Changes in environmental variables of sediment
The sediment temperature values remained homogenous throughout the study area (Table 3) and the spatial difference was statistically insignificant (Table S1). Electrical conductivity of sediment samples varied from 2085.50 to 5246.40 μs. Slightly alkaline pH was observed along the study area and the spatial variation was statistically significant. Sulphate concentration ranged from 7.05 to 24.09 mg/g. Corg and TOM were in the range of 0.28 to 2.29% and 0.48 to 3.95%, respectively. The labile part of organic matter ranged from 0.06 to 0.12% and comprised of CHO (1.12 – 2.87mg/g), protein (2.13 – 6.76mg/g) and lipid (1.98 – 5.3mg/g). The environmental variables such as pH, sulphate, EC, CHO, protein, lipid, LOM, Corg, TOM, LOM (% of TOM), sand, silt and clay showed a significant spatial variation (p<0.001) (Table S1). The granulometric analysis of the sediment samples revealed the predominance of sand (Fig. 2).
Changes in environmental variables of overlying water
Similar to sediment temperature, the seawater temperature also remained homogeneous and spatial variation were insignificant (Table 4). pH values ranged from 7.35 to 9.42, salinity from 25.43 to 28 PSU and EC values ranged between 39390- 43196μs/cm. TDS ranged from 25.09 to 28.06ppm. Along the study area, DO ranged from 1.22mg/L to 6.12mg/L (Fig. 3). The environmental variables such as EC, salinity, sulphate and water depth showed significant spatial variations (p<0.001) (Table S2).
Faunal composition and diversity indices of macro and meiobenthic communities
A total of 319 macrobenthic organisms were isolated, which belonged to 27 species of 5 phyla, including polychaeta, oligochaeta, amphipoda, decapoda, and gastropoda in the offshore sediments of the study area, along the SW coast of Tamil Nadu (Table S3). The relative abundance of macrobenthos (Fig. 4) was represented by polychaeta (76.41%) as dominant taxa followed by gastropoda (21.69%). In the case of polychaeta, Pisionidens sp. was abundant, followed by Polygordius sp. and Arabella sp. Pyramidellidea sp. was the predominant taxa in Gastropoda followed by Physidea, Turritellidae, and Cerithideae. Most of the diversity indices showed significant correlation with TOM and Corg content. The maximum Shannon-Weinner diversity index value was 0.8 (S23-Manavalakurichi-MK), whereas species richness, species evenness index and Simpson index were 1.44, 1.0 and 0.67 respectively (Table S4).
A total of 2525 benthic meiobenthos were isolated which belonged to 60 species and 5 phyla including foraminifera, polycheata, nematoda and amphipoda (Table S5). Relative abundance of meiobenthos (Fig.4) showed foraminifera (86.85%) as dominating taxa followed by polycheata (11.08%). In polycheata, dominant species was Polygordius sp. followed by Pisionidens sp. In nematoda, predominant species was Daptonema sp. In the phylum foraminifera, the most abundant was Elphidium sp. followed by Quinqueloculina sp. and Ammonia sp. The Shannon-Weinner diversity index was 0.94 (S25-Kadiapattinam-KP), whereas, species richness, species evenness index and Simpson index were 0.87, 1.0 and 0.58 respectively (Table S6).
Effect of environmental variables on benthic diversity
Redundancy analysis (Fig. 4) showed that CHO, a part of the TOM, was the highest determining factor for explaining most variation in the composition of the macrobenthos of all the 63 sampling sites, followed by temperature and protein concentration. The relative abundance of gastropods and amphipods increased with CHO concentration, whereas that of polychaetes increased with the increase of LOM (% of TOM), which indicates the affinity of polychaetes to LOM. Decapoda and oligochaeta showed an increasing trend with increasing protein concentration. Similar to macrobenthos, RDA showed that CHO, a part of the TOM followed by salinity was the highest determining factor for explaining most variation in the composition of the meiobenthos of the 63 sampling sites (Fig. 5). The relative abundance of polychaeta and foraminifera increased with CHO concentration.
Ecological health analysis
A total of 22 environmental variables were used as the input parameters for BIOENV analysis (Table 5). The results clearly shows that the combination of five variables such as pH, LOM (%), TOM (%), clay% (sediment) and EC (water) explained the best match (rho =0.899) influencing the benthic distribution in the study area.
AZTI Marine Biotic Index (AMBI) and Multivariate-AZTI Marine Biotic Index (M-AMBI)
Percentage composition of various ecological groups (EG) of polychaete species were recorded at various stations in the study (Table 6). A total of 14 polychaete species were identified in all the sampling stations. Among the 14 polychaete species, 2 species (20%) were assigned in EG-I, 3 species (30%) in EG –II, 4 species (40%) in EG-III, and only one species (10%) in EG-V. The remaining 3 species were not assigned in any ecological groups available in the AMBI classification. Moreover, there were no polychaete species under the group of EG-IV (Fig. 7). The AMBI index values of the stations TP, IN, MI, MU, PT, RT, ST, PA and KK showed very high (7) value, which highlights the anthropogenic disturbances in this area. Pollution tolerant polychaete species (EG-III) Myrianida sp. Eunice sp. and Syllids sp. and EG-II species (species unaffected by pollution) such as Polygordius sp. and Pisione sp. were observed in the study area (Fig. 8).
The low value of M-AMBI index (0.015) indicates that the stations TP, IN MI, MU, PT, RT, ST, PA and KK had a “bad” ecological status revealing the anthropogenic disturbance. The M-AMBI values (0.35-0.366) of the stations (MA and KO) clearly illustrated the poor ecological status with slightly increased diversity value (0.211-0.337) and richness value (0.2). The station CO had the M-AMBI value of 0.447 which shows the “moderate” ecological status with the diversity value of 0.961. The “good” M-AMBI values (0.876-0.931) recorded in the stations MK and KA indicates the “high” ecological status with increased diversity (2.56 to 2.82) and richness (0.7-0.8). The AMBI index value (1.05-1.89) of the stations MK and KA indicates the undisturbed and slightly disturbed environmental condition which also shows the presence of pollution-sensitive species (Polygordius sp. and Pision sp.) and other ecological groups of species (Fig. 9). The AMBI value of other three stations CO, MA and KO was “0” which demonstrates the undisturbed condition and the presence of pollution sensitive species in this area.