RELATIONSHIPS BETWEEN BENTHIC BIOTA AND HYDROLOGICAL INDEXES IN NEW-ZEALAND STREAMS

1. The objective of this study was to identify the most ecologically r elevant hydrological indices for characterizing hydrological regimes i n Neu Zealand streams. To do this we related measures of periphyton ch lorophyll a, ash-free dry mass (AFDM), species richness, and diversity and invertebrate density, species richness and diversity, to thirty-f our hydrological variables derived from daily flow records at eighty-t hree sites. The hydrological variables included some describing averag e flow conditions, now variability, floods, and low-flow characteristi cs. 2. A principal components analysis showed that the interrelationsh ip between many of the hydrological variables was high, and most varia bles correlated significantly with Principal Component 1 (PC1). The fl ood frequency variables formed a distinct component of the flow regime and were the main contributor to PC2. 3. We found that both the avera ge flow conditions and some measure of variability were significantly related to most of the biological variables, and these individual hydr ological variables were more strongly correlated to the biological mea sures than the composite principal components. Only four of the thirty -four flow variables were significantly correlated (P < 0.05) with mea sures of periphyton biomass (chlorophyll a and AFDM), whereas twenty-f our variables were correlated with periphyton diversity. Conversely, t hirty-one of the thirty-four now variables were correlated with total invertebrate density, whereas only four variables correlated with dive rsity. 4. We selected the flood frequency (FRE3), where a flood is def ined as flows higher than three times the median flow, as the most eco logical 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 cont rol of the biota which was commensurate with current stream ecosystem theory. Periphyton biomass decreased with increasing FRE3, whereas inv ertebrate density had an increasing/curvilinear relationship with FRE3 . Periphyton species richness and diversity decreased with increasing FRE3.