Net Productivity of Emergent Vegetation at Horn Point Salt Marsh

Abstract

Analyses of monthly standing crop, daily rates of production, and variations in yearly productivity for 5 production for the Spartina patens/Distichlis spicata mixture were conducted over two consecutive growing seasons for a Chesapeake Bay brackish marsh. Regression models for plant height and dry weight biomass were generated for all that the relationship between height and dry weight within each species is the same for all seasons of the year except in the species Spartina alterniflora and Phragmites australis. Positive correlation coefficients ranged from .27 for S. alterniflora to .96 for P. australis with the other species having intermediate value. Overall, production at Horn Point is lower than most other values in the literature with the 2-year average for S. alterniflora (676 g/m2) being 1/2 the average for the Atlantic Coast but the 2-year average for S. patens (628 g/m2) being slightly higher than its Atlantic Coast average. On a square meter basis, the primary producers rank in the following order of importance for the two year average of standing crop: Typha angustifolia (985 g/m2), Phragmites australis (892 g/m2), S. alterniflora/Amaranthus cannabinus (676 g/m2), S. patens/D. spicata (628 g/m2), and Hibiscus moscheutos (531 g/m2). However, the most important zones in terms of areally weighted production (in metric tons) for 1973 at Horn Point Marsh are the S. patens/D. spicata (7.61), H. mocheutos (5.07), S. alterniflora/A. cannabinus (3.22), P. australis (0.659), and T. angustifolia/H. moscheutos (0.644). In the brackish marsh (S. patens/D. spicata) exclosure experiments demonstrated that almost 100% of the net primary production (NPP) passes through the detritus food chain but in the contiguous fresh marsh (H. moscheutos) 37% of the NPP is utilized by the grazing food chain. Underground production for S. patens/D. spicata was determined by an experimental approach involving transplantations of underground material and a dry weight shoot:root ratio of 1:16 was determined over a twelve month period. An efficiency rate for conversion of visible solar radiation to plant production in 1974 ranged from 0.11% for H. moscheutos in the Typha/Hibiscus zone to 1.12% for the Typha angustifolia/Hibiscus moscheutos mixture.