Artificial Crab Burrows Facilitate Desalting of Rooted Mangrove Sediment in a Microcosm Study

Nathalie Pülmanns,Inga Nordhaus,Ulf Mehlig,Karen Diele

doi:10.3390/jmse3030539

Abstract

Water uptake by mangrove trees can result in salt accumulation in sediment around roots, negatively influencing growth. Tidal pumping facilitates salt release and can be enhanced by crab burrows. Similarly, flushing of burrows by incoming tidal water decreases sediment salinity. In contrast to burrows with multiple entrances, the role of burrows with one opening for salinity reduction is largely unknown. In a microcosm experiment we studied the effect of artificial, burrow-like macro-pores with one opening on the desalting of mangrove sediment and growth of Rhizophora mangle L. seedlings. Sediment salinity, seedling leaf area and seedling growth were monitored over six months. Artificial burrows facilitated salt release from the sediment after six weeks, but seedling growth was not influenced. To test whether crab burrows with one opening facilitate salt release in mangrove forests, sediment salinities were measured in areas with and without R. mangle stilt roots in North Brazil at the beginning and end of the wet season. In addition, burrows of Ucides cordatus were counted. High crab burrow densities and sediment salinities were associated with stilt root occurrence. Precipitation and salt accumulation by tree roots seem to have a larger effect on sediment salinity than desalting by U. cordatus burrows.

Highlights

Mangrove trees are typically exposed to saline tidal waters
The final model indicates that the pattern of sediment salinity over time in the plant containers differed between the two treatments
This study is the first to show that artificial crab burrows with one opening facilitated salt release from rooted sediment in a microcosm experiment

Summary

Introduction

Mangrove trees are typically exposed to saline tidal waters. Most mangrove species are able to exclude salt during water uptake and/or to regulate ion concentration via salt glands [1,2]. The drawback of excluding salt during water uptake is the consequential accumulation of salt in the sediment surrounding the roots [3]. The accumulated salt around the roots may reach concentrations that limit further water uptake, leading, in the worst case, to lethal conditions for the tree [3]. Salt can be washed out by tidal pumping [8,9] or by tidal flushing of animal burrows [10,11,12] Mangrove sediments usually feature a high proportion of clay or silt with low hydraulic conductivity and low diffusion coefficients for salt [4,5,6], resulting in low diffusion rates and thereby a slow removal of accumulated salt by groundwater flow [7].

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