The range and the average of grain size and TOC in sediments for November and December are presented in Table 1.
At the Chapora station, sand was more than silt and clay concentration in sediments for both months. Also, it was dominant (more than 85 %) at the Moira station. However, in the Zuari Estuary, clay was higher than sand and silt in sediments for November and December months. The sand concentration decreased by more than 10 % in the Chapora Estuary and was compensated by an increase in silt from November to December months. In the Moira River, sand showed a slight increase and finer sediments exhibited a slight decrease in concentration from November to December months. In the Zuari Estuary, silt increased from November to December, while a slight decrease in sand and clay was observed. At all three stations, not much change in TOC was observed between the months.
The coarser sediments were predominant near the mouth of the Chapora Estuary. This might be the result of strong currents which act on the bed. It causes resuspension of fine-grained particles and are carried by waves and tides towards middle estuary (Nasnodkar and Nayak 2015). It is well construed that hydrodynamics at the mouth region owing to intense tides and waves is stronger and facilitate the retention of coarser sediments (Siraswar and Nayak 2011). Such estuarine processes seem to have deposited fine (clay) particles at middle region of the Zuari Estuary. In contrast, the sediment core collected from the Moira River was largely composed of sand particles. The presence of coarser sediments is expected at this station, as the energy associated with the river runoff is high to support the deposition of the coarser sediments. Therefore, the sediment transport and deposition at three stations was governed by hydrodynamic.
Among the three stations, the variation in sediment grain size between November and December months was higher at the Chapora station than at Zuari and Moira stations. The observed variation between the months at the Chapora station might be due to the change in energy associated with waves and tides and the disturbance at the surface sediments by the action of fishing trawlers passing via or operating at the mouth region of the Chapora Estuary.
Total metals in sediments
An average value of metals at sampling stations for the months of November and December are presented in Table 2a. A considerable variation in concentration of Mn and Zn was observed between the two months at Chapora and Moira stations. At the Zuari station, Mn level in sediments varied from November to December. The rest of the metals did not show much variation between the months at the three stations. Change in metal concentration in sediments from November to December might be due to the additional input of those metals to estuary or the effect of physicochemical processes which regulate the adsorption and mobilization of metals. The fate (speciation) of mobilized metal is influenced by conditions of the overlying water, especially hydrogen ion concentration, salinity, Eh and number of suspended solids (Li et al. 2013). Further, the metal level in sediments was related to shale value (Turekian and Wedepohl 1961). Zn in November at the Chapora station while, Zn, Cu and Co for both the months at the Zuari station surpassed the shale value (Table 2a) indicating metal input via both natural and anthropogenic sources. Further, CF revealed moderate contamination of Zn at the Chapora Station (November), while at the Zuari Station Zn, Cu and Co showed moderate contamination (Table 2b).
Total metals abundance in sediments
In order to understand the abundance of total metal concentration among three stations isocon diagrams were plotted between the sampling stations (Fig. 2). For November, total Mn, Cu and Co level was high at Zuari while Fe and Zn at Chapora. Later in December, total metals (Fe, Mn, Zn, Cu and Co) were higher in concentration at Zuari than Chapora Station. The comparison of total metals between Zuari and Moira stations for November and December revealed comparatively higher metal concentration at the Zuari Station. The Chapora Station exhibited comparatively higher level of total metals in comparison to Moira Station for November and December, with an exception of slightly higher total Fe concentration at the Moira Station for December. Among the sampling stations, concentration of total metals was more at the Zuari Station. This is attributed to an enhancing rate of metal input through mining activities at the Zuari Station in comparison to a gradual release of metals via agricultural, industrial and domestic activities at Chapora and Moira stations.
Metal species in sediments
The metals associated with different sediment fractions were studied based on anthropogenic input and their pollution signatures established through total metal analysis and CF. An average of species of Fe, Mn, Zn, Cu and Co in sediments at Chapora, Zuari and Moira stations F1: exchangeable, F2: carbonate, F3: Fe-Mn oxide, F4: organic matter/sulfide bound and F5: residual is graphically illustrated in Fig. 3.
Fe and Cu were significantly present in the residual fraction at Chapora (November and December). Additionally, Cu was around 10 and 16 % in the organic bound fraction in November and December, respectively. In the case of the Zuari station, Fe (88.90 % in November and 91.99 % in December) and Cu (95.49 % in December) were abundant in the residual fraction. Although, Cu was highest in an environmentally dormant fraction (residual), but was also significantly high in the organic/sulphide fraction (27.42 %) in November. Fe also recorded the highest value in an environmentally dormant fraction at the Moira station (70.25 % in November and 72.37 % in December). Amongst the environmentally active fractions (bioavailable), it was significantly held in the Fe-Mn oxide fraction (22.83 % in November and 26.30 % in December) in both months.
Mn was abundantly held in its residual form for November (82.44%), while in December it reported highest value in the carbonate fraction (40.23 %) at Chapora. Also, its concentration was significant in the Fe-Mn oxide fraction (22.85 %) for December. On the other hand, at the Zuari station, its concentration was highest in the Fe-Mn oxide (47.28 % and 56.84 % in November and December, respectively) fraction and was also significantly associated with the exchangeable fraction (21.83 % in November and 12.23 % in December). In Moira, Mn was highest in residual (36.84 %) and Fe-Mn oxide (47.93 %) fractions for November and December, respectively. The exchangeable fraction recorded significant level of Mn for both the months (23.03 % in November and 15.95 % in December), in addition to Fe-Mn oxides (32.31 % for November).
The residual fraction showed highest value of Zn for both the months at the Chapora station. In November, a significant concentration of Zn (30.48 %) was reported bound to Fe-Mn oxides and was reduced to almost half a concentration by December (14.71 %). Zn was largely available in its residual form (78.54 % in November and 82.40 % in December) at the Zuari Station and was also considerably held in the Fe-Mn oxide fraction (14.61 % in November and 13.05 % in December). Although, Zn was highest in the residual fraction (46.69 % in November and 59.69 % in December) at the Moira station but, was also significantly held in Fe-Mn oxide form (28.32 % in November and 26.22 % in December).
At the Chapora (63.89 % in November and 59.74 % in December) and the Zuari (57.06 % in November and 56.95 % in December) stations, Co exhibited highest value in the residual fraction. Both the months reported considerable concentration of Co in Fe-Mn oxide (19.97 % in November and 22.26 % in December) and organic/sulphide (10.10 % in November and 10.48 % in December) bound fractions at the Chapora station. On the other hand, it was significantly held in the Fe-Mn oxide (35.36 % in November and 31.53 % in December) fraction at the Zuari station. The Fe-Mn oxide fraction showed highest of Co for November (38.99 %), while for December, the residual fraction (42.60 %) reported maximum concentration in sediments of Moira. However, it was significantly held onto Fe-Mn oxides (29.42 %) in December. Besides, Co was also considerably held in exchangeable (10.14 % in November and 14.35 % in December) and organic/sulphide bound (14.08 % in November and 13.50 % in December) fractions.
The metals viz., Fe and Cu at Chapora, Zuari and Moira stations in November and December, along with Mn (November) at Chapora, Zn (December) at Moira and, Zn and Co at Chapora and Zuari were predominantly high in the residual fraction. In general, metals associated with the residual fraction are immobile as are strongly held in the mineral lattice structure (Wang et al. 2002). The presence of metals in the environmentally dormant fraction (residual) which is relatively stable is recognized as the input derived from natural sources (Sundaray et al. 2011). Therefore, are considered non-bioavailable and have no effects on the environment (Chakraborty et al. 2015; Nasnodkar and Nayak 2017). A metal form that is capable of affecting the environment is one present as the bioavailable fraction, which includes exchangeable, carbonate, Fe-Mn oxides and organic matter/sulfides. Upon fluctuations in pH, salinity, Eh, bioturbation, etc., in estuaries, those metals are likely to mobilize from sediments to the overlying water, thereby become bioavailable and might have significant effects on the environment (Noronha and Nayak 2016). So are called the ‘’effective fraction’’. In the present study as well, metals (Zn and Co in November, and Mn, Zn, Cu and Co in December at the Chapora station; Mn, Zn, Cu and Co in November and Mn, Zn and Co in December at the Zuari station; Fe, Mn, Zn and Co in both November and December at the Moira station) were considerably or significantly high in certain environmentally active (bioavailable) fractions. Thus, these metals suggested bioavailability to sediment-associated biota and are likely to harm the environment.
Among the bioavailable fractions, the Fe-Mn oxide fraction seemed to have influenced the maximum number of metals (viz., Fe, Mn, Zn and Co) adsorption on sediments at three stations for both months. The processes such as metals adsorption, flocculation, and co-precipitation with Fe-Mn oxyhydroxides perform a chief role in retention of non-residual fraction of metals in sediments (Li et al. 2016). Besides, Cu and Co in November and December (Chapora); Mn and Cu in November (Zuari); and Co in November and December (Moira) were considerably or significantly held onto organic matter/sulphides which was attributed to the complexing nature of organic matter with metals in the estuarine environment (Yu et al. 2010). The metals that are adsorbed onto Fe-Mn oxides and organic matter are able to mobilize from sediments with a change in reduction or oxidation processes (Sundaray et al. 2011). Thus, ultimately increase the bioavailability of metals to the benthic biota. Furthermore, the labile fraction (exchangeable and carbonate) holds the metals less firmly which are susceptible to alteration in ionic composition and pH. The considerable or significant concentration of metals with the exchangeable (Mn in November and December at the Zuari station; Mn and Co in November and December at the Moira station) and carbonate (Mn in December at the Chapora station, Zn in November at the Moira station) fractions indicated their possible mobilization and subsequent bioavailability.
The bioaccumulation of metals by Cassosstrea spp. showed a difference in retention of metals in different body organs and between the November and the December months at Chapora and Zuari stations (Table 3).
Fe at the Chapora Station was highest in gills for the months November (970 ppm) and December (1860 ppm) as compared to adductor muscle and digestive gland. The concentration of Fe increased from November to December in the gills and digestive gland, while it decreased in the adductor muscle. The Fe level in the bivalve at the Zuari Station was highest in the digestive gland (450 ppm) and adductor muscle (1157 ppm) in November and December months, respectively. It increased from November to December in gills and adductor muscle, while decreased in the digestive gland.
Mn was highest in gills and digestive gland (around 22 ppm) in November at the Chapora Station. In December it was highest in the adductor muscle (37 ppm). It showed a drastic and a gentle decrease from November to December in gills and digestive gland, respectively. However, its concentration doubled from November to December in the adductor muscle. At the Zuari Station, Mn was highest in the digestive gland (26 ppm) and gills (33 ppm) for November and December, respectively. It exhibited an increase from November to December in gills and adductor muscle and showed a slight decrease in the digestive gland.
The concentration of Zn was highest in the gills for both November (790 ppm and 723 ppm at Chapora and Zuari Stations, respectively) and December (923 ppm and 1262 ppm at Chapora and Zuari Stations, respectively) months in comparison to adductor muscle and digestive gland. Gills and digestive gland showed an increase in the level of Zn, while revealed a drastic decrease from November to December at the Chapora Station. On the other hand, an increase in Zn level was observed from November to December in all the body parts at the Zuari Station. The metals viz., Cu and Co exhibited low concentration in gills, adductor muscle and digestive gland at Chapora and Zuari Stations. At both stations, Cu was highest in the gills for November (8 ppm and 5 ppm at Chapora and Zuari Stations, respectively) and December (7 ppm and 16 ppm at Chapora and Zuari Stations, respectively) months. The change in Cu level from November to December was not so significant at the Chapora Station, while a slight increase was noted in gills from November to December at the Zuari Station. The level of Co was negligible for November in all body parts and reported the highest concentration of 8 ppm in gills for December at the Chapora Station.
The bivalve Polymesoda spp. collected from the Moira Station showed the highest concentration of metals for November (2719 ppm, 5 ppm, 425 ppm, 18 ppm and 4 ppm of Fe, Mn, Zn, Cu and Co respectively) and December (1680 ppm, 3 ppm, 35419 ppm, 1.45 ppm and 3 ppm of Fe, Mn, Zn, Cu and Co respectively) in gills than that in adductor muscle and digestive gland. Overall, the concentration of Fe (all studied body tissues) and Zn (gills) was significantly high in the bivalve.
The concentration pattern of metals in tissues of Cassosstrea spp. and Polymesoda spp. showed discrepancies in metal accumulation at three stations. The feeding habits of species, their trophic levels and the contamination gradients of metal sources might be the reasons for such variations in metal level (Jayaprakash et al. 2015). High concentration of Fe in gills, adductor muscle and digestive gland revealed organic matter increase and metal released through the anthropogenic activities (Satheesh-Kumar and Kumar 2011). The significant Fe concentration in digestive gland of both the species at three stations indicated an enhanced rate of feeding and metabolic activity (Filipovic-Marijic and Raspor 2014). Next to Fe, concentration of Zn was considerably high in the gills of the bivalves. Zn is rapidly assimilated through food as a vital element in bivalves (Dallinger et al. 1987). Fe, Mn, Zn and Cu were higher in gills (at most stations) than adductor muscle and digestive gland. This was attributed to (i) their ion exchange from water through the chief route (gills) due to large surface area facilitating swift metal diffusion (Dhaneesh et al. 2012). (ii) Presence of a mucous layer assists quick metal accumulation (Sarkar 2018). (iii) The amount of water filtered by gills than adductor muscle and digestive gland. (iii) Free interaction with the adjacent water leading to rapid ingestion and bioaccumulation (Pringle et al.1968).
Metals in bivalves varied at Chapora, Zuari and Moira stations. The variation in metal accumulation in bivalves at three stations might be due to the existence of different hydrodynamic conditions at these stations. The sampling station at the Chapora was close to the mouth region, while was at the middle estuarine and upper regions of Zuari and Moira, respectively. These stations experience different energy conditions and physicochemical factors governing the processes associated with the retention of metals and their bioavailability. Ultimately, regulate the ecological conditions which perhaps differed at the three sampling stations. Additionally, the differential source of metals at three stations might be another factor that regulated the metal accumulation in bivalves. For instance, the Zuari Estuary is reported to have received a significant concentration of metals through the catchment area operated ferromanganese mining activities (Noronha and Nayak 2016; Gadkar et al. 2019). On the contrary, the rest of the two water bodies receive metal input through industrial, agricultural and domestic activities.
Interrelating metal bioavailability and bioaccumulation
In order to understand the relation between metal bioavailability and total metal bioaccumulation isocon diagrams were plotted between the sampling stations (Fig. 4 and 5). In November, bioavailable Fe, Mn and Co were more at Zuari, whereas bioavailable Zn and Cu were slightly higher at Chapora. Fe and Zn in an edible biota was more at the Chapora station than the Zuari Station. Rest of the metals namely, Mn, Cu and Co were also slightly more in concentration at the Chapora station. During December, bioavailable Fe, Mn, Zn and Co were higher at the Zuari Station, while Cu was relatively more at the Chapora Station. Edible biota retained more of Fe, Zn and Co in tissues at the Chapora Station, whereas bioaccumulation of Mn and Cu was slightly higher at the Zuari Station. All metals exhibited higher bioavailability at the Zuari Station than the Moira Station for November and December. However, there was a change in metal bioaccumulation pattern between the two months. Zuari Station revealed higher bioaccumulation of Mn and Zn during November, while Mn and Cu were relative more accumulation in an edible biota during December. At Moira Station, bioaccumulation of Fe, Cu and Co was more during November, while an edible biota retained more of Fe, Zn and Co during December. The Chapora station showed enhanced bioavailability of metals in comparison to the Moira Station during November and December. As far as bioaccumulation is concerned, Mn and Zn were more during November, whereas Mn, Cu and Co were higher during December at the Chapora Station than the Moira Station. During November, an edible biota showed higher bioaccumulation of Fe, Cu and Co at the Moira Station. Further, bioaccumulation of Fe and Zn was more at the Moira Station for December.
Among the sampling stations, concentration of bioavailable metals was more at the Zuari Station. This was attributed to higher mobility of metals with respect to varying salinity and pH with higher tidal influence in Zuari Estuary compared to Chapora and Moira stations. It is interesting to understand relation between total metal bioaccumulation and bioavailable metals in sediments as it is ultimate factor for bioaccumulation in an edible biota. However, present study to a larger extent revealed non-linear relationship between bioavailable metals in sediments and total metal bioaccumulation indicating selective preference of metals by the different species of edible biota.
The sampling at November and December months showed a variation in net metal concentration in the bivalves (Table 3). The changes in pH, Eh, salinity, ionic composition are most frequent in the aquatic bodies where there is a mixing of seawater with the river water. The variations in these factors within the aquatic bodies can trigger the adsorption and desorption of metals between the sediment and the water. Such processes might have regulated the change in metal concentration between November and December months in the bivalves. Further, the study revealed different combinations of metal concerning dissolved ion (Table 4), its bioavailability (Table 5a) and net bioaccumulation (Table 3). For instance, first, the metals such as Fe (Zuari), Mn (Zuari) and Zn (Moira) exhibited a decrease in bioavailability (sediments) and dissolved concentration (water) from November to December, while registered an increase in the net bioaccumulation. Secondly, Fe (Chapora) and Co (Chapora) showed an increase in bioavailability and net bioaccumulation, although not much variation was observed between the months in the dissolved concentration. Third, Fe (Moira) and Mn (Chapora and Moira) exhibited an increase in bioavailability from November to December, while registered a decrease in net bioaccumulation and dissolved level. Fourth, Zn (Chapora) indicated an increase in dissolved concentration, while there was a decrease in its bioavailability and net bioaccumulation from November to December. Fifth, Cu (Chapora) revealed an increase in dissolved level, bioavailability and so the net bioaccumulation of metals from November to December. Sixth, Co (Moira) exhibited not much variation in dissolved level and bioavailability from November to December, whereas showed a slight decrease in the net bioaccumulation. Seventh, Co (Zuari) exhibited not much variation in dissolved level and net bioaccumulation and reported an increase in the bioavailability from November to December. Eight, Zn (Zuari) showed an increase in dissolved level and net bioaccumulation, while indicated a decrease in its bioavailability from November to December. Ninth, Cu (Zuari) exhibited not much difference in dissolved level and showed a decrease and increase in bioavailability and net bioaccumulation from November to December, respectively. In general, the interlinkage of dissolved metal ion (water), bioavailability (sediments) and bioaccumulation (bivalves) suggested that mere metal bioavailability does not cause net bioaccumulation of metals. The physiology of the bivalves (trophic location, life span, size of the body, mode of ingestion, gender, reproducing status of species) affecting their differential response to different metals in the aquatic system might have regulated the net metal bioaccumulation (Mustafa and Guluzar 2003). The BSAF and modified BSAF construed the same (Table 5b). Although metals viz., Fe and Mn were present in the significant quantity in sediments, the bivalves showed the selective preference to Zn and/or Cu and/or Co at the studied stations, except for Fe at the Moira station. The BSAF and the modified BSAF revealed significant preference of bivalves for Zn as the macro-concentrator for both November and December months at Chapora, Zuari and Moira stations attributed to its application in biological activities. Moreover, as per the BSAF, the bivalve species at the Moira station was also the macro-concentrator (November) and micro-concentrator (December) of Fe, and micro-concentrator of Co (November and December). According to the modified BSAF, at the Chapora Station bivalve was macro-concentrator and micro-concentrator of Cu (November and December) and Co (December) respectively, at the Zuari Station bivalve was micro-concentrator (November) and macro-concentrator (December) of Cu, while at the Moira Station bivalve species was macro-concentrator (November) and micro-concentrator (December) of Co.
Comparison of metal concentration in bivalves with the standard values
The net (sum of gills, adductor muscle and digestive gland) metal concentration in bivalves was compared with the standard metal limit in marine biota prescribed by FAO/WHO (2004), WHO (1989) and presented by Charbonneau and Nash (1993) to understand metal contamination and bivalves suitability for human consumption (Table 3). Fe, Mn and Zn (all three sampling sites), while Cu (Zuari-December) and Co (Chapora-December) in bivalves exceeded the standard limit value. Furthermore, MPI for Chapora (77.68 for November and 132.93 for December), Zuari (42.96 for November and 97.84 for December) and Moira (72.09 for November and 96.84 for December) stations was higher and thus, indicated metal contamination with toxic effects on the bivalves and their non-suitability for the human consumption.