Effects of urban wastewater on crab and mollusc assemblages in equatorial and subtropical mangroves of East Africa

Abstract

a b s t r a c tMangrove forests are known to accomplish crucial ecosystem functions and services. They are nursery areas for fish, prawns and crabs, which provide coastal communities with a variety of food, timber and chemicals, and protect coasts from catastrophic events, such as tsunamis. Recently, a novel ecological service has been proposed for mangrove systems, namely natural wastewater treatment wetlands. This hypothesis was based on experimental data collected mainly in Chinese mangrove systems, which proved that mangrove soils were efficient in absorbing nutrients. Moreover, sewage loading seemed harmless to both plants and benthic communities in these systems. However, before promoting the use of natural mangroves as pollution buffers, or constructed mangrove wetlands as sewage treatment facilities, more data are needed on their overall tolerance to organic loading. Differences in macrobenthos patterns were thus investigated between peri-urban mangroves and sites not affected by sewage disposal in East Africa. We assessed differences in epifaunal assemblages, comprising crabs and molluscs, employing multivariate ACI unbalanced analyses to compare peri-urban mangrove swamps with those characteristic of non-urban mangroves with similar ecological traits. The sampling design was spatially nested, replicates being assessed at equatorial (southern Kenya) and subtropical (southern Mozambique) sites. The results manifested a consistent increase in crab biomass at the peri-urban sites in both Kenya and Mozambique. Moreover, the peri-urban systems were richer than the non-urban mangroves, both in terms of fiddler crabs (Uca spp.) which feed on benthic microalgae and bacteria, and sesarmids, such as Perisesarma guttatum and Neosarmatium meinerti, which feed on both substratum and leaf litter. The abundance of gastropods, in contrast, decreased significantly, especially in Kenya, mainly due to the disappearance of the mud whelk Terebralia palustris. The results thus indicate that, in East African mangrove systems, domestic wastewater has detectable effects on crabs and molluscs, suggesting their usefulness as bioindicators of its effects in mangroves. Transformed benthic patterns at the peri-urban sites indicated the need for further study of the actual potential of natural mangrove forests to absorb pollution in sewage treatment.

Genetic erosion in the snailLittoraria subvittata(Reid, 1986) due to mangrove deforestation

“…This is caused by increased predation risk, disease and extreme environmental factors (Beasley et al, 2005). Wastewater inflow was reported to cause lower abundances of gastropods in East African coastal areas (Cannicci et al, 2009). Clear-cutting and selective logging of Avicennia germinans in the southern Caribbean resulted in decreased abundances of the gastropod Neritina virginea, which is likely associated with microclimate alteration, increased insolation, induced canopy gaps and the promotion of predation (Amortegui-Tores, Taborda-Marine & Blanco, 2013) Ecological factors such as food availability and habitat quality may impact larval dispersal capabilities, which influence intrapopulation genetic variation and gene flow (Nanninga & Berumen, 2014).…”

Effect of Anthropogenic Activities on Mangrove Crab Diversity in Cameroon Atlantic Coast

“…Studies concerning the effects of industrial and/or domestic wastewater on crab diversity are recent [4,20,21]. The consequences of deforestation on mangrove fauna have not yet been documented in West Africa.…”

“…More recently, the beneficial effect of Uca spp. activities on mangrove soil biogeochemistry can be strongly impaired by organic waste discharge was showed [4]. including climate change, have receive much attention of late mainly because mangrove deforestation is occurring at a rate of 1-2% per year, which implies that most forests will disappear within century [15].…”

“…Among the resident mangrove fauna, brachyuran crabs are undoubtedly the most important component, high in species number, abundance and biomass compared with many other animals [3,4]. About 6,793 valid species, including 93 families and 38 subfamilies have so far been discovered worldwide [5].…”

Molluscs’ Composition and Distribution in Mangroves of the Cameroon Central Coast

“…They help in filtering pollution and the quality of coastal waters [3], and in the natural treatment of swamp wastewaters [4]. Mangroves also fix atmospheric CO 2 and thus contribute to the fight against global warming [5].…”