Detecting Anthropogenic Stress in an Ecosystem: 3. Mesoscale Variability and Biotic Indices

• Published in: Environmental forensics Vol. 6; no. 4; pp. 371 - 384
• Main Authors: Chenery, Anna M., Mudge, Stephen M.
• Format: Journal Article
• Language: English
• Published: Taylor & Francis Group 01.12.2005
• Subjects: barrier; biotic index; macrofaunal communities; PCA; sewage; Portugal, Faro, Ria Formosa
• ISSN: 1527-5922; 1527-5930
• DOI: 10.1080/15275920500351825

Previous work in the Ria Formosa Lagoon has revealed that it is possible to use macrofauna community structure as an indicator of contamination effects. Communities at sites adjacent to sewage outfalls indicated contamination but no pollution gradients were observed. In this study, macrofauna within a sub-section of the earlier work were investigated to determine the extent of mesoscale (10-100 m) variability and new indices of biological community structure. The results were evaluated using univariate (Marine Biotic Index; BI) and multivariate statistical methods (PCA). The computer program Barrier (usually used with genetic data) was also applied to detect "barriers" between spatially referenced samples. PCA identified the most significant factor effecting the distribution of the organisms in the study area to be the mud content of the sediment (% < 63 μm). Communities at sandy sites consisted of suspension feeders and detrital browsers, whilst communities at muddy sites were composed mainly of deposit feeders. The second principal component produced a vector characterized by a contamination gradient. The effects of the sewage discharge appeared to be concentrated at the outfall as it had the highest TOC (10.4%) content and was the only site to be characterized by the BI as slightly polluted. With the use of the BI and Barrier two gradients were observed in the transition of sites from an unpolluted to a polluted state. These gradients appeared not to follow the flow of the channel as expected (high % TOC of sites along the channel), and instead extended west and south from the sewage source. In conclusion, weak gradients were observeable; the multiple approaches of BI, PCA and Barrier analyses in concert were able to confirm the sewage dispersion route through the area.