Assessing the ecological integrity of boreal streams: a comparison of functional and structural responses

Functional (leaf litter breakdown) and structural (macroinvertebrate metrics) attributes of nine boreal streams showed predictable changes along a nutrient enrichment gradient. Of the functional metrics studied, the percentage leaf mass remaining after 118 days in coarse mesh bags was best correlated with the nutrient gradient. However, none of the functional metrics performed as well as the macroinvertebrate (structural) metrics studied here. Somewhat disconcerting was the finding that the importance of decomposition in coarse (k(c), values) and fine (k(f) values) mesh bags, and hence interpretation of the degree of impairment, changed with time (during the 118 day study). Our findings indicate that caution should be exercised if ratios of k-values for coarse and fine mesh bags are used to assess the ecological integrity of boreal streams. The poor performance of functional metrics in boreal streams may be attributed to slow rates of decomposition, resulting in the need for relatively long incubation times to increase power to detect change if/when it occurs. This conjecture was supported by the finding that the strongest correlation between leaf litter breakdown in coarse mesh bags and degree of impairment was noted after 118 days of incubation. Long study (incubation) time has, however, serious implications in biomonitoring since the risk of not recovering leaf-litter bags increases with time.