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Relation of the morphology of stick-like bryozoans at Friday Harbor, Washington, to bottom currents, suspended matter and depth

Published online by Cambridge University Press:  08 February 2016

Thomas J. M. Schopf ,
Kenneth O. Collier  and
Barbara O. Bach
Thomas J. M. Schopf
Affiliation:
Department of the Geophysical Sciences, University of Chicago, 5734 S. Ellis Avenue, Chicago, Illinois 60637
Kenneth O. Collier
Affiliation:
Department of the Geophysical Sciences, University of Chicago, 5734 S. Ellis Avenue, Chicago, Illinois 60637
Barbara O. Bach
Affiliation:
Department of the Geophysical Sciences, University of Chicago, 5734 S. Ellis Avenue, Chicago, Illinois 60637
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Abstract

Stick-like bryozoans from 1 to 5 cm tall, and of varying degrees of colony robustness, are common in deposits from Ordovician through Cenozoic in age; their occurrence has been used to infer strength of bottom currents. Three species which are modern analogues of this mode of growth occur abundantly at subtidal depths near Friday Harbor, Washington; we studied one of these forms, the calcitic cyclostome Heteropora pacifica. Branch diameter was used as an indication of colony robustness because it can be measured in both living and fossil material. Branch diameter decreases significantly with depth (in collections from approximately 9, 15 and 30 m depth) at each of 4 transect localities.

At each locality, currents 6 to 10 cm above the bottom were measured continuously in water depth of ≃ 15 m, for several hours at a time. Under these conditions, the maximum sustained current through the several hour tidal cycle was 70 to 90 cm/sec at one locality, and 20 cm/sec or less at each of the other 3 localities. Suspended matter was determined at heights of 0.2, 0.5 and 1.0 m above the bottom. The amount of suspended matter 0.2 m above the bottom was 40 to 45 mg/cm2/day at the locality with the high current (depth of 15 and 30 m), and usually 5 to 10 mg/cm2/day at the localities with the low current (depth 15 and 30 m). Despite these several fold differences in current strength and suspended matter between localities, for any given depth bryozoan branch diameter does not vary. Branch breaking strength (23 MN/m2) and elastic modulus (30 GN/m2) were measured using small (.16 mm) strain gauges affixed to dry branches of Heteropora. These values are typical of carbonate skeletons of corals. Although Heteropora branch diameter (and colony robustness) decreases significantly with depth, this trend probably is unrelated to current strength.

Type
Articles
Information
Paleobiology , Volume 6 , Issue 4 , Fall 1980 , pp. 466 - 476
Copyright
Copyright © The Paleontological Society 

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