Relationships between benthic biota and hydrological indices in New Zealand streams

Abstract

1. The objective of this study was to identify the most ecologically relevant hydrological indices for characterizing hydrological regimes in New Zealand streams. To do this we related measures of periphyton chlorophyll a, ash‐free dry mass (AFDM), species richness, and diversity and invertebrate density, species richness and diversity, to thirty‐four hydrological variables derived from daily flow records at eighty‐three sites. The hydrological variables included some describing average flow conditions, flow variability, floods, and low‐flow characteristics.2. A principal components analysis showed that the interrelationship between many of the hydrological variables was high, and most variables correlated significantly with Principal Component 1 (PC1). The flood frequency variables formed a distinct component of the flow regime and were the main contributor to PC2.3. We found that both the average flow conditions and some measure of variability were significantly related to most of the biological variables, and these individual hydrological variables were more strongly correlated to the biological measures than the composite principal components. Only four of the thirty‐four flow variables were significantly correlated (P < 0.05) with measures of periphyton biomass (chlorophyll a and AFDM), whereas twenty‐four variables were correlated with periphyton diversity. Conversely, thirty‐one of the thirty‐four flow variables were correlated with total invertebrate density, whereas only four variables correlated with diversity.4. We selected the flood frequency (FRE3), where a flood is defined as flows higher than three times the median flow, as the most ecological useful overall flow variable in New Zealand streams because it explained a significant amount of the variance in four out of the six main benthic community measures, and it had a clear mechanism of control of the biota which was commensurate with current stream ecosystem theory. Periphyton biomass decreased with increasing FRE3, whereas invertebrate density had an increasing/curvilinear relationship with FRE3. Periphyton species richness and diversity decreased with increasing FRE3.