Elemental Status and Dynamics in Temperate (New Zealand) and Semi-arid Mangroves (New Caledonia)
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Mangrove ecosystems demonstrate a high capacity to accumulate macronutrients and trace metals and play a significant role in global oceanic nutrient budget and water quality. With growing pressures from global change, this role is now at stake as mangroves experience increasing shifts in their biogeochemistry, distribution and productivity. In particular, the simultaneous influences of sea-level rise, increasing aridity and anthropogenic pressures worldwide are projected to have a significant impact on mangrove element cycles. The aim of this thesis was to improve our understanding of global change effects on macronutrient (C, N, P, K, Mg, Ca, S) and trace metal (Fe, Mn, Al, Ni, Cu, Zn, Cr, Co) dynamics and their transfers within the mangrove soil-plant continuum. In order to do so, this research focuses on the distribution of these elements and their interactions in situ, in contrasted climate conditions and in different physiographic contexts in temperate New Zealand and semi-arid New Caledonia. Worldwide data showed that despite a decrease in mangrove biomass with increasing latitude, the various elemental sources and biogeochemical processes in mangrove ecosystems lead to a strong heterogeneity in elemental concentrations in plants and soils at every spatial scale. This point is further demonstrated in a case study in temperate Avicennia marina (Forsk.) Vierh subsp. australasica (Walp.) J. Everett estuarine mangroves in New Zealand, where soils were found to be a significant sink of C, macronutrients and trace metals. Although this research highlights the role of temperate mangroves as an efficient filter for terrigenous materials, findings also showed that this capacity significantly decreases with increasing length of immersion, tidal energy and reduction-oxidation potential (Eh). Thus, there is cause for concern over sea-level rise and erosion subsequent to mangrove stand removal. As we move to a warmer and drier climate in New Caledonia, elevation measurements showed dramatic centimetre-scale variations of soil properties and elemental contents along a semi-arid toposequence. Strong evapotranspiration in landward areas results in a 200 and 400% increase in pore-water salinity and Eh compared to seaward areas. This difference in Eh was magnified during the dry season and coincided with a loss of trace metal content compared to mangrove soils at the lowest elevations. In addition, plant component analyses demonstrated the magnitude of the influence of soil Eh, salinity and total sodium variations on trace metal and macronutrient soil-plant transfers along semi-arid mangrove gradients. The effect of a recent increase in tidal range on elemental distribution was also investigated at the two extremities of that same toposequence. Increase in soil elevation and a depletion of trace metals in fringe mangrove stands suggest that while mangroves may partly mitigate sea-level rise by vertical accretion or root accumulation, the effect of tidal pumping, perturbation and weathering on the pool of nutrients and trace metals in soils may remain significant. At the highest elevations, soil surface analyses in salt-flats recently colonized by A. marina give an insight into the ability of this pioneer species to transform hypersaline soils in a substrate favourable for plant growth. These results together with total Na contents analyses in A. marina‘s tissues showed that the success of this species in colonizing arid mangrove soils is due as much to its capacity to modify soil structure, nutrient contents and water-holding capacity as to its ability to tolerate salinity stress. Further research in different study sites adjacent to mining and aquaculture activities showed how variations of physico-chemical properties and OM cycling lead to polarized responses to labile OM and trace metal loads in mangrove plants and soils along semi-arid intertidal gradients. Results suggest that increased pollution, aridity and sea-level rise are likely to increase P accumulation in mangrove soils and nutrient export from soil towards plant biomass and litterfall. This research also indicates a potential decline in mangrove ability to accumulate OM, N and K in soils in landward areas and trace metals in seaward areas. The findings of this work contribute significantly to the understanding of macronutrient and trace metal dynamics in temperate and semi-arid mangroves. This work also provides insights into the implications of drought intensification on plant nutrition and their response to salinity and metal stresses.