Benthonic Communities Research Articles

Bimini Lagoon: Model Carbonate Epeiric Sea: ABSTRACT

Bimini Lagoon contains a wide variety of marine environments, most of which have readily recognizable counterparts in the larger epeiric seas of the geologic past. Located on the northwest margin of the Great Bahama Bank, Bimini Lagoon is a shallow area with End_Page 468------------------------------ depths exceeding 5 feet in very few places; the area is 8 square miles. It is divided into an inner and an outer lagoon. The outer lagoon opens to the Gulf Stream through a narrow, deep natural channel. The outer lagoon also opens onto the Great Bahama Bank proper across wide, shallow flats, exposed at low tide. Tidal range in the lagoon is 2-3 feet; large volumes of water enter and leave the lagoon with each tidal cycle. In the lagoon, bottom communities, sediments, and currents are closely interrelated. Distribution of the communities is controlled by current action, and by the degree of tidal exchange of water with the Gulf Stream; the nature of the sediment is determined by the organism community present. Most of the tidal exchange into the lagoon takes place through the deep channel. Current velocities exceed 2 knots. Consequently the channel is floored by bare rock, sorted gravel, and coarse sand. The margin of the channel is marked by an abrupt rise and current velocities of about 1½ knots. The strong current over the channel margin promotes luxuriant growth of Thalassia, which acts as a sediment trap, and prevents erosion. The sediment is fine-grained and poorly sorted. Away from the channel, the current velocity is less than 1 knot, Thalassia is sparse, and wave action affects the nature of the sediment. Many benthonic communities can be recognized. The inner lagoon is an isolated hypersaline water mass. Current velocities are very low. Most of the bottom is covered by bare sand with a few species of algae, but the richest molluscan faunas occur here. End_of_Article - Last_Page 469------------

北海道宗谷管内のホタテガイの資源とホタテガイ漁場における底生生類群集について

The author investigated the resources of the scallop, Patinopecten yessoensis JAY, and the benthonic communities of the scallop fishing grounds during the period from May to August in 1962 and 1963 at Sarufutu, Tonbetu, and Esasi in Soya District. On the basis of these results he discussed the fluctuation of the scallop resources after 1960 in these three regions (Table 1), and the composition of the benthonic communities of the scallop fishing ground in June of 1964 at Tonbetu (Table 2). The results obtained are as follows: 1. The scallop resources were estimated by strip-fishing method (Fig. 1) and DeLury's method in these three regions (Table 1). 2. In these three regions the scallop resources tend to decrease gradually, and it seems to be caused by following two factors: 1) Fewer appearance of the youger scallop (Fig. 2); According to the investigations of Hokkaido Fisheries Experimental Station, the younger scallop were always at the head of age distribution after 1955 at Tokoro and Monbetu, where are the main fishing grounds of scallop at present in Okhotsk Sea, Abasiri Dist., Hokkaido. 2) Overfishing; Some years ago, fishing amounts of the scallop were decided on the basis of all estimated resources included them of the outside of main fishing ground. 3. The benthonic communities of all the stations at Tonbetu were dominated by Asterina pectinifera, Asterias nippon, and Strongylocentrotus sp. respectively. The population densities of the scallop were fairly lower than these species at each station (Table 2). 4. According to the correlation method (Motomura, 1935, '52), which was applied to the benthonic composition, the benthos of twenty stations at Tonbetu were divided by those correlation coefficient series into five groups as follows; Group 1: Asterina pectinifera Community. Group 2: Asterina pectinifera-Asterias nippon Community. Group 3: Asterias nippon-Asterina pectinifera Community. Group 4: Strongylocentrotus sp. Community. Group 5: Asterina pectinifera-Asterias nippon-Strongylocentrotus sp. Community. The relation between the scallop communities and above mentioned groups is not clear (Fig. 3).

Petrology and Origin of the Lower Tertiary Formations of New Jersey

ABSTRACT The sediments of lower Tertiary age in New Jersey have been divided into four formations: the Hornerstown Marl, The Vincentown Sand, the Manasquan marl, and the Shark River Marl. This writer has recognized three of these units in outcrop: the Hornerstown, Vincentown, and Manasquan. The lithologic and paleontologic character of these formations suggests that they represent sedimentation in a regressive phase of an early Tertiary marine cycle. The available data suggests that marine currents were effective in distributing the sediments much of the time. Reducing conditions prevailed in the bottom sediments during the time the Hornerstown Formation was being deposited and locally during the time of deposition of the Vincentown and Manasquan Formations. The Vincentown Formation may have b en deposited in a shallow bay, parts of which were a haven for a cosmopolitan benthonic community. The outcropping Manasquan Formation was deposited in a near-shore, shallow water environment and may represent the climax of the regression.

The Palaeoecology of the Top Hosie Shale (Lower Carboniferous) at a Locality Near Kilsyth

Summary The sizes, conditions of preservation and environment of some 5000 fossils collected from two feet (measured vertically) of a calcareous shale of Lower Carboniferous age are analysed and the lithology of the sediment described. Where possible, the modes of life of the fossil species are determined and ecological comparisons made with related living species. It is shown that many of the fossils are “drift” shells or fragments, which were washed into their present position in the shale. Some shells, however, have maintained the position occupied by the living animal. Two benthonic fossil communities are recognized, comparable with the living benthonic communities described by C. G. J. Petersen ( Rep. Danish Biol. Stat. 1918). A mud-surface community, consisting principally of Posidonia corrugata , arenaceous foraminifera and the ostracod Waylandella cuneola , lived in the relatively quiet depositional environment of the basal four inches of shale. Increase in the strength of currents sweeping the substratum, as evidenced by coarser sediment and shell fragments, is correlated with the disappearance of this community and the introduction of a community of burrowing forms which characterize the following 20 inches of shale. This new community consists chiefly of the burrowers Lingula squamiformis, Nuculopsis gibbosa and Sanguinolites costellatus , certain surface animals more variable in abundance than the burrowers, and a microfauna similar to that of the Posidonia band. I. Introduction The work described in this paper was undertaken in an attempt to elucidate the palaeoecology of benthonic fossil communities, and to gain a better understanding of the depositional environment of