The interrelatonship between seagrasses, Zostera marina and Halodule wrightii, and the physical and chemical properties of sediments in a mid-Atlantic coastal plain estuary near Beaufort, North Carolina, U.S.A.

A study was undertaken to evaluate the interrelationship between the growth of seagrasses, Zostera marina and Halodule wrightii, and the physical and chemical properties of sediments in a coastal plain estuary near Beaufort, North Carolina. Sediment profiles in vegetated and unvegetated substrates were analyzed for % silt-clay, organic matter, exchangeable ammonium, ammonium dissolved in pore waters, total nitrogen, Eh and pH. Plant growth for both species was evaluated in a year long study (1978-1979) of biomass and canopy surface area. A technique was developed for measurement of the in situ net production of leaves, rhizomes and roots of Z. marina, and used to estimate the net productivity of this plant growing in sediments having different physical and chemical properties.

In sediments underlying a cover of seagrass the measured parameters were larger in magnitude than in unvegetated profiles and appear to have accumulated in the upper 15 to 18 cm. A gradient analysis of sediment profiles within and between different habitats showed that the magnitude of the physical and chemical properties varied according to the location in the vegetation. The largest pools of nitrogen, the finest sediment texture, and the greatest organic matter content were in stations associated with the mid-bed regions of seagrass meadows; areas having a cover of grass established for the longest period of time. The lowest values for each measured variable were observed in unvegetated substrate; areas not directly influenced by the seagrass. Intermediate values were found in edge and patch stations; substrates most recently colonized by the vegetation. These data were discussed in regard to contemporary theories of ecosystem development and ecological succession.

Seasonal measurements of plant biomass showed that the seagrass communities consisted of both monospecific and mixed species meadows. In mixed communities the species exhibited a bimodal seasonal pattern of abundance. Z. marina dominated the community in winter, spring and early summer, while H. wrightii was more abundant in late summer and early fall. This bimodal pattern of species abundance may be important in maintaining an extended period of influence in mixed stands of seagrass.

Z. marina allocated more net production to roots in coarse textured sediments, low in organic matter, and deficient in nitrogen relative to other stations. Total shoot net production was highest in sediments with the finer texture, greatest organic matter content and largest pools of nitrogen.

General conclusions from this study are: 1) once established, seagrasses appear capable of modifying the sediment texture, organic matter content, and the magnitude of the intermediate pools in the cycle of nitrogen; 2) nitrogen accumulates beneath the vegetation suggesting that these areas are sinks; however, functional recycling mechanisms appear to be operating as suggested by the larger magnitude of remineralized nitrogen in the vegetated profiles; and 3) although the establishment of seagrasses are not necessarily restricted by the sediment properties measured in this study, total shoot net production and the allocation of net production varied as a function of the physical and chemical properties of the sediments.

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