Density-Dependent Effect On Grazing And Success Of Seed Generated Seagrass (Zostera Marina L.) Plants

Principal Investigator (s):

Scott W. Nixon
Stephen Granger
Brian Maynard
Malia Schwartz

Work Accomplishments

Tasks for the Period.

From January 2000 through July 2000, our project focused on the continuation of experiments initiated in the fall of 1998 and 1999. This report will focus on five topics. First, the final measurements of a seeding density experiment begun in 1998. Second, observations of the germination and success rate of seeds and seedlings planted at four different densities and in sediments with four concentrations of organic material. Third, an investigation of the effect of water velocity on the morphology of developing seedlings. Fourth, studies of variation in seed germination and variation in seedling survival between seeds planted in field locations and those planted in the aquarium. And fifth, the design and initial construction of a planting machine for the mechanized seeding of Zostera marina L. seeds at the depth and densities which we have determined will achieve the highest seedling success.

Seed Density Experiment:
This experiment was designed to investigate seedling density and sediment organic content on seedling survival. Zostera marina L. seeds were planted at four different densities (500, 1000, 2000 and 4000 seeds m-2) in high-organic sediment from Lower Narragansett Bay, and low-organic sediment from Rhode Island Sound. During the first growing season, test plots seeded at the highest densities achieved the greatest number of healthy seedlings; however, at these densities we observed the greatest loss of seedlings during senescence in August. Plots seeded at lower densities lost fewer seedlings during this time. As a result, we reported that counts of shoot density taken in the 4000 and 2000 seeds m-2 test plots, during August 1999, showed significant seedling loss, with a lower rate of loss in the 500 and 1000 seeds m-2 test plots

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We have continued our counts of seedling emergence through the spring and terminated the experiment in July 2000. Many seed germination studies stop at the end of initial germination and therefore little is known about the long-term survivability of seedlings. This experiment demonstrated that healthy seedlings produced from seed can successfully over-winter and grow through a second season.

Our analysis of the total above- and below-ground biomass produced in each plot after two growing seasons will determine a seeding density that will produce the optimum number of seedlings and plant biomass (a measure of habitat value) for the fewest number of seeds. Proper seeding densities, as determined from this project, will be used to aid in the design of the seeding machine in Year 3.

Sediment Organic Content Experiment:
The results from our initial seeding density experiments displayed significant differences in germination, lateral shoot production, node production, and canopy height between the different sediment types (table 1, see figure&table.xls). This result was compelling and warranted further investigation of the effects of sediment organic content on germination and survival of eelgrass seeds and seedlings.

We lined 0.016 m3 trays with 4 different levels of organic material (0.1, 0.7, 2.0, 3.1 measured by % loss on ignition at 500oC), and seeded triplicate treatments at 500, 1000, 2000 and 4000 seeds m-2. We had two main goals for this reporting period. First, to monitor seedling emergence until the initial stage of germination ended some time in early May 2000. Second, after ending the experiment in July 2000, to compare total above-ground biomass to below-ground rhizome material and investigate lateral shoot and node production. The relationship of sediment organic content and the ultimate success of seedlings determined from these experiments will guide us in the selection of the most appropriate restoration site and ultimately aid in the determination of the appropriate seeding density to be used at that site.

Water Velocity Experiment:
The goal for this project during the reporting period was to design and run a
water velocity experiment to determine the effects of water velocity on seedling development. The experiment was to begin in April and run through June 2000. Plants were germinated and raised over the prior winter in a range of water velocities. Measurements of morphological changes such as leaf length, production and growth etc. were scheduled for regular intervals from spring through the summer. Again, this information was to guide us in choosing appropriate field test sites during the third year.

Field Vs. Aquarium Germination and Seedling Success:
To date, our field trials with eelgrass seed have shown lower overall success than our aquarium plantings. This project was designed to investigate the effects of seeding density, sediment type, and the time of field transplanting on seeds and seedlings in aquaria and field locations. Our previous results showed differences in successful germination and seedling survival between the two sediment types, the four seeding densities, and between aquarium and field plantings.

Pots were filled with two sediment types (high and low organic) and seeded at the same four densities used in the previous density experiment. After seeding, the pots were held in flowing seawater tanks until transplanted to the field. One transplant occurred in December 1999, shortly after planting. A number of pots were kept in the aquaria to determine if allowing the seedlings to grow in a protected environment before transplantation would increase their likelihood of survival. In February 2000, and again in April, pots were taken from the aquaria and transplanted to the field. During the April transplant, 32 pots from each of the two earlier field plantings were excavated, as were 32 pots from the aquaria plantings. They were then sieved and the number of remaining ungerminated seeds or seed coats determined, to ascertain whether lower field germination was due to seed loss, or if other factors were involved. Our aim is to achieve a similar germination success, lateral shoot production and leaf development in field plots as we see in our experimental tanks; the difficulties encountered in transferring our experiments to field sites has guided us in our collaboration with Brian Maynard’s development of a seeding machine.

Work Plan to Accomplish Tasks.

Seed Density Experiment:
Counts of seedlings were conducted in early June and at the end of the experiment in July. Completion of the project occurred at the end of the second year, and involved a count of surviving plants in each plot and a comparison of above-ground plant biomass to below-ground rhizome material between experimental plots. The tanks were drained and 20 plants randomly subsampled from each plot. We measured total leaf surface area, individual plant biomass, epiphyte coverage and internode length (first 6 nodes were measured) for each plant. After subsampling, the tank was drained and the remaining plants in each plot excavated. Above ground material was separated from the roots and rhizomes, pooled by plot, dried and weighed separately. At the time of this report, samples are still being processed; results will be included in our next progress report.

Sediment Organic Content Experiment:
During this reporting period, we counted seedling emergence in May 2000, and measured maximum leaf length (canopy height), in June 2000, finally terminating the experiment in July 2000. Six randomly selected plants were sampled from each treatment and measurements of total leaf surface area recorded. The remaining plants were excavated, and the sediments rinsed from the rhizome material. Many of the rhizomes remained intact allowing us to trace the history of the individual seedling back to the original cotyledon. As a result, the number of lateral shoots produced by the seedling during one growing season could be recorded for each plant. All shoots were counted and separated from the below-ground rhizome material, dried and weighed for a comparison of above- and below-ground biomass between treatments.

Water Velocity Experiment:
During the winter and early spring, we constructed three experimental circular tanks (6.5 m x 1.5m), two with paddle wheel driven flow, and a third (control) with no flow. The tanks were filled with UV sterilized, filtered (5µm cartridge) seawater. The paddle wheels are driven by two electric motors, set at 4.5 rpm and 13 rpm. A timer reversed the direction of paddle rotation every 6 hours; in this way, the plants experienced alternating water motion simulating the flood and ebb of tides. Moving from the center of rotation toward the outside of each tank water velocities range from 2 to15 cm sec-1 in one tank and from15 to 30 cm sec-1 in the other. Pots are placed at three different radial distances, exposing plants to 6 different velocities in the two tanks. To decrease turbulent flow and sediment scouring, a circular disk has been installed to create a false bottom in each tank. Holes machined in the plates hold each pot containing sediment and seeds flush to the surface of the disk. Our tests of this design indicate that sediment will not erode at the test velocities used.

As described in the previous report, seeded plastic pots were held in flowing seawater tanks through the initial germination stage. In April 2000, the pots were transferred to the experimental tanks. By May 2000, we noticed that many of the seedlings were losing pigment and turning a whitish to light brown color. The effect was seen across all treatments, hence a velocity effect was ruled out as the cause. By June 2000, many of the plants had been lost and the experiment was terminated. Further investigation led us to believe that the loss of plants may have been due to high water temperatures (>28oC). Our experimental design relied on a closed seawater system in order to reduce epiphytic fouling of the plants. Water changes occurred weekly during the course of the experiment, where 75% of the tank volume was exchanged. In May 2000, a refrigeration system was put in place to cool the water; however, this proved inadequate. In July 2000, the experimental tanks were redesigned to allow for a flow-through seawater system. We feel this modification will address overheating in the tanks, but will necessitate that we regularly remove nuisance algae from the tanks.

Field Vs. Aquarium Germination and Seedling Success:
Work during this reporting period focused on the spring transplant of pots from aquaria to the field site. Pots seeded in December and allowed to germinate in a controlled flow-through seawater tank were transplanted to our test site next to an existing eelgrass bed located in the West Passage of Narragansett Bay. In April 2000, a subsample of pots from each December and February transplant group were excavated from both the field site and the aquaria, retaining the plants and sediment. Pots were sieved through a stainless steel screen (500 µm mesh). The number of plants in each pot and the number of viable or rotted seeds were counted. All plants were separated from the below-ground material, pooled by treatment, dried, and weighed to attain a total above-ground biomass. The number of emerged seedlings was monitored monthly in the aquaria pots and less frequently on those in the field. Maximum leaf length was measured on the aquarium plants in February and May 2000, and on the field plants in May 2000.

Concerns or Difficulties.

We have indicated changes in experimental design of the water velocity experiment and do not foresee any difficulties that cannot be solved. We have struggled with the University purchasing system to lease a pump needed in the development of our seeding machine. However, I believe that we have found the means to test the pump without actually purchasing it. Our only concern is the limited time (due to the end of this funding cycle) we will have to observe the success and persistence of the test beds. We have discussed the prospect of following the persistence of our seed trials in the field for a few years by making periodic surveys to document them, when time and finances allow. We will make any new data from these trials available to CICEET.

Anticipated Success in Meeting Project Objectives in Scheduled Project Period.

We do not anticipate any difficulties in meeting our objectives during the third year. Our success will be dependent upon selecting field sites that are favorable to the propagation of eelgrass from seed. It will be extremely important that we use the information gathered from our experiments to determine the most favorable seed application rate and proper depth to ensure high germination.

Preliminary data

We have included some of our preliminary data earlier in the text. Many of the plant samples taken from our test plots are still being processed and results will be presented in the next progress report.

Tasks and Activities for the Next Report Period.

Tasks for the Next Report Period.

We will focus our efforts toward final modifications to the seeding machines and the design of a series of field trials that will determine not only the success of this approach but the costs associated with its application in large scale restoration efforts.

Work Plan to Accomplish Tasks.

We will perform a number of preliminary test with the planting machines under controlled conditions in our large seawater tanks. After determining the appropriate seed application rate and making any necessary adjustments to the machine design, we will begin a series of field and aquaria plantings. We have several years of experience in raising seeds in the type of sediment that we are likely to encounter in the field. The information we have gathered during our experiments will be used as baseline data for germination success and will act as a reference for our field trials. Test plots located in the seawater tanks will be seeded at the same time as our field trials. The comparison between seedling emergence in the field and in our tanks will provide the needed information to determine if year-to-year environmental variability has a role in the outcome of our trials

We will locate two test plots in Narragansett Bay and/or Ninigret pond (Charlestown, R.I.) by means of surface buoys and bounded by permenent PVC posts. Field tests of the seeding machines will occur in October and again in December 2000. Diver observations will document the emergence and health of the seedlings until the end of this project in August 2001.

Concerns or Difficulties

We do not anticipate any problems that will prevent the completion of our objectives.

Expenditures

Our expenditures have been within our anticipated budget, although we have incurred unexpected expenses associated with the machining and fabrication of the seeding sled. However, I believe we can produce and test our two seeding machines with our current funding