Macrobenthic survey of Navigation Pool No. 8 of the Upper Mississippi River, with special reference to ecological relationships

The macrobenthic fauna of Navigation Pool No. 8 of the Upper Mississippi River was inventoried and correlated with the average physical-chemical conditions encountered in the pool. Forty-one sampling areas, numbered in order of increasing current velocity, were delineated on the basis of characteristic chemical and physical properties during the summer of 1975. A trend toward decreasing eutrophy occurred from area 1 to area 41. Benthos samples were collected twice during the summer in each of the study areas. One hundred forty-four taxa were found in the 616 Ponar-dredge collections. Over half of the collected taxa were insect nymphs and larvae. A total of 90,693 representatives of the Phyla Platyhelminthes, Nematoda, Annelida, Arthropoda, and Mollusca were counted, weighed, and identified. Oligochaetes were by far the most ubiquitous and dominant macroinvertebrates. Greatest oligochaete densities were respectively recorded during Sampling Periods I and II in areas 3 (17,306.10/m2) and 18 (16,609.98/m2) and in areas 1 (10,302.11/m2) and 18 (10,894.18/m2). The qualitative and the quantitative compositions of the benthic communities varied among the 41 study areas. Habitat preferences of particular benthic forms were reflected in the distributional relationships between the macroinvertebrates and the physical-chemical conditions. Benthic production, in terms of the total wet weight/m2 and the mean number of macroinvertebrates/m2 in each area, was generally greater in the more eutrophic areas. The more eutrophic areas supported fewer taxa. These taxa generally consisted of pollution-tolerant organisms, such as oligochaetes and certain chironomids, which were capable of burrowing into the depositional-type substrates. More taxa and greater numbers of gill breathers and filter feeders, such as caddisflies, mayflies, stoneflies, and dipterans, were collected from the less eutrophic areas.