Spartina alternifloraDieback Recovery Correlates with Increased Acetylene Reduction Activity in Saltmarsh Sediments

Spartina alterniflorasalt marshes occasionally exhibit areas where the plants have all died off for no apparent reason. These dieback areas remain devoid of plant growth for months to years. An analysis of microbial acetylene reduction activity (ARA) and porewater chemistry of four such diebacks has been carried out on marshes in the Cooper River estuary, South Carolina, U.S.A. Four different diebacks were studied and sediments in all of them showed low rates of ARA, relative to sediments from the control areas which were populated by healthyS. alternifloraplants. The pore water of all the dieback sediments had higher salinities than the controls when this study began, but this trend was not consistent over the 18-month study period. Furthermore, dissolved ammonium and sulfide in the pore water, microbial biomass, organic matter and macro-organic matter content did not show a consistent trend differentiating the control and dieback sediments over this study period. During the course of this study, one of the diebacks recovered and the recolonisation of the barren area with healthy plants correlated with a full recovery of sediment ARA. The return of ARA to the sediments of this recovered marsh could not be correlated, however, with any of the porewater parameters measured (including NH4+), nor with any of the sediment characteristics. It is concluded that the lower levels of bacterial ARA in the diebacks may be due to the lack of essential factors associated with healthy spartina plants.