Metals in coastal seagrass habitats: A systematic quantitative literature review

Abstract Seagrasses provide crucial ecosystem services in the coastal environment, but are under increasing threat associated with anthropogenic activities. Seagrass meadows effectively trap suspended sediment, a key vector for trace metal transport, by altering local hydrodynamic conditions around their leaves and stems. As a result, seagrass meadows often have different geochemical sediment characteristics compared to adjacent bare sediments and may accumulate higher concentrations of trace metals. The aim of this systematic review was to identify research trends and knowledge gaps in the relationships between metals and seagrasses. The systematic search of the literature identified a total of 191 relevant articles. Although seagrasses have a broad geographic distribution, the highest diversity and endemism exists in the Indo-Pacific region, yet most studies (40%) were conducted in the Mediterranean Sea. There were also taxonomic gaps with some common species groups, including Amphibolis and Thalassodendron spp., being poorly studied. The reviewed studies examined the relationship between seagrass and 39 metal elements. There were no studies examining rare-earth elements (REEs) or platinum-group elements (PGEs), both of which are critical classes of metals in emerging technologies. Furthermore, the review determined that all 191 studies focused on the impact of metals on seagrasses, while only six also considered the effect of seagrasses on metal geochemistry in sediments. We conclude that further research is necessary to address these key gaps in knowledge to better understand the role(s) of seagrasses in the distribution, immobilization, and release of trace metals in coastal ecosystems. Graphical abstract

The primary transport mechanism for most contaminants harmful to seagrasses is catchmentderived runoff into estuaries and nearshore coastal ecosystems (Chilton et al., 2021), which are important habitats for most seagrasses (Short et al., 2001). Inputs of nutrients and suspended sediments can lead to negative impacts, such as eutrophication and/or reduction of light availability (Hamilton et al., 2016;Li et al., 2019;Nixon, 1995), and hypoxic or anoxic events (Cui et al., 2022;Gooday et al., 2009). Seagrasses adapt to eutrophic conditions through increased respiration, reduced rates of photosynthesis, low growth or shorter leaf lengths, and reduction in the number of leaves per shoot (Holmer & Bondgaard, 2001). Moreover, higher sediment loadings and increased turbidity, driven by dredging and coastal erosion, can cause reduced light penetration (Cui et al., 2022;Erftemeijer et al., 2012), which impacts seagrass photosynthesis and growth (Paterson et al., 2008;Zabarte-Maeztu et al., 2021) and in turn reduces primary productivity and seagrass depth distributions (Erftemeijer & Robin Lewis, 2006;Tang & Hadibarata, 2022).
Seagrass meadows can trap suspended sediments due to the alteration of local hydrodynamic conditions around their leafy benthic canopy (Fonseca et al., 2019;Hendriks et al., 2008). The increased drag associated with seagrass leaves can result in decreased bed-water flow and the dissipation of wave and current energy (Luhar et al., 2010). These conditions favor the deposition of suspended particulates in seagrass habitats (Barcelona et al., 2021;Sanchez-Vidal et al., 2021). Sediment deposition can alter the geochemistry of seagrass meadows due to the accumulation of fine sediment containing high concentrations of organic carbon (Garzon-Garcia et al., 2017;Strom et al., 2011), nutrients (Ikeda et al., 2009), and trace metals (Strom et al., 2011;Toshihiro et al., 1998). To date, the majority of studies on seagrass-inhabited sediments have focused on ecosystem metabolism and nutrient dynamics (i.e., carbon turnover, nitrogen and phosphorus cycling) and the microbial activities that mediate these processes in seagrass rhizosphere sediments (Ferguson et al., 2017;Nielsen et al., 2001;Pag es et al., 2011;Welsh et al., 2000). Only a few studies, however, have attempted to determine how seagrasses influence trace metal sequestration or release, despite the fact that seagrass colonized sediments could play a significant role in the trapping and cycling of trace metals from urbanized catchments.
Metals are common, persistent (non-biodegradable) coastal contaminants, which are potentially bioaccumulated by biota, causing acute and chronic toxicity (Buzzi et al., 2022;Wang et al., 2015). Metal contaminants such copper (Cu) and lead (Pb) are well known for their potential toxic impacts on marine biota (Ramesh et al., 2015). More recently, however, other metal contaminants such as rare earth elements (REEs) and platinum group elements (PGEs) have received increased interest as contaminants of emerging concern (CEC) due to their increased usage in emerging technologies (Batley & Campbell, 2022;Chaukura et al., 2016). Numerous marine organisms including fishes, mussels, and seagrass are exposed to high risks of metal pollution and bioaccumulation (Chan et al., 2021;Qiu et al., 2011;S anchez-Quiles et al., 2017), which in turn can influence human health (Brouziotis et al., 2022). Consequently, there have been increasing efforts to monitor metal concentrations in marine organisms and the environment (e.g., water and sediment) to provide crucial information on water quality and the evaluation of environmental risk (Hong et al., 2020;S anchez-Quiles et al., 2017).
Seagrass meadows represent a unique ecosystem for metal accumulation and cycling due to their ability for capture, uptake and remobilise dissolved and particulate metal species. Furthermore, seagrass can directly influence biogeochemical processes in the sediment that influence metal sequestration/mobilization dynamics via the process of radial oxygen loss from their root systems (de la Cruz Jim enez et al., 2021;Jovanovic et al., 2015;Wang et al., 2018). Additionally, the sediment of seagrass meadows may represent valuable archives of metal pollution over centennial time scales (Lafratta et al., 2019).
This systematic quantitative literature review aims to investigate trends in study aims, geographic and taxonomic distribution patterns, the range of metals investigated and general methodological approaches to study metal pollution in seagrass ecosystems worldwide, and how theses have shifted over time. More specifically, we aim to examine where and when research has been conducted, which seagrass species and trace metals were investigated, the range of methodological approaches used, and the nature of the relationships examined. This information is critical to identify key research trends and knowledge gaps to inform future research and management in coastal seagrass habitats.

Systematic quantitative literature review
The Systematic Quantitative Literature Review (SQLR) method (Pickering & Byrne, 2014) was selected for the current review as a critical literature analysis guide. The SQLR is a robust method to survey scientific literature, select relevant articles, quantify the number of studies on a particular topic, and identify research gaps within the literature (Carlini et al., 2021;Humphries et al., 2021;Pickering & Byrne, 2014). Data, which are coded into a database via the SQLR process, can include bibliographic information, research location, methodologies adapted, the research aim, and the results of each study. This review method bridges the gap between traditional narrative literature review and meta-analysis and identifies research trends and gaps in knowledge. As a result, the SQLR is a reliable, quantifiable, and reproducible method for effectively researching the literature (Pickering & Byrne, 2014).

Data collection and sorting
Following Pickering and Byrne (2014), searches of four commonly used online databases (Scopus, Web of Science, ScienceDirect, JSTOR) were conducted in October 2022 to identify peer-reviewed articles that focused on seagrass and metal contaminant research. The key search terms were "seagrass" AND ("metal Ã ") in Scopus (n ¼ 169), Web of Science (n ¼ 399), ScienceDirect (n ¼ 130), and JSTOR (n ¼ 3). In total, 701 relevant articles were initially identified from all four databases. Selected articles were transferred to a custom database (Endnote), where duplicates were excluded (n ¼ 229). Then, all articles that only contained the key terms in indexes and references or only consisted of conference reviews and abstracts were excluded (n ¼ 150). Next, publications not related to both seagrass and metals were removed (n ¼ 68). These included articles focusing on non-seagrass species such as mangroves, freshwater macrophytes and other plants and animals, and nonmetal contaminants. The 191 remaining articles were manually entered into Excel for systematic quantitative analysis and full-text screening.

Literature coding
The 191 articles were assessed and coded in Excel according to criteria including article bibliographic information, geographic information, seagrass taxonomy, metal elements examined, methodology employed, and study approaches (i.e., did the article focus on the effect of seagrass on metal dynamics or on the effect of metals on seagrass) (Supplementary material). The criteria used for coding are summarized in Table 1. These data enabled the analysis of geographic, taxonomic, methodological, and thematic patterns. Trace metal Aluminium (Al), Actinium (Ac), Arsenic ( Study types Field experiment, laboratory experiment, modeling Sample types Plant components or whole plants, water, sediment Data patterns Diel/spatial/temporal/seasonal variations, historical records, lepidochronological year, and specific events such as weather (e.g., storms, dry and wet season) Target measurement Environmental monitoring and/or assessment, physiological responses of seagrasses, mobilization and/or redistribution of metals in seagrass sediment, uptakes and/or losses, metal speciation and/or fraction Study approach Study approach Approach 1: Use of seagrass as a biomonitor/bioindicator Approach 2: Trace metal fluxes (uptakes or losses)/ calculation of kinetics of trace metal uptake or loss by seagrasses, or release from sediments Approach 3: Physiological/health responses of seagrass to trace metals Approach 4: Understanding the influence of seagrass on trace metal accumulation in the sediments and/or metal speciation within sediments 3. Results and discussion

Publication year
The 191 articles in this review were published between 1976 and 2022 ( Figure 2). There were relatively few articles published annually in the first 30 years (< 5), which gradually increased to a maximum of 17 articles in 2019 ( Figure 1). This may be due to the increasing awareness of the important role of seagrass as a bioindicator in estuarine and coastal ecosystems during the last 20 years (Mart ınez-Crego et al., 2008;Oliva et al., 2012) or recognition of their importance in sequestering carbon (i.e., seagrass account for approximately 10-15% of global oceanic carbon mitigation) (Duarte et al., 2012). The negative ecological and socio-economic consequences from the decline of seagrass habitats may have also led to increased research on seagrass restoration.

Geographic trends
The 191 studies examined in this review were conducted in 35 different countries across five geographic regions ( Figure 2). In Europe, where over 40% of studies were conducted, research was  mostly carried out in Italy (n ¼ 31), France (n ¼ 17) and Spain (n ¼ 13), with some also in Greece (n ¼ 10), Montenegro (n ¼ 4) and Portugal (n ¼ 2). Asia (27%) was the second most studied region including China (n ¼ 17), Indonesia (n ¼ 12), India (n ¼ 8), Malaysia (n ¼ 4), Saudi Arabia (n ¼ 2), Egypt (n ¼ 1), Japan (n ¼ 1), Jordan (n ¼ 1), Korea (n ¼ 1), Kuwait (n ¼ 1), and Taiwan (n ¼ 1). In Oceania (15%), studies were conducted in four countries/regions: Australia (n ¼ 26), Palau (n ¼ 1), Micronesia (n ¼ 1), and Fiji (n ¼ 1). Eleven studies were conducted in North and South America (11%), in the United States (n ¼ 7), Mexico (n ¼ 7), Brazil (n ¼ 5), Guadeloupe (n ¼ 1), Puerto Rico (n ¼ 1), and Venezuela (n ¼ 1). Only four studies were conducted in Africa (4%), in Tunisia (n ¼ 3), Morocco (n ¼ 3), Algeria (n ¼ 1), and South Africa (n ¼ 1). Eight studies were conducted in multiple countries. Almost half of the study locations were concentrated in temperate zones, accounting for 47% (n ¼ 90) of articles. Studies conducted in tropical and subtropical zones made up 26% and 27%, respectively, of the articles reviewed. These geographic trends provide information about where seagrass vegetated areas are found globally, but also highlight which countries have actively studied the relationship between seagrasses and trace metals. The majority of studies reviewed focused on seagrass in European waters. This result reflects the most dramatic declines in seagrass meadows, which have occurred in the Mediterranean region since 1950 due to rapidly increasing populations, significant industrial development, and extensive land use change in Mediterranean countries, especially the northern Mediterranean region (Ru ız et al., 2009;Vera-Herrera et al., 2022). For example, the estimated areal extent of decline for Posidonia oceanica, which is the most common and widespread species in Mediterranean waters, is between 13% and 50% of the area of seagrass coverage since 1960 (Marb a et al., 2014). As a result, countries such as Spain, France and Italy have conducted intensive studies on seagrass decline and environmental contaminants in their waters (Boudouresque et al., 2021;Ru ız et al., 2009). While the research focus to date has been primarily in Mediterranean waters; the Gulf of Mexico, Caribbean, and the entire Atlantic coastline of Europe and Africa, which have large extent and are ecologically important seagrass habitats (McKenzie et al., 2020), have relatively few studies on seagrass and trace metals. In addition, there are relatively few studies in Southeast Asia (not including China) and the tropical Indo-Pacific region despite these regions having the greatest taxonomic diversity, distribution, and abundance of seagrass in the world (Short et al., 2007;Spalding et al., 2001). More research on the relationship between seagrasses and trace metals in the coastal regions of Africa, South America, Europe, and the Atlantic and Pacific coasts of North America is urgently required.
Posidonia oceanica meadows are limited to regions including the Mediterranean, northwest Africa and the Aral, Black, and Caspian Seas (Short et al., 2007). It is the most dominant and ecologically important seagrass species (Larkum, Drew, et al., 2006;Traganos et al., 2022) in the Mediterranean Sea (Telesca et al., 2015). In Mediterranean waters, P. oceanica has become a target species requiring protection and management over the past two decades Personnic et al., 2014). Furthermore, it was identified as a priority habitat by the European Union's Habitat Directive (92/43/CEE) in 1992. Hence, the ecological roles and significance of P. oceanica in the Mediterranean region have attracted intensive research attention since the 1990s. While Cymodocea, Zostera, and Halophila spp. are the next most commonly studied genera in this review, the remaining genera, such as Thalassia, Enhalus, Halodule, and Ruppia spp., have been relatively poorly studied despite their large geographical extent around the world (Short et al., 2001;Veettil et al., 2022;Yu et al., 2018). Short et al. (2011) showed that the global populations of many Zostera spp. (e.g., Z. capensis, Z. noltii, and Z. marina) and Posidonia spp. (e.g., P. australis and P. oceanica) have decreased, while A. antarctica and all species of Ruppia and Syringodium have been stable or have even increased. Hence, the taxonomic trends in this review appear to demonstrate that research has been more focused on threatened species rather than species with more stable populations. In addition, some of the less well-studied species, such as H. ovalis and T. hemprichii, are also important ecosystem indicators throughout coasts and estuaries of Eastern Africa, Southern Asia, all of Northern Australia, and the Indo-Pacific region (Mishra & Farooq, 2022;Zhang et al., 2014). These species have a high sensitivity to changes in environmental conditions, especially elevated temperatures and decreased water quality (Webster et al., 2021;Zulfikar et al., 2020). Additional studies on these species are needed to ensure effective environmental management and protection of these bioindicator seagrass species in these regions.

Metals
A total of 39 metals were included in the articles identified in this review (Figure 4). The most common elements analyzed were Cu (n ¼ 138), Pb (n ¼ 133), Cd (n ¼ 124), and Zn (n ¼ 121). The next most common metals measured were Cr (n ¼ 82), Ni (n ¼ 74), Fe (n ¼ 62), Mn (n ¼ 53), and As (n ¼ 47). The number of articles that analyzed Al, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, and Zn has shown noticeable increases over the last three decades while B, Be, Ga, Mo, Y, and Zr were only examined in studies published after 2010. A total of 157 articles examined multiple metal elements with only 34 articles investigating a single metal.
Metals such as Cd, Cr, Cu, Pb, and Zn have been extensively studied, especially since the industrial revolution, due to their toxicity, widespread use, and increasing industrial and municipal discharge containing high concentrations of these metals (Han et al., 2002;Musyoka et al.,  2013; Ozaki et al., 2019). For example, Cu has been mostly used in electrical equipment and construction, and its production has increased over the last few decades (Graedel et al., 2004;He & Small, 2022), resulting in increasing quantities of Cu entering the environment (Galster & Helmreich, 2022;Northey et al., 2017). Zn occurs naturally, but environmental concentrations are rising above these natural levels due to the addition of Zn through human activities such as mining, coal and waste combustion and steel processing (Hutton & Symon, 1986;Lee et al., 2018;Masekoameng et al., 2010). Such anthropogenic activities have led to increased metal pollution in the environment (Qian et al., 2015;Tchounwou et al., 2012) producing a serious problem worldwide (Ozaki et al., 2019). Concerns about effluent containing various metals from human activities have increased due to their non-biodegradable, often high toxicity (Musyoka et al., 2013) and the difficulty involved in abating concentrations using conventional sewage and wastewater treatment systems (Gagnon & Saulnier, 2003).
Fe, Mn, and Cu are essential metal elements for seagrass growth and, consequently, quantification and distribution of their concentrations in sediments has been well-studied (Marb a et al., 2008;Tahril et al., 2019). However, excessive concentrations of essential metals can have a negative effect on seagrass growth and health (Ali et al., 2019;Singh et al., 2016). Accordingly, the important roles of several particular essential metals (e.g., Fe and Mn) and potential risks of nonessential metals (e.g., Cd and Pb) in estuarine and coastal environments (Ozaki et al., 2019;Qian et al., 2015) are a major focus for physiological and ecotoxicological studies in seagrass ecosystems (Blinova et al., 2010;Bols et al., 2001;Flemming & Trevors, 1989).
There are a significant number of important metals that were not included in the studies in this review (Figure 4). These metals include the REEs, which include scandium (Sc) and the lanthanides (i.e., lanthanum (La) through lutetium (Lu)); PGEs, which include platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), and osmium (Os); and emerging critical metals such as antimony (Sb), tungsten (W), germanium (Ge), and tantalum (Ta). Many of these metals have been identified as contaminants of emerging concern that pose future environmental health risks for both ecosystems and humans (Kulaksiz & Bau, 2013;Rauch & Morrison, 2008). For example, plants and animals exposed to increased concentrations of REEs in the environment have a higher risk of accumulating these REEs (Crocket et al., 2018;Galhardi et al., 2020) and their subsequent consumption by humans can cause detrimental impacts on human health such as neurotoxicity and organ damage (Brouziotis et al., 2022;Guerra-Vargas et al., 2020). With the rapid development of new technologies, high anthropogenic demand for a wide range of metals for a variety of applications and products, and industrial activities, the aforementioned uncommon metal elements can accumulate in waste and wastewater and be discharged into surrounding areas such as lakes, rivers, and eventually coastal waters . The increased inflow of these elements into aquatic ecosystems can alter their natural biogeochemical cycles in the environment (Arienzo et al., 2022;Paffrath et al., 2020) and may increase the risk of toxicological impacts on resident biota and humans alike. Future research should examine the relationship between seagrasses and poorly studied metal elements such as REEs, PGEs, and other critical metals.

Methodological trends
Metal concentrations were analyzed primarily in the tissues (n ¼ 181), and to a lesser degree in sediment (n ¼ 94) and overlying water (n ¼ 40) samples. The collected data were used to examine spatial (n ¼ 15), temporal (n ¼ 5), and seasonal (n ¼ 26) patterns. Five studies examined the data in relation to lepidochronological year. No study investigated diel variations in association with metal concentration of seagrass, water, or sediment, however, five studies examined diel variations of seagrass photosynthetic function/activity after exposure to different concentrations of metals. Long-term variations (i.e., greater than one year of observation) were analyzed in six studies. Five studies examined the data patterns for before and after specific events such as storms, floods, or dry/wet season conditions.
The majority of studies (n ¼ 153; 80%) in this review involved measuring the total concentration of metals in seagrass, sediment, or water samples in the environment. Six studies involved in-situ experiments while 31 studies were laboratory experiments. All 31 laboratory experiments and five field-based studies had similar aims to investigate metal concentrations in seagrass, sediment and/or water samples or the physiological responses under controlled conditions and/or experimental conditions in situ, respectively. One field study was conducted by transplanting seagrass from a pristine area without sewage and industrial waste inputs to a highly polluted bay to observe the impact of metals in sediment on seagrasses (Lee et al., 2019). One study focused on modeling seagrass growth under conditions of metal contamination (Lin & Sun, 2015).
A large number of studies in this review measured the concentration of metal(s) in seagrass tissues. These data can provide an estimation of the extent to which the environment is exposed to metal contamination with seagrass acting as an environmental indicator (Al-Najjar et al., 2021;Govers et al., 2014). However, the behavior of metal elements in coastal areas are also closely linked to sediment and water (Tahril et al., 2019). Hence, the determination of total metals only in seagrass tissue can limit assessment of the environmental risk as the data do not reflect the mobility, reactivity, or bioavailability of metals (Tack & Verloo, 1995;Violante et al., 2010). In addition, metals tend to be associated with fine suspended sediments that ultimately end up depositing on the benthos, leading to much higher elemental concentrations (often 10 to 100fold) in sediment compared to water and seagrass tissue (Temara et al., 1998). Hence, further study investigating different metal phases in sediment, water and seagrass tissues are needed to understand the relationships between seagrass and metal dynamics.

Study approach
All 191 studies were classified into four different approaches based on the target measurements used to achieve the study aims. The majority (n ¼ 152; 80%) included the determination of total metal concentrations in plant tissues, sediment, and/or water (Approach 1). Eleven studies determined the kinetics of metal uptake and/or loss by seagrasses, or release from sediments (Approach 2). Observations of physiological responses, such as health condition indices and metabolic changes, were included in 29 studies (Approach 3). Only six studies attempted to investigate the influence of seagrasses on trace metal accumulation and/or speciation in the sediment (Approach 4).
Seagrass meadows and their ecosystems have received much research attention during the last few decades due to their crucial ecological roles and rapid declines worldwide (Nordlund et al., 2018;Phillips & Milchakova, 2003;Short et al., 2007) Presently, the main causes for seagrass decline are anthropogenic impacts, which include sustained pressures from eutrophication, increased turbidity and contaminants (Hemminga & Duarte, 2000). These can lead to negative impacts, including deterioration of seagrass health, alteration of community species composition, dieback, and reduction in spatial coverage (Cambridge et al., 1986;El Zrelli et al., 2017;Short & Wyllie-Echeverria, 1996). Hence, the majority of studies in the review have primarily focused on how metal contaminants impact seagrass ecosystems by examining the accumulation of metals by seagrasses, seagrass physiological responses associated with exposure to metal pollutants, and the utility of seagrasses as biological indicators of trace metal contamination in the environment. Investigating the influence of metals on seagrass meadows is important for the environmental management and conservation of seagrass ecosystems (Kilminster et al., 2015;Turner & Schwarz, 2006;Unsworth et al., 2014). However, the influence of seagrasses on metal dynamics in coastal waters/sediments has not been well studied. Although some studies included in this review examined trace metal fluxes between the benthos and overlying water in seagrass colonized sediments, no studies considered the effects of the seagrass on sediment geochemistry and metal speciation in seagrass colonized sediments.
The physical and chemical conditions in seagrass-colonized areas are modified according to the metabolic activities and the structure of the seagrass meadow (Mateo et al., 2006). Seagrasses have a strong overall impact on carbon and nutrient cycling in coastal regions in both the overlying water and sediment due to the burial of organic and inorganic particles by morphological characteristics of leaves and nutrient uptake by the seagrass roots during photosynthesis (Glud, 2008;Holmer & Bondgaard, 2001). Hence, seagrass-vegetated areas provide different biogeochemical environments compared to unvegetated areas (Hebert et al., 2007;Mazarrasa et al., 2021;Stockdale et al., 2009). However, the impact of these biogeochemical differences on trace metal geochemistry is poorly studied.
Seagrasses can transport gases from their above-ground tissues (i.e., leaves) to their belowground tissues (i.e., roots and rhizomes) via a network of internal pores known as lacunae (Larkum, Orth, et al., 2006). This lacunal system facilitates radial oxygen loss (ROL) from roots/rhizomes and allows seagrass to oxygenate their surrounding benthic environment (de la Cruz Jim enez et al., 2021;Jensen et al., 2005). ROL in the rhizosphere of seagrass beds effectively extends the oxic zone deeper into the sediment and facilitates both the biotic and abiotic oxidation of other reduced compounds such as ferrous iron and manganese (II) (Pedersen et al., 1998). Fe and Mn oxides are general highly reactive minerals found in sedimentary environments, influencing the fate and transport of many metals (Brown & Parks, 2001;Frierdich & Catalano, 2012). Fe and Mn (hydr)oxide minerals produced by oxidation related to ROL control the distribution and speciation of trace metals via chemical mechanisms involving adsorption, incorporation, and electron transfer (Brown et al., 1999;Brown & Parks, 2001;Zachara et al., 2001).
Seagrass rhizospheres are highly heterogenous and characterized by shifting mosaics of redox zonation (Kankanamge et al., 2020;Pag es et al., 2012). The dynamic redox conditions driven by seagrass ROL are potentially also a major influence on trace metal speciation and mobility in rhizosphere sediments. For example, Fe and Mn oxy(hydr)oxides precipitate under oxic conditions during the day and can act as effective sorbents for many trace metals (Kankanamge et al., 2020;Weiss et al., 2004), whereas oxidation of metal-sulfide phases may release previously precipitated trace metals to the porewater (Li et al., 2022;Teuchies et al., 2012). Conversely, more reducing conditions at night would favor reductive dissolution of Fe and Mn (hydr)oxides and the release of adsorbed metals and the precipitation of new metal-sulfide phases. Therefore, the influence of ROL on geochemical processes in seagrass sediment has dynamic, cascading effects on the biogeochemistry of various elements including nutrients and metals over diel light cycles (Brodersen et al., 2017;Soana, 2013). Our current understanding of these processes is poor. Unraveling the geochemical functions of seagrasses in estuarine ecosystems, especially their role in the dynamics of various trace metals, is vital to the successful management of these systems into the future.

Study limitations
While we used four databases for our systematic search, we did not conduct manual forward or backward citation searches, which may have resulted in our missing some articles on this topic. However, the use of multiple databases and broad initial search terms means that our search was comprehensive and that our results are a good reflection of the overall trends currently present in the literature on this topic. Our review also only included articles published in English, which means we have missed some articles published in other languages, such as Chinese, Korean or Spanish, and thus studies from some regions may be under-represented.

Conclusion
Dramatic declines in global seagrass coverage driven by increased anthropogenic pressures have contributed to increased research on trace metals and seagrasses in recent years. However, there are still substantial knowledge gaps that require attention. Most studies were conducted in Mediterranean waters and to a lesser extent the Gulf of Mexico and the Caribbean. There were very few studies for the Atlantic Ocean coasts of Africa and South America, despite high biodiversity and abundance of seagrasses in these regions. Research to date has predominantly focused on Posidonia and Cymodocea spp., whereas other species with large distributions and high ecological significance, such as Amphibolis and Thalassia spp., have been relatively poorly studied.
The most commonly studied metals were Cd, Zn, Cu, and Pb, however, emerging metal contaminants with high potential risk to human and environmental health, including REEs and PGEs, were not studied in any of the reviewed articles with the exception of one study examining Y. While many studies have examined how trace metal contaminants influence seagrasses, we have a poor understanding of the impact of seagrass meadows on metal speciation and mobility in rhizosphere sediments, which can ultimately control the transport and fate of metals in these vulnerable coastal ecosystems.

Funding
This research was supported by a Griffith University PhD scholarship to HL.