Relationships in ecological health between connected stream and estuary ecosystems

Abstract Estuaries are the endpoints of rivers, which shape their physical and biological characteristics, yet few tools are available to link the health of estuaries to their upstream sources. As a first step towards the use of macroinvertebrate indices for setting ecological health thresholds in streams that also protect estuaries, we examined relationships between the ecological health of streams and their receiving estuaries over a wide geographic scale in New Zealand. Eight macroinvertebrate biotic indices known to respond to key physico-chemical stressors (fine sediment, organic enrichment and metals) were used. We explored whether correlation strength between the stream and estuary indices varied with survey time difference (i.e., the number of years the stream survey was conducted prior to the estuary survey) and estuary type at the catchment/estuary scale. Significant positive correlations (rho = 0.30–0.78) were detected between many of the stream and estuary indices responding to fine sediment and (either) organic enrichment. Index correlation strength tended to increase with increasing survey time difference (usually up to at least a decade), indicating a temporal lag in ecological health between the streams and estuaries. Correlation strength also varied across estuary type and there was at least one moderate correlation for each of three types (deep and shallow drowned estuaries, tidal lagoons) representing a variety of geomorphological characteristics. We also used linear mixed models to determine the influence of estuary and stream characteristics, as well as survey time difference, on index relationship strength. The relationships were influenced by estuary site location and volume, and survey time difference. For spatially integrated management purposes, our results are a step towards demonstrating that macroinvertebrate indices could be used to set ecological thresholds for streams and that also consider protection of estuary health from impacts of fine sediment and organic enrichment. Overall, our study highlights the value of using biotic indices to compare the ecological health of connected ecosystems.