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

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

The influence of crab burrows on sediment salinity in a Rhizophora-dominated mangrove forest in North Brazil during the dry season

“…This refutes our hypothesis of lower sediment salinity at areas with higher density of crab burrows during this time of year. The result corroborates the findings of Pülmanns et al (2015) in the rainy season, when sediment salinity in "rooted" areas was also higher than in "gap" areas (27 and 31 in gap areas and rooted areas, respectively, at the end of the rainy season) Findings by Smith III (1987), demonstrating a salinity of 57.5 in areas with high amounts of aerial roots versus 55.2 in gaps, are also in concordance with the present results. A microcosm experiment by Pülmanns et al (2016), showed lower salinity in treatments with (41) than without (47) artificial burrows after 6 months (both treatments started with sediment salinity of 37.5 at the first centimeter),…”

“…The tides at the study area are semidiurnal with a range between 3 and 5 m. The forest at the study site is located in the high intertidal zone which is not flooded during neap high tides (Pülmanns et al, 2015). The region has two very distinct seasons.…”

“…However, the first authors that have addressed specifically the relation between burrows with one opening from larger crabs, like Ucides cordatus, and salt contents in rooted sediments were Pülmanns et al (2015). In a laboratory microcosm study, salinity in mangrove sediment with artificial crab burrows with one opening was significantly lower than in sediment without burrows.…”

“…However, no evidence for a desalination effect in rooted areas was found for natural Ucides burrows in a field study conducted by the same authors during the pronounced North Brazilian rainy season. Pülmanns et al (2015) suggested that any such effect of the burrows may have been masked by leaching of salt through precipitation. They predicted that the burrows' influence on sediment salinity would be revealed during the dry season.…”

Relationship Between Tree Size, Sediment Mud Content, Oxygen Levels, and Pneumatophore Abundance in the Mangrove Tree Species Avicennia Marina (Forssk.) Vierh

“…In addition, small-scale topographical variations (±15 cm) have been shown to influence pneumatophore abundance and length [15]. Macropores, or cavities in soils, increase drainage of water and help facilitate transport of solutions, salt, and other particles in sediments [16,17]. These macropores are usually created by penetration by plant roots, burrowing activities by soil invertebrates, and aggregation of soil particles [18][19][20].…”