Long range transport and carbon and nitrogen dynamics of floating seagrass wracks in Greater Florida Bay
Introduction
Seagrass ecosystems serve to interconnect marine and terrestrial ecosystems through passive and active transport of nutrients, carbon, detritus, prey and consumers (Heck et al., 2008). An important element of this interconnection is through the production and transport of seagrass detritus from one habitat to another. As seagrass meadows age, senesce, and interact physically with the environment around them, leaves may break off from the beds and either sink to the seafloor surrounding the plants, or float to the surface. Due to the high primary productivity and turnover rates of seagrass leaves (Zieman et al., 1989), considerable amounts of leaves are shed from the meadows and can be transported away from the beds by currents and waves (Davis III et al., 2004; Mateo et al., 2006; Duarte and Krause-Jensen, 2017). Large aggregations of floating vegetation called “seagrass wrack” can be formed (Dierssen et al., 2015) and the biomass is ultimately exported to the seafloor or washed ashore on beaches. Many studies have quantified nutrient subsides from wrack to beach communities (Coupland et al., 2007; Dugan et al., 2011) and the deep sea (Vetter and Dayton, 1998, 1999), but less is known about the habitat structure and nutrient dynamics of floating seagrass wrack, particularly in surface waters devoid of nutrients (Duarte and Krause-Jensen, 2017).
Although reports are few, different species of seagrass appear to export different amounts of biomass ranging from 0 to 100% of total production (Mateo et al., 2006). The morphology and buoyancy of the leaves can determine the duration of the floating wrack with long, bulky leaves sinking soon after shedding and light, thin leaves staying afloat for longer period of time (Mateo et al., 2006). For example, a study on adjacent beds of broad flat-leafed Thalassia testudinum Banks & Sol. ex Koenig (turtle grass) and thin cylindrical-leafed Syringodium filiforme Kuetz (manatee grass) from a site in the U.S. Virgin Islands found that T. testudinum exported only 1% of its leaf production, while S. filiforme exported 60–100% of its biomass (Zieman et al., 1979). Our study evaluates floating wrack produced in Greater Florida Bay which is home to large seagrass meadows of both T. testudinum and S. filiforme (Fourqurean and Robblee, 1999; Fourqurean et al., 2001; McPherson et al., 2011; Gilerson et al., 2013).
Weather, tides, and the degree of bed exposure can determine the intensity of the physical forces that serve to export seagrass leaves from the beds to distant locations offshore (Thomas et al., 1961; Davis III et al., 2004; Mateo et al., 2006). Dierssen et al. (2015) found that strong southerly winter winds in Greater Florida bay advected considerable amounts of seagrass wrack comprised predominantly of S. filiforme from the dense meadows in Greater Florida Bay to oligotrophic Atlantic Ocean waters. Over time, the leaves became more aggregated into patches and could be found in long windrows produced by downwelling lobes of Langmuir circulation. In addition, the wrack observed floating over the continental shelf contained aggregates of leaves occurring in whorled structures. During the winter storm season, considerable litter from mangroves and seagrass beds can be advected from southeastern Everglades National Park (Davis III et al., 2004). Off the Tasmanian coast, Thresher et al. (1992) similarly found offshore transport of seagrass detritus coincided with strong winterly storms (Thresher et al., 1992). Winds and turbulence associated with the 1960 Hurricane Donna produced over 1 million kg of T. testudinum wrack washed ashore along the beaches in Biscayne Bay (Thomas et al., 1961).
Floating at the sea surface, seagrass wrack can serve as a habitat or a metabolic “hot spot” similar to other floating vegetation such as floating macroalgae Sargassum sp. (hereafter Sargassum). In tropical waters, pelagic Sargassum wrack is an important home to many species of organisms, including juvenile fish species, many invertebrate species including shrimp, crabs, and nudibranchs, and epiphytic organisms like hydroids, bryzoans, and algae (Dempster and Kingsford, 2004), as well as recreationally and commercially important fish species like mahi, snapper, and grouper (Coston-Clements et al., 1991). Thresher et al. (1992) is one of the only studies to report on the trophic impacts of floating seagrass wrack. Through indirect lines of evidence, they report that microbial decomposition of floating seagrass played a pivotal role in the coastal planktonic food chain (Thresher et al., 1992). This study aims to bridge this knowledge gap on trophic impacts of wrack by examining the degradation rate and capacity for nutrient regeneration of Florida Bay wracks composed of a variety of primary producers. Travel time of wracks and the shedding rate of seagrass beds were also examined to inform understanding of the life cycle of a floating seagrass wrack.
Section snippets
Study region
Field observations and collections were conducted in Greater Florida Bay during January 2014 with experimental work conducted at the Keys Marine Laboratory on Long Key, Florida, United States. Greater Florida Bay is a shallow estuary grading into a tropical lagoon, influenced by the Gulf of Mexico and the Everglades (Fig. 1). The south Florida region, which includes Greater Florida Bay and the Atlantic Ocean side of the Florida Keys, hosts over 10,000 km2 of seagrass. Thalassia testudinum has
Transport of wrack
Seagrass wrack was observed in large quantities within Greater Florida Bay as well as on the Atlantic Ocean side of the Keys during January. Wrack on the sea surface was observed to be 97% S. filiforme and <3% T. testudinum (Dierssen et al., 2015). Drifter buoys deployed over the dense seagrass beds north of Long Key provided an estimate of the distance that leaf detritus formed in Greater Florida Bay traveled on the sea surface due to winds and currents (Fig. 3). The first buoy deployment
Discussion
Through a combination of field and laboratory experiments, our results show that floating seagrass wrack can be considered a relatively long-lived ecosystem “hot spot” providing habitat and carbon and nitrogen to the oligotrophic ocean comparable to floating mats of Sargassum. Here we discuss the fate, remineralization, and ecological implications of exported seagrass wrack.
Acknowledgements
This work was funded by the U.S. National Aeronautics for Space Administration Ocean Biology and Biogeochemistry program (NNX13AH88G) to H. Dierssen and the University of Connecticut. We thank A. Chlus, J. Godfrey, and B. Russell and the staff at Keys Marine Laboratory for assistance in conducting field experiments. We also acknowledge NASA's Ocean Biology Processing Group for processing and distribution of the MODIS imagery and the University of South Florida for maintaining the wind
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All authors contributed equally to this work.