Seagrass ecosystems of India as bioindicators of trace elements

Seagrasses are considered as efficient bioindicators of coastal trace element contamination. This chapter provides an overview on the trace element accumulation, tolerance and biomonitoring capacity of the various seagrass species distributed along the coast of India. A total of 10 trace elements are reported in seagrasses, 11 in sediment and nine in the water column from India. From the 11 seagrass species studied, 60% of research have focused on Syringodium isoetifolium , Cymodocea serrulata , Cymodocea rotundata and Halophila ovalis . 78% of seagrass trace element research in India is from Palk bay and Gulf of Mannar (GOM), Tamil Nadu and 16% from Lakshadweep Islands. Out of the 10 trace elements, Cd, Cu, Pb and Zn are the most studied in seagrass, Fe, Mn, Ni and Pb in sediment and Cu, Fe, Mg, Ni and Zn in the water column. Accumulation capacity of various trace elements in seagrass were species-specific. S. isoetifolium have the highest concentration of Cd and Mg at Palk bay and Lakshadweep Islands respectively. The concentration of Cu was higher in C. serrulata at GOM. Halodule uninervis and Halophila decipens have the highest concentration of Co, and Cr, Ni, Pb and Zn from Lakshadweep Islands. The highest concentration of Fe and Mn were highest in Halophila beccarii and H. ovalis from the coast of Goa and Palk bay respectively. Threshold levels (>10 mg L -1 ) of Cd, Cu, Pb and Zn were observed for C. serrulata , H. ovalis , H. uninervis and T. hemprichii , that can affect the Photo System -II of these seagrasses and exert cellular stress leading to seagrass loss and die-off. High concentration of these elements can exert negative impacts on seagrass associated trophic assemblages and ecosystem functioning. Seagrasses of India can be utilized as bioindicators of coastal trace element contamination but the associated toxicity and human health risks needs further investigation.


1.Introduction
Seagrass ecosystems are distributed worldwide covering the five important bioregions of the world oceans except Antarctica (Hemminga and Duarte, 2002;McKenzie, 2020). Seagrasses form complex interlinkage between saltmarsh and mangrove ecosystems that are important in maintaining a wide range of ecological functions (Medina-Gómez, 2016;Mishra and Apte, 2020). This inter-linkage forms complex food webs that support both herbivore grazing and detrital food chain and provides habitat and nursery for various species . Seagrasses provide 24 different types of ecosystem services (Nordlund et al., 2016), which includes habitat and nurseries for commercially important fish population and endangered sea cows , carbon sequestration and storage ((Duarte et al., 2013) , shore line protection from storm surges and prevention of coastal erosion (Ondiviela et al., 2014;Potouroglou et al., 2017) and regulation of nutrient cycles (Costanza et al., 2017) that are critical in the functioning of seagrass dependent trophic levels. These ecosystem services support millions of coastal communities by supplying livelihood and food security (Nordlund et al., 2018;Unsworth et al., 2017). Like coral reefs seagrass ecosystems are also declining worldwide (Waycott et al., 2009) due to various anthropogenic factors, but the most relevant factors include habitat modification, dredging, wastewater discharge, nutrient enrichment, fishing, coastal developmental activities and boat anchoring (Lewis and Richard, 2009). These various anthropogenic activities act as a source of various anthropogenic chemicals and trace elements that enters into the marine ecosystem (Machowski et al., 2019;Serrano et al., 2011).
Trace elements as the name suggests, occurs in very low concentrations in the marine environment. At these low levels, trace elements are not toxic and play a critical role in marine ecosystem functioning (Avelar et al., 2013;Mishra et al., 2019). Among these trace elements some are non-essential and toxic to organisms (As, Cd, Cr, Hg and Pb), whereas others act as essential micronutrients (Cu, Mn and Zn), provided that their concentrations do not exceed the threshold levels (Millero et al., 2009;Stockdale et al., 2016). These trace elements pose serious risk to seagrass eco-physiology, because of their persistent nature in the marine sediment. Once accumulated in the seagrass roots their bioavailability increases (Bonanno and Borg, 2018;Govers, 2014) and under future ocean acidification and anthropogenic pollution scenarios their concentration and toxicity is predicted to increase (Mishra et al., 2019;. Once bioavailability increases, trace elements get absorbed into the root plasmalemma at the root: soil interface and are translocated to the leaves via rhizomes. Once threshold levels of trace elements are reached, it affects both root cellular structure and plant photosynthesis (Ambo-rappe et al., 2011;Prange and Dennison, 2000). Consequently, once concentrated in the seagrass tissues, through bioaccumulation these trace elements can move up the food chain through seagrass associated organisms and get biomagnified at higher trophic levels (Roberts et al., 2008; and pose serious risk for humans through marine food intake. This chapter aims to provide state of the art information about trace element concentration in the seagrass ecosystems of India and their bioindicator potential. In the early 1990's, studies on trace element accumulation patterns in seagrasses of India started, when Jagtap, (1983) first reported about the trace element levels in the seagrass Halophila beccarii. Thereafter, in the last few decades considerable amount of data have been generated on trace elements in various seagrass species of India Nobi et al., 2010;Sachithanandam et al., 2020;Sudharsan et al., 2012a;Thangaradjou et al., 2013). However, these studies have focused on few locations of India; mostly in the Palk bay and Gulf of Mannar (GOM) region of Tamil Nadu and the islands of Lakshadweep and Andaman and Nicobar, even though seagrasses have a pan India distribution. These studies have mostly recorded the trace element levels in water-seagrass or sediment-seagrass or seagrass only without focusing on accumulation capacity or seagrass bioindicator potential.

Distribution and ecology of Indian Seagrasses
Seagrass ecosystems, has a pan India distribution covering both the west and the east coast, including the islands of Andaman and Nicobar and Lakshadweep (Fig.1). These seagrass ecosystems of India are also part of South Asia (including other countries such as Pakistan, Sri-Lanka, Bangladesh and Maldives) and South-East Asia [due to Andaman and Nicobar Islands (ANI)] in the Indian Ocean region (Fortes et al, 2018;Patro et al, 2017)India has a record of 16 seagrass species belonging to three families, i.e., Hydrocharitaceae, Cymodoceaceae and Ruppiaceae. These 16 seagrass species are part of the 19 seagrass species found in South-east Asia (Prathep et al., 2011). These 16 seagrass species of India cover an area of 516.59 Km 2 up to a depth limit of 21 m (Bayyana et al., 2020;Geevarghese et al., 2018).
These various seagrass species of India occupy sandy, muddy or mixed habitat, in the intertidal region to increased depth (Parthasarathy et al., 1991). For example, small seagrass species like Halophila beccarii, Halophila ovalis are found in the muddy or sandy-muddy habitat of the intertidal region (Parthasarathy et al., 1991), whereas other seagrass species like Thalassia hemprichii and H. beccarii are found associated with mangroves (Jagtap et al., 2003;Mishra and Apte, 2020;Mishra and Mohanraju, 2018). Consequently, bigger seagrass plants like Enhalus acoroides are found at increased depths ( Patankar et al, 2018). However, this distribution of seagrass plants is dependent upon various limiting factors such as turbidity, light penetration, nutrient availability (Arumugam et al., 2013) that affect seagrass photo-physiology and reproductive processes (Patankar et al., 2018;Mishra and Apte, 2020b). Secondly, this distribution of seagrass species is also influenced by presence of other seagrasses or mangroves or coral reefs that determines distribution patterns and ecological connectivity with sourrounding ecosystems (Apte et al., 2016;Mishra and Apte, 2020).
The presence of various seagrass species at the land and sea-interface makes them suitable bioindicators of coastal metal contamination (Bonanno and Borg, 2018;Mishra et al., 2019). This suitability of seagrass as bioindicators have been extensively used by the European Water Framework Directive using the endemic seagrass Posidonia oceanica and Cymodocea nodosa of the Mediterranean Sea (Bonanno and di Martino, 2016;Bonanno and Orlando-Bonaca, 2018;Bonanno and Raccuia, 2018). However, in India there are few studies exploring the potential of seagrass as bioindicator of coastal pollution (Gopi et al., 2020;Govindasamy and Azariah, 1999;Sudharsan et al., 2012b). This chapter will provide valuable information about the various metal studies that have been carried out using different seagrass species, the efficiency of seagrass in accumulating trace elements, toxic effects of these trace elements on seagrass physiology above the threshold levels and the bioindicator potential of seagrass to these trace elements. 2.Trace element in coastal water, sediment and seagrasses 2.1Trace element in water column above seagrass meadows In general, it is thought that the trace element concentration is higher in the water column and readily available for the leaves of the seagrass for uptake. However, it has been observed, that is not the case always (Bonanno and di Martino, 2017). The accumulation of trace elements from the water column is species-specific among seagrasses and depends on the plant physiology and the nature of the trace element, i.e., toxic or essential (Millero et al., 2009;Mishra et al., 2019). In India, the trace element studies of the water column above seagrass ecosystems are very less compared to that of the sediment and seagrasses. Only 4 studies have reported the nine out of 11 elements (As, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb, Zn) reported in sediment and 10 in seagrass. However, on the west coast trace elements like Cd, Cr, Pb and Zn are not reported (Table 1). These studies on trace elements are restricted to five locations, i.e., Palk bay and GOM, Tamil Nadu, Goa, Maharashtra, and Lakshadweep Islands, consisting of only eight seagrass species.
Out of the nine trace elements, Mg was the most studied trace element in the water column of seagrass ecosystems in India, and Mn the least (Fig.2a). The concentration of Mg in the water column was highest in the seagrass meadows of Cymodocea rotundata, Syringodium isoetifolium, Halodule uninervis, Thalassia hemprichii and Halophila ovalis of Lakshadweep Islands. The concentration of Fe and Mn was highest in Halophila beccarii from the west coast in Goa. The water column above T. hemprichii meadows have higher concentration of Cd and Pb in Palk bay, whereas the water above S. isoetifolium meadows have higher concentration of Cu and similar levels of Pb with T. hemprichii (Table 1). The concentration of Cr, Ni and Zn was similar among the water column of Cymodocea serrulata, S. isoetifolium and Enhalus acoroides meadows at GOM. The trace element concentration in the water column of various seagrasses followed a decreasing pattern, Mg>Zn>Fe>Cr>Cu>>Mn>Ni>Pb>Cd (Table 1). In the water column, Mg concentration was very low on the east coast withing a range of 0.16 to 2.06 mg kg -1 , while that on the west coast was 550-fold higher ( Table 1). The Mg concentration (18318 mg kg -1 ) in the water column of Lakshadweep Islands was highest in the coastal waters of India. The Zn levels were in the range of 0.11 to 11.6 mg kg -1 , with higher levels in the water column of GOM, Tamil Nadu. The Cd concentration were in the range of 0.02 to 0.15 mg kg -1 , while that of Cr are 0.31 to 2.03 mg kg -1 on the east coast. Copper levels in the water column were 2-fold higher in the east coast than that of the west coast. Iron concentration were higher in the coastal waters of Goa. Manganese and Ni levels were similar among the both coasts. The concentration of Pb were 0.007 to 0.13 mg kg -1 and that of Zn were 0.11 to 11.6 mg kg -1 (Table  1). Trace elements such as Co and Hg are not reported in the water column, even though they are reported from the sediment and seagrass tissues ( Fig.2b and c).
The source of trace element in the coastal waters of India are mostly through riverine input, which varies according to the monsoon dependent seasonal runoff and subsequent erosion from river catchment area (Tripathy et al., 2014). Consequently, local anthropogenic discharge from industrial and domestic waste water also leads to input of these trace elements into the coastal waters (Libin Baby et al, 2017;Nobi et al., 2010;Thangaradjou et al., 2009;Thangaradjou and Bhatt, 2018). Other than these inputs, release of trace elements from the sediment to the water column within the seagrass ecosystems also plays an important role in varying concentration of trace elements in the water column (Govindasamy and Azariah, 1999;Libin Baby et al., 2017). The low concentration of most of the elements like Cd, Cr, Cu, Ni and Pb is a result of settling of the organic matter content that inflows with the land run-off. Seagrass ecosystems are considered as efficient ecosystem engineers and they help in settling a small fraction of this organic matter content on their leaf surface or into the sediment, thus reducing water turbidity and enhancing their photosynthetic activity (Gillis et al., 2017;Guannel et al., 2016). This seasonal and local variation of input of trace elements are reflected in seagrass ecosystems (Govindasamy and Azariah, 1999). This seasonal influence of high concentration of Co, Cd, Cu, Fe, Ni, Mn and Zn in the water column has been observed at GOM, TN (Govindasamy and Azariah, 1999) and the east coast of India (Vinithkumar et al., 1999).

Trace metals in the sediment of seagrass meadows
A total of 11 trace elements has been reported in the sediment of seagrass meadows of India, including As and Co that has not been reported in the water column of seagrass meadows (Fig.2b). In sediment, the trace element concentration of Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn were multifold higher than their water column values, except Mg which was higher in the water column . The concentration of trace elements in the sediment of seagrass meadows followed the decreasing pattern of Fe>Mg> Mn> Cr> Ni>Cu>Zn>Co>As>Pb>Cd (Table 2).
Five locations of India on both the east and west coast within seagrass meadows have been used for trace element studies, i.e., Palk bay and GOM, Tamil Nadu, Goa, Maharashtra, Lakshadweep and Andaman and Nicobar Islands (ANI). The sediment of seven seagrass species, such as S. isoetifolium, T. hemprichii, C. serrulata, C. rotundata, H. beccarii, H. ovalis, H. uninervis have been used for trace element studies (Govindasamy and Azariah, 1999;Jagtap, 1983;Libin Baby et al., 2017;Thangaradjou et al., 2013;Untawale and Jagtap, 1984;Vinithkumar et al., 1999). On the east coast the sediment within seagrass meadows of GOM had higher levels of As than the seagrass sediment of Palk bay region of Tamil Nadu (Table  2). However, the highest concentration of As in the sediment of seagrass meadows were recorded from ANI. This higher concentration of As can be due to the volcanic origin of this island, where As enters the coastal ecosystem through seasonal land run-off, as these islands are far from industrially polluted (Nobi et al., 2010;Sachithanandam et al., 2020). Other than Arsenic, Co, Cr, Cu, Mg, Mn, Ni and Pb concentration in the sediment of seagrass meadows are the highest in ANI ( Table 2). The concentration of Fe in the sediment were higher on the west coast at Vijayagiri and Ratnagiri of Maharashtra within the H. beccarii meadows (Table  2). However, these Fe values in the sediment of H. beccarii are more than three decades old and this high concentration of Fe in H. beccarii sediment compared to other seagrass ecosystem of India can be due to its presence within close proximity of mangrove sediments, which act as sink of trace elements (Apte et al., 2016;Mishra and Kumar, 2020). Though most of the trace element levels in the sediment of S. isoetifolium, T. hemprichii and C. serrulata meadows of Palk bay have similar levels in their sediment (Govindasamy et al., 2013;Libin Baby et al., 2017;Thangaradjou et al., 2013) the concentration of Cu and Zn are 20-fold lower in the sediment of T. hemprichii meadows . On the west coast, the sediment of H. beccarii meadows were found with high levels of Cu at Vijayagiri and Ratnagiri, Maharashtra (Jagtap, 1983). There is a clear evidence that the sediment of seagrass meadows act as a sink of various trace elements. Consequently, the continuous persistence of these trace elements (particularly trace elements like As, Cu, Pb) can result in potential toxicity to seagrass rhizosphere and the seagrass associated biota (Ambo-rappe et al., 2011;Richir, 2016;Richir and Gobert, 2014). However, for the trace elements to be toxic, it has to be bioavailable to the seagrass root systems and reach above threshold levels. This bioavailability depends on trace elements mobility in the sediment, their chemical speciation (Usero et al., 2005) and sediment characteristics such as pH, organic matter content and redox potential (Yang and Ye, 2009

Role of sediment characteristics in making trace elements bioavailable
The sediment within seagrass meadows act as a storehouse of trace elements, where the influx of land run-off and anthropogenic chemicals recharge this storehouse. Other than this input, trace element recycling happens within the seagrass meadows (Sanz-Lázaro et al., 2012), which releases trace element bound to the fine grain sediment fraction of seagrass meadows. pH of the sediment and the overlying water plays a major role in release of this sediment bound trace metals, as low pH can alter the metal speciation and favor the release of metals from sediment pore waters (Atkinson et al., 2007;Simpson et al., 2005) that are generally not bioavailable. The trace metals in the water column above the sediment, are absorbed on to sediments where redox stratification of metal bound particles with depth occurs (Basallote et al., 2020(Basallote et al., , 2014Eggleton and Thomas, 2004), until resuspension of these particles happens due to physical processes and bioturbation. Resuspension with oxygenated overlying waters results in metal speciation in the dissolved phase (Simpson et al., 2005), making the metals bioavailable in pore waters (Batley et al., 2004). Once released from pore waters into water column, these metals are bioavailable to seagrass and associated organisms till precipitation of these metals are initiated by the fine fraction (<63 micron) of the sediments suspended in water column (Zoumis et al., 2001).

2.4.TE accumulation in seagrasses
A total of 10 trace elements are reported in seagrass tissues of India, excluding As (Fig.2c). Six seagrass species from Palk bay, seven from GOM and eight from Lakshadweep and H. beccarii from Goa and Maharashtra (Table 3). For trace element in seagrass, Palk bay region is the most studied region followed by Lakshadweep islands, whereas GOM and ANI have similar levels of studies (Fig.3a). In general, S. isoetifolium is the most studied seagrass for various trace element levels followed by C. serrulata and C. rotundata (Fig.3b). There are only 4 studies in India, which have studied trace elements in water, sediment and seagrass Jagtap, 1983;Jagtap and Untawale, 1984;Libin Baby, et al., 2017) and there are six studies including the above four, which have reported about trace element levels in sediment and seagrasses (Nobi et al., 2010;Vinithkumar et al., 1999) and the rest of studies have reported only about the trace elements in seagrass ecosystems, excluding the trace elements in water or sediment.   The accumulation capacity of trace elements in the various seagrass species are different, which is reflected by the highest concentration of each trace element observed in a different seagrass species. For example, Cd concentration were highest in the tissues of S. isoetifolium in Palk bay, with similar levels of Cd in H. pinifolia and H. uninervis of Lakshadweep Islands. The concentration of Mg, were also highest in S. isoetifolium of Lakshadweep Islands (Table 3). The concentration of Cu in seagrasses of India were highest in C. serrulata, at GOM, even though the highest levels were reported from mixed seagrass species of ANI (Nobi et al., 2010;Thangaradjou et al., 2014). The H. uninervis of Lakshadweep Islands had the highest concentration of Co, whereas Cr concentration were highest in H. decipens of Lakshadweep islands. However, the highest levels of Cr were reported from ANI, but the authors have not specified any seagrass species (Arumugam et al., 2013;Nobi et al., 2010;Thangaradjou et al., 2013). Other than Cr, the concentration of Ni, Pb and Zn levels in H. decipens were also the highest in India. The concentration of Fe was highest in the tissues of H. beccarii, at GOA, whereas the highest concentration of Mn was recorded from H. ovalis at Palk bay (Table 3).This accumulation capacity of different seagrass species of India, clearly indicates trace element accumulation in seagrass is a species-specific phenomenon and the various seagrass species of India are potential indicators of different trace element concentration in the environment. However, regarding the kind of investigated organs/tissues of seagrass most of the trace element studies in India have used the whole seagrass plants, except Kannan et al., (2011) andImmaculate et al. (2018), who have reported trace element levels in the leaves of C. rotundata, C. serrulata, T. hemprichii, S. isoetifolium, H. pinifolia and E. acoroides from GOM, Tamil Nadu.

Effects of trace elements on seagrass physiology
In seagrasses the concentration of elements varies within the tissues; leaves, rhizomes and roots. Where roots accumulate the maximum concentrations and the leaves accumulates less (Bonanno and Orlando-Bonaca, 2018;Mishra et al., 2019) as higher metal concentration in leaves can lead to trace metal toxicity and damage photosynthetic apparatus of seagrass (Govers, 2014;Prange and Dennison, 2000). However, in India, trace metals and their toxicity on seagrass physiology or growth have not been reported. Globally, there are some toxicity assessment of trace elements on seagrass physiology of C. serrulata (Aljahdali and Alhassan, 2020; Prange and Dennison, 2000), H. ovalis (Ambo-rappe et al., 2011;Prange and Dennison, 2000), H. uninervis (Prange and Dennison, 2000) and T. hemprichii (Lei et al., 2012) which can be compared to the seagrass species of India. Trace elements such as Cd (10 mg L -1 ), Cu (1-10 mg L -1 ), Pb (10 mg L -1 ) and Zn (10 mg L -1 ) are toxic to C. serrulata, H. ovalis, T. hemprichii and H. uninervis photosynthetic apparatus; Photo System-II (PS-II). Other than damaging PS-II, Cu concentrations reduced leaf growth and width of H. ovalis and amino acid levels in C. serrulata and H. uninervis (Prange and Dennison, 2000;Ambo-Rappe et al., 2011). Zinc toxicity reduced photosynthetic pigments of T. hemprichii (Lei et al., 2012). The antioxidant activity of H. ovalis and C. serrulata were decreased by Cd and Pb toxicity (Ambo-Rappe et al., 2011; Aljahdali and Alhassan, 2020).
For the above mentioned four seagrass species in India, Cu concentration are 6-fold and 1.7-fold higher than toxic levels in the tissues of C. serrulata and at GOM and Palk bay, Tamil Nadu (Govindasamy et al., 2013;Immaculate et al., 2018;Libin Baby et al., 2017). In Lakshadweep Islands, H. uninervis and T. hemprichii have 1.8-fold and 1.3-fold higher Cu levels than toxic concentrations (Gopinath et al., 2011;Thangaradjou et al., 2013;Untawale and Jagtap, 1984), whereas H. ovalis has 2-fold higher Cu levels than toxic levels at Palk bay region (Gopi et al., 2020;Kannan et al., 2011). Lead levels are 1.2-fold and 2.3-fold higher in T. hemprichii and H. uninervis at Lakshadweep islands Gopinath et al., 2011;Thangaradjou et al., 2013). T. hemprichii and H. uninervis have 3-fold higher Zn levels than toxicity levels Gopinath et al., 2011;, whereas C. serrulata has 3 to 5-fold higher levels of Zn concentration which can exert toxicity on its PS-II at Palk bay and GOM, Tamil Nadu and at Lakshadweep islands Sudarshan et al., 2012;Thangaradjou et al., 2013;Gopi et al., 2020). Trace element levels above toxic concentration for these seagrasses clearly suggests that, these four seagrass species are under stress from metal toxicity, which needs further research and attention from the scientific community of India.
High concentration of trace elements in these seagrass species will result in trophic transfer of these elements and exert toxicity to the associated trophic assemblages (de los Santos et al., 2019;Prange and Dennison, 2000), such as gastropods, molluscs, fish and invertebrates that depend on seagrass for direct and indirect food sources (Manikandan et al., 2011). Consequently, metal toxicity can lead to seagrass population loss and die offs, which will have negative consequences on the coastal ecosystem functioning.

4.Future scenarios and metal toxicity on seagrass
Global changes, such as ocean acidification due to increased CO2 concentrations, and low pH, will affect the trace metal chemistry, speciation and their bio-availability (Millero et al., 2009;Zeng et al., 2015) and can have possible negative impacts on the seagrass ecosystem. Low pH can increase bioavailability of trace elements bound to seagrass sediment, and even increase their concentrations as trace metal speciation in seawater is strongly dependent on seawater chemistry, with several metals known to be sensitive to speciation changes within the pH range projected for near-future (Byrne et al., 1988;Richards et al., 2011). Changes in ocean carbon chemistry may also alter the behaviour of metals bound to sediments, influencing metal fluxes from contaminated sediments (Millero et al., 2009;Zeng et al., 2015). Low pH is predicted to increase the toxic free ion concentration of metals in coastal waters by as much as 115% in the next 100 years (Lewis et al., 2016;Millero et al., 2009). Saying that most of the studies on metal concentrations has been focused on marine animals (e.g. marine invertebrates, mussels, planktons, fish larva) with very few studies on seagrass ecosystems, which needs to be addressed in India.

5.Conclusions
Globally, seagrasses are used as bioindicators of coastal contamination Orlando-Bonaca, 2018, 2017;Lewis and Richard, 2009). In India, though seagrass is found to be efficient indicators of the environmental concentration of trace elements in their tissues, they have not been used as bioindicators of coastal pollution. However, the National Action Plan for seagrass ecosystems that have been launched in 2018 (Koshy et al., 2018) plans to address these issues, and provides guidelines that will use this bioindicator potential of vast seagrass ecosystems of India to facilitate their conservation and management issues.