Microplastic abundance in urban vs. peri-urban mangroves: The feasibility of using invertebrates as biomonitors of microplastic pollution in two mangrove dominated estuaries of southern Africa.

Microplastic (MP) prevalence has been well documented, however, knowledge gaps exist for African mangrove forests. This research is the first to compare MP pollution (using FT-IR analysis) in an urban (Durban Bay) and peri-urban (Mngazana Estuary) mangrove forest in South Africa, across different compartments. MP pollution (typology, abundance, and distribution) was quantified in estuarine surface water, sediment and the soft tissue of three keystone species (Austruca occidentalis, Chiromantes eulimene and Cerithidea decollata) in relation to disturbances acting on these systems. MP averages ranged from 99 to 82 MPs/kg sediment, 177 to 76 MPs/L water and 82 to 59 MP/g-1 DW in biota. Overall fibres were the dominant MP type across all compartments. The three invertebrate species exhibited MP bioaccumulation, however, significant differences were observed between MP concentrations in the soft body tissue of invertebrates and abiotic compartments, providing evidence that they are not effective biomonitors of MP pollution.

Quantification and characterisation of microplastics ingested by selected juvenile fish species associated with mangroves in KwaZulu-Natal, South Africa.

Though the number studies on microplastic ingestion by fish is growing, data on fish species characteristic of the South African coastline are scarce. This study quantified and characterised (physically and chemically) microplastics ingested by four species of juvenile fish (viz. Oreochromis mossambicus [Peters, 1852], Terapon jarbua [Forsskål, 1775], Ambassis dussumieri [Cuvier, 1828] and Mugil sp.), within four mangroves along the east coast of South Africa. Microplastics were isolated from whole fish using a proteinase K digestion method, and then quantified and characterised in terms of shape, chemical nature (plastic type), colour and length. Fibres (68%) and fragments (21%) were the dominant shapes found. Of the 174 fish sampled, 52% contained microplastic particles, with 0.79 ± 1.00 particles per fish. The average number of particles per fish did not differ significantly across species within sites and across sites but was higher than in juvenile fish of other species sampled in oceanic habitats. The main plastic types collected using 10 µm filters and identified with Fourier Transform Infrared Spectroscopy (FTIR), were rayon (70.4%), polyester (10.4%), nylon (5.2%) and polyvinylchloride (3.0%). Particle length ranged from 0.1 to 4.8 mm, averaging 0.89 ± 0.77 mm, but irrespective of length, particles were mostly blue in colour. This study provides evidence that juvenile fish inhabiting mangroves are consuming significant quantities of microplastics. Importantly, it should be noted that rayon, though the most abundant plastic type found, is a semi-synthetic fibre made from regenerated cellulose that is commonly reported in studies of this nature. The habitats studied serve as nurseries for numerous fish species; however, more detailed studies are needed to assess whether microplastic ingestion could compromise the health of these fish or whether these effects are dependent on species, feeding habit and/or plastic type.

Oil induces chlorophyll deficient propagules in mangroves.

In Australia, some trees of the mangrove, Avicennia marina, growing in a chronic oil polluted site, produce chlorophyll deficient (albino) propagules. We tested the hypothesis that albinism was due to an oil-induced mutant allele that controls photosynthesis. We determined whether there are genetic differences between normal and chlorophyll deficient propagules. Four gene regions (nuclear 18S-26S cistron; chloroplast - trnH-psbA, rsp16 and matK) were sequenced and analysed for normal and albino propagules. Mutations occurred in both nuclear (ITS) and coding chloroplast (matK) genes of albino propagules. There were 10 mutational differences between normal and albino propagules in the matK samples. Analysis of molecular variation (AMOVA) of the matK dataset indicated highly significant genetic differentiation between normal and albino propagules. Our study suggests for the first time that PAHs from a chronic oil polluted site resulted in mutations in both nuclear and chloroplast genes, resulting in the production of albino propagules.

Mangrove expansion and salt marsh decline at mangrove poleward limits.

Mangroves are species of halophytic intertidal trees and shrubs derived from tropical genera and are likely delimited in latitudinal range by varying sensitivity to cold. There is now sufficient evidence that mangrove species have proliferated at or near their poleward limits on at least five continents over the past half century, at the expense of salt marsh. Avicennia is the most cold-tolerant genus worldwide, and is the subject of most of the observed changes. Avicennia germinans has extended in range along the USA Atlantic coast and expanded into salt marsh as a consequence of lower frost frequency and intensity in the southern USA. The genus has also expanded into salt marsh at its southern limit in Peru, and on the Pacific coast of Mexico. Mangroves of several species have expanded in extent and replaced salt marsh where protected within mangrove reserves in Guangdong Province, China. In south-eastern Australia, the expansion of Avicennia marina into salt marshes is now well documented, and Rhizophora stylosa has extended its range southward, while showing strong population growth within estuaries along its southern limits in northern New South Wales. Avicennia marina has extended its range southwards in South Africa. The changes are consistent with the poleward extension of temperature thresholds coincident with sea-level rise, although the specific mechanism of range extension might be complicated by limitations on dispersal or other factors. The shift from salt marsh to mangrove dominance on subtropical and temperate shorelines has important implications for ecological structure, function, and global change adaptation.