing crops are excessive. Mat-forming filamentous algae are normal components of many 

 aquatic ecosystems, including prairie streams, and there is no evidence from this study that 

 standing crops o^ Rhizoclonium in Big Dry Creek are excessive. Also, criteria have not been 

 established for determining when algal growth in prairie streams is excessive. 



fristead o^ Rhizoclonium, Tribonema dominated the sample collected at The Trees. 

 Tribonema is a filamentous chrysophyte that prefers cool waters. Also present and frequent at 

 The Trees was Cladophora, a filamentous green alga that prefers cooler temperatures than 

 Rhizoclonium. Besides Tribonema, three genera of algae were found exclusively at The Trees: 

 Lyngbya. Vaucheria and Spirogyra. Lyngbya is a nitrogen-fixing cyanophyte and its presence 

 here may indicate nitrogen-deficient waters. Vaucheria, a filamentous chrysophyte, requires 

 steady flows of cool water. It is common in springs and spring brooks. Spirogyra, or "pond 

 scum", is a free-floating filamentous green alga that is common in quiet backwaters. The Trees 

 site supported the most diverse and the most unique assemblage of non-diatom algae. 



Samples from near the sewage plant and at Highway 200 contained the fewest number of 

 nondiatom algae (3). This may be a sign of stress, perhaps resulting from wastewater discharges 

 and/or non-point sources of pollution. The absence of cyanobacteria (aka blue-green algae) at 

 Highway 200 and their rarity downstream may be in response to nitrogen loading. Genera of 

 cyanobacteria that have heterocysts, including Calothrix, Lyngbya, and Nodularia, are capable of 

 fixing atmospheric nitrogen and have a competitive advantage when dissolved nitrogen is in 

 short supply. Heterocysts are thick-walled cells specialized for fixing nitrogen. Genera of 

 cyanobacteria that lack heterocysts (e.g., Oscillatoria) are capable of fixing atmospheric nitrogen 

 under anaerobic conditions. 



^1 Diatoms (Table 4) 



