Page 48

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