Mangrove Trace Metal Biogeochemistry Response to Global Climate Change

Abstract

This review discusses observed impacts from different climate change-driven pressures on mangrove's role in modulating trace metal transfer at the land-ocean interface. It contributes to the literature in a global context and shows mangroves as mitigators or providing positive feedback to metal mobilization. Most chalcophile metals2+ accumulate in mangrove soils associated with sulfides while high sedimentation rates avoid their oxidation. Exudation of oxygen by roots fixates Fe, which co-precipitates metals as oxyhydroxides in the rhizosphere. These two biogeochemical processes reduce trace metal availability to plants and their mobility within estuaries. However, climate change-driven pressures alter this geochemical equilibrium. Increasing atmospheric CO2 and temperature, and the intensity and frequency of extreme climatic events, have proved to affect mangrove functioning and cover, but no direct observation on the impact on metal biogeochemistry is presently available, whereas sea level rise and saline intrusion impacts on the fate of metals have already been observed. Sea level rise increases erosion, that dissociates deposited sulfides releasing metals to the water column. Released metals adsorb onto suspended particles and can re-deposit in the estuary or are exported to continental shelf sediments. Saline intrusion may oxidize deeper sediment layers releasing metals to porewaters. Part of the mobilized metals may remain in solution complexed with DOM and have their bioavailability increased, as shown by high bioaccumulation factors and biomagnification and high metal concentrations in the estuarine biota, which results in higher human exposure through fisheries consumption. Since erosion occurs preferentially at the sea border and higher sedimentation at the higher reaches of the estuary, triggering mangroves migration landward, spatial gradients are formed, and shall be taken into consideration when planning mitigation or adaptation strategies. These observations suggest disruption of traditional humans dwelling in mangrove dominated coastlines by increasing contamination of coastal fisheries, often the principal protein source for those groups and an important source of income. Further research into the environmental and socioeconomic impacts of climate change driven alterations to metal biogeochemical processes in mangroves as contaminant levels are expected to increase.

Heavy metal pollution status and deposition history of mangrove sediments in Zhanjiang Bay, China

“…Organic matter and pH can alter the distribution of HMs in sediments by affecting their bioavailability (Peng et al, 2009). In short, the accumulation, migration and transformation of HMs in sediments are a complex process, which is affected by the various biogeochemical properties from sediments and climate drivers (Zhang et al, 2014;Lacerda et al, 2022). However, for the process of HM deposition in mangrove cores with long-time series, we should pay more attention to the macro-control of anthropic factors and natural processes on the distribution of HMs.…”

“…Although the dry as the above-described can promote the precipitation of HM pollutants in mangroves, the longer rainy season and daily tidal erosion in the Zhanjiang Bay may lead to more severe HM loss from mangrove sediments. Moreover, sea level rise and tidal erosion will lead to the oxidation of reducing minerals (sulfides) and release the accumulated HMs into the water (Lacerda et al, 2022). It's worth noting that the loss of mangroves means that a large area of exposed beach is exposed on the seaward side.…”

Mercury Sources, Emissions, Distribution and Bioavailability along an Estuarine Gradient under Semiarid Conditions in Northeast Brazil

“…The influence of mangroves in the Hg biogeodynamics in tropical and subtropical ecosystems has been reported in recent publications, and, irrespective of the geographical location, suggests a similar and consistent pattern. In summary, there is an export of particulate Hg during the short rainy season, whereas in the dry season, particulate Hg accumulates in the estuary and most Hg exported to the sea is soluble reactive Hg [ 1 , 6 , 14 , 15 ].…”

Biodegraders of Large Woody Debris Across a Tidal Gradient in an Indonesian Mangrove Ecosystem

“…The shift in activity of wood degrading organisms will have significant consequences for mangrove forests where landward accretion is not possible, e.g., due to coral-rock plateaus, as found in the Sombano and Langira sites from this study, or where topographical or infrastructure barriers occur (Ward and Lacerda, 2021). This alteration in the spatial distribution of carbon in mangroves is likely to be regionally variable as a result of species differences in LWD degraders (Cragg, 2007), as well as linked to variations in the impacts of sea level rise globally and linked to tidal variation (Ward et al, 2016;Lacerda et al, 2022). Additionally, as LWD biodegrading guilds play a key role in the structure and function of mangrove forests and adjacent ecosystems, the altered proportion of wood degrading organisms will result in different LWD biodegradation processes, which will affect the sediment deposition and export of POM and DOM from mangrove forests (Robertson and Daniel, 1989).…”