Vertical structure of seed banks and the impact of depth of burial on recruitment in two temporary marshes

Abstract

The structure of the seed bank (including Chara oospores), in relation to depth within the sediment and disturbance, was studied in two Rh6ne delta temporary marshes for two years. The seeds of all species were concentrated in the top 2 cm of sediment with very low numbers beeing found below 4 cm. When an exclosure eliminated disturbances of the sediment by animals, the vertical repartition of seeds at site 2 was more pronounced than outside the exclosure.In experiment 1, the emergence capacity of seeds from different depths and buried under layers of sterile equivalent to those in the field was measured. Depending of the species, 22 to 98 ~o of the seeds germinated from unburied seeds in the top 2 cm. Only 1 ~o of the oospores of Chara (from site 2) at 2 to 4 cm depth in the sediment emerged.In experiment 2, surface seed bank samples were placed under 0, 2 or 4 cm sterile sediment depth. The samples contained numerous recent seeds and the emergence percentage reached 41 ~o (for Ruppia maritima). Only the seeds of Zannichellia spp failed to germinate from a depth of 2 cm or more. The emergence percentage from 2 cm depth or more was always lower than at the surface. These experiments showed that both burial and ageing of seeds decrease germination capacity.The majority of the active seeds located at the surface germinate when the marsh is flooded. Seeds located between 2 and 4 cm can be brought back to the surface by disturbances and play the role of a reserve involved in maintenance of populations that go without seed production for one or some years.Nomenclature: Tutin et al. (1964Tutin et al. ( -1980Talavera et al. (1986) for Zannichellia; Cotillion (1957) for the charophytes.

Burial limits to the emergence of aquatic plant propagules

“…Surprisingly, the emergence of P. ochreatus seed was limited to depths less than half that of C. globularis oospores which were l/30th the mass. However, this result is in keeping with Bonis & Lepart's (1994) findings that charophyte oospores emerged from depths (40 mm) in excess of the larger propagules of vascular species tested. They suggested that elongation by the single-celled protonema following oospore germination was a more energy efficient method of emergence than multicellular growth exhibited by angiosperm seedlings.…”

“…Comparable investigations of burial limits for emergence of submerged seedlings are few, however Bonis & Lepart (1994) also describe a declining seedling response with burial depth ranging from 0 to 40 mm for propagules of four taxa that included one charophyte. In contrast, van den Berg et al (1999) found that germling response increased with burial depth over 0-5 mm for oospores of a Chara species, collected from Lake Veluwemeer, the Netherlands.…”

“…Propagule establishment depends firstly on germination cues being received by the propagule at its position within the substrate (Parker et al 1989). Second, the germinated propagule must contain sufficient reserves to traverse the physical obstacle posed by the overlying substrate (Bonis & Lepart 1994). The outcome of both processes is best described as "emergence".…”

Growth and Reproduction of Ruppia maritima in the Northern Gulf of Mexico

“…Ruppia maritima has a broad distribution (the Mediterranean: e.g., Bonis and Lepart, 1994; both sides of the Atlantic: e.g., Orth and Moore, 1988;Figuerola et al, 2002; the Pacific: e.g., Bigley and Harrison, 1986) and is common in the northern Gulf of Mexico (Eleuterius, 1987;Cho and Poirrier, 2005). Other species of SAV are also present in the northern Gulf of Mexico (e.g., Thalassia testudinum and Halodule wrightii; Eleuterius, 1987), but R. maritima's broad salinity tolerance allows it to inhabit areas with heavy freshwater input such as Lake Pontchartrain (Cho and Poirrier, 2005) and the nearshore habitats around Mobile Bay (Johnson et al, 2006), making it a dominant species in many of these areas.…”

“…Seeds germinate from the winter through early spring and recruitment is frequently from seeds (Cho and Poirrier, 2005). Seeds can apparently remain dormant and viable in the sediments for several years and germinate when conditions are appropriate, though more deeply buried seeds (reviewed in Kantrud, 1991;Bonis and Lepart, 1994) and older seeds (Bonis and Lepart, 1994) are less likely to germinate. The species acts much like an annual plant in many years, although it can also regenerate in the spring from overwintering rhizomes (Dunton, 1990;Cho and Poirrier, 2005).…”