Tidally driven exchange makes mangrove forests alternately act as sources and sinks of nitrogen and phosphorus for adjacent estuaries, but the foundational creek-scale, species-resolved nitrogen and phosphorus flux quantification (Wang et al. 2021, Zhangjiang Estuary, China) rests on spring and summer campaigns during 2016-2018 at a single creek, infers the controlling microbial processes rather than measuring them, and leaves the annual net balance undetermined. The open problem is to determine the seasonal-to-annual net exchange of nitrogen and phosphorus species across mangrove-estuary interfaces: quantify autumn and winter exchange and close annual net flux budgets; quantitatively attribute the observed lateral fluxes to specific microbial pathways (anammox, dissimilatory nitrate reduction to ammonium, denitrification, iron-redox-coupled phosphorus release) and to physical transport; assess how episodic events (storms, extreme rainfall) alter the exchange; and establish whether the source-sink pattern generalizes across mangrove systems with different geomorphology, tidal regimes, and species composition, including the chronic effect of sustained upstream nutrient loading on mangrove retention capacity.

Background

Mangrove forests are intertidal tropical and subtropical forests whose tidal creeks flush the forest platform twice daily, exchanging water and dissolved and particulate material with the adjacent estuary. The classical "outwelling" hypothesis holds that wetland-derived organic matter and nutrients exported on ebbing tides subsidize coastal food webs; whether a given mangrove system is a net nutrient source or sink for its estuary, species by species, is a quantitative question that controls how mangroves participate in estuarine eutrophication. The relevant nutrient species are dissolved inorganic nitrogen (DIN, the sum of nitrate NO3-N, nitrite NO2-N, and ammonium NH4-N), dissolved reactive phosphorus (DRP, the immediately bioavailable dissolved phosphate pool), dissolved organic nitrogen and phosphorus (DON, DOP), and total phosphorus (TP, the sum of dissolved and suspended particulate phosphorus).

The direct precursor work was done at a tidal creek of the Yunxiao National Mangrove Nature Reserve on the Zhangjiang Estuary, Fujian, China. Wang et al. (2019) combined water sampling and sediment incubations to identify mineralization and denitrification as the major processes making these mangrove sediments an ammonium source and nitrate sink, but did not quantify lateral fluxes or all pathways. Wang et al. (2021) then quantified creek-scale lateral fluxes: hourly nutrient concentrations measured at the creek mouth were multiplied by hourly water discharge produced by FVCOM (the Finite-Volume Community Ocean Model, an unstructured-grid hydrodynamic model used here to compute the water volume crossing the mangrove-estuary boundary) and integrated over tidal cycles. They found the mangrove consistently exported NH4-N and DRP while importing NO3-N, that DON and DOP switched from net export in spring to net import in summer, that the mangrove was a net sink of DIN, dissolved total nitrogen, and TP on the sampled cycles, and that upstream aquaculture and domestic effluents discharged on ebb tides substantially increased nutrient export. The same authors and others subsequently measured the candidate microbial controls in sediments of this and nearby systems: denitrification (microbial reduction of nitrate to dinitrogen gas, a permanent nitrogen loss), anammox (anaerobic ammonium oxidation, in which ammonium and nitrite combine to dinitrogen), and DNRA (dissimilatory nitrate reduction to ammonium, which retains nitrogen in the ecosystem as ammonium), all typically quantified with 15N isotope-tracer incubations, and salt-marsh versus mangrove differences in these rates at the same creek. Iron-redox-coupled phosphorus release — dissolution of iron-bound sediment phosphorus under reducing conditions, releasing DRP to porewater — is the analogous suspected control on phosphorus export. Tidal pumping (the asymmetry by which ebbing porewater-rich creek water exports dissolved constituents and flooding estuarine water imports them) is the dominant physical transport mechanism.

The Wang et al. (2021) flux record, however, comprised only spring and summer campaigns during 2016-2018 at one creek, so autumn and winter exchange and the annual net fluxes remain undetermined, the pathway contributions to the fluxes were inferred rather than partitioned, episodic events were not sampled, and generality to other mangrove settings is untested — the limitations that define this problem.

Problem Statement

Determine the seasonal-to-annual net exchange of nitrogen and phosphorus across mangrove-estuary tidal interfaces and its controls, resolving jointly the linked questions left open by the spring-summer-only creek-scale flux record of Wang et al. (2021):

  1. All-season fluxes and annual budgets. Quantify the lateral tidal exchange of the nitrogen species (nitrate, nitrite, ammonium, dissolved organic nitrogen) and phosphorus species (dissolved reactive phosphorus, dissolved organic phosphorus, particulate phosphorus) during autumn and winter — currently unexplored at any mangrove creek — and combine all four seasons into defensible annual net flux budgets, so that the source-sink classification of the mangrove (source of ammonium and dissolved reactive phosphorus, sink of nitrate, net sink of dissolved inorganic nitrogen and total phosphorus on the sampled cycles) can be stated, or corrected, at the annual scale.

  2. Pathway attribution of the fluxes. Quantitatively partition the observed lateral fluxes among the microbial and physical processes that produce and consume them — mineralization, nitrification, denitrification, anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA), iron-redox-coupled phosphorus release, and advective or porewater transport — replacing the current inference-based attribution with a measured partition.

  3. Episodic and anthropogenic controls. Assess how episodic events (storms, extreme rainfall) alter the lateral exchange, and determine the chronic, cumulative effect of sustained upstream nutrient inputs (aquaculture wastewater, domestic sewage) on the mangrove's retention capacity over multi-year scales, beyond the single-campaign effluent reconstruction performed to date.

  4. Generality. Establish whether the dynamic source-sink pattern holds across mangrove systems that differ in geomorphology, tidal regime, and species composition, under a comparable flux-quantification protocol.

These four components jointly define one objective: a closed, mechanistically attributed, annual-scale nutrient exchange budget for mangrove-estuary interfaces. The components are scientifically linked — a representative annual budget depends on episodic-event coverage, and the pathway attribution applies to the same observed lateral fluxes. Components 1-3 target the mangrove-estuary tidal interface under study, and component 4 tests whether the resulting pattern holds across the class of mangrove systems that differ in geomorphology, tidal regime, and species composition — those three axes describe how systems in that class differ rather than a requirement to sample every combination.

References

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