Origin, transport and fate of organic matter in Florida Bay: A biomarker record of historical environmental changes

The Everglades are undergoing the world largest wetland restoration project with the aim of returning this system to hydrological conditions in place prior to anthropogenic modifications. Therefore, it is essential to know what these pristine conditions were. In this work, molecular marker (biomarker) distributions and carbon stable isotopic signatures in sediment samples were employed to assess historical environmental changes in Florida Bay over approximately the last 4000 years. Two biomarkers of terrestrial plants, particularly for mangroves (taraxerol and C29 n-alkane), combined with two seagrass proxies (the Paq and the C25/C 27 n-alkan-2-one ratio) revealed a sedimentary environmental shift from freshwater marshes to mangrove swamps and then to seagrass dominated marine ecosystems, likely as a result of sea-level rise in Florida Bay since the Holocene. The maximum values for the Paq and the C 25/C27 n-alkan-2-ones occurred during the 20th century, suggesting that the greatest abundance of seagrass cover is a recent rather than a historical, long-term phenomenon. The greater oscillation in frequency and amplitude for the biomarkers after 1900 potentially reflects an ecosystem under increasing anthropogenic stress. Several algal biomarkers such as C20 highly branched isoprenoids (HBIs), C 25 HBIs and dinosterol indicative of cyanobacteria, diatom and dinoflagellate organic matter inputs respectively, increased dramatically in the latter part of the 20th century and were attributed to recent anthropogenic changes in Florida Bay. ^ The highlight of this work is the development of HBIs as paleo-proxies. As biomarkers of diatoms, the C25 HBIs in the core from the central bay displayed the highest concentration at mid depth, reflecting strong historical inputs of diatom-derived sedimentary OM during that period. In fact, the depth profile of C25 HBIs coincided quite well with historical variations in diatom abundance and variations in diatom species composition in central Florida Bay based on the results of fossil diatom species analysis by microscopy. This study provides evidence that some C25 HBIs can be applied as biomarkers for certain diatom inputs in paleoenvironmental studies. The sources of C20 and C30 HBIs and their potential applicability as paleo-proxies were also investigated and their sources assessed based on their δ13C distributions. ^

Molecular characterization of Cladium peat from the Florida Everglades: biomarker associations with humic fractions

The accumulation and preservation of peat soils in Everglades freshwater marshes and mangrove swamps is an essential process in the ecological functioning of these ecosystems. Human intervention and climate change have modified nutrient dynamics and hydroperiod in the Everglades and peat loss due to such anthropogenic activities is evident. However, not much is known on the molecular level regarding the biogeochemical characteristics, which allow peat to be preserved in the Everglades. Lipid biomarkers trapped within or bound to humic-type structures can provide important geochemical information regarding the origin and microbial transformation of OM in peat. Four lipid fractions obtained from a Cladium peat, namely the freely extractable fraction and those associated with humin, humic acid, and fulvic acid fractions, showed clear differences in their molecular distribution suggesting different OM sources and structural and diagenetic states of the source material. Both, higher plant derived and microbial lipids were found in association with these humic-type substances. Most biomarker distributions suggest an increment in the microbial/terrestrial lipid ratio from the free to humin to humic to fulvic fractions. Microbial reworking of lipids, and the incorporation of microbial biomarkers into the humic-type fractions was evident, as well as the preservation of diagenetic byproducts. The lipid distribution associated with the fulvic acids suggests a high degree of microbial reworking for this fraction. Evidence for this 3D structure was obtained through the presence of the relatively high abundance of α,ω-dicarboxylic acids and phenolic and benzenecarboxylic compounds. The increment in structural complexity of the phenolic and benzencarboxylic compounds in combination with the reduction in the carbon chain length of the dicarboxylic acids from the free to fulvic fraction suggests the latter to be structurally the most stable, compacted and diagenetically altered substrate. This analytical approach can now be applied to peat samples from other areas within the Everglades ecosystem, affected differently by human intervention with the aim to assess changes in organic matter preservation.

Biomarker Assessment of Spatial and Temporal Changes in the Composition of Flocculent Material (Floc) in the Subtropical Wetland of the Florida Coastal Everglades

Flocculent material (floc) is an important energy source in wetlands. In the Florida Everglades, floc is present in both freshwater marshes and coastal environments and plays a key role in food webs and nutrient cycling. However, not much is known about its environmental dynamics, in particular its biological sources and bio-reactivity. We analysed floc samples collected from different environments in the Florida Everglades and applied biomarkers and pigment chemotaxonomy to identify spatial and seasonal differences in organic matter sources. An attempt was made to link floc composition with algal and plant productivity. Spatial differences were observed between freshwater marsh and estuarine floc. Freshwater floc receives organic matter inputs from local periphyton mats, as indicated by microbial biomarkers and chlorophyll-a estimates. At the estuarine sites, the floc is dominated by mangrove as well as diatom inputs from the marine end-member. The hydroperiod (duration and depth of inundation) at the freshwater sites influences floc organic matter preservation, where the floc at the short-hydroperiod site is more oxidised likely due to periodic dry-down conditions. Seasonal differences in floc composition were not consistent and the few that were observed are likely linked to the primary productivity of the dominant biomass (periphyton in the freshwater marshes and mangroves in the estuarine zone). Molecular evidence for hydrological transport of floc material from the freshwater marshes to the coastal fringe was also observed. With the on-going restoration of the Florida Everglades, it is important to gain a better understanding of the biogeochemical dynamics of floc, including its sources, transformations and reactivity.