Non-Diatom Algae (Table 4) 



Both of the standard composite samples that were collected in September 2003 were 

 dominated by diatoms and contained healthy numbers of non-diatom genera. Cladophora was 

 also a dominant near the mouth, but it was missing from the sample collected at Johnstone's 

 (Table 4). The Cladophora collected near the mouth in September was senescent, as it was here 

 in July and August (see above). Next to diatoms, green algae accounted for the most diversity 

 and the most biomass at both stations. An occasional cyanobacterium was present at both sites. 

 The non-diatom algal assemblages at Johnstone's and near the mouth were very similar. 



Diatoms (Table 5) 



Five of the major diatom species in the Shields River are sensitive to organic pollution 

 (pollution tolerance class 3). One of these {Cymbella excisa) was most abundant at Johnstone's 

 and two {Encyonopsis spp.) were most abundant near the mouth. Two of the sensitive species 

 {Achnanthidiwn minutissimum and Denticula kuetzingii) were about equally abundant at both 

 sites (Table 5). Two of the major diatom species are somewhat tolerant of organic pollution 

 (pollution tolerance class 2). One of these was most abundant at Johnstone's and the other was 

 most abundant near the mouth. None of the major diatom species in the 2003 samples from the 

 Shields River are most tolerant of organic pollution (pollution tolerance class 1). 



Diatom metrics indicate good biological integrity, minor impairment, and full support of 

 aquatic life uses at both sites where composite samples were collected in 2003 (Table 5). The 

 only apparent deviation from excellent biological integrity was a few abnonnal diatom valves at 

 each station and slightly elevated percentages of Achnanfhidium minutissimum, which indicates 

 chemical, physical, or biological disturbance. The stress causing this disturbance and the 

 abnormal diatom valves may be natural in origin. An assessment of minor impairment resulted 

 from a comparison of metrics to criteria for mountain streams and criteria for plains streams. 

 The two sites were virtually identical in terms of diatom floristics and ecological conditions. 



