Improving region-wide ecological condition of wadeable streams: Risk analyses highlight key stressors for policy and management

Abstract Unbiased estimates of the current state of target ecosystems and identification of potential causal factors are key to managing stressors over large scales, and for guiding policy and decision makers to set realistic targets and expectations in light of economic pressures. A probability survey design for 176 target wadeable, perennial streams mapped on developed land in the Waikato Region, New Zealand, sampled over three austral summers (2013 to 2015), was used to i) estimate the extent to which “Poor” stressor and biological condition states occur, ii) determine the co-occurrence likelihood of “Poor” biological and environmental condition, and iii) identify and estimate the relative importance of key environmental stressors. The probability survey design also allowed the quantification of uncertainty around mean estimates of extent and risk. These analyses reveal that between 25 and 50% of mapped target stream length can be considered to be in “Poor” condition based on biological indices derived from macroinvertebrate and fish community data. For assessed stressors, Poor condition was estimated for 10 to 50% of the target stream network depending on the stressor. Poor biological condition was likely to co-occur with Poor stressor condition for 10 of the 12 assessed stressors for macroinvertebrate indices, and 5 of the 12 stressors for the fish index. These analyses identify that management actions targeted at improving instream habitat quality, particularly reducing fine sediment deposition, when applied across the entire stream network are likely to yield the most widespread improvement in biological condition indices. Our findings also highlight the importance of extending policy development beyond a singular focus on water quality if ecosystem health objectives are to be met.