Webb, Frederick J., Jr. (ed.) / Proceedings of the Fourteenth Annual Conference on Wetlands Restoration and Creation : May 14-15, 1987
Bloom, Stephen A.
Seagrass zonation: test of competition and disturbance at Seahorse Key, Florida, pp. 48-62 PDF (6.4 MB)
SEAGRASS ZONATION: TEST OF COMPETITION AND DISTURBANCE AT SEAHORSE KEY, FLORIDA Stephen A. Bloom Ecological Data Consultants, Inc. P.O. Box 542 Archer, Florida 32618 ABSTRACT The zonation of Halodule wrightii and Thalassia testudinum (which occur in adjacent, virtually non-overlapping monocultures in the intertidal and shallow subtidal respectively at Seahorse Key, Florida) was subjected to experimental manipulations. Transplantation was done above, within and between beds, and mean blade length (as a measure of plant health) was monitored. Upper limits appeared to be set by exposure (desiccation and/or heat) stress while the lower limit of Halodule was not set physiologically since transplants into clearings in the Thalassia bed flourished. Experimental evaluations of shading, leaf abrasion, short term allelopathy and root-root interactions were performed to isolate potential competitive mechanisms. Thalassia outcompeted Halodule via long-term root interactions. Herbivory were modeled by cropping at the border zone and was capable of decreasing the ability of Thalassia to exclude Halodule and enabling Halodule to invade. Other potential forces which could alter the outcome of the competition were discussed. INTRODUCTION Subtropical grassbeds along the coast of the Gulf of Mexico often exhibit zonation of the shallow, intertidal shoal grass, Halodule wrightii Aschers., and the deeper, subtidal turtle grass, Thalassia testudinum Banks ex Konig (Humm 1956; Phillips 1960, 1962; Moore 1963; Keller & Harris 1966; den Hartog 1970; Iverson & Bittaker 1986) though mixed beds are known to exist (Voss & Voss 1985; Humm 1956; Strawn 1961). Little experimental work has been done on the zonation of Halodule and Thalassia (but see Phillips 1960 as an early approach). The purpose of this research was to explore experimentally the causal mechanisms of the zonation of Halodule and Thalassia and to examine phenomena which could affect the zonation. STUDY SITE All experiments were performed on the south beach of Seahorse Key (83 04'West and 29 06'North), a small island 5 kilometers offshore from Cedar Key, Florida on the Gulf Coast. Stations were established at approximately 60 cm (Upper Beach), 30 cm (Middle Beach), 15 cm (Halo- dule bed) and -15 cm (Thalassia bed) from the mean tide level. These correspond to 2, 10, 30 and 80 meters respectively from the extreme 48
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