Linking centennial scale anthropogenic changes and sedimentary records as lessons for urban coastal management.

Coastal eutrophication and urban flooding are increasingly important components of global change. Although increased seawater renewal by barrier openings and channelizing are common mitigation measures in coastal lagoons worldwide, their effects on these ecosystems are not fully understood. Here, we evaluated the relationships between human interventions in the watershed, artificial connections to the sea, and the sediment burial rates in an urban coastal lagoon (Maricá lagoon, Southeastern Brazil). Sediment accretion along with nutrient and carbon burial rates were determined in two sediment cores representing the past ∼120 years (210Pb dating) and associated with anthropogenic changes as indicated by historical records and geoinformation analyses. Lagoon infilling and eutrophication, expressed by the average sediment accretion, TP, TN, and OC burial rates, respectively, increased ∼9-18, 13-15, 11-14 and 11-12-fold from the earliest (<1950) to the most recent (2000-2017) period. These multi-proxy records confirm mechanistic links between deforestation, urbanization, and untreated sewage discharges. In addition, our findings reveal artificial connections to the sea may contribute to lagoonal eutrophication and infilling, particularly when not integrated with sewage treatment and forest conservation or reforestation in the watershed. Therefore, increased seawater renewal by physical interventions commonly considered as mitigation measures may in contrast cause severe degradation in coastal lagoons, causing harmful consequences that should be not neglected when implementing management practices.

Linking eutrophication to carbon dioxide and methane emissions from exposed mangrove soils along an urban gradient.

Mangroves are one of the most important but threatened blue carbon ecosystems globally. Rapid urban growth has resulted in nutrient inputs and subsequent coastal eutrophication, associated with an enrichment in organic matter (OM) from algal and sewage sources and substantial changes in greenhouse gas (GHG) emissions. However, the effects of nitrogen (N) and phosphorus (P) enrichment on mangrove soil OM composition and GHG emissions, such as methane (CH4) and carbon dioxide (CO2), are still poorly understood. Here, we aim to evaluate the relationships between CO2 and CH4 efflux with OM composition in exposed soils from three mangrove areas along watersheds with different urbanization levels (Rio de Janeiro State, Brazil). To assess spatial (lower vs. upper intertidal zones) and seasonal (summer vs. winter) variability, we measured soil-air CO2 and CH4 fluxes at low spring tide, analyzing elementary (C, N, and P), isotopic (δ13C and δ15N), and the molecular (n-alkanes and sterols) composition of surface soil OM. A general trend of OM composition was found with increasing urban influence, with higher δ15N (proxy of anthropogenic N enrichment), less negative δ13C, more short-chain n-alkanes, lower C:N ratio (proxies of algal biomass), and higher epicoprostanol content (proxies of sewage-derived OM). The CO2 efflux from exposed soils increased greatly in median (25/75 % interquartile range) from 4.6 (2.9/8.3) to 24.0 (21.5/32.7) mmol m-2 h-1 from more pristine to more urbanized watersheds, independent of intertidal zone and seasonality. The CO2 fluxes at the most eutrophicated site were among the highest reported worldwide for mangrove soils. Conversely, CH4 emissions were relatively low (three orders of magnitude lower than CO2 fluxes), with high peaks in the lower intertidal zone during the rainy summer. Thus, our findings demonstrate the influence of coastal eutrophication on global warming potentials related to enhanced heterotrophic remineralization of blue carbon within mangrove soils.

Tropical forests as drivers of lake carbon burial.

A significant proportion of carbon (C) captured by terrestrial primary production is buried in lacustrine ecosystems, which have been substantially affected by anthropogenic activities globally. However, there is a scarcity of sedimentary organic carbon (OC) accumulation information for lakes surrounded by highly productive rainforests at warm tropical latitudes, or in response to land cover and climate change. Here, we combine new data from intensive campaigns spanning 13 lakes across remote Amazonian regions with a broad literature compilation, to produce the first spatially-weighted global analysis of recent OC burial in lakes (over ~50-100-years) that integrates both biome type and forest cover. We find that humid tropical forest lake sediments are a disproportionately important global OC sink of ~80 Tg C yr-1 with implications for climate change. Further, we demonstrate that temperature and forest conservation are key factors in maintaining massive organic carbon pools in tropical lacustrine sediments.