Diatom species traits vary in response to inorganic versus organic acidity in Adirondack streams

Background/Question/Methods

Streams in the Adirondack region of New York are acidified by both inorganic acid deposition and organic acidity in the form of dissolved organic carbon (DOC), but the impacts of these two types of acidity on biological communities are not well understood. Acid deposition mobilizes aluminum in streams to toxic levels, but DOC mitigates this process by forming biologically unavailable complexes with aluminum. The ratio of inorganic to organic aluminum (Alim:Alom) is correlated with the proportion of inorganic to organic acidity in streams, and high ratios of Alim:Alom are considered stressful to biota. Stressful environments are predicted to select for smaller and short-statured species which tend to exhibit higher stress tolerance. We tested the prediction that higher inorganic acidity would associate with a shift in body size organization and guild composition of diatom communities. Diatom cell size and ecological guilds were examined in streams with a broad gradient of Alim:Alom, ranging from 0.09 to 5.7. Ecological guild classifications, based on position in the periphyton mat, included low profile, high profile, and motile species.

Results/Conclusions

Our results showed that average cell size was significantly smaller in streams with preponderance of inorganic acidity (i.e., Alim:Alom >1), which had greater abundance of small, low profile species. Conversely, streams with higher organic acidity (i.e.,  Alim:Alom<1) had greater abundance of large, high profile species, which may be indicative of lower stress or higher nutrient input associated with the transport of DOC into streams. Motile species had low overall abundance and did not differ among stream types, indicating that motility is not beneficial in either acidified habitat. These results support predictions that diatom traits respond to stressful environments and provide evidence that different types of acidity (inorganic versus organic) may have different impacts on community composition. Future research should focus on developing indices to distinguish inorganic from organic acidity, which would be important to acid-sensitive regions, such as the Adirondacks.