Sediment accumulation in a shallow-water meadow carpeted by a small erect bryozoan

A marine meadow covered 80-90% by a turf of the erect bryozoan Cellaria occurs over an area > 10(4) m(2) west of Banjole Island (Croatia) in the northeastern Adriatic Sea, Surrounding areas are bare to patchily covered by isolated clumps of bryozoans, sponges, and ascidians attached to scattered shell and rock debris. Silt and clay comprise > 90% of the mass of sediment within the meadow, and Cellaria branch fragments are the next-most-abundant component. From 13-cm to at least 25-cm depth in the meadow sediment coralline algal branch fragments are abundant, and silt and clay comprise over 50% of the sediment's mass. In contrast, sediment accumulated in the bare area adjacent to the meadow is predominantly coarse carbonate sand composed of diverse skeletal grains. Mud comprises < 50% of the mass of this sediment. Concentration of mud in the sediment below the meadow apparently is due primarily to baffling by the erect bryozoans and earlier by the coralline algae, because composition and texture of the muds and fine sand are identical within and outside the meadow. The proportion of bryozoan skeletal fragments to mud in the tipper sediment is similar to that of erect delicate bryozoans to sediment in shallow ramp buildups of Late Ordovician age in the southern Appalachians and of fenestrate bryozoans to mud plus cement in bryozoan-rich cores of Carboniferous mounds. The Paleozoic mounds also are surrounded by coarser sediment. By analogy with the Cellaria meadow, trapping and deposition of muds by erect bryozoans can be inferred for many of these Paleozoic mounds, even though bryozoans are a relatively small proportion of the total sediment. (C) 2001 Elsevier Science B.V. All rights reserved.