Variation in benthic invertebrate abundance along thermal gradients within headwater streams of a temperate basin in Japan

To better understand the impacts of water temperature variation on stream invertebrates, we sampled benthic assemblages and measured water temperature monthly in headwater streams located in northeast Japan during 2011 and 2012. The sites in headwater streams had similar hydraulic and topographical settings, were free of man-made structures, and ranged in elevation from 100 to 850 m above sea level. The results of month-by-month analysis on the relationship between temperature and Plecoptera density revealed that the abundance declined with highly elevated temperatures in mid-summer (July, August). The strongest relationship between variation in Plecoptera abundance (June to October in 2012), however, and water temperature occurred during early summer (June). Thus, early summer temperatures contribute to greater seasonal increase in abundance, suggesting that snapshot data may be unreliable for detecting consistent relationships between the abundance and temperature. The degree of similarity in benthic fauna between sites was significantly negatively correlated with differences in water temperature but not correlated with geographical distance. These results suggest that faunal composition varies along the water temperature gradient rather than habitat connectivity in temperate headwater streams.