Benthic biofilms in riverine systems: A sink for microplastics and the underlying influences.

Rivers are known as major pathways for transporting microplastics from terrestrial areas to the marine environment. However, the behavior of microplastics in terms of retention and transport within riverine systems remains unclear. While considerable efforts have been made to investigate the water column and sediment, limited attention has been given to understanding the interplay between microplastics and benthic biofilms. Therefore, this study aimed to examine the distribution of biofilm-trapped microplastics along the CaoE River and identify the factors influencing the immobilization of microplastics by benthic biofilms. The findings of this study revealed that benthic biofilms served as a sink of microplastics in the CaoE River, with an average abundance of 575 items/m2 in tributaries and 894 items/m2 in the main stream. The dominant shape of microplastics was fiber, while the primary polymer type was polyethylene terephthalate. The distribution of microplastics exhibited significant spatial heterogeneity, as indicated by their abundance and characteristics. In order to reveal the intriguing phenomenon, variations of influencing factors were estimated, including physicochemical characteristics of water, extracellular polymeric substances of benthic biofilms, and microbial communities of benthic biofilms. A partial least squares path modeling analysis was performed using these variables, revealing that water velocity and microbial diversity of benthic biofilms were the key factors influencing the interaction between microplastics and benthic biofilms. In summary, this study provides substantial evidence confirming the crucial role of benthic biofilms in the immobilization of microplastics, which expands concerns about microplastic pollution in the riverine systems. Furthermore, uncovering the underlying influences of microplastic-biofilm interactions will facilitate the development of effective strategies for the control and management of microplastic pollution.

Microplastics in the riverine environment: Meta-analysis and quality criteria for developing robust field sampling procedures.

Current sampling approaches for quantifying microplastics (MP) in the riverine water column and riverbed are unstandardised and fail to document key river properties that impact on the hydrodynamic and transport processes of MP particles, hindering our understanding of MP behaviour in riverine systems. Using ten criteria based on the reportage of the catchment area, river characteristics of sampling sites and approach, we reviewed the sampling procedures employed in 36 field-based river studies that quantify MP presence in the water column and benthic sediment. Our results showed that a limited number of studies conducted reliable sampling procedures in accordance with the proposed quality criteria, with 35 of the 36 studies receiving a score of zero for at least one criterion, indicating the omission of critical information relating to the study's sample size and the physical and hydraulic characteristics of the sampled river. On the other hand, a good number of studies adequately documented the spatial information of the sampling sites, the vertical location of sample collection, and sampling equipment used. An idealised MP sampling approach is presented to ensure that future studies are harmonised and variables underpinning MP transport in rivers are reported. In addition, a meta-analysis on MP particle characteristics from these studies found that concentrations in the riverine water column and benthic sediment are highly variable, varying by five and seven orders of magnitude respectively, and are heavily dependent on the sampling equipment used. Polypropylene (PP), polyethene, (PE), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) were the most frequently reported MP polymers, while irregular-shaped particles, fibres, spheres, and films were the most commonly reported shapes in the river studies. These results highlight the urgent need to standardise sampling procedures and include key contextual information to improve our understanding of MP behaviour and transport in the freshwater environment.

Assessing the potential ecological risk of Co, Cr, Cu, Fe and Zn in the sediments of Hooghly-Matla estuarine system, India.

Hooghly-Matla estuarine system along with the Sundarbans mangroves forms one of the most diverse and vulnerable ecosystems in the world. We have investigated the distribution of Co, Cr, Cu, Fe and Zn along with sediment properties at six locations [Shamshernagar (S1), Kumirmari (S2 and S3), Petuaghat (S4), Tapoban (S5) and Chemaguri (S6)] in the Hooghly estuary and reclaimed islands of the Sundarbans for assessing the degree of contamination and potential ecological risks. Enrichment factor values (0.9-21.6) show enrichment of Co, Cu and Zn in the intertidal sediments considering all sampling locations and depth profiles. Geo-accumulation index values irrespective of sampling locations and depth revealed that Co and Cu are under class II and class III level indicating a moderate contamination of sediments. The pollution load index was higher than unity (1.6-2.1), and Co and Cu were the major contributors to the sediment pollution followed by Zn, Cr and Fe with the minimum values at S1 and the maximum values at S5. The sediments of the Hooghly-Matla estuarine region (S4, S5 and S6) showed considerable ecological risks, when compared with effect range low/effect range median and threshold effect level/probable effect level values. The variation in the distribution of the studied elements may be due to variation in discharge pattern and exposure to industrial effluent and domestic sewage, storm water and agricultural run-off and fluvial dynamics of the region. The study illuminates the necessity for the proper management of vulnerable coastal estuarine ecosystem by stringent pollution control measures along with regular monitoring and checking program.

Macroinvertebrate communities in riverine systems of buffer areas of protected wildland, rangeland and city areas: implications for conservation of riverine systems on urbanising watersheds.

Riverine systems in developing countries continue to be degraded by anthropogenic pressures such as urbanisation. The responses of biota in watersheds surrounding a drainage divide may provide critical information that is required to protect the ecological condition of riverine systems. This study assessed the spatial variation of selected environmental variables together with macroinvertebrate communities in upper reaches of riverine systems across different land use categories of the Bulawayo region. Based on an a priori selection criterion, studied sites were grouped following an urban-wastewater disturbance gradient comprising of (i) heavily polluted city sites, (ii) moderately disturbed rangeland sites and (iii) less disturbed sites of the buffer areas of protected wildland. Most of the studied environmental variables and the macroinvertebrate community assemblages were significantly (ANOVA, p < 0.05) different and degraded within the city areas. In this study, the variance of environmental variables known to be associated with organic pollution like increased nutrients, embeddedness by particulates, salinity, COD, conductivity, turbidity and reduced dissolved oxygen was found to be related with the variation of macroinvertebrate communities across the studied sites. Besides affirming the effectiveness of macroinvertebrate-based bioassessment approaches, the results of our study demonstrate that an aggressive riverine protection policy that prohibits the discharge of poor-quality urban effluents and preservation of the less disturbed riverine systems needs to be part of the larger urban planning and regulatory framework in urbanising watersheds.

Marine and giant viruses as indicators of a marine microbial community in a riverine system.

Viral communities are important for ecosystem function as they are involved in critical biogeochemical cycles and controlling host abundance. This study investigates riverine viral communities around a small rural town that influences local water inputs. Myoviridae, Siphoviridae, Phycodnaviridae, Mimiviridae, Herpesviridae, and Podoviridae were the most abundant families. Viral species upstream and downstream of the town were similar, with Synechoccocus phage, salinus, Prochlorococcus phage, Mimivirus A, and Human herpes 6A virus most abundant, contributing to 4.9-38.2% of average abundance within the metagenomic profiles, with Synechococcus and Prochlorococcus present in metagenomes as the expected hosts for the phage. Overall, the majority of abundant viral species were or were most similar to those of marine origin. At over 60 km to the river mouth, the presence of marine communities provides some support for the Baas-Becking hypothesis "everything is everywhere, but, the environment selects." We conclude marine microbial species may occur more frequently in freshwater systems than previously assumed, and hence may play important roles in some freshwater ecosystems within tens to a hundred kilometers from the sea.

Influence of diet in the accumulation of organochlorine compounds in herons breeding in remote riverine environments.

The composition of organochlorine compounds (OCs), pentachlorobenzene (PeCB), hexachlorobenzene (HCB), hexachlorocyclohexanes (HCHs), DDTs and polychlorobiphenyls (PCBs), has been analyzed in eggs from cattle egret (Bubulcus ibis) and little egret (Egretta garzetta), two species of herons (family Ardeidae), nesting at the same remote riverine environment (Aiguabarreig, Ebro River). These two species were selected to evaluate the importance of diet in the accumulation of OCs. Cattle egret essentially feeds on dry grassy habitats and follow cattle or other large animals whereas little egret feeds on fish, amphibians and crustaceans captured in shallow waters. The δ(15)N and δ(13)C isotopic composition of the sampled eggs was studied and the results were consistent with these species feeding habits. In both species, the compounds accumulated the most were the less volatile and more lipophilic, e.g. PCB congeners of higher chlorination, DDT and metabolites. The distinct foraging species preferences were reflected in significant higher concentrations in little egret than cattle egret of all pollutant groups analysed. These differences were statistically significant for DDTs and PCBs (p < 0.015 and p < 0.047, respectively), e.g. the p,p'-DDE and PCB concentrations were 6 and 4.5 times higher, respectively, in the former than the latter. This strong contrast indicates that in remote environments aquatic riverine ecosystems are more efficient OC reservoirs than the terrestrial ecosystem.