Understanding factors that control seagrass reproductive success in sub-tropical ecosystems

Abstract

Seagrasses are submerged marine plants that provide essential ecosystem functions, but are declining in abundance worldwide. As angiosperms, seagrasses are capable of sexual reproduction, but also propagate asexually through clonal rhizome growth. Clonal growth was traditionally considered the primary means for seagrass propagation. Recent developments in genetic techniques and an increasing number of studies examining seagrass population genetics, however, indicate that sexual reproduction is important for bed establishment and maintenance. Few studies have investigated the reproductive biology and ecology of sub-tropical seagrass species, although this information is necessary for effective management and restoration. This work investigates the influence of pore-water nutrients on flowering, water flow on seed dispersal, consumption on seed survival, and describes the reproductive phenology in Texas for the two dominant seagrass species in the Gulf of Mexico: turtle grass (Thalassia testudinum) and shoal grass (Halodule wrightii). These species exhibit distinctive reproductive seasons that span summertime months, but reproductive output varies spatially and temporally. Results of an in situ nutrient enrichment experiment indicate that turtle grass produces fewer flowers (but more somatic tissue) when exposed to high pore-water ammonium than when exposed to low pore-water ammonium, suggesting that nutrient loading has the potential to reduce seagrass reproductive output. Seed consumption may also limit reproduction and recruitment in some areas, as laboratory feeding experiments show that several local crustaceans consume shoal grass and turtle grass seeds and seedlings, which do not survive consumption. Dispersal experiments indicate that seed movement along the substrate depends on local water flow conditions, is greater for turtle grass than shoal grass, and is related to seed morphology. Under normal water flow conditions in Texas, turtle grass secondary seedling dispersal is relatively minimal (< 2.1 m d⁻¹) compared to primary dispersal, which can be on the order of kilometers, and shoal grass secondary seed dispersal can be up to 1.1 m d⁻¹, but seeds are likely retained in the parent meadow. Results from this work can be used when developing seagrass management, conservation and restoration actions and provide necessary information concerning a life history stage whose importance was historically under-recognized.