Contributions of benthic microalgal biofilms to sediment organic carbon stocks across a salt marsh gradient

Benthic microalgal (BMA) communities contribute significantly to food webs, nutrient cycling, and carbon flows in intertidal habitats. However, the contribution of BMA to saltmarsh carbon stocks (‘blue carbon’) is unclear. BMA and sediment total organic carbon (TOC) stocks were measured in an east coast American Atlantic saltmarsh, revealing key relationships between biofilm biomass, carbohydrate, and carbon content. BMA biomass (chlorophyll a) was highest in Sporobolus stands and mudflat habitats, with diatoms the dominant algal group, with cyanobacteria more important in upper saltmarsh sites. Habitat-specific differences in biofilm properties (biomass, carbohydrates, photopigments, near-infrared spectra) corresponded to differences in overall contributions to sediment TOC. Carbohydrates contributed between 8%-23% of sediment TOC, with the highest levels in Sporobolus and mudflat habitats. BMA biomass and colloidal carbohydrate were significantly correlated, except on lower shore sandflats. The greatest relative contribution of colloidal carbohydrate to %TOC was in upper marsh and tidal channel habitats (1%). Mudflats had the highest %TOC (up to 5% dry weight), but TOC stocks (2000 g C m-2 to a depth of 10 cm) were highest in Sporobolus habitats. A modelling approach, based on LIDAR and sediment measures, determined a BMA carbon contribution of 1.3%-8 % of sediment TOC, with the lowest values in Sporobolus and mudflat habitats. Upscaling from m2, incorporating habitat heterogeneity, gave median values of 14-16 tonnes TOC ha-1 for the North Inlet Estuary saltmarshes, of which BMA contributed 0.06-0.08 tonnes C ha-1. This approach could permit BMA contributions to blue carbon to be estimated across other saltmarshes.