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Seasonal variation of heavy metals in seagrasses along Thondi coast, Palk Bay, India

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Overview

Pasumpon and Vasudevan (2021) measured Mn, Cu, Zn, Pb, Cd, and Cr by atomic absorption spectrophotometry in four live seagrass species (Syringodium isoetifolium, Halodule pinifolia, Cymodocea serrulata, Halophila ovalis) and in dead/decayed offshore seagrass deposition at Thondi coast, Palk Bay, southeast India, across four seasons of 2019–2020. Mn was the most concentrated metal across all species and seasons (peak 15.62 ± 1.02 mg/kg in S. isoetifolium, summer); Cr was the least concentrated (down to 0.002 mg/kg in H. ovalis, monsoon). S. isoetifolium bioaccumulated heavy metals to higher levels than the other three species. Heavy metals were significantly higher in summer (P < 0.05) and declined through pre-monsoon to monsoon; offshore decayed seagrass deposition carried higher metal concentrations than live seagrass for Mn (16.01 ± 2.01 mg/kg), Pb (0.35 ± 0.02 mg/kg), and Cd (0.54 ± 0.04 mg/kg). The paper is an environmental-bioindicator study; seagrasses are not directly consumed by humans, so it routes as marine-bioindicator context for the heavy-metals literature rather than as a food-contamination measurement.

Key numbers

All concentrations are mg/kg in seagrass tissue; samples dried at 110 °C and ground to fine powder before digestion; basis is dry weight by convention (samples were dried prior to weighing and digestion; the paper does not state the basis explicitly).

Water quality parameters at Thondi coast (Table 1): pH 7.6; water temperature 32 °C; atmospheric temperature 35 °C; salinity 36%; dissolved oxygen 7.4 mg/L.

Manganese (Mn), mg/kg, mean ± SD by species × season (Table 2):

  • S. isoetifolium: summer 15.62 ± 1.02; pre-monsoon 13.54 ± 0.92; monsoon 11.91 ± 0.91; post-monsoon 14.76 ± 1.10
  • H. pinifolia: summer 8.01 ± 1.23; pre-monsoon 6.22 ± 0.98; monsoon 7.01 ± 0.25; post-monsoon 7.62 ± 0.14
  • C. serrulata: summer 6.01 ± 1.2; pre-monsoon 4.98 ± 0.85; monsoon 4.07 ± 0.72; post-monsoon 5.17 ± 0.21
  • H. ovalis: summer 7.92 ± 1.25; pre-monsoon 7.24 ± 0.91; monsoon 5.19 ± 0.21; post-monsoon 6.98 ± 0.34
  • Concentration order across species: S. isoetifolium > H. pinifolia > H. ovalis > C. serrulata (text §3.2)

Copper (Cu), mg/kg (Table 2):

  • S. isoetifolium: 7.41 ± 0.84; 6.92 ± 0.51; 7.04 ± 0.83; 5.81 ± 0.47
  • H. pinifolia: 5.92 ± 0.21; 5.11 ± 0.32; 5.42 ± 0.14; 5.81 ± 0.09
  • C. serrulata: 3.98 ± 0.14; 3.01 ± 0.12; 3.72 ± 0.19; 3.49 ± 1.01
  • H. ovalis: 3.60 ± 0.27; 3.71 ± 0.16; 3.39 ± 0.28; 3.10 ± 0.39
  • Concentration order across species: S. isoetifolium > H. pinifolia > C. serrulata > H. ovalis (text §3.2)

Zinc (Zn), mg/kg (Table 2):

  • S. isoetifolium: 1.49 ± 0.12; 1.87 ± 0.08; 2.01 ± 0.04; 1.62 ± 0.04
  • H. pinifolia: 1.47 ± 0.04; 1.92 ± 0.02; 2.29 ± 0.07; 2.01 ± 0.04
  • C. serrulata: 0.92 ± 0.04; 1.11 ± 0.08; 1.36 ± 0.02; 0.87 ± 0.12
  • H. ovalis: 0.64 ± 0.02; 1.30 ± 0.03; 1.52 ± 0.04; 1.07 ± 0.01
  • Concentration order across species: H. pinifolia > S. isoetifolium > H. ovalis > C. serrulata (text §3.2)

Lead (Pb), mg/kg (Table 2):

  • S. isoetifolium: 0.32 ± 0.06; 0.41 ± 0.05; 0.26 ± 0.01; 0.28 ± 0.03
  • H. pinifolia: 0.19 ± 0.01; 0.21 ± 0.01; 0.19 ± 0; 0.11 ± 0.01
  • C. serrulata: 0.26 ± 0.04; 0.31 ± 0.04; 0.37 ± 0.02; 0.19 ± 0.05
  • H. ovalis: 0.12 ± 0.02; 0.18 ± 0.03; 0.07 ± 0.004; 0.12 ± 0.005
  • Pre-monsoon peak in S. isoetifolium 0.41 ± 0.05 mg/kg; monsoon low in H. ovalis 0.07 ± 0.004 mg/kg
  • Concentration order across species: H. ovalis < H. pinifolia < C. serrulata < S. isoetifolium (text §3.2)

Cadmium (Cd), mg/kg (Table 2):

  • S. isoetifolium: summer 0.02 ± 0.002; pre-monsoon BDL; monsoon 0.047 ± 0.003; post-monsoon 0.040 ± 0.009
  • H. pinifolia: summer 0.007 ± 0.0; pre-monsoon 0.014 ± 0.001; monsoon BDL; post-monsoon 0.011 ± 0.001
  • C. serrulata: summer 0.017 ± 0.01; pre-monsoon 0.02 ± 0.004; monsoon 0.013 ± 0.003; post-monsoon 0.009 ± 0.001
  • H. ovalis: summer 0.004 ± 0; pre-monsoon BDL; monsoon BDL; post-monsoon 0.003 ± 0
  • Cd was at or below detectable levels in much of the dataset; maximum 0.047 ± 0.003 mg/kg in S. isoetifolium, monsoon
  • The paper does not report the BDL numerical threshold

Chromium (Cr), mg/kg (Table 2):

  • S. isoetifolium: 0.07 ± 0.004; 0.10 ± 0.007; 0.14 ± 0.009; 0.09 ± 0.004
  • H. pinifolia: 0.081 ± 0.008; 0.050 ± 0.006; 0.031 ± 0.006; 0.047 ± 0.002
  • C. serrulata: 0.004 ± 0; 0.011 ± 0.002; 0.013 ± 0.008; 0.009 ± 0
  • H. ovalis: 0.009 ± 0.001; 0.010 ± 0.002; 0.002 ± 0; 0.013 ± 0.003
  • Cr maximum 0.14 ± 0.009 mg/kg in S. isoetifolium, monsoon; Cr minimum 0.002 mg/kg in H. ovalis, monsoon
  • The abstract reports “the least metal observed (0.002 mg/kg)” referring to the H. ovalis monsoon Cr value; concentration order across species: H. ovalis < C. serrulata < S. isoetifolium < H. pinifolia (text §3.2)

Offshore decayed seagrass deposition (text §3.3, no separate table): Mn 16.01 ± 2.01 mg/kg (highest of the metals); Pb 0.35 ± 0.02 mg/kg; Cd 0.54 ± 0.04 mg/kg; Cr “least during all seasons” (no numerical value reported). Levels were higher in summer with gradual decrease through pre-monsoon, monsoon, and a slight increase in post-monsoon. The text states heavy metals were higher in offshore decayed seagrass than in the live seagrass species.

One-way ANOVA (text §3.2): Mn and Cu variations across species were significant in summer (P < 0.05); Zn variations across species were highly significant in summer (P < 0.001); Cd variations across species were significant in summer (P < 0.05); Cr variations across species were not significant in any season (P < 0.05 cutoff).

Pearson correlations between metals within species (Table 3, selected significant pairs):

  • S. isoetifolium (P < 0.05): strong negative Zn–Mn (r = −0.984); strong negative Cd–Pb (r = −0.982); strong negative Cr–Mn (r = −0.983)
  • H. pinifolia (P < 0.01): strong positive Cu–Mn (r = 0.993); strong negative Cr–Zn (r = −0.997)
  • C. serrulata (P < 0.05): strong positive Pb–Zn (r = 0.950); strong negative Cr–Mn (r = −0.961)
  • H. ovalis (P < 0.05): strong negative Cd–Zn (r = −0.927)

Cluster analysis (Fig. 2, Ward linkage): Two main clusters. Cluster A grouped C. serrulata, H. ovalis, and H. pinifolia (across all four seasons each) into three sub-clusters. Cluster B contained S. isoetifolium alone (across all four seasons). The dendrogram supports the finding that S. isoetifolium bioaccumulates heavy metals distinctly from the other three species.

Methods (brief)

Sample collection: live seagrasses (four species above) collected at low tide from the intertidal zone of Thondi coast, Palk Bay (9°4′N, 79°4′E), Tamil Nadu, southeast coast of India, sampled across summer, pre-monsoon, monsoon, and post-monsoon seasons from April 2019 to March 2020. Dead/decayed offshore seagrass samples were collected from the seashore. Species identification followed Ramamurthy et al. (1992) and Thangaradjou and Bhatt (2018). Water quality parameters (pH, atmospheric temperature, surface water temperature, dissolved oxygen, salinity) were measured at the sampling site during the study.

Sample preparation: washed in seawater on collection to remove debris; transported to laboratory in separate polythene bags. Both live and decayed seagrasses were washed in tap water then distilled water to remove sand and mud. Moisture removed with blotting paper, then samples dried in shade. Dried seagrasses further dried at 110 °C and ground to fine powder.

Digestion: 1 g aliquots weighed and digested overnight with 20 mL concentrated nitric acid. Samples then mixed with 10 mL concentrated nitric acid and perchloric acid in the ratio 4:1. Complete dryness of the acid mixture attained by evaporating at 125 °C on a hot plate. After complete dryness, residue mixed with 20% concentrated nitric acid and made to 50 mL solution. Solution filtered and centrifuged at 4000 rpm. Supernatant used for analysis.

Instrumentation: Shimadzu AA-7000 atomic absorption spectrophotometer (Japan) at wavelengths Cd 228.80 nm, Cr 357.90 nm, Cu 324.75 nm, Pb 217.00 nm, Mn 279.50 nm, Zn 213.85 nm. Heavy metal standards from Merck Genei, Bangalore. Quality assurance and control followed the US EPA protocol for metal analysis. The paper does not report a certified reference material, recovery percentages, method detection limit, or numerical BDL threshold.

Statistical analysis: SPSS Software v20. One-way ANOVA to assess significance of variation among heavy metals and seagrass species. Pearson correlation analysis and cluster analysis (Ward linkage dendrogram) to assess relationships between metals within species and similarities among species.

Limitations: per-cell sample size (replicates per species-season) not stated; no certified reference material or recovery data reported; method detection limit and numerical BDL threshold not reported; speciation not performed (total metal only — Cr-VI vs Cr-III not distinguished; tAs and Hg not assayed); single coastal location (Thondi only); the paper does not report concurrent measurements in surrounding seawater or sediment that would allow bioaccumulation factor calculation.

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