UNDERGROUND BIOMASS PROFILES AND PRODUCTIVITY IN ATLANTIC COASTAL MARSHES

An underground biomass profile and productivity study involved year-long sampling programs in 18 stands of salt marsh plants in Georgia, Delaware, and Maine. As the result of the monthly or bimonthly marsh coring program three types of underground biomass profiles were found. In the first, the concentration of macro-organic matter (MOM) was uniform with depth; the notable example of this type was creekbank Spartina alterniflora in the southern part of the coast. A second type had a high MOM concentration at the surface which decreased with depth. This, the most common type of profile, was exemplified by Spartina patens, S. alterniflora from the high marsh along the southern coast (Georgia), and creekbank S. alterniflora from the northern part of its range (Maine). The third type of profile was seen where a large rhizome mat developed 15-20 cm below the surface. Spartina cynosuroides and Phragmites communis were typical examples of this type of profile, resulting in a low biomass at the surface, a higher biomass somewhat below the surface, and a low concentration at depth. The annual maxima and minima of MOM biomass were used to calculate annual increments, which can be considered minimum annual production values. These productivity values ranged from a low of 80 g C/m2 for creekhead S. alterniflora in Maine to a high of 1690 g C/m2 for Juncus gerardi in Maine. The mean for all plant stands was 650 g C/m2. Since the average carbon content of the MOM was 35.3%, this corresponds to 1850 g dry weight/m2 per year. As a measure of the relative activity of the total pool of macro-organic material in the soil, turnover times were calculated by dividing the total macro-organic matter by the annual increments. Within the MOM pool there are several components with turnover times varying from days to years. The turnover time for the entire pool ranged from 18 months in two Georgia salt marsh plant stands to 224 months for one in Maine. In the two instances where values for a species could be compared between Maine and Georgia, the turnover time was shorter at the more southerly site. These results can probably be attributed to slower microbial decay rates in the cooler climate. In Georgia and Maine where the turnover values for a species were determined for two elevations, the time was shorter at the lower elevation. SUFFICIENT SALT MARSH aboveground plant part dynamics and primary production studies have been reported in the literature to warrant several comprehensive reviews (Keefe, 1972; Turner, 1976). In spite of the fact that most of the fixed carbon reserves of salt marshes are in the soils (Gallagher, 1974) the underground portions of salt marsh plant dynamics has received relatively little attention (Valiela, Teal and Pearson, 1976; Stroud, 1976; de la Cruz and Hackney, 1977). In 1 Received for publication 24 March 1978; revision accepted 5 October 1978. This work was supported in part by grants from the Sapelo Island Research Foundation, Inc., Contract No. DACW3973-C-01 10 as part of the Dredged Material Research Program administered through the U.S. Army Engineer Waterways Experiment Station, and National Science Foundation grant OCE75-20842-A02. 2 Present addresses: U.S. Environmental Protection Agency, Corvallis Environmental Research Laboratory, 200 SW 35th Street, Corvallis, Oregon; and Department of Botany, University of Georgia, Athens, Georgia. Contribution No. 363 from the University of Georgia Marine Institute, Sapelo Island Georgia. the study reported here we conducted a sampling program aimed at identifying the types of underground macro-organic matter (material not passing a 1-mm sieve) profiles found within and between species in a given marsh and along the latitudinal gradient of the eastern coast of the United States. Further, we designed the program to examine the seasonal dynamics of the quantity of macro-organic matter (MOM), from which we computed minimum values for underground production. METHODS AND MATERIALS-Samples were collected along the U.S. Atlantic Coast gradient from March 1972 to April 1975 from sites in Georgia, Delaware and Maine (Table 1). In Georgia, Spartina cynosuroides was sampled on a small island near the mouth of the Altamaha River where water salinity was usually less than 3%o. Borrichia frutescens, Distichlis spicata, Spartina patens, and Sporobolus virginicus were sampled in marshes developed along creeks behind the dune complex on the eastern