Date of Award

1985

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

Vallisneria americana grass beds of Lake Pontchartrain contribute approximately 200 g dry weight/m('2)/yr primary production, a continuous release of detrital material, and rapid nutrient recycling. A harvest method was used to estimate production. The addition of litterfall to harvest production raised annual production estimates by up to 28%. When litterfall was corrected for decomposition, the production estimates were raised by up to 85% over harvest estimates alone. Litterfall and decomposition increased toward shore and occurred throughout the year providing a continuous supply of energy and nutrients. Grass bed production varied along the coastline and was greatest at mid-depths. Values of the environmental factors were most extreme near and far from shore. Severe environmental conditions are related to the patterns of biomass and production. The bell-shaped depth distribution pattern is perpetuated annually by survival of grass and by dormancy of fruit during the winter at mid-depths. The majority of fruit are produced at mid-depths where biomass and production are greatest. The surviving plants or new shoots start growing first at mid-depths, and later radiate out via the rhizome system to revegetate the entire grass bed. Although depth distribution patterns are similar along the coastline, the grass bed extends almost twice as far from shore at the control area as at the nitrogen enriched area. Water clarity was significantly greater at the control area than at the nitrogen enriched area. Light penetration may determine the compensation depth of the grass bed. Seasonal fluctuations of leaf and root and rhizome nitrogen simulated by GROW are very similar to field experimental values. GROW was most sensitive to the forcing functions light, temperature, and sediment nitrogen. GROW was validated using initial conditions from the nitrogen enriched grass bed and had correlations better than 80% between simulated and measured leaf and root and rhizome nitrogen. Once validated, GROW was run under normal and reduced light conditions. When light reaching the sediment surface was reduced by 50%, the Vallisneria leaf and root and rhizome nitrogen declined to zero within a year. This suggests that conditions that reduce water clarity could cause the loss of the grass beds.

Pages

286

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