Blue carbon and nutrient stocks in salt marsh and seagrass from an urban African estuary

Halophytes in estuaries are effective sinks of carbon, nitrogen, and phosphorus. Blue carbon (BC) is carbon stored in coastal habitats such as mangroves, salt marshes and seagrass. The objectives of this study were to firstly assess the biomass and sediment C stocks in salt marsh (Spartina maritima and Salicornia tegetaria) and seagrass (Zostera capensis) habitats of the Swartkops Estuary, South Africa. Secondly, we applied the nutrient pollutant indicator (NPI) to assess the nutrient status of the estuary. Six replicate sediment cores of 1 m (summer) and 0.5 m (winter) per plant species were collected at six sites. Six replicates for biomass of each species were harvested at each site during summer and winter. Biomass and nutrient dynamics showed that there were distinct seasonal differences in the magnitude of C, N, and P stored in the plants. The sediment was the dominant C pool and differed spatially with creek sites storing more C. Out of the three species, S. maritima stored the most C (224 ± 19.1 Mg C ha^-1 in sediment, 16.7 ± 1.2 Mg C ha^-1 in biomass), followed by S. tegetaria (207 ± 3.5 Mg C ha^-1 in sediment, 4.3 ± 0.4 Mg C ha^-1 in biomass) and Z. capensis (180 ± 18.4 Mg C ha^-1 in sediment, 2.1 ± 0.7 Mg C ha^-1 in biomass). The N:biomass ratios were low in both seasons for Z. capensis, S. maritima and S. tegetaria (0.08, 0.02 and 0.04, respectively) indicating consistent eutrophic conditions in the estuary. Resultantly, the plants displayed a significantly lower below-ground standing biomass highlighting the potential variations of BC storage in eutrophic estuaries.