Ontogenetic photosynthetic changes, dispersal and survival of Thalassia testudinum (turtle grass) seedlings in a sub-tropical lagoon

Abstract

Northward expansion of Thalassia testudinum (turtle grass) in Laguna Madre is occurring faster than can be explained by rhizome growth. We hypothesized that seedling establishment can account for the measured rates of meadow expansion and that seedling carbohydrate reserves are utilized until the plant is photosynthetically self-sufficient. To address seedling establishment, we estimated seed output, seedling dispersal and survival. Carbon dynamics were calculated from measurements of biomass allocation, non-structural carbohydrate carbon reserves and photosynthetic parameters in relation to T. testudinum seedling age. Potential seed production calculated for 1996 was consistent with field observations and was estimated at 66±14 seeds m−2 bare area. Fruits can be positively buoyant for up to 10 days, while seeds were generally buoyant for <1 day. Water current measurements, made at about the time of seed release, indicate a positive net transport of 1.5 km d−1 to the north. Seedling survival in laboratory culture after 6 months was 96% compared to 11% in the field after 1 year. The average root:rhizome+seed:leaf ratio changed from 0:11:1 for a 1 week old plant to 1:3:1 for a 15 month old plant. Seedlings used to determine whole plant photosynthesis ranged in age from about 1 week (0.25 months) to 15 months. Gross Pmax increased from 80 to 220 μmol O2 gdw sht−1 h−1, while whole plant respiration decreased from 170 to 60 μmol O2 gdw sht−1 h−1. As the photosynthetic parameters changed, the average non-structural carbohydrate carbon (NSCC) reserves of the seeds decreased from 24 to 3.0 mg NSCC plant−1. Subsequent increases in NSCC were the result of rhizome development. Daily carbon balance, assessed using Hsat periods of 8–18 h d−1, predicts that T. testudinum seedlings become photosynthetically self-sufficient between 2 and 6 months. The unique characteristics of T. testudinum, including seed buoyancy, high seed production and survival rates, coupled with ontogenetic changes in carbon allocation and production imply that sexual reproduction can be important in the long distance dispersal and colonization for this species.