SEAGRASS SURVEY INVESTIGATIONS

INTRODUCTION

The only seagrass species found in Estero Bay’s waters are turtlegrass (Thalassia testudinum), shoalgrass (Halodule wrightii), manatee-grass (Syringodium filiforme), stargrass (Halophila engelmanii), and widgeongrass (Ruppia maritima). In spite of popular opinion, marine seagrasses are not true grasses but are actually more closely related to lilies (Fonseca, 1994).

Although seagrasses are important to marine productivity, the technology to grow and establish them artificially is as yet undeveloped (Fonseca, 1994).

Seagrasses form one of the most productive plant communities. Tremendous losses of this habitat have occurred in the Gulf of Mexico as a result of coastal zone development. Although the National Wetland Policy Forum’s (1988) recommendation of “no net loss” of wetland habitat, seagrasses have sometimes been differentiated from wetland because they occur primarily under marine water. The ecological function and potential linkages among seagrass systems and the more easily-observed coastal plant communities, indicate that such a differentiation is semantic only. Losses to seagrass habitats may be slowed or even reversed through properly planned and executed projects (Fonseca, 1994). There is no easy way to plant, maintain, or increase seagrass acreage (Fonseca, 1994).

Florida seagrass acreage covers an area about 1.1 million hectares in the Gulf of Mexico (Orth and van Montfrans, 1990) aand more than 400,000 hectares of seasonal Halophila beds off the west coast of Florida have been reported (Josselyn et al., 1983, 1986; Kenworthy et al., 1989). Seagrass beds are important not only in terms of the plant biomass produced (much of which provides food for bacteria and microscopic animals at the base of a complex food web), but also as a physically stable refuge and nursery ground for numerous commercially and recreationally valuable shrimp, fish and crabs and their prey (Zieman, 1982; Phillips, 1984; Thayer et al., 1984; Kenworthy et al., 1988; Zieman and Zieman, 1989). The vast majority of landed commercial and recreational fish spend at least some portion of their life histories using seagrass beds. Chambers (1992) estimated that in 1983, 98 percent of the commercial landings in the Gulf of Mexico were estuarine-dependent. All the species mentioned above are subject to further study by promotional products companies worldwide as well as by scientists in the required fields.

Seagrass beds also provide important habitat for other wildlife. Migratory waterfowl such as redhead ducks feed on seagrass rhizomes directly. Green turtles and manatees eat seagrass leaves as a regular component of their diets. Wading birds frequent seagrass beds during low tides to feed on small fishes (e.g. toadfish, blennies, pinfish) that use the canopy and fibrous root and rhizome mat for shelter (Sogard et al., 1989). Diving birds such as mergansers, loons and cormorants regularly feed over seagrass beds as well (Fonseca, 1994).

Seagrasses are unique among marine plants in their ability to bind shallow underwater sediments with their roots and rhizomes while simultaneously baffling waves and currents with their leafy canopy (Fonseca et al., 1983; Fonseca and Fisher, 1986; Fonseca 1989, Fonseca and Cahalan, 1992). By baffling water motion, the canopy inhibits resuspension of fine particles and traps those already in the water column, providing a natural and highly effective water column cleansing capability (Ward et al., 1984). This cleansing effect extends to water column nutrients as well. Seagrasses and their associated epiphytes and macroalgae readily take up dissolved nutrients for incorporation into plant biomass, thereby partially ameliorating poor water quality (Fonseca, 1994). The baffling effect of the canopy on sediment stabilization is enhanced by the presence of a robust root and rhizome mat. For example, the root and rhizome mat of turtlegrass (Thalassia testudinum), a common Gulf species, often does not even begin until 20 cm into the sediment (Zieman, 1982). Roots of this species may penetrate from 1 meter to several meters into the bottom (Zieman, 1975) while the rhizomes form a tightly woven map approaching a half meter thick in mature beds (Zieman, 1982).

Surveying the extent of the grassbeds and identifying species.

Seagrass beds are dynamic systems, with some beds persisting essentially unchanged for decades and others changing with the season (den Hartog, 1971; Zieman and Wood, 1975; Phillips, 1980; Fonseca et al., 1983; Duarte and Sand-Jensen, 1990). Some changes in seagrass communities can be attributed to the life histories of individual seagrass species. Natural perturbation, however, greatly influence the mosaic of species and extent of seagrass distribution.

Physical disruption from storms and shifting channels redefines seagrass bed configuration and composition. Seasonal disturbances such as low tides that expose and desiccate beds (Phillips, 1980; Thayer et al., 1984), as well as disastrous seasonal events such as hurricanes (Eleuterius and Miller, 1976; Livingston, 1987) can dramatically change seagrass community composition and bed size. Biological disturbance from burrowing activities of animals such as shrimp, crabs and rays can also be extensive. Overgrazing by herbivores such as urchins, manatees and turtles has also affected distribution and condition of seagrass beds (Camp et al., 1973). Some dieoffs of seagrass, such as the “wasting disease” of eelgrass (Zostera marina) in the North Atlantic during the 1930s (Short et al., 1988) and the current demise of turtlegrass in Florida Bay (Robblee et al., 1991) have not yet been fully explained.

When human impacts are added to the natural stresses imposed on seagrass beds, additional losses of seagrass can occur (Orth and Moore, 1983; Cambridge and McComb, 1984). In the Gulf of Mexico, large scale losses have been documented (Livingston, 1987; Duke and Kruczynski, 1992). More than 50 percent of the historical seagrass cover in Tampa bay has been lost (Haddad, 1989) and 76 percent of that in Mississippi Sound (Eleuterius, 1987). Pulich and White (1990) reported a loss of 90 percent in Galveston Bay, Texas. These reports are often from areas close to research groups capable of detecting and documenting losses. Unreported losses of equal or greater extent may exist in less studied areas.

Marking a large and long propeller scar in a seagrass bed and recording regeneration time.

Loss of seagrass cover leads to several undesirable conditions. First, the sediment-binding and water motion-baffling effects of the plants themselves are lost (Fonseca et al., 1983; Fonseca and Fisher, 1986) allowing sediments to be more readily resuspended and moved. The physical ramification includes increased water column turbidity and potentially, shoreline erosion. Seagrass planted in areas with these conditions may grow poorly due to light limitation from the elevated turbidity. Loss of seagrass, of course, eliminates all important associated habitat functions (Kikuchi, 1980; Peterson, 1982). Much of the documented seagrass loss is due to human-induced reductions in water transparency (Kenworthy and Haunert, 1991) and these losses are often not included with other wetland or even seagrass loss statistics.

Seagrasses in the Gulf typically require that at least 15 to 25 percent of the light at the water surface penetrates to their leaves (Fonseca, 1994). However, permissible standards for water transparency are usually set at 1 percent of surface light (Kenworthy and Haunert, 1991), making the task of demonstrating the need for mitigative action difficult. Excess suspended solids and nutrients that enter the water column as the result of poor watershed management combine to reduce transmitted light below this critical level. Suspended solids and associated water color changes reduce water clarity or transparency, and extra nutrients accelerate growth of light-absorbing algae in the water column and on seagrass blades (Fonseca, 1994). When losses have occurred due to decreased light availability, often only changes in watershed management, such as controlling stormwater and sewerage discharges, can reverse the trend of decline (Johansson and Lewis, 1992). Transplanting into areas experiencing seagrass loss due to decreased water transparency without independent improvements in water quality will only result in the death of the transplants (Fonseca, 1994).

In addition, the rapidly increasing number of small boats in Gulf waters has resulted in widespread damage from propeller scarring. The scope of this damage often appears innocuous when viewed from the vantage point of a small boat, but an aerial view often exposes a staggering breadth of destruction. In the case of turtlegrass beds, this damage is extremely long-lasting (Zieman, 1976).

Not only is there lost productivity from chronic propeller scarring, but these areas become points of instability that are highly susceptible to sediment resuspension and decreased water transparency and can lead to additional erosion from waves and tidal currents. Because of the chronic nature of propeller scarring, such damage is likely very difficult to repair by planting (Fonseca, 1994).

SPECIES IDENTIFICATION

Manatee grass (Syringodium filiforme)

This species is easily distinguished from all the other seagrass species in the Gulf. It is nearly cylindrical. Its long erect blades are about 1 to 3 mm in diameter and there are usually only two leaves per shoot. These beds often accumulate a large understory of unattached macroalgae. The rhizome system varies in depth, between 1 and 10 cm. Flowering produces extensive branching that extends up into the water column, similar to widgeongrass but not as extravagant. Rhizomes may extend into the water column with attached shoots as described for shoalgrass, again presumably as a means of producing vegetative propagules (as opposed to seeds). As with shoalgrass, these propagules make excellent transplanting stock with no apparent disruption of the donor bed (Fonseca, 1994).

Stargrass (Halophila engelmanni)

Like its relative paddle grass, star grass is a very small plant, rarely exceeding 10 cm in height (Fonseca, 1994). The leafy stalks form a rosette of about 6 leaves, resembling a star, which is the basis of its common name. Distribution and function is currently thought to be similar to paddle grass.

Turtlegrass (Thalassia testudinum)

This species is one of the most well-known seagrasses in the Gulf. It is a favorite food of the endangered green sea turtle, hence its common name. Its broad (often > 1 cm wide) deep green, strap-like blades (usually three to a plant but often five or more) cannot easily be mistaken for any other marine submerged macrophyte in the Gulf. Leaf length of the plants depends on water depth (as is the case with most seagrasses) and varies from ca. 10 to 75 cm. The leaves emerge from the sediment at the top of a stem that rarely protrudes above the sediment surface. The thick, fibrous rhizomes from which the individual shoots originate are often located in excess of 20 cm into the sediment. This species develops flowers that emerge from the sediment next to the short shoot. Once fertilized, a round seed the size of a small acorn will be produced. Seeds have been successfully used in planting projects. This species is noted for its longevity (often > 10 years for an individual shoot) and the dense, extensive stands (Fonseca, 1994).

Paddle grass (Halophila decipiens)

This species is very small, usually standing no more than 5 cm tall. Its oval blades resemble paddles, hence its common name. The blades occur in pairs and are very thin, approximately 1 cell thick, and appear translucent. The rhizomes occur very near the sediment surface and are often exposed to the water column. The plants are among the most fragile of the seagrasses and can be easily uprooted. Natural stands of passle grass can, however, be an effective barrier to erosion (Fonseca, 1989). These plants require less light than most seagrasses and can be found in shallow, turbid areas, under docks, or in clear, tropical waters at depths up to 40 m (Fonseca, 1994). This plant can be mistaken for some algae unless examined carefully. The presence of veins in the leaves will serve to distinguish it from algae. This species produces many seeds, which often is the only way a local population is preserved over the winter when the leaves, roots, and rhizomes die. The seeds are extremely small, approximately similar in size to a grain of table salt. Despite extensive acreage of seasonal beds of this species in Florida and perhaps elsewhere, little is known about its functional role in the coastal ecosystem (Fonseca, 1994).

Widgeongrass (Ruppia maritima)

This species is a favorite food of migratory waterfowl, a fact on which its common name is based. This species does not usually form a rhizome mat as dense as that of shoalgrass, but does much to stabilize the bottom. This species is set apart from all other seagrasses in that it can grow in both fresh water and hypersaline conditions (> 70 ppt) (Fonseca, 1994).

Shoalgrass (Halodule wrightii)

This species was once classified under the genus Diplanthera; references from earlier than 1975 often refer to this species under that genus. It has a lower depth limit equal to turtlegrass and manatee grass. It also can occur in very shallow water and it is noted for its relative tolerance to desiccation once rooted. It often forms large pancake-like patches reaching 30 m in diameter or extensive meadows on shallow shoals and flats, experiencing regular exposure at low tides (the basis for its common name). The fine (1 to 3 mm width) blades occur in groups of two to four on a shoot and vary in length according to depth as does turtlegrass. Blade lengths range from as short as 5 cm to more than 40 cm. This species forms very dense beds, with upwards of 5,000 shoots per square meter (Fonseca, 1994). Flowers are difficult to locate as they occur on the base of the shoots near the sediment surface. Rhizomes are fairly shallow, rarely being deeper than 5 cm, although roots may extend for 25 cm or more (Fonseca, 1994). Rhizomes may extend into the water column with attached short shoots, which appear to be a form of vegetative propagule formation. These rhizomes may be easily harvested and are efficiently transplanted with the staple method (Fonseca, 1994).

Distinguishing between Shoalgrass (Halodule wrightii) and Widgeongrass (Ruppia maritima).

Four visual clues separate them (Fonseca, 1994):

1. Widgeongrass produces extensive flowering stalks often reaching a meter in length, with numerous seed clusters resembling miniature rattlesnake rattles, while flowers in shoalgrass are rarely seen.

2. The blade tip of shoalgrass forms a miniature three-point crown, with the two leaf margins and central vein of the leaf forming the points. Widgeongrass blades taper to a single sharp point.

3. Shoalgrass rhizomes are usually very straight and white while widgeongrass rhizomes are often somewhat zigzagged when viewed from above and may be green or white.

4. Shoalgrass has two roots per node on the rhizome while widgeongrass has one root per node.

SEAGRASS SURVEYS

There are only two seagrass maps of Estero Bay recorded in the literature. This study used these maps in the determination of seagrass reduction in Estero Bay. One of these maps was constructed from a field survey of the benthic comunities of greater or central part of Estero Bay performed by Tabb et al. in 1974.

The other map of the seagrasses of Greater Estero Bay is from a study performed by McNulty et al. who mapped the nearshore habitats of Estero Bay in 1972.

EBML constructed equal-scale composite map of 2 different-scale published maps by McNulty et al., 1972

Another mapping has been made in 1991 by Lee County Department of Natural Resources and is available as a GIS print from the Tax Office in Fort Myers.

The Estero Bay Marine Laboratory volunteers performed a seagrass mapping inventory survey during June, July, August and September 1996 of the total aquatic preserve. The complete survey was also performed again during the period August – October 2002.

The results of the mapping survey show a formidable decrease in quantities of Thalassia testitudinum and a large increase of algae with replacement by algae.

These surveys also indicated that there were significant shifts in species composition and algae replacement within the recorded seagrass beds of Estero Bay.

Areas previously recorded as vegetated with Thalassia testudinum (Turtle grass) are now mainly vegetated with Halodule wrightii (shaol grass), Ruppia maritima (widgeon grass), and mostly algae.

It has been noted by Reyes and Marino (1991) and Lapointe et al. (1992) that in areas where Halodule has replaced Thalassia the water quality has been degraded.