Wood decay rates and macroinvertebrate community structure along contrasting human pressure gradients (Waikato, New Zealand)

Abstract

Mass loss rates of wooden sticks (Betula platyphylla Sukaczev) and macroinvertebrate community composition were measured in the Waikato Region of northern New Zealand at 53 running water sites draining native forest catchments (reference), or characterised by urban, industrial, agroforestry or hydrological stressors. Overall wood substrates retained 80% of initial dry mass over 2–3 month incubation periods, equating to average daily decay coefficients (k_d) of 0.0027 or 0.0002 when adjusted for degree days (k_dd). There were no statistically significant differences in decay rates between stressor types and native forest reference sites. In contrast, macroinvertebrate richness and compositional metrics varied significantly from reference conditions for several stressor types. Wood decay rates were weakly related to macroinvertebrate community composition, while k_dd was weakly but significantly related to % ‘worms’, %EPT* abundance and Quantitative Macroinvertebrate Community Index. The results indicate weak correspondence between macroinvertebrate indicators and wood decay rates, with spatial and temporal factors potentially masking stressor responses. Before wood decay can be widely used as an indicator of lotic ecosystem health there needs to be an improved understanding of environmental factors affecting breakdown rates.

Keywords:

• functional indicator
• organic matter breakdown
• urban stream
• large river
• agriculture
• abstraction
• benthos

Acknowledgements

Roger Young (Cawthron Institute), Zoe Dewson and Alex James (Massey University), and John Quinn and Aslan Wright-Stow (NIWA) provided some of the wood decay and macroinvertebrate data used in this synthesis. Johlene Kelly, Jessica Costell, Manas Chakraborty, Arved Schwendel, Steve Moore and Kerry Costley assisted with field deployment and retrieval of sticks, and macroinvertebrate identifications. Helpful comments on a draft manuscript were kindly provided by John Quinn, Alex James, Roger Young, and two anonymous referees.