Concentration and partitioning of metals in intertidal biofilms: implications for metal bioavailability to shorebirds

Heavy metals in wetlands of southwestern India: from sediments through invertebrates to migratory shorebirds

Heavy metal pollution in Indian wetlands is rising due to industrial, agricultural and urban development activities. Shorebirds occupy upper trophic levels and are therefore especially vulnerable to heavy metal pollution. We evaluated the concentration of heavy metals (zinc, copper, cobalt, chromium, lead and cadmium) in 22 common species of migrant shorebirds (220 shorebird dropping samples) with diverse foraging behaviors, in their different prey (55 prey samples) and in the sediments (90 sediment samples) in different habitat types (mudflats, mangroves and sand beaches) between 2019 and 2021. Further, we analyzed a total of 10 biofilm samples from mudflats and mangroves. We detected relatively low concentrations of heavy metals in the sediments (Zn concentration range: 9.11-40.91 mg/kg; Cu: 5.74-21.38 mg/kg; Co: 2.00-4.04 mg/kg; Cr: 4.05-41.03 mg/kg; Pb: 1.02-7.19 mg/kg; Cd: 0.56-4.35 mg/kg). However, we measured relatively high concentrations of heavy metals in invertebrate prey species (Zn concentration range: 84.72-224.74 mg/kg; Cu: 26.63-170.36 mg/kg; Co: 13.98-14.42 mg/kg; Cr: 14.78-98.16 mg/kg; Pb: 18.95-157.29 mg/kg; Cd: 9.33-60.56 mg/kg). In addition, we found high concentrations of heavy metals in shorebird droppings (Zn concentration range: 41.33-58.8 mg/kg; Cu: 31.42-52.11 mg/kg; Co: 36.34-55.68 mg/kg; Cr: 52.3-68.21 mg/kg; Pb: 25.94-43.13 mg/kg; Cd: 5.53-16.4 mg/kg). It is evident that concentration of heavy metals increased successively moving from sediment to prey to shorebird species, likely through trophic transfer. The biofilm samples contained very high concentrations of Cr, Pb and Cd (22.64, 28.09 and 18.46 mg/kg respectively) which could be harmful to biofilm grazing shorebirds. Since bioaccumulation of heavy metals entail risks in living species, we suggest that increasing concentrations may detrimentally affect physiological processes in invertebrates and shorebirds. There is an urgent need to identify the sources of pollution and to reduce the discharge of heavy metals and other pollutants into coastal and inland wetlands.

Microplastic pollution in aquatic environments may facilitate misfeeding by fish

Numerous recent studies have documented ingestion of microplastics (MPs) by many aquatic animals, yet an explanation for misfeeding by fish remains unexplained. Here we tested the hypothesis that biofilm (biofouling) on MP surfaces due to exposure in the aquatic environment facilitates misfeeding in fish. Spherical polystyrene (PS) was cultured for 0-22 weeks in a freshwater environment to grow a biofilm on the MPs. Goldfish were employed in a simple feeding experiment with and without provision of genuine food at ecologically relevant MP concentrations. Absorbance (ABS), which is a proxy for biofilm formation, increased exponentially within three weeks of initiation and reached a plateau after approximately five weeks. Although fish did not swallow the MPs, "capture" occurred when food pellets were in the vicinity and significantly increased in probability with aging. Duration of capture also increased significantly with increasing aging. These results suggest that drifting of MPs in aquatic environments may facilitate fish misidentification of MPs as edible prey.

Biofilm facilitates metal accumulation onto microplastics in estuarine waters

In aquatic environments, plastic debris accumulates chemical pollutants from the surrounding water, potentially altering the fate of xenobiotics in these ecosystems. The effects of biofouling on the potential for plastic to sorb environmental pollutants remain poorly understood. In this study, we test the hypothesis that concentrations of metals are directly related to biofilm accumulation on microplastics submerged in natural estuarine waters. Two types of pre-production plastic pellets (polylactic acid (PLA) and low-density polyethylene (LDPE)) and glass pellets, were suspended for up to 28 days in an urbanized estuary (San Francisco Bay, California) to investigate how biofilm affects the accumulation of metals on these materials. During the initial weeks of the experiment, biofilm growth differed between locations, but after 28 days, PLA and LDPE had similar amounts of biofilm at the two field sites. Biofilm was the only significant predictor variable for Ba, Cs, Fe, Ga, Ni and Rb, and simple regressions of these metals after one month of submersion predicted much of the variability in the data (respective adjusted R² values: 0.46, 0.90, 0.86, 0.81, 0.87, 0.90; p < 0.001). For other metals influenced by location or substrate material, multivariate analysis showed that increases in metal concentrations were predicted by increases in biofilm for Cu, Pb, Al, K, U, Co, Mg (p < 0.001) and Mn (p < 0.01). This work highlights the role of biofilm in facilitating metal accumulation on plastic debris and contributes to current understanding of the underlying processes that influence the behavior of microplastics as aquatic contaminants.

Mercury levels in common (Actitis hypoleucos) and green (Tringa ochropus) sandpipers from west-central Iran

Mercury concentrations were examined in the liver, kidneys, and tail and breast feathers of common and green sandpipers from Zayanderud Dam in west-central Iran. The aim was to provide indirect information about habitat contamination. Tail feathers of both species had higher mercury levels compared to other tissues. Moreover, tissues of common sandpipers had significantly higher mercury concentrations compared to tissues of green sandpipers. Male specimens of both species had higher values of mercury compared to females. The pattern of larger body size-higher mercury body burden was not completely true in the current study. Smaller and shorter common sandpipers had higher mercury concentrations compared to taller and heavier green sandpipers. At the intraspecific level, body weight was positively correlated with mercury concentrations in tissues of common sandpipers. Based on the data presented here, it appears that these sandpipers, especially common sandpipers, are at potential risk from the toxic effects of mercury.

Trace elements in Pacific Dunlin (Calidris alpina pacifica): patterns of accumulation and concentrations in kidneys and feathers

Trace element concentrations were measured in Pacific Dunlin (Calidris alpina pacifica) to identify factors that influence accumulation and to assess toxicity risks. We report concentrations of cadmium, copper, and zinc in kidneys as well as copper, lead, mercury, selenium and zinc in feathers. Relationships between element concentrations and Dunlin age, sex, bill length, habitat preference, trophic level, and sample group were investigated with regression analyses. Stable isotope ratios of carbon and nitrogen in Dunlin muscle tissue were used to determine habitat preference and trophic level, respectively. Cadmium concentrations in kidneys were significantly related to habitat preference: [Cd] in estuarine foragers >[Cd] in terrestrial foragers. Cadmium accumulation was age-dependent as concentrations increased significantly within 10 months of hatch dates but not afterward. Concentrations of cadmium and zinc in kidneys as well as lead and mercury in feathers were below those known to cause deleterious effects in birds. In contrast, selenium concentrations in feathers (range: 2.1-14.0 µg/g) were often at levels associated with toxicity risks (>5 µg/g). Toxicity thresholds are not available for copper in kidneys or copper and zinc in feathers; however, measured concentrations of these elements were within documented ranges for sandpipers. Future studies should assess potential impacts of selenium on embryonic development in Dunlin and other sandpipers. Risk assessments would yield more conclusive results for all elements if impacts under ecologically relevant stresses (e.g. development in the wild, migration, predation) were better understood.