Date of Award

1973

Document Type

Thesis

Degree Name

Bachelor of Science (BS)

Department

Geology

First Advisor

Dr. Frank R. Karner

Abstract

Living and recent dead mollusks were sampled by SCUBA in Upper LaSalle Lake, northwestern Minnesota, in 1972 along belt transects paralleling the depth contours and normal to the shoreline. Three aquatic plant associations were found in the lake – dense, finely-branching forms in shallow water; less dense and a greater percentage of broad-leaved plants in intermediate depths; and a sparse density of a finely-branching forms in deep water. Two live species of mussels (in order of decreasing abundance) – Lampsilis radiata luteola (Lamarck, 1819), and Anodonta grandis Say, 1829 – were found at maximum depths of 5 and 6 m, respectively. Fossils of Lampsilis radiate luteola, Anodonta grandis and Anodontoides ferussacianus (Lea, 1834) were recovered in core samples from maximum depths of 6, 6, and 2 m, respectively. Five species of live snails (in order of decreasing abundance) Amnicola lirnosa, Physa gyrina, Physa integra, Gyraulus parvus and Valvata tricarinata – were found in densities which generally decreased from 1 to 3 m and increased from 4 to 6 m. Eleven species of fossil snails (in order of decreasing abundance) – Amnicola limosa, Valvata tricarinata, Gyraulus parvus, Fossaria obrussa, Physa gyrina, Helisoma anceps, Promenetus exacuous, Helisoma campanulatum, Physa integra, Lymnaea stagnalis jugularis and Amnicola lustrica – had densities which generally decreased from 1 to 3 m and increased from 4 to 6 m, except at a sampling station where sediment was being moved to 3 m and the largest fossil densities also occurred at this depth. Low adult/juvenile ratios at 1 m and 4-6 m and high ratios at 2-3 m for fossils of Amnicola limosa, Valvata tricarinata and Gyraulus parvus may possibly reflect seasonal migration.

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