Primary Research Paper Recovery of the macroinvertebrate community below a wastewater treatment plant input in a Mediterranean stream Jesu´s D. Ortiz*, Euge`nia Martı´ & M. A ` ngels Puig Center of Advanced Studies of Blanes (CEAB-CSIC)Camı´ dÕacce ´s a la cala St. Francesc, 14, 17300 Blanes, Girona, Spain (*Author for correspondence: Tel.: 34-972-336-101, Fax: 34-972-337-806, E-mail: jortiz@ceab.csic.es) Received 20 June 2004; in revised form 4 February 2005; accepted 12 March 2005 Key words: biotic integrity, diversity, functional feeding groups, Mediterranean stream, nutrient enrichment, sewage Abstract We sampled chlorophyll a, benthic organic matter, and benthic macroinvertebrates in June 2001 in La Tordera stream (Catalonia, NE Spain), receiving a wastewater treatment plant (WWTP) input. Samples were collected in six equidistant transects in three reaches located upstream (UP), few m below (DW1), and 500 m below the WWTP input (DW2). Our first objective was to assess the effects of the point source on the structure and functional organization of the benthic macroinvertebrate community. Our second objective was to determine if the self-purifying capacity of the stream implied differences between the communities of the DW1 and the DW2 reaches. The WWTP input highly increased discharge, nutrient concentrations, and conductivity and decreased dissolved oxygen. At the DW1 and the DW2 reaches, taxa richness, EPT taxa (Ephemeroptera, Plecoptera, and Trichoptera), and Shannon diversity decreased and gatherer relative density increased relative to the UP reach. At the UP reach, CPOM and FPOM standing crops were similar, whereas at the DW1 and the DW2 reaches CPOM was two times higher than FPOM. Detailed analysis showed that major changes in the benthic community occurred abruptly between 80 and 90 m downstream of the point source (middle of the DW1 reach). At this location, chlorophyll a concentration, density of macroinvertebrates, taxa richness, and scraper relative density increased, whereas gatherer rel- ative percentage decreased. The macroinvertebrate community at the DW2 reach was comparable to that at the second middle of the DW1 reach (DW1B). The macroinvertebrate community at the DW1B and the DW2 reaches were quite similar to that at the UP reach, indicating that the recovery capacity of the stream from nutrient enrichment was high. Introduction Domestic sewage effluents represent one of the most common causes of degradation of water quality in stream ecosystems (Paul & Meyer, 2001). The effects of point sources attain special relevance in Mediterranean ecosystems where water is scarce. The actual Framework Directive of the European Community highlighted the need of considering biologic quality to provide information for the efficient and effective design of future monitoring programs (Council of the European Communities, 2000). Karr & Dudley (1981) emphasized the need of considering biotic integrity in assessments of aquatic ecosystems, including taxa composition, diversity, and functional organization of living organisms. Using their approach will provide a broader understanding of the processes going on in altered streams. However, it is not clear what patterns should be expected below wastewater treatment plant (WWTP) effluents. Previous studies reported changes in taxa composition that implied a decrease in taxa richness and an increase in dominance because sensitive taxa were eliminated and resistant taxa were enhanced Hydrobiologia (2005) 545:289–302 Ó Springer 2005 DOI 10.1007/s10750-005-3646-z