SEA GRASSES OF THE NORTHERN

GULF COAST

HAROLD J. HUMM

Department of Botany, Duke University

ABSTRACT

On the basis of observations in Mississippi Sound and elsewhere, it is

known that at least five species of marine monocotyledonous plants commonly known as "sea grasses" occur in abundance along the northern

Gulf coast and may be virtually continuous between Florida and Aransas,

Texas. An annotated Jist of species and key are included.

INTRODUCTION

Because it is usually assumed that marine spermatophytes, commonly know as "sea grasses," are rare along the northern coast of the

Gulf of Mexico between Bay County, Florida, and Aransas County,

Texas (Thorne, 1954), it seems worthwhile to record certain observations made in Mississippi Sound while the writer was in residence

at the Gulf Coast Research Laboratory, Ocean Springs, Mississippi,

during summer months from 1950 to 1955. Additional observations

made in the Alligator Harbor area, Franklin County, Florida, and

elsewhere are included.

Grateful acknowledgment for the opportunity to work at the Gulf

Coast Research Laboratory is made to Dr. R. L. Caylor, founder and

first director.

ANNOTATED LIST OF SPECIES

The following annotated list deals with five species of sea grasses

that occur in abundance in Mississippi Sound, and a sixth that is to

be expected outside the barrier islands at depths ranging from 15

to 50 feet.

Ruppia maritima L., widgeon grass, the only species previously

known to be abundant in Mississippi Sound, occurs principally along

the mainland shore, especially in bayous where the salinity is usually

less than half that of inshore Gulf waters.

Thalassia testudinum Konig is apparently the most abundant of any

of the sea grasses in Mississippi Sound. It occurs along the inside of

the barrier islands extending from depths of 1.5 to 2 feet along the

beaches, outwards to depths of 8 feet or more, with occasional interruptions. It also occurs in patches or bands along the outer beaches

of the barrier islands, where the inner edge of the growth is in deeper
306 Bulletin of Marine Science of the Gulf and Caribbean [6(4)

water than in the sounds and bays. The outer or deeper edge of

Thalassia beds is evidently determined primarily by light penetration

and thus it occurs in deeper water outside the barrier islands than in

the sounds. Apparently this species has not been reported to occur

between Florida and Aransas Pass.

Cymodocea manatorum Ascherson (Syringodium filiforme Ktitzing), manatee grass, grows interspersed with Thalassia, usually in

lesser abundance than Thalassia. It was not found in pure stands. In

general, it occurs farther out from the beach than does Thalassia. Its

relative abundance is easily underestimated because of its filiform

leaves.

Diplanthera wrightii (Ascherson) Ascherson (Halodule wrightii

Ascherson) occurs in interrupted patches along the inner beaches of

the barrier islands, and probably along the outer beaches also. It

grows in pure stands closer to the beach than any other species. Evidently it will tolerate higher water temperatures and longer exposure

to the air than the others. Beds of Diplanthera are sometimes higher

than the surrounding sand, indicating that it has a tendency to hold

or accumulate sand. Farther out it is sometimes interspersed with

Thalassia where the development of Thalassia is subnormal. In areas

of vigorous development of Thalassia and Cymodocea, Diplanthera is

usually absent. Apparently it has not been reported previously between

Tampa Bay, Florida, and Aransas, Texas.

Halophila engelmannii Ascherson seems to be the least abundant

of the five species. It has been found only within beds of Thalassia or

of Thalassia and Cymodocea. Though inconspicuous because of the

taller leaves of its two associates, it is often present in abundance. It

can best be observed by means of suitable air-supplied diving equipment which permits the observer to glide horizontally over the sea

grass beds and to brush aside the taller leaves of Thalassia and Cymodocea. Since it grows partially shaded by the other sea grasses, it

evidently tolerates lower light intensity and hence may be expected to

occur in deeper water. Taylor (1928) obtained Halophila by dredge

at a depth of over 220 feet at Dry Tortugas, while his shallowest

record was about 14 feet. In all areas observed by the writer, Halophila occurs within a foot of the shallowest location of Thalassia.

Apparently this species also has not been reported previously between Tampa Bay and Aransas, Texas.

Halophila bail/onis Ascherson, although apparently not reported

previously north of Dry Tortugas in the Gulf, has been collected re-
1956] Humm: Sea Grasses 307

peatedly by dredge and aqua-lung in 20-30 feet of water 10 to 12

miles southeast of Alligator Point in the open Gulf. It has also been

taken by dredge at simIlar depths off Pensacola. It seems highly probable that this species occurs also at many localities along the northern

Gulf coast between Florida and Texas and it may be almost continuous. Because it evidently occurs at greater depths than any of the

other species listed here, cOllecting it is difficult and usually accidental.

Taylor (1928) dredged it from about 16 to 90 feet at Dry Tortugas.

In its northern range it seems to be confined to depths exceeding

20 or 25 feet.

DISCUSSION

There is reason to believe that the distribution of Thalassia, Cymodocea, Diplanthera, and two species of Halophila is essentially continuous around the entire Gulf coast. There is little doubt as to their

abundance without major interruption between the Florida west coast

and Mississippi Sound. Off St. Marks Light, Wakulla County, Florida,

there are literally square miles of bottom that are completely covered

with mixed stands of Thalassia, Cymodocea, and Halophila en~elm~nnii from a depth of about two feet to 15 feet or more. Outside

these beds Halophila bail/onis occurs. The bottom of Alligator Harbor,

Franklin County, is almost entirely covered with the same species and

there are patches of Diplanthera in addition.

Shifting sands and river mouths are the two major factors responsible for minor interruptions in the distribution of sea grasses.

South Shoals is a ridge of shifting sand that extends from the beach

at Alligator Point seaward for a distance of about seven miles. Even

in this area there are scattered patches of sea grasses. Larger interruptions may occur opposite mouths of major rivers but because of

the remarkably gentle slope and the resultant width of the continental

shelf in the northern Gulf, sea grasses are able to grow many miles

off shore where salinities remain sufficiently high despite the rivers.

Furthermore, there is usually a tongue of high salinity water beneath

river effluent that extends much nearer shore than might be expected.

In general the outer edge of sea grass beds seems to be determined

by light penetration, whereas the inner edge seems to be determined

by wave action and shifting sands or, in some places, by the level of

extreme low tides. Sea grasses are often exposed at spring low tides,

after which a killing of the leaves is evident.

The importance of sea grasses as basic producers along the Gulf

coast cannot be over-emphasized, as has been pointed out by Voss
308 Bulletin of Marine Science of the Gulf and Caribbean [6(3)

and Voss (1955). They published a map showing the distribution of

Thalassia around the Florida coast with the exception of western

Florida. It is clearly evident that the attached marine algae are responsible for but a small fraction of the productivity carried on by the

St:a grasses in the Gulf. Many ecological problems of these important

plants await study.

KEY TO THE SEA GRASSES OF THE GULF OF MEXICO

In order to encourage further observations on the distribution of

sea grasses in the GUlf of Mexico, a key to the six species discussed

herein is included. Only Ruppia may be expected in water of low

salimty and even in fresh water. The others are more stenohaline and

probably do not occur where salinities fall much below 25 % for considerable periods of time.

1. Leaves flat, of the same width throughout ..... . . . . . . . . . . . . . . . . . . .. 2

Leaves terete or, if flat, widest at or above the middle . . . . . . . . . . . . . .. 4

2. Width 5 to 10 mm Thalassia testudinum

Width 1 to 2 mm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3

3. Leaves 1-]112 mm wide, the tip with a single point, an expansion at the

base; stems often in part upright and above the bottom .. Ruppia maritima

Leaves usually 1112-2 mm wide, no expansion at base but with ligulate

sheath, leaf tips with 2 or 3 points; stems horizontal, covered by bottom

material except where exposed by erosion Dip/anthera wrightil

4. Leaves terete, about 2 mm in diameter Cymodocea manatorum

Leaves flat, widest near or above the middle . . . . . . . . . . . . . . .. 5

5. Leaves in pairs at each node Ha/ophi/a baillonis

Leaves in a whorl of 4 to 8 at the tip of a short erect stem .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Halophila engelmannii

BIBLIOGRAPHY

BERNATOWICZ, A. J.

1952. Marine monocotyledonous plants of Bermuda. Bull. Mar. Sci. Gult

& Carib., 2 (1) : 338-345.

MUENSCHER, W. C.

1944. Aquatic Plants of the United States. x + 374 pp., 154 figs., 400

maps. Comstock Publ. Co., Ithaca, N. Y.

SETCHELL, W. A.

1920. Geographical distribution of the marine spermatophytes. Bull. Torrey

Bot. Club, 47 : 563-579.

TAYLOR, W. 1<..

1928. The marine algae of Florida with special reference to the Dry Tor·

tugas. Carnegie Inst. Wash. Publ. 379, Papers from the Tortugas Lab.

25 : 1-219.

HORNE, R. F.

1954. Flowering plants of the waters and shores of the Gulf of Mexico.

Fish. Bull., U. S. 89 : 193-202.

Voss, G. L., AND NANCY A.

1955. An ecological survey of Soldier Key, Biscayne Bay, Florida. Bull. Mar

Sci. Gulf & Carib., 5(3) : 203-229.