Seed bank, biomass, and productivity of Halophila decipiens, a deep water seagrass on the west Florida continental shelf

Abstract

One of the largest contiguous seagrass ecosystems in the world is located on the shallow continental shelf adjacent to the west coast of Florida, USA and is comprised of seasonally ephemeral Halophila decipiens meadows. Little is known about the demography of the west Florida shelf H. decipiens, which may produce 4.56 × 10⁸ g C day⁻¹ or more during the peak growing season. We documented seagrass distribution, biomass, and productivity, and density of sediment seed reserves, seedlings, flowers and fruits on the southeastern portion of the west Florida shelf by sampling along a transect at three stations in 10, 15, and 20 m water depth. Biomass, flower, fruit, seedling, and seed bank densities tended to be highest at stations in 10–15 m water depth and lowest at 20 m. Flowers and fruit were most prevalent during summer cruises (June and August 1999, July 2000). Seedling germination occurred during summer, fall (October 1999), and winter (January 2000) sampling events, with the highest seedling densities present during the winter. Seed bank density remained consistent through time. A Category I hurricane with sustained winds of 120 km h⁻¹ passed over the stations, but only limited impact on H. decipiens biomass was observed. The presence of a persistent seed bank provides for recovery after storm disturbance, annual reestablishment of populations, and continual maintenance of the 20,000 km² of deep water seagrass habitat present on the west Florida shelf.

Introduction

One of the largest, open-water, contiguous seagrass ecosystems in the world occurs on the shallow continental shelf in the eastern Gulf of Mexico adjacent to the west coast of Florida, USA (CSA Inc., 1985, CSA Inc., 1987, CSA Inc., 1991; Iverson and Bittaker, 1986, Fourqurean et al., 2001). This area is referred to as the west Florida shelf (WFS) and extends from Cape San Blas south to Cape Sable and the Florida Keys (Fig. 1, inset). The WFS encompasses 75% of the total continental shelf area of the Gulf of Mexico and with its associated shallow water estuaries (<10 m water depth) it was estimated to contain approximately 9100 km² of seagrass coverage or about 76% of the total coverage in the Gulf of Mexico (Iverson and Bittaker, 1986). On relatively deeper portions of the shelf, between 10 and 30 m, there is estimated to be an additional 20,000 km² of seasonally ephemeral Halophila decipiens habitat interspersed with unvegetated soft sediments and diverse benthic communities of calcareous and fleshy macroalgae, crustacean burrows, and hard bottom outcrops dominated by sponges and soft corals (CSA Inc., 1985, CSA Inc., 1991Phillips et al., 1990, Fourqurean et al., 2001). The non-seagrass habitats harbor motile invertebrates and fishes that utilize adjacent H. decipiens meadows (Phillips et al., 1990). This region of the Gulf of Mexico supports some of the richest coastal fisheries in the United States (Mahmoudi et al., 2002). Depending on climatic conditions and severe storms, portions of this seagrass ecosystem may completely disappear from the shelf in winter, as is the case in other ephemeral seagrass populations (CSA Inc., 1987, CSA Inc., 1991Williams, 1988, Jupp et al., 1996, Kenworthy, 2000). Although shallow water seagrass populations have been well-studied, little is known about the demography, reproductive biology and life history mechanisms which maintain the deeper water seagrass ecosystems throughout the world (Kenworthy et al., 1989, Lee Long et al., 1996, Erftemeijer and Stapel, 1999).

H. decipiens occurs in tropical and sub-tropical seas worldwide (den Hartog, 1970, Waycott et al., 2002). It grows in many types of unconsolidated substrate, from soft, silty sediments to coarse shell hash, and is capable of stabilizing the seafloor (Fonseca, 1989). Like many of the larger taxa, H. decipiens provides a source of organic matter, habitat structure and food for benthic and pelagic organisms, including sea turtles and dugongs (Ross, 1985, Josselyn et al., 1986, Dirnberger and Kitting, 1988, Kenworthy et al., 1989, Lanyon et al., 1989, Preen, 1995, Russell et al., 2003). It is distributed from the shallow subtidal to depths of >40 m, where it forms extensive habitat in lagoons, on continental shelves, and in other deep water environments (den Hartog, 1970; CSA Inc., 1985, CSA Inc., 1991; Kenworthy et al., 1989, Lee Long et al., 1996, Fourqurean et al., 2001). As is the case for other members of the genus, H. decipiens is a ruderal species adapted to surviving in diminished and fluctuating light regimes and thrives in environments where disturbances such as herbivory, bioturbation, and storms make conditions unsuitable for maintaining perennial populations of the competitively superior, larger, and longer-lived seagrasses (Josselyn et al., 1986; Continental Shelf Associates, Inc., 1987; Williams, 1988, McMillan and Soong, 1989, Duarte, 1991, Kenworthy et al., 1993, Kuo and Kirkman, 1995, Preen, 1995, Jupp et al., 1996, Lee Long et al., 1996, Kenworthy, 2000).

H. decipiens is monecious and forms flowers and fruits at the base of the leaf pairs, at or just beneath the sediment surface. Seeds are negatively buoyant (McMillan, 1988a), and are thus ideally suited to percolate into the sediment. In laboratory settings, Halophila spp. seeds have germinated after 1–2 years of storage under dark conditions (McMillan, 1991, McMillan and Soong, 1989), suggesting that members of this genus could rely on a buried seed bank to re-establish populations. Although H. decipiens may propagate vegetatively by fragmentation (Hall, 2002), annual populations likely reestablish through germination of seeds maintained in a persistent sediment seed bank. This hypothesis is suggested by observations of ephemeral H. decipiens populations in Florida, the Virgin Islands, Panama, and Australia (Williams, 1988, McMillan and Soong, 1989; Continental Shelf Associates, Inc., 1991; Kuo and Kirkman, 1995, Kuo et al., 1995, Kenworthy, 2000). Seed banks are a valuable trait for recruitment of plants living in seasonally fluctuating environments or unpredictable habitats where disturbance is common (McMillan and Soong, 1989, Fenner, 1995, Inglis, 2000a, Kenworthy, 2000), typical growing conditions for H. decipiens.

Although several descriptive studies and many independent observations have corroborated the seasonality of the large west Florida shelf seagrass ecosystem, no one has yet examined the demography and reproductive ecology of H. decipiens and the specific mechanisms that result in persistent deep water (10–20 m) populations. The objective of this paper is to quantify and describe H. decipiens biomass, productivity and reproductive ecology along a depth gradient transect in the southern region of the west Florida shelf, extending from the shallowest to deepest ranges of the plants’ distribution. Specifically, we examine variability in the sediment seed bank, seedling density, flowering, fruiting, productivity and biomass to gain a better understanding of the mechanisms controlling the distribution and abundance of this widely dispersed species.