Above- and below-ground production, biomass and reproductive ecology of Thalassia testudinum (turtle grass) in a subtropical coastal lagoon

Abstract

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 ± 136 g DW (dry weight) m -2 yr -1 ) was calculated from monthly measurements of leaf and rhizome production made using marking techniques. Leaf growth exhibited a seasonal pattern; monthly production ranged from 8 to 95 g DW m -2 mo -1 , equivalent to 614 ± 71 g DW m - yr -1 , Rhizome growth was seasonal, and areal below-ground production ranged between 14 and 40 g DW m -2 mo - 1 , equivalent to 339 ± 65 g DW m -2 yr -1 . 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 m -2 , with below-ground tissues accounting for 80 to 90 % of the total. There was no seasonal pattern in the below-ground 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 -2 and on average each fruit contained 2 seeds. The annual flowering event represents a substantial resource (e.g. carbon and nitrogen) investment, which 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.