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Nutrients effects on mangroves

Nutrient pollution, specifically nitrogen (N), has already surpassed the threshold of what is considered sustainable[19]. Excessive N in catchment runoff threatens the ecological integrity of rivers, mangroves, seagrass, and coral reefs[1].

Mangroves on Poyungan Creek, K'gari. Photo by Gary Cranitch © Queensland Museum

Quick facts

Mangroves can lose their capacity to remove nitrogen
from the water column as a result of excess nutrients and low oxygen[21].

Mangroves can be severely degraded due to excess nutrients, but this degradation cannot be determined solely by looking at the trees. Signs of degradation are first observed in the soil, where the microbial community is the first to respond to increased nutrients. As a result of the increase in the number and change in types of soil microorganisms, soils become low in oxygen, which has cascading effects on the biogeochemistry of the wetlands, including in their capacity to process nitrogen. Soils low in oxygen are also unsuitable for most benthic fauna, such as crabs.

Acute and chronic effects of prolonged exposure to excess nutrients in mangroves

Increased nutrients can have an effect on the soil microbial community, both in terms of abundance and composition, and these changes can then modify the biogeochemical processes in the soil[3][20]. For example, high nutrient levels have been associated with increased carbon decomposition and the release of phosphorus into the water column[18].

Higher levels of nitrogen and phosphorus in mangrove leaves can make mangroves more susceptible to insect pests[10]. These chronic changes only become apparent during extreme climatic events when large areas of forests can suddenly die[12].

High nutrient levels, especially nitrogen, can increase plant growth and boost primary production[8]. However, the growth of branches and above ground biomass comes at the expense of root growth, making mangrove trees unstable and vulnerable to losing canopy or dying during periods of low rainfall or tropical storms[13].

The effect of nutrients on mangroves depends on the concentration of nutrients and the exposure time. Mangrove trees can show high-stress levels due to chronic low oxygen in the soil[14][15].

Nutrient concentrations recorded that negatively impact mangroves

The concentrations of excess nutrients affect a variety of mangroves processes, and can vary depending on the nutrient type and examples shown in Table 1 below. Concentrations higher than 0.1 mg/L of DIN (NO3 + NH4+), 0.04 mg/L of SRP, and 0.4 mg/L of TN negatively impact mangroves[7].

Table 1. Summary of nutrient concentrations (as nitrate (NO3-), ammonia (NH4+), dissolved inorganic nitrogen (DIN), total nitrogen (TN), soluble reactive phosphorus (SRP) and at which an adverse effect was measured on mangroves.
Effect NO3- NH4+ DIN TN SRP Reference
Soil nutrient processing modified 0.14±0.12 0.04±0.02 0.18± 0.36±0.02 (Molnar et al., 2013)[17]
Bacteria density increase and dissolved organic carbon exchange modified 0.05±0.02 0.06±0.01 0.11±0.03 0.04±0.02 (Adame et al., 2012)[2]
Increase phytoplankton blooms 0.17±0.01 0.15±0.09 0.32±0.10 0.04±0.22 (Constanzo et al., 2004)[9]
Changes in crab behaviour 0.17±0.06 1.72±0.13 1.89± 3.38±0.20 (Bartolini et al., 2009)[7]
Increase N2O emissions 0.09±0.08 0.37±0.13 (Allen et al., 2011)[6]

Excess organic matter and nutrients increase bacteria abundance, which in turn consume the carbon and oxygen within the soil[15]. The conditions of low oxygen have cascading effects on the biogeochemical functions of the mangroves. For example, soils with low oxygen have high methane emissions (a powerful greenhouse gas), reduced habitat for fish[16], export of excess phosphorus from the soil to the water column[18]and reduced capacity of the wetlands to process nitrogen[17]. High ammonia levels are toxic to aquatic fauna, affecting the respiration of mangrove crabs[7].

Capacity of mangroves to “assimilate” nitrogen

Mangroves can remove N from the water, primarily through denitrification. The conditions required are extensive vegetation cover, slow water flows, available soil organic carbon (> 2%), nitrate concentrations of around 0.10 mg/L, and, importantly, mildly anoxic soils ( -100 to +200 mV)[4]. Additional processes for N removal are plant growth, or the accumulation of N as woody biomass, soil accumulation of particulate N, and anammox or the conversion of ammonia to nitrogen gas[5]. In some conditions, mangroves can also transform nitrate to ammonia through dissimilatory nitrate reduction to ammonium (DNRA), which can result in the export of ammonia to waterways[11].

For mangroves to act as nitrogen sinks, nutrient loads (concentration and frequency of discharge) cannot exceed those that the mangroves can assimilate and should not cause damage to the forest. Mangroves can lose their capacity to remove nitrogen from the water column as a result of excess nutrients and low oxygen[22].

The threshold of nutrient inputs into mangroves before they start losing their capacity to act as sinks has been estimated at 10 kg/ha for phosphorus and 25 kg/ha of nitrogen per year[22].

To reduce the impacts of nutrients on mangroves, treatment wetlands, or those specifically designed to reduce nutrients, should be considered before the water high in nutrients is discharged into the mangroves.

References

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Last updated: 6 May 2025

This page should be cited as:

Department of Environment, Science and Innovation, Queensland (2025) Nutrients effects on mangroves, WetlandInfo website, accessed 8 May 2025. Available at: https://wetlandinfo.des.qld.gov.au/wetlands/management/pressures/nitrogen/mangroves/

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