Correlations between hydrology, benthic macroinvertebrate, and diatom metrics for Central Texas streams

Background/Question/Methods

Monitoring biological communities has been shown to be an effective tool in assessing water quality degradation in streams. Benthic macroinvertebrate communities represent a month-to-year temporal scale and a broad range of trophic levels in the middle of the stream food web.  Diatoms, a benthic algal community, represent a days-to-months temporal scale and are at the base of the aquatic food web.  Changes in flow can alter the structure and function of these communities outside of other anthropogenic influences that they are intended to measure.  The specific components of hydrology that influence the composition of benthic macroinvertebrate and diatom communities were examined in Central Texas by comparing hydrologic metrics to benthic macroinvertebrate and diatom metrics from 1994 to 2009, using Kendall-tau correlations.  Flow data from US Geologic Survey gauges on 14 creeks in Austin, TX, were used to calculate 48 hydrologic metrics, grouped by water year and then by multi-year periods that had relatively similar impervious cover or development levels.

Results/Conclusions

The number of significant correlations between diatom and hydrology metrics was increased substantially when the hydrology data was composited into homogeneous development periods.  This was not the case with the macroinvertebrate metrics, suggesting that the conditional hydrology was more influential for diatoms than macroinvertebrates.  However, rate of change in flow was negatively correlated to abundance of intolerant organisms (p<0.05, r>0.3) and the number of intolerant taxa in the macroinvertebrate community (p<0.05, r>0.3).  Among the diatom metrics, the pollution tolerance index was correlated to the most hydrologic metrics including the average length of high flow pulses, mean flow rate, and the fraction of total flow that is base flow.  Results suggest that larger changes in the flow rate are related to lower abundance and diversity of intolerant organisms in Austin creeks.  Other measures of the benthic macroinvertebrate community were not greatly influenced, while the diatom measures indicated poorer quality of biological health.  This information should be considered with respect to development density and protection of natural flow regimes through best management practices.  Future work could identify thresholds in specific hydrology metrics at which ecosystem health would be preserved.