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On December 6, 198 1 an oil spill of 160 barrels (25 440 liters) occurred in a small southeast Texas stream. Water quality changes, other than the presence of oil, were not evident until six months later when water temperature increased and stream flow ceased. This resulted in decreased dissolved oxygen and increased carbon dioxide concentrations. Responses of the benthic macroinvertebrate community included an increase in density of oligochaetes, decrease in numbers and taxa of chironomids, with eventual complete elimination, and low community diversity (d). Decrease in oil concentration resulted in reversal of these responses. No clean water taxa were collected and complete recovery had not occurred 26 months after the spill when the study was terminated.

The sequence of colonization, species diversity (d), biomass, and productivity of macroinvertebrates on artificial substrates was determined in a relatively constant environment freshwater canal . Three substrates were removed weekly during a 16 week test period . Community structure of benthic macroinvertebrates was determined at the onset and end of the test period and compared with substrate community structure .Calopsectra sp . was the dominant early colonizing organism ; Dicrotendipes sp . and Dicrotendipes modestus were also abundant . In later collections other chironomids, ephemeropterans, gastropods, oligochaetes, amphipods, and trichopterans occurred . Trichopterans were generally dominant in numbers and biomass in later collections . A total of 104 taxa were collected on the substrates .Collection diversity (a) began at 3 .42 (week 1), decreased to 2 .72 in the third week, and then continually increased for the remainder of the test period to 4 .43 . Cumulative diversity had a similar trend, decreasing from 3 .42 (week I) to 3 .06 in the fourth week, and then increasing to 4 .05 . Neither collection a or cumulative a reached an asymptote in the 16 week period .Collection biomass began at 0 .0867 g/o .26 m 2 for week one and was 1 .0575 g/0.26 m' at the end of the test period . Biomass increased linearly for seven weeks, fluctuated widely until week 14, then increased sharply for the remainder of the test period . Productivity ranged from -3 .42 g/m~/wk in the eighth week to 5 .10 g/m 2 /wk in the last collection . Biomass and productivity were greatly affected by the presences or absences of a relatively few large organisms .