Effects of Hydrology on Red Mangrove Recruits

Introduction

Coastal wetlands along the Gulf of 

Mexico have been experiencing significant 

shifts in hydrology and salinity levels over 

the past century as a result of changes in 

sea level and freshwater drainage patterns. 

Local land management in coastal zones 

has also impacted the hydrologic regimes 

of salt marshes and mangrove areas. Parks 

and refuges in south Florida that contain 

mangrove forests have, in some cases, 

been ditched or impounded to control 

mosquito outbreaks and to foster wildlife 

use. And while mangroves dominate the 

subtropical coastlines of Florida and thrive 

in saltwater environments, little is known 

about how they respond to changes in 

hydrology under managed or variable tidal 

conditions. USGS researchers designed 

a study to evaluate the basic hydrological 

requirements of mangroves so that their 

health and survival may be more effectively managed in controlled impoundments 

and restored wetlands.

Mangroves are commonly found in 

the intertidal zone (between low and high 

tides) in a rather broad spectrum of hydrologic settings. Because they thrive at the 

interface of land and sea, mangroves are 

subject to changes in freshwater flow (flow 

rate, nutrients, pollutants) and to marine 

influences (sea-level rise, salinity). Salinity has long been recognized as a controlling factor that determines the health and 

distribution of mangrove forests. Field 

and experimental observations indicate 

that most mangrove species achieve their 

highest growth potential under brackish 

conditions (modest salinity) between 10 

and 20 parts per thousand (ppt). Yet, if 

provided with available propagules, successful regeneration, and limited competition from other plants, then mangroves can 

survive and thrive in freshwater systems 

as well. 

Because little is known about the 

growth and survival patterns of mangrove 

species relative to changing hydrology, 

USGS scientists conducted greenhouse 

and field experiments to determine how 

flooded or drained patterns of hydrology 

would influence growth of the red mangrove, Rhizophora mangle (fig. 1). Red 

mangrove propagules (recruits) of select 

sizes and genotypes (i.e., genetically similar groups) were planted both in greenhouses and in the field. Seedling growth 

was monitored in both studies on a quarterly basis for over a year; measurements 

included shoot growth, seedling height, 

and a final harvest of plant biomass. 

Greenhouse Experiment

A greenhouse study mimicked both 

impounded and tidal hydrology for mangrove recruits planted in deep pots of sand 

and peat (fig. 2). Hydrologic treatments 

included permanent flooding and tidal 

cycling. All study treatments followed the 

same automated and simultaneous tidal 

schedule, a daily ebb and flow of 6-hour 

durations. Water treatments included 

freshwater (<0.01 ppt salinity) and brackish conditions (15 ppt).

Results

Differences between salinity and 

hydrological treatments in this experiment 

were insignificant and otherwise masked 

by plant response to substrates and nutrients. These experimental findings suggest 

that both substrate and nutrient quality 

play a much greater role than the factors 

U.S. Department of the Interior USGS Fact Sheet FS-029-03

U.S. Geological Survey March 2003

Fig. 1. Tom Doyle, a USGS scientist, measured seedling height and diameter of red mangroves to determine 

differences in growth rates as a result of hydrologic conditions.

Fig. 2. In the greenhouse study, red mangrove 

seedlings were arranged in tanks and received 

hydrologic treatments of varying salt and fertilizer 

concentrations.
Potted recruits of red mangrove 

were planted into both fixed and floating 

nursery structures designed to mimic tidal 

and static water levels above and below 

mean tide. The floating nurseries served 

as an experimental control because the 

water level remained the same in both tidal 

and impounded waters. Fixed structures 

of hydrology and salinity in determining 

mangrove growth and form (i.e., secondary branching).

Field Trials

Field trials were conducted in 

coastal waters and impoundments of Ding 

Darling National Wildlife Refuge on Sanibel Island, Florida (fig. 3). Site locations 

included a protected bay under continuous tidal influence and two controlled 

impoundments, one of which remained at 

a static level while the other was allowed 

to flush with tides 3 months after initial 

planting. Unlike the greenhouse experiment, all potted recruits were in an organic 

peat substrate. 

were set to mean tide height so that potted 

plants were either flooded at high tides 

or drained at low tides, depending on the 

actual tides themselves. Floating nurseries, on the other hand, maintained steady 

water level and soil saturation conditions 

by moving up and down with the changing 

tide or fluctuations in impoundment water 

levels. 

Results

Red mangrove recruits exposed 

to tidal fluctuation experienced greater 

growth than those confined to static water 

levels. In the first 3 months, there were 

no significant differences between the 

impounded, fixed sets and all floating 

groups, including the tidal site. Thereafter, however, recruits subjected to tidal 

fluctuations (i.e., the impoundment opened 

to tidal influence and the fixed site in the 

bay) showed positive growth.

Whether in the open impoundment 

or the uncontrolled bay site, red mangrove recruits subjected to tidal exchange 

exhibited growth elongation significantly 

greater than the closed impoundment. In 

addition, there were no significant differences in growth start or success among 

genotypes or propagules of varied initial 

sizes. Although susceptibility to insect 

attack and dieback was greater in the 

elevated soil group, plant survival was 

not significantly affected by hydrologic 

treatment. These findings suggest that 

hydroperiod—the rate and level of tidal 

exchange—plays a much more important 

role in determining mangrove growth and 

success than previously documented. 

Conclusion

These field and greenhouse experiments show that salinity may have less 

effect on mangrove growth and development than hydrologic conditions and 

substrate quality, particularly in freshwater 

and brackish zones. The lack of plant 

response to hydrology in the greenhouse 

study may be attributed to the regular 

inundation cycle that essentially kept soils 

saturated. These experiments also provide 

a framework to isolate causal relationships 

within a multifactorial design under both 

controlled and field conditions.

The hydrology and salinity experiments conducted by the U.S. Geological 

Survey have provided insight into the degree that hydrologic fluctuations and substrate quality augment mangrove growth 

and survival. This empirical evidence will 

be used to upgrade MANGRO, an existing 

mangrove forest simulation model. The 

refinement of MANGRO, in turn, will help 

private, State, and Federal land managers 

assess how large-scale water management 

projects and restoration alternatives may 

affect mangrove habitat and distribution in 

south Florida. 

For more information, contact

T. W. Doyle

U.S. Geological Survey

National Wetlands Research Center

700 Cajundome Blvd.

Lafayette, LA 70506

337-266-8500

thomas_doyle@usgs.gov

http://www.nwrc.gov

Fig. 3. In the field experiments, fixed and floating nurseries of planted red mangrove seedlings were monitored 

for plant growth and survival in controlled impoundments and open bay sites.