James E. Kaldy

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

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.

Factors Controlling Seagrass Revegetation onto Dredged Material Deposits: A Case Study in Lower Laguna Madre, Texas

Abstract Our objective was to evaluate the influence of water quality and sediment chemistry on the survival and growth of Halodule wrightii transplanted onto unconsolidated dredged materials in Lower Laguna Madre, TX. Subsequent to transplanting activities, we measured environmental conditions and seagrass parameters at transplant and natural beds over a 1-yr period. Although water quality characteristics at the transplant and comparison sites were compatible with seagrass growth, transplants failed to survive for more than a few months. Seagrasses at natural sites received high light (>6000 mols m−2 y−1) and exhibited typical patterns of annual growth, biomass and density as well as sediment chemical parameters. In contrast, the estimated annual quantum flux of 2500 to 3200 mols m−2 y−1 at the transplant sites was near the minimum light requirements for H. wrightii. The marginal light environment was a consequence of high turbidity from wind-driven sediment resuspension. Sediment erosion at the transplant site also resulted in a 30 cm increase in water depth. Sediment NH4+ concentrations at the transplant sites were at or above the maximum values for Texas seagrasses (up to 600 µM). Although NH4+ is generally considered a nutrient, recent evidence suggests that moderate to high NH4+ concentrations can be toxic to below ground tissues. We hypothesize that substrate loss, chronic stress from elevated sediment NH4+ levels coupled with minimal light caused the demise of the H. wrightii transplants. Consequently, this work illustrates the importance of site history and sediment bio-geochemistry as factors that control the success of seagrass transplanting efforts.

Variation in Macroalgal Species Composition and Abundance on a Rock Jetty in the Northwest Gulf of Mexico

Seasonal variation in algal species composition and biomass was examined on a rock jetty at Port Mansfield, Texas, in the northwest Gulf of Mexico between July 1993 and May 1994. Ten sites were established and monitored to examine spatial and temporal variation in biomass and species composition. Sampling depths varied among sites, but generally ranged from 0.3 to 1.0 m below mean sea level (MSL). Over 30 algal species were collected during the study period, with the overall species composition dominated by rhodophytes, especially Bryocladia spp., Gelidium crinalelPterocladia bartlettii complex, Hypnea spp., Centroceras clavulatwn and Polysiphonia spp. The calculated Cheney ratio of 5.0 indicates a mixed algal flora with strong tropical affinity that can be classified as sub-tropical. All sites exhibited greater than 50% similarity of algal species, indicating a relatively homogeneous algal distribution. Data on the relative abundance of algal species at each site indicate that most species were present year-round. There was little seasonal change in relative contribution to overall biomass by each of the three algal groups; red algae accounted for 94% of the average annual biomass compared to 5% for green algae and 1% for brown algae. However, there were major seasonal differences in total algal biomass. Average biomass was lowest in August 1993 (405 gdw m~) and highest in May 1994 (1691 gdw m~). The low biomass during the late summer probably reflects the effect of increased environmental stress (e. g., desiccation and high temperatures) on plant productivity and possibly, increased herbivory by sea urchins. The high biomass of algae recorded on the north jetty at Port Mansfield indicates that these plants represent a substantial food and habitat resource to marine animals, both invertebrates and vertebrates, which is available year-round.

The light requirements for growth and photosynthesis in seagrasses with emphasis on Texas estuaries : a literature survey

During the last 20 years, seagrass communities throughout the world have experienced decreased productivity and distribution. These declines have often been attributed to decreased water transparency as a result of turbidity or shading by epiphytic algae. Epiphytic shading is often an indication of nutrient enrichment caused by anthropogenic inputs. Although both epiphytes and turbidity occur as natural phenomena, human activities can exacerbate existing natural conditions with adverse effects on seagrass communities. The objectives of this study were (a) to review the existing literature and data available on the effect of natural and anthropogenic factors on the underwater light environment; (b) to examine the relationship between light and seagrass distribution and productivity; and (c) to make recommendations on how to protect seagrass habitats in Texas bays and estuaries. To meet these goals, we have examined the available literature, emphasizing the physiological response of seagrasses to light and temperature. By using data and observations collected on a variety of species from around the world we may be better able to define the light requirements of Texas seagrasses. A knowledge of the minimum annual light requirements for seagrass growth is necessary to maintain the current distribution of Texas species. This information will also be required in the development of a management plan that permits the expansion and establishment of new seagrass habitat.