The past year has been exciting for the Virginia Coast Reserve Long-Term Ecological Research Program at the University of Virginia.
Thirty-three LTER subprojects were initiated during the year, ranging from literature reviews to long-term experiments. Most of the field projects were designed to provide survey data on basic ecosystem properties (e.g., spatial variability in soil organic matter and soil nutrient pools) for which little or no field work was devoted to designing, perfecting and implementing sampling methods appropriate to long-term studies in a harsh environment. Plans for the forthcoming field season include new initiatives in stable isotope analysis, population genetics and microevolutlon, physiological ecology of evergreen shrubs, and plant demography.
GIS activities progressed on several fronts. Semi-rectified versions of the 1974 vegetation maps compiled by Cheryl McCaffrey for Hog, Rogue, Wreck, Cobb, Little Cobb, and Cedar Islands have been complied using a ERDAS GIS. The GIS images have a resolution of 2x2 m. Final rectification to the UTM coordinate system is expected when copies of the original aerial photographs are received in January.
A geomorphological data layer was developed for Hog Island. Aerial photographs from 1985 (1:12,000 scale) were projected onto a base map, classified based on visible geomorphological and vegetative features, and digitized using ERDAS to a 10 m resolution. The resulting GIS image has 20 classes of landscape elements ranging from active beach to low saitmarsh. A corresponding data matrix exhibits the relationship between the landscape elements and the dominant processes responsible for geomorphological change in each element (e.g., storm overwash, aeolian deposition). Field work during summer 1989 will test the validity of the process matrix.
A botanical dataset based on information compiled by Cheryl McCaffrey has been entered into the VCR/LTER database. The dataset consists of two data files. The first contains a preliminary listing of vascular plant species known to occur on any of the Virginia barrier islands, along with common names, origin (introduced, native), and growth form (shrub, tree, herb). The second file contains a listing of each island on which a species occurs, the vegetation types in which it occurs, and its relative abundance. Both data flies will expand as additional species are collected on the islands, Above-ground primary production of marsh cordgrass (Spartina alterniflora) is being estimated on a series of permanent plots. The methodology, developed by Jim Morris at the North Inlet LTER site, utilizes length-weight relationships of permanently marked plants. Sediment pore-water chemistry and hydrology are being monitored at the same sites. Preliminary results suggest that both morphology and primary production of Spartina are strongly affected by pore-water conditions, particularly salinity. Ultimate controls appear to include a combination of local climatology, terrestrial run-off and seasonal sea level fluctuations. Grazing by crabs on Spartina appears to be significant in certain sections of the coastal marshes. Emphasis will be placed during the coming year upon quantifying the Impact of this herbivore grazing.
Four projects were initiated by the microbial ecology group during the past year, including three decomposition studies (each concerned with a different temporal scale) and a study of variation in water-quality along the box transect between the mainland and Hog Island. Studies of decomposition are usually conducted on time scales ranging from several weeks to months and years. We have initiated studies with time scales ranging from several days (short-term) to months (intermediate-term) to years (long-term). In the short-term experiments, litter dry weight loss and the growth and activity of microorganisms were examined in detail during the first 2 weeks of decay and compared with later stages of decay. Litter bags containing eelgrass (Zoster marina) were submerged to the sediment-water interface, and were sampled dairy for 14 days and after 4 and 6 weeks.
Measurements of detritus weight loss and microbial oxygen consumption, abundance, biomass, activity, and growth rate were made on each sample. Weight loss was rapid during the first 14 days, with 20% of the original dry weight disappearing in the first 24 hours. Bacterial abundance increased with time while biomass increased at a slower rate as a result of a decrease in average cell size. Microbial activity (oxygen consumption and respiration of acetate) and bacterial growth rates were consistent with detrital weight loss, and peaked within 24 hours. These results support the hypothesis that the initial litter weight loss is due to leaching of readily soluble materials but that rapid microbial assimilation of the dissolved matter results in the reentry of much of the carbon and energy into the detrital food web as microbial cells.
An intermediate length (one year) study of Spartina alterniflora decomposition was begun in March 1988. Root decomposition and root growth into the litter bags is being studied at depths ranging from the marsh surface to -40 cm using 50-cm long litter bags inserted vertically in the marsh sediment. Results from the first 7 months indicate different trends in the rates of weight loss between the creek bank and the marsh interior. Decomposition rates on the marsh surface have exceeded those of the buried plant material, but no clear differences in rates have been observed between litter buried 0- 10 cm and 30-40 cm below the surface. These results, along with measurements of sediment moisture and platinum electrode potential, suggest that variation in the rates of belowground composition may be related to differences in the degree of sediment aeration. We are currently testing this hypothesis in laboratory simulations with controlled drainage regimes.
A long-term study of the acorn position of Spartina foliage and roots was initiated on a geologically active section of Paramore Island in June 1988. This experiment takes advantage of the dynamic nature of the barrier island environment, particularly the burial of saltmarshes on the interior side of the island by sand which Is transported across the island by wind 1and oceanic overwash. The rate of sand movement over the marshes behind Parramore is as rapid as 5 meters per year. The buried marsh grass emerges on the ocean side (front) of the island as soon as 5-10 years after burial, often showing little evidence of decay.
To examine the rate of decomposition during the interval between marsh burial and emergence, litter bags were buried at the interface between salt marsh and island geomorphological active zone. These bags will be sampled once each year until they emerge on the beach. Additionally, cores taken along a transect normal to the beach face will provide information on the nature of change in the plant material as the island passes over it.
Water quality monitoring at 10 permanent stations from the mainland saltmarshes to Quinby Inlet was started in July 1988. These stations are sampled monthly for bacterial abundance, microbial activity (respiration and incorporation of acetate), dissolved organic carbon (DOG), particulate organic carbon (POC), pH, salinity, oxygen concentration, turbidity, temperature, and sediment characteristics. Summer results show the greatest bacterial abundance and microbial activity in the mainland marsh creeks with declining abundance and activity with increasing distance from the mainland marshes to a minimum in the inlet. Bacterial abundance and microbial activity in the back-barrier island marsh creeks are substantially greater than at non-marsh stations but are significantly less than in the mainland marsh creeks. Our current hypothesis is that the influence of the mainland (greater nutrient input, greater sediment loads, etc.) is responsible for higher microbial abundance and activity in the landward marshes as compared with the island marshes. Continued monitoring will provide more information regarding seasonal fluctuations in microbial processes occurring in the water column.
Ecological modeling on the VCR/LTER site has been strongly oriented to developing a basis for inter-site comparisons using models (Shugart 1988, Shugart and Urban 1988, Shugart et al., in press) and in augmenting site- specific data collection. This orientation is currently focused on vegetation models that simulate the interaction of plants of differing life forms (grasses, shrubs, and trees). The philosophy behind this approach and some of the initial results have been published in a recent workshop in Germany that discussed the use of long-term ecological research in both global and local studies (Shugart et al., in press). This work has been funded in part by the VCR/LTER Program and in part by the LTER coordinating committee grant. This project is a collaborative effort with scientists at the Central Plains and Konza Prairie LTER sites as well as with other collaborators throughout the LTER network. Bill Lauenroth (Central Plains) is spending his sabbatical at the University of Virginia and has been actively involved in this inter-site modeling project.
For additional information contact Ray Dueser, Dept. of Environmental Sciences, University of Virginia, Charlottesville, VA 22903.
References:
Shugart, H.H. 1988. The Role of Ecological Models in Long-Term Ecological Studies. p. 90-109. in G.E. Likens (ed.), Long-Term Studies in Ecology. Springer Verlag, New York.
Shugart, H.H. and D.L. Urban. 1988. Scale, Synthesis, and Ecosystem Dynamics. p. 279-290. in L.R. Pomeroy and J.J. Alberts (eds), Concepts of Ecosystem Ecology. Springer Verlag, New York.
Shugart, H.H., G.B. Bonan, D.L. Urban, W,K. Lauenroth, W.J. Parton, and G.M. Hornberger. in press. Computer models and long term ecological research. In P.G. Risser (ed.), Long-Term Ecological Research and Global Ecology. Scope Series, John Wiley, New York.