Macroinvertebrate Abundance and Biomass: 2007 Data, BPA-51; Preliminary Report, February 10, 2009..

Holderman, Charles

United States. Bonneville Power Administration.

关键词: Biomass;    Abundance;    Symmetry;    Statistics;    Asymmetry;   

DOI  :  10.2172/962140
RP-ID  :  P110274
RP-ID  :  39133
RP-ID  :  962140

美国|英语

来源: UNT Digital Library

【 摘 要 】

Four Excel files containing information on the 2007 macroinvertebrate data were initially provided to Statistical Consulting Services (SCS) by EcoAnalysts on 1/27/2009. These data files contained information on abundance and biomass data at the level of taxonomic groups. The data were subsequently reformatted and compiled, and aggregated for analysis by SCS. All descriptions and analyses below relate to this compiled data. Computations were carried out separately for each site over all sample periods. Basic summary information for both the abundance and biomass data is presented in Print Out No.2. The 14 sites varied widely in their minimum, mean, maximum and variance values. The number of observations ranged from 10 to 18. Some large abundance values (abundance > 40,000) were noted for sites KR6 and KR13. A more detailed summary of each site is given in Print Out No.3. Site KR3, for example, had a mean abundance of 6914 with a sample size of 17. The variance was 4591991 and the standard error of the mean was 1643. The skewness value, a measure of symmetry for the frequency distribution, was moderately large at 1.29 indicating an asymmetric distribution. Biomass for KR3 had a mean value of 0.87 g/m{sup 2} with 17 observations. The variance was 0.8872 and the standard error was 0.228 g/m{sup 2}. Skewness for biomass was also high at 1.29. Further examination of the quantiles and frequency plots for abundance and biomass also indicate considerable skewness. The stem and leaf diagram (frequency plot) for abundance in KR3 shows most of the data centered on smaller values with a few very large counts. The distribution for biomass has a similar pattern. Statistical tests for normality are significant for both response variables in KR3, thus, the hypothesis that the data originates from a symmetric normal distribution is rejected. Because sample size estimation and statistical inference assume normally distributed data, a transformation of the data is required prior to further analysis. As was the case for previous years, the natural logarithm was chosen as a transformation to mitigate distributional skewness. Abundance and biomass for the remaining sites were also notably skewed, therefore, these data were also log transformed prior to analysis. Summary information for the transformed data (referred to as L-abun and L-bio for abundance and biomass, respectively) are given in Print Out No.4. For site KR3, the logarithmic transformation reduced skewness value for biomass to -0.66. The distributions of abundance and biomass in the other sites also generally showed improvement as well. Hence, all subsequent statistical analyses reported here will be based on the log transformed data.