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Environmental contextMangroves dominate at the interface between land and sea, especially along tropical and subtropical coasts. To gain a better understanding of how mangroves respond to various environmental stress factors, we investigated the use of monomethylalkanes as potential chemical tracers for black mangroves. The application of these chemical tracers could elucidate how black mangroves respond to environmental stress such as sea level rise in mixed mangrove environments. AbstractA series of iso- and anteiso-monomethylalkanes (MMAs) with carbon numbers from C23 to C35 and C14 to C34 respectively were detected in Avicennia germinans. These compounds were present in varying amounts up to 54.1, 1.0 and 3.4µg g–1 dry weight in the leaves, bark and the crustose lichens attached to the bark of A. germinans respectively. These MMAs were not detected in the leaf waxes of Rhizophora mangle and Laguncularia racemosa, but were detected in significantly lower abundances (2–6% of that in A. germinans leaf wax) in the bark and lichen of R. mangle. Significant odd-carbon number distributions and even-carbon number distributions were observed for long chain (C ≥ 25) iso- (maximising at C31) and anteiso-MMAs (maximising at C32) respectively in A. germinans leaf wax. However, no obvious carbon number preferences were detected for bark and lichen. The long chain (LC) iso- and anteiso-MMAs in A. germinans leaf waxes were found to be enriched in 13C by 0.3–4.3 and 0.7–4.2 per mille (‰) compared to the n-alkanes with the same carbon numbers respectively across the salinity gradient of 19.7–32.0 practical salinity units (psu). In comparison, the LC iso- and anteiso-MMAs were found to be more depleted in D by 6.1–55.1 and 7.3–57.0 ‰ compared to the n-alkanes with same carbon numbers respectively. The results imply that A. germinans could be another important source of iso- and anteiso-alkanes in sediments and soils, and that these compounds could potentially be used as biomarkers for this species in mixed mangrove environments.

Wetland ecosystems are often characterized by self-organized landscape patterning, driven by abiotic and biotic factors. In the Florida Everglades, natural sheet flow is hypothesized to have distributed sediments to form the pattern of linear emergent ridges and submerged sloughs. Drainage and barriers to flow have degraded these microtopographic features. As part of the Comprehensive Everglades Restoration Plan, the Decompartmentalization Physical Model is a landscape-scale experiment to evaluate ecosystem responses to restored sheet flow by increasing freshwater inputs and removing barriers to flow. To test the proposed mechanism that flow rebuilds ridge-slough microtopography by remobilizing slough sediments into ridges, four molecular markers capable of distinguishing ridge, slough, and microbial sources were evaluated in flocculent benthic sediments (floc) and advected sediments (collected in traps) during preflow, high-flow, and postflow conditions over 4 years. The combined use of the four biomarkers, namely, the aquatic proxy (Paq), C 20 highly branched isoprenoids, kaurenes, and botryococcenes, showed compositional patterns that clearly distinguished ridge and slough organic matter. Of these molecular parameters, the Paq was the most reliable in distinguishing among organic matter sources. Long-term patterns in floc Paq at ridge and slough sites indicate a general increase, indicative of preferential mobilization of slough material. The Paq values for advected sediments are also strongly associated with slough environments, supporting temporal trends in floc samples. Our results tentatively confirm the hypothesis that increased flow in degraded ridge-and-slough wetlands, and associated sediment transport, is a potentially viable mechanism to restore historic patterns of microtopography.Plain Language Summary We examined how experimental flows within the Decompartmentalization Physical Model (DPM) altered the movement of benthic sediment (floc) in ridge and slough habitats, an important mechanism for restoring pattern and topography of the historic Everglades ridge-and-slough landscape. The approach is novel in utilizing molecular organic biomarkers that identify sources of organic matter from ridges and sloughs to infer the movement of floc. Results so far are promising in suggesting that sediment redistribution does occur and may over longer time periods help rebuild topography, an important objective of large-scale flow restoration projects such as those proposed in the Comprehensive Everglades Restoration Plan (CERP).