Production on marshes adjacent to the upper Patuxent estuary was 1,000 to 1,500 grams dry weight per square meters per year. Approximately 6 to 9 percent of the annual production was exported to the estuary as particulate carbon. Production was comparable to other marsh systems but export (as percentage of production) was less, probably due to poor tidal exchange. In spite of the relatively low percentage of their fixed carbon contributed to the estuary, the marshes provide about one-third of the total carbon budget in the upper Patuxent. A large portion of the marsh carbon enters the estuarine system in early spring when levels of algal primary production are low. Substantial production of the calanoid copepod, Eurytemora affinis occurs with detrital carbon as the apparent food base. Feeding experiments indicate that E. affinis can reproduce when fed a diet of detritus enriched with bacteria and protozoa, or when fed only protozoa. Diets of detritus and micro-organisms alone were seldom equal to algal controls.