GrowtH PattErNS of SHoal GraSS hALodULE wRiGhTii aNd MaNatEE GraSS SYRiNGodiUM FiLiFoRME iN tHE wEStErN GUlf of MEXiCo

IntroductIon Seagrass beds are a valuable resource because of the numerous roles they play in coastal systems. Seagrass blades provide habitat for abundant micro– and macro–algal communities, which in conjunction with the seagrass, contribute substantially to primary productivity in estuarine systems (Heck and Valentine 2006). The blades and roots stabilize sediments, which improves water clarity and increases light penetration, further increasing primary production (Zieman 1982, Gacia and Duarte 2001). Seagrass beds may also provide increased growth rates, critical habitat, and predation refuges for a variety of organisms (e.g., Irlandi and Peterson 1991, Hemminga and Duarte 2000) and have an increased abundance and diversity of juvenile and adult fish and other epibenthic organisms (Summerson and Peterson 1984, Heck et al. 1995). In Texas, like most of the western Gulf of Mexico (GOM), shoal grass (Halodule wrightii), manatee grass (Syringodium filiforme), and turtle grass (Thalassia testudinum) are the most common seagrasses (Zieman 1982, Quammen and Onuf 1993, Withers 2002). Eighty percent of seagrass beds in Texas currently occur in the Laguna Madre (hereafter LM), a hypersaline lagoon that separates a coastal barrier island (Padre Island) from the Texas mainland (Pulich 1998, Tunnell and Judd 2002). Seagrasses were once common in many Texas bays, but have disappeared or declined in coverage in many areas due to anthropogenic causes (Pulich and Onuf 2007). Fortunately, the loss of seagrasses in Texas bays was offset by the increase in seagrass abundance in the LM due to the moderation of salinity after dredging of the Gulf Intracoastal Waterway (Quammen and Onuf 1993). Shoal grass can survive and grow in salinities from 5–80 (McMillan and Moseley 1967) and this seagrass dominated the LM for decades because of its ability to withstand these extreme salinities (Withers 2002). Shoal grass harbors a diverse resident fauna (Tolan et al. 1997) and the migratory redhead duck (Aythya americana) population depends on shoal grass in the LM for food during the winter season (Cornelius 1977). Shoal and manatee grass are currently present in the upper LM, with shoal grass historically being dominant (Quammen and Onuf 1993) and extensively studied (e.g., Dunton 1994, 1996). However, manatee grass is increasing in coverage in much of the LM and is steadily replacing shoal grass in this system (Quammen and Onuf 1993, Pulich and Onuf 2007). In other areas such as Corpus Christi Bay, shoal, manatee, and turtle grass have coexisted for the past 20 yrs (Czerny and Dunton 1995, K. Dunton pers. comm.). Although manatee grass is becoming increasingly abundant in Texas bays, its growth characteristics have not been measured in the western GOM. Changes in seagrass species composition can have significant community effects (Micheli et al. 2008), but the effects of a transition from shoal to manatee grass in the LM have not been extensively studied (but see Tolan et al. 1997). The goals of our study were to measure growth patterns of these two seagrass species in two locations in the western GOM that vary in salinity, epiphyte loads, and nutrient inputs.