Dynamics of interdepression benthic communities

TitleDynamics of interdepression benthic communities
Publication TypeJournal Article
Year of Publication1952
AuthorsEggleton FE
JournalTransactions of the American Microscopical Society

1. A promising opportunity to study the dynamics of littoral benthic communities, apart from shoreward influences, is presented by the existence of a large, relatively isolated area of littoral bottom lying between three regions of deep-water in the multi-depression basin of Douglas Lake, Michigan. That portion of the lake floor is designated as the interdepression area. 2. Thirty-three stations were established within the interdepression area in such a manner that they were distributed effectively over the five different major types of contemporaneous surface bottom deposits, which are characterized as (1) clean sand, (2) muddy sand, (3) sandy mud, (4) soft, black ooze, and (5) brown, pulpy peat (gyttja). 3. At these stations physico-chemical determinations and measurements were made and, by use of a Peterson dredge, samples of the bottom taken for study of the biotic communities. A majority of the field work was done during three successive summers, but some additional field study and much of the laboratory work of identification of the material and analysis of the data was conducted in subsequent years. 4. The origin, nature, and evolution of the pattern, of surface bottom deposits are described and the existence of small, isolated areas of clean sand, soft black ooze, and gyttja are explained in the light of observations made during this investigation, as well as those by Scott and later by Wilson. 5. The roughly concentric zonation of present-day superficial bottom deposits is described, illustrated, and subsequently explained as a result of the interplay of the dynamics of waves and wind-induced currents modified by the position of headlands, by shore-line sinuosities, and by the configuration of the basin itself. These forces continue to develop the slowly but constantly changing pattern of the lakes bottom deposits using both Algonquin and post-Algonquin depositions as well as both allochthonous and autochthonous detritus as construction materials. 6. The contention that flotation of sand by the surface film on a lake is an important factor in the formation of its bottom deposits is supported. 7. Physico-chemical conditions are shown not to be significantly different from surface to bottom in the water superimposed upon the benthic habitats of the interdepression area which nowhere extends below a depth of 20 feet. 8. The benthic fauna is a quantitatively rich and qualitatively diversified assemblage within which the phyla Platyhelminthes, Nemathelminthes, Annelida, Mollusca, and Arthropoda were most abundantly represented. Within these five metazoan phyla, animals were distributed among 9 classes, 15 orders, 30 families, 52 genera and 67 species. In addition, animals from other groups were present, but either they were only occasionally taken or, for various reasons, were not identified to the lower taxonomic categories. 9. Rooted aquatic plants included various species from the genera Anacharis, Chara, Myriophyllum, Najas, Potamogeton, Scirpus, and Utricularia. In addition, the blue-green alga, Dichotomosiphon tuberosus appeared in thick blankets, lying flat upon the soft, black ooze at certain stations. 10. Although observations were made on certain Protozoa, the phylum as a group, was not included in the scope of the present investigation. 11. Porifera were collected but they were not abundant; only two species were identified. 12. A few specimens of Pelmatohydra oligactis were the only representatives of the Coelenterata taken. 13. The phylum Platyhelminthes was represented by a few rhabdocoeles and by an abundant and widefy distributed population of the triclad, Dugesia tigrina. 14. The nemathelminths were, surprisingly, rather scarce. Only the genera Hydromermis and Dorylaimus were represented in the fauna. 15. Polyzoans were likewise much less numerous than was expected. Plumatella repens was the only species taken. 16. Annelida, on the other hand, were among the most common animals in a majority of the benthic communities. Members of the family Tubificidae far outnumbered all other members of the phyl m. Several other oligochaetes, as well as four genera of leeches, were present at various stations. 17. Both qualitatively and quantitatively, the Mollusca, as a phylum, were exceeded only by the Arthropoda. Nine genera and fifteen species of aquaticsnails, together with three species of unionids and two genera of sphaeriids comprised the molluscan fauna. 18. The Arthropoda far outnumbered any other phylum within the interdepression fauna. Indeed, in certain habitats the arthropods exceeded all other animals combined. Crustaceans and insects vied for first place in abundance, generally, but the former actually exceeded the latter only on areas of sandy mud and at two depths. 19. Hyalella azteca, along with representatives from certain genera of the insects, were clearly the numerically dominant invertebrates in many benthic communities. 20. The hydrachnids, represented in the interdepression fauna by 8 families, 12 genera, and 19 species, were widely distributed, but never abundant in anyone community. A list of the species is included. 21. Extensive consideration is given to the interplay of dynamic factors in the control of distribution and abundance of the constituent species within the various benthic communities. 22. Within the interdepression area, there appears to be no direct relationship between depth, per se, and either distribution or abundance of the benthos, although depth does secondarily produce a marked effect on both aspects of the biota through its influence on light penetration and especially on water movement. 23. Normal limits of toleration of the benthic species to variations of physico-chemical factors in the superimposed water, fall well within the limits of those variations as they occur in the indepression area. 24. Both qualitatively and quantitatively the invertebrate benthic fauna increases as the abundance and variety of the plants increase. 25. Abundance and diversity of the benthic biota are correlated with the type of bottom deposit. The greatest numbers of individuals and the most kinds of both plants and animals occur on areas of sandy mud, followed in order of decreasing abundance by regions of soft, black ooze, muddy sand, clean sand, and brown, pulpy peat (gyttja). 26. The maximum abundance of both individuals and species of benthic invertebrates occurs at intermediate depths, where a luxuriant rooted, aquatic vegetation grows on an area of sandy mud. 27. Seasonal variations in the invertebrate fauna modifies the composition of the benthic communities, but to a minor degree during the mid-summer period, as compared with the same fluctuations during other seasons of the year.