Export of detritus from eelgrass (Zostera marina) beds near Beaufort, North Carolina, USA

Abstract

Eelgrass Zostera rnarjna and shoalgrass Halodule wrightii occupy about 93 km2 within the 532 km2 estuarine system near Beaufort, North Carolina, but account for a major portion of total primary production. To estimate exchange with adjacent areas, we measured export of detritus (including live and dead seagrass blades, roots and rhizomes, macroalgal fronds. Spartina alterniflora stems, and particulate organic matter) from 2 sheltered embayment-type grass beds and a n open water bed. We also estimated export from a much larger part of the estuary (Bogue Sound). Rates of export of Z. marina detritus from a n embayment-type bed at Phillips Island ranged from 4.9 to 6.0 g ash-free dry weight mo-' m-2 during summer 1976, equivalent to a total of 195 to 240 kg mo-' for the bed, or to 456 to 558 metric tons mo-' for the 532 km2 estuary, assuming seagrasses cover 93 km2. Rates of export of Z. marina from the Harkers Island (exposed) and Middle Marsh (embayment) beds were 1 3 and 1 g afdw mo-' m-2, respectively, equivalent to 1200 and 112 t mo-' for the whole estuary. These differences in export between embayment and open water types of seagrass beds are due to entrapment of floating detritus by embayment beds. The amount of detrital material exported from Bogue Sound was a n extremely small portion of the total estimated biomass, due to influences of basin shape. When total amounts of material exported are compared with other sources it can b e concluded that seagrass beds contribute important amounts of detrital material to the carbon cycle of thls estuary.