An Ecological Study of the Microbenthic Fauna of Two Minnesota Lakes

Abstract

This report is based on a general study of a neglected and poorly-known ecological group, the lacustrine microbenthos. The microbenthos may be defined as the whole assemblage of microscopic organisms living within and on the surface of the bottom sediments. Further definition is based on limnological sampIing procedure, the microbenthos being that part of the bottom community which is not retained by the coarse metal screens usedl in ordinary bottom-fauna studies. In this sense the microbenthos is analogous to the nannoplankton, while the macroscopic bottom animals, collectively termed macrobenthos, are analogous to the net plankton. Results of the only previous detailed and intensive ecological study concerned primarily with the microbenthos were published by Moore (1939). His paper included an historical bibliography to which should be added at least the papers of Imel (1915), Berg (1938), and Lindeman (1941). The data for the present paper were obtained from two Minnesota lakes. Twelve collections were made in the north arm of Lake Itasca, Clearwater County (R 36 W, T 143 N) during July and August, 1948. Eighty-six benthic collections were made from midwinter, 1948, to midsummer, 1949, in Crystal Lake, Hennepin County (R 21 W, T 29 N, Section 15). Of the total collections, 55 were qualitative, involving 73 ooze-sucker samples, and 43 were quantitative, based on 112 core samples. The molluscs, insects, malacostracans, and larger annelid worms were noted when found, but were considered macroscopic and beyond the realm of this investiagtion. Special emphasis was placed on species distribution with respect to physicochemical factors in the lacustrine environment throughout the seasons. An appraisal of the relationships of the microbenthic assemblage to other lake biota was attempted by examining occasional plankton samples and correlating them with data from bottom collections.