Changes in periphyton communities in a watershed receiving passive treatment for acid mine drainage over an 11 year period

Background/Question/Methods

A 70 km² area in the headwaters of Slippery Rock Creek in Western Pennsylvania is impacted by over 70 acid mine drainage (AMD) discharges that represent an average of 30% of the total flow out of the watershed.  The discharges load approximately 1289 kg/day of acidity and 282 kg/day of iron into the watershed.  Fifteen major reclamation projects have been carried out within the drainage involving construction of passive treatment systems that treat approximately 30 discharges totaling 2.8 million m³/y.  In passive treatment, AMD is diverted through limestone and/or organic substrates in an anaerobic environment before entering an aerobic wetland to increase alkalinity and remove metals.  Seven stream sites were sampled on 23 dates from 1996-2007 to monitor water chemistry; 2 in unimpacted reference streams, 4 downstream of treatment systems, and 1 untreated for AMD.  Previous results showed a significant increasing trend in pH and alkalinity at stream sites below treatment during the 11 year period.  Dissolved metals generally decreased at the site downstream from all treatment.  The objective of this aspect of the study was to examine the response of periphyton communities to improvements in water quality, and determine whether sites downstream of AMD treatment have recovered sufficiently to resemble communities at unimpacted reference sites.  Epilithic periphyton was scraped from a defined area on 3 rocks in riffle areas at the 7 stream sites on the same dates as for water chemistry samples.  Scrapings were combined into a single composite sample for each site on each sample date.  Algal taxa were identified and counted to determine community composition.

Results/Conclusions

There was a significant increasing trend in diatom species richness at 2 of the 4 treated sites over the 11 year period, with no increase at reference or untreated sites.  MDS ordination of communities indicated that within fall and spring seasons, community composition at the 2 low-order treated sites became progressively more similar to communities at the low-order reference site over the 11 year period.  This change was associated with a decrease in the acidophilic taxa Brachysira vitrea, Eunotia minor, and Eunotia exigua, and an increase in Achnanthidium minutissimum.  There were no significant temporal trends in community composition at the reference, untreated, and 2 high-order, treated stream sites.  The limited, post-treatment recovery of periphyton communities probably results from continued poor substrate quality (metal precipitates), and insufficient reduction for some dissolved metals, especially during storm events.