Outdoor mesocosm experiments were used to examine the response of eelgrass communities to excess nutrient loading and reduced light that simulated coastal eutrophication. A series of replicated manipulations conducted between 1988 and 1990 demonstrated the effects of reduced available light and increased loading of nitrogen plus phosphorus on habitats dominated by eelgrass Zostera marina L. Shade and nutrients each significantly affected eelgrass growth, morphology, density, and biomass. WC found no significant interactions between the effects of shade and the effects of nutrients on any plant characteristics except leaf length. The growth rate of individual eelgrass shoots was linearly related to light, increasing throughout the range of available light. Biomass and daily biomass increase, or areal growth, were also linearly related to light, but specific growth showed no response to light. Shoot density increased with the log of light.Excess nutrient loading was shown to significantly reduce eelgrass growth and bed structure through stimulation of various forms of algae that effectively competed with eelgrass for light. The absence of significant interactions between the effects of shade and nutrients on eelgrass density, growth, and biomass suggests that the negative effect of algae on eelgrass occurs primarily through the reduction of light (i.e. shading). The outcome of nutrient enrichment was a shift in plant dominance from eelgrass to three algal forms: phytoplankton, epiphytic algae, and macroalgae. We quantified the effects of eutrophication and demonstrated that increased nutrient loading results in less light for eelgrass and that eclgrass growth linearly decreases with reduced light.

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Above-and below-ground growth, biomass, phenology and reproductive effort in the seagrass Thalassia testudinum were monitored monthly for 2 yr in the Lower Laguna Madre, Texas. Annual whole plant production (953 i 136 g DW (dry weight) m-2 yr-l) was calculated from monthly measurements of leaf and rhizome production made using marking techniques. Leaf growth exhblted a seasonal pattern; monthly production ranged from 8 to 95 g DW m-' mo-', equivalent to 614 * 71 g DW m-' yr-l.Rhlzome growth was seasonal, and area1 below-ground production ranged between 14 and 40 g DW m" mo-', equivalent to 339 2 65 g DW m-' yr-'. On an annual basis, rhizome production accounted for 35% of total plant production. Seasonal leaf and rhizome growth patterns were correlated with underwater irradiance. daylength and temperature. Total biomass ranged between 750 and 1500 g DW with below-ground tissues accounting for 80 to 90 % of the total. There was no seasonal pattern in the belowground biomass of T. testudinum; variability was a result of environmental heterogeneity. Flowering was variable between years; 13 to 30% of the shoots flowered and about 15% of total above-ground biomass was allocated to reproduction. Flowering phenology was positively correlated with underwater daylength. During 1996, maximum fruit abundance ranged between 20 and 70 fruits m-' and on average each fruit contained 2 seeds. The annual flowering event represents a substantial resource (e.g. carbon and nitrogen) investment, whlch may influence individual plant production. Seasonal fluctuations in environmental parameters are the primary factors controlling seagrass growth rates and production. Determination of total plant productivity must take into account seasonal patterns, reproductive costs and the large fraction of production occurring in the below-ground tissues.

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