. The Canadian field-naturalist. Natural history; Sciences naturelles. 2005 Catling: Dragonfly Density at a Bioindicator 235 24 20 - 18 - 12 - 8 - 4 - BIOLOGICAL OXYGEN DEMAND 40 - 30 - 20 H 10 - _J 0 H E 10 - —T— 3,4 0 - SUSPENDED SOLIDS 8 - 7 - 6 - 5 - 4 - TOTAL NITROGEN 5»6 !,2 5 - 4 - —T— 3,4 —~r~ 5,6 3 - 2 - 0 1,2 —I— 5,6 TOTAL PHOSPHOROUS. 3,4 5,6 1,2 3,4 Treatment ponds (in sequence) —T— 5,6 Figure 1. Mean measurements of chemical parameters at Embrun, eastern Ontario, based on sampling of the six ponds on three separate dates (22 October 2001, 26 February 2002, 15 October 2002). differ
. The Canadian field-naturalist. Natural history; Sciences naturelles. 2005 Catling: Dragonfly Density at a Bioindicator 235 24 20 - 18 - 12 - 8 - 4 - BIOLOGICAL OXYGEN DEMAND 40 - 30 - 20 H 10 - _J 0 H E 10 - —T— 3,4 0 - SUSPENDED SOLIDS 8 - 7 - 6 - 5 - 4 - TOTAL NITROGEN 5»6 !,2 5 - 4 - —T— 3,4 —~r~ 5,6 3 - 2 - 0 1,2 —I— 5,6 TOTAL PHOSPHOROUS. 3,4 5,6 1,2 3,4 Treatment ponds (in sequence) —T— 5,6 Figure 1. Mean measurements of chemical parameters at Embrun, eastern Ontario, based on sampling of the six ponds on three separate dates (22 October 2001, 26 February 2002, 15 October 2002). different species, but average chemical conditions asso- ciated with dragonfly populations can be estimated from Figures 1 and 2. The first lagoons were poorer in species diversity and numbers in the spring than in the fall possibly due to decreased rate of decomposition and reduced oxygen during the winter months resulting in extreme condi- tions that eliminated much of the fauna in the initial ponds. Nymphs in early spring may provide the best indication of water quality due to direct association with water following a period of stress. However, adults ap- pear to be able to recognize water quality and some species tend to occur only on the cleaner ponds (Fig- ure 2). Overall the increase in species diversity in the final ponds is twice that of the ponds receiving waste- water. This study suggests that the improving water quality in a series of sewage ponds is reflected by increasing odonate species diversity and increasing numbers of individuals of both aquatic larvae and aerial adults. The use of the indicator species is particularly promis- ing and can be used in conjunction with diversity and numbers. The potential to use dragonflies as a bioindi- cator of the efficiency of sewage ponds requires addi- tional study to confirm this suggestion over a broad area. It has two obvious advantages over chemical tests: (1) It includes reference to a time period (the la
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