The seawater temperature at the surface and near-bottom ranged from 29.2 to 29.9 (avg. 29.6 ± 0.2) ºC and 29.2 to 29.6 (avg. 29.5 ± 0.1) ºC, 28.9 to 30 (avg. 29.5 ± 0.3) ºC and 27.6 to 28.4 (avg. 28 ± 0.3) ºC, 25.4 to 26.9 (avg. 26.1 ± 0.5) ºC and 24.8 to 25.4 (avg. 25.1 ± 0.2) ºC, 29.0 to 29.6 (avg. 29.2 ± 0.2) ºC and 28.8 to 29.3 (avg. 29.1 ± 0.1) ºC during PM I, PreM, MON and PM II respectively (Table I). The difference in the average surface seawater temperature was 3.5 ºC and at the bottom it was 4.4 ºC during different seasons. The seawater salinity at surface ranged between 34.4 ± 0.3 and 35.9 ± 0.1 and at near-bottom it ranged between 34.7 ± 0.2 and 36 ± 0.4 and it varied with the seasons (Table I). The dissolved oxygen concentration varied with the seasons and between surface and bottom water (Table I). The concentration of DO in bottom water was low during all the seasons, and such a trend was more prominent during the monsoon season (average DO was 1.1 ± 1.2 mg. L− 1), indicating hypoxic conditions at most of the stations. During PM II, at OFW stations, the DO concentration of the near-bottom seawater was higher than the surface seawater (Table I).
The sediment texture was dominated by sand followed by clay and silt (Fig. 2) and their composition varied with the seasons. A comparison between the PM I (2011) and PM II (2012) indicated that PM I had higher percentage of sand when compared to PM II (Fig. 2A-D) indicating an inter-annual variation in the sediment texture. Different areas within the port showed variation in the sediment texture with the seasons, as stations 14 and 17, had lower percentage of sand (47 and 62% respectively) during PM I, and during PM II the percentage of sand was 92 and 83% respectively at these stations. In PreM, stations 3, 9, and 18 were dominated by sand (nearly 80%), however, at other stations the sediment was dominated by clay and silt. During MON, the percentage of sand was nearly 45% followed by clay and silt (Fig. 2A-D). Sediment texture significantly varied between the stations and seasons (Two-way ANOVA; P < 0.001) (Table II).
The ternary diagram depicted ten classes of mixed sediment: clay, silty-clay, clayey-silt, silt, sandy-silt, silty-sand, sand, clayey-sand, sandy-clay, and combination of sand-silt-clay respectively (Fig. 2E). During PM I, the sediment was dominated by combination of sand-silt-clay and clayey-silt. During PreM season sand-silt-clay content was dominant along with clayey-sand, sandy-clay, sandy-silt, clayey-silt, silty-clay and silty sediment at different stations (Fig. 2E). The combination of sand-silt-clay was dominant along with silty-clay and clayey-silt during MON. During PM II, the sediment was dominated by sand-silt-clay at most of the stations and at few stations sediment was characterized by silt and sandy-silt.
A significant variation in the organic carbon (OC) content (0.13 to 4.14%) was observed with stations and seasons (Fig. 2A-D) (Two-way ANOVA; P < 0.001) (Table II). The OC was minimum during PreM (ranged from 1.43 to 2.77%) (Fig. 2B), and during PM I it ranged from 1.7 to 2.78% (Fig. 2A). The maximum variation in the OC was observed during MON (0.4 to 4.12%) and PM II (0.13 to 4.14%) (Fig. 2D).
A significant variation in the sediment chlorophyll-a was observed with the seasons and stations (Two-way ANOVA; P < 0.001) (Table II). The average chlorophyll-a of near-bottom seawater during PM I was 1.3 ± 1 mg.m− 3 and it was lower then sediment chlorophyll-a (Table I). A wide variation in the near-bottom seawater and sediment chlorophyll-a was observed with the seasons. During PreM, the average sediment chlorophyll-a was 7.7 ± 2.5 mg. m− 2, and the stations located in the vicinity of the channel showed higher chlorophyll-a in the near-bottom seawater. During monsoon the variation in the chlorophyll-a content in near-bottom seawater and sediment was maximum (Table I). The stations located in the LCA and OFW had higher chlorophyll-a content in the near-bottom seawater when compared to stations located in the HCA (Table I). During this season average sediment chlorophyll-a was 8.2 ± 3.9 mg. m− 2, and higher concentration of chlorophyll-a was observed at stations located in HCA. During PM II, the average chlorophyll-a of near-bottom seawater was 8.2 ± 3.9 mg.m− 3 and in the sediment the chlorophyll-a was 7.3 ± 2.7 mg. m− 2 (Table I).
Seasonal variation in the nutrient concentration in near-bottom seawater and sediment pore water was observed (Supplementary Fig. 1). During PM I, the concentration of ammonium and silicate from the sediment (pore water) showed large variation with respect to the stations. However, during other seasons the nutrient concentrations did not show variation in both near-botttom seawater and sediment (pore water), except ammonium.
A total of 61 macrobenthos taxa belonging to 8 phyla were recorded during the study. The number of taxa were 9, 32, 21 and 47 during PM I, PreM, MON, and PM II respectively. The abundance of macrobenthic organisms varied significantly between the stations and seasons (Fig. 3; Spplementray Table I). The abundance was 1063 no.m− 2, 9440 no.m− 2, 3835 no.m− 2, and 16139 no.m− 2 during PM I, PreM, MON and PM II respectively (Fig. 3A-D; Supplementary Table I). The biomass of macrobenthic organisms also varied significantly with the seasons (Two-way ANOVA; P < 0.001) (Table II). The macrobenthic taxa comprised of polychaetes, crustaceans, molluscs, oligochaetes, sipunculans and nemerteans. Polychaetes were the most abundant organism’s contributing to the total abundance during PM I, MON and PM II, however, during PreM season the amphipod, Ampelisca sp. was abundant (Supplementary Table I). A significant variation in the abundance of polychaetes was observed between PM I (1001 no.m− 2) and PM II (15277 no.m− 2). At st-18 (situated in the channel) maximum abundance of macrobenthic organisms was observed during all the seasons (Fgiure 3 A-D; Supplementary Table I).
During PM I, Prionospio sp., Cossura sp. and Polydora sp. dominated and they contributed ~ 75% to the total macrobenthic community (Fig. 4; Supplementary Table I (A)). Macrobenthic organisms belonging to the family Cirratulidae and the Chaetognatha, Serratosagitta sp., (31 no.m− 2 in both LCA and HCA areas) also contributed to the total abundance (Supplementary Table I (A)). The Prionospio sp. was dominant in HCA (34%) and OFW (100%), however, in LCA, Cossura sp. was dominant (48% contribution) followed by Prionospio sp., and this points out that Prionospio sp. was distributed in all three areas of the port. Minimum abundance of macrobenthic organisms during this season was observed in the OFW area when compared to other two areas within the port (Fig. 3; Supplementary Table I (A)). During PreM, the amphipod, Ampelisca sp., was dominant followed by Magelona capensis, Pelecypod, and Cossura sp. (Supplementary Table I (B)). The abundance of Isopod and Tharyx filibranchia was comparatively high during PreM. During this season 7192 no. m− 2 macrobenthic organisms were reported from the HCA area, and Ampelisca sp. contributed 62% to the total macrobenthos, whereas, in LCA (1987 no.m− 2) and OFW (262 no.m− 2) the to total abundancewas comparatively low (Fig. 3B; Supplementary Table I (B)). In LCA, Cossura sp. and Pelecypod were dominant, whereas, in OFW, Cossura sp. was dominant followed by Gastropod, during this season, Pelecypods were reported in all three areas of the port. In the MON season, the abundance of polychaetes was 3835 no. m− 2 MON, and Prionospio sp. was dominant (47%) followed by Cossura sp., Tharyx filibranchia, and Pelecypod (Supplementary Table I (C)). During this season also the abundance of macrobenthic organisms was high in the HCA (2772 no.m− 2) and Prionospio sp. was dominant (contributed 63%) along with Pelecypods and Magelona capensis. Whereas, the abundance was comparatively low in LCA (755 no.m− 2) and OWF (308 no.m− 2), and in both these areas Cossura sp. was dominant (Supplementary Table I (C)).
Overall, the maximum abundance of macrobenthic organisms (16139 no. m− 2) was observed during PM II (Supplementary Table I (D); Fig. 3D). The polychaetes, Mediomastus capensis (6545 no. m− 2), Magelona capensis (2264 no. m− 2) and Tharyx filibranchia (1925 no.m− 2) were dominant along with Prionospio pinnata and Cossura sp. (Fig. 4; Supplementary Table I (D)). The abundance of macrobenthic organisms was higher in HCA (9024 no.m− 2) compared to LCA (6191 no.m− 2) and OFW (924 no.m− 2) area (Supplementary Table I (D)). Mediomastus capensis was dominant in the LCA and HCA and contributed 41% and 43% respectively to the total abundance, whereas in OFW area, Tharyx filibranchia was dominant (Supplementary Table I (D)). The macrobenthic diversity index values were maximum during PM II and PreM season (Table IIIA). The Shannon–Weiner index (H′) was also high during these seasons (4.9 and 4.5 during PM II and PreM respectively). Species diversity was maximum during PM II followed by PreM, PM I and MON (Table IIIA) indicating significant seasonal variation in the community structure of macrobenthos. The maximum number of species were encountered in HCA during PreM and PM II season (Table IIIB).
Relationship between environmental variables and macrobenthic community
Cluster analysis indicated stations clustering in different groups based on the abundance of macrobenthic organisms (Fig. 5A-D), and the CCA analysis indicated the correlation between the abundance of macrobenthos speceis and physico-chemical variables and sediment characteristics in different regions of the port during different seasons (Fig. 5E-H). The CCA axes 1 and 2 (Eigenvalues 1 and 0.799 during PM I; 1 and 0.707 during PreM; 1 and 1 during MON and 1 and 0.765 during PM II respectively) explained the relationship between the macrobenthos and physico-chemical variables and sediment characteristic (Fig. 5E-H) between different areas of the port (LCA, OFW, HCA). In general, during all the seasons, Eigenvalues were greater than 0.5 indicated relatively good dispersal of species along different axes (ter Braak 1986). The CCA derived correlation between macrobenthic abundance and physico-chemical and sediment characteristics was 54.9% during PM I, 38.6% during PreM, 45.9% during MON and 47.4% during PM II (Fig. 5E-H). During PM I, stations 15, 17, and 18 (located in HCA) indicated differences in the community structure between the stations (Fig. 5A). The st-15 had relatively low DO and supported higher abundance of Nereis sp., whereas, at st-17, the higher abundance of Ancistrosyllis sp. was influenced by higher percentage of sand and moderate silt, along with PW silicate, sediment OC, salinity and temperature (Fig. 5E). At st-18 higher abundance of Cossura sp. and lower abundance of Magelona capensis was weakly correlated to DO and silt. In this season, OFW stations (st-13 and 14) formed group I, and these stations had higher abundance of Prionospio sp., and were weakly correlated with ammonium and near-bottom seawater temperature. The group II, represented by st-2 and st-8 located in LCA area (Fig. 5A) were dominated by Cossura sp., and represented by high sediment OC, while in the same areas, group III stations (3, 4, 5 and 6) had higher abundance of Cossura sp., Prionospio sp., Cirratudiae, Tharyx filibranchia followed by Serratosagitta sp. In both these groups (II and III) macrobenthic abundance was correlated to the concentration of DO, temperature, PW phosphate, PW ammonium, OC and percentage of clay (Fig. 5E ).
During PreM, four clusters were observed and few stations did not group to form clusters (Fig. 5B). In the OFW, st-14 had higher abundance of Tharyx filibranchia influenced by high percentage of sand and nutrients (PW ammonium and silicate). The st-11 and st-12 from this area in group I (Fig. 5B) dominated by Cossura sp. followed by Gastropods (Fig. 5F). In HCA, every station had different dominant macrobenthic organism (Mediomastus capensis at st-9, Cossura sp. at st-17, and amphipod at st-18), and their abundance was correlated with the near-botttom seawater temperature, salinity, nitrate and percentage of silt (Fig. 5F). In LCA, at st-2 high clay and OC content was directly correlated with Tharyx filibranchia which was dominant at this station. Station 1 in this area had higher abundance of Pelecypods and it was correlated with silt. However, in group II, at st-3 and st-7 (Fig. 5B) Pelecypoda was dominant and the percentage of silt was high (Fig. 5F). The st-4 and st-8 (group III) dominated by Cossura sp. followed by Nemertea and Gastropods, and they were influenced by sediment texture (Fig. 5F). In group IV (st-5 and st-6) Cossura sp. and Magelona capensis were dominant which was correlated to the concentration of nutrients and near-bottom seawater temperature and salinity (Fig. 5B&F).
During MON, at st-18 (HCA) Prionospio sp. was high in abundance followed by Pelecypoda, Magelona capensis, macrobenthos belonging to Class Insecta and their abundance was correlated to the sediment characteristics (texture and OC), PW phosphate and silicate (Fig. 5C&G). The st-15 and st-19 (group I) in the HCA (Fig. 5B) had moderate abundance of Cossura sp. indicating none of the environmental variables or sediment charactristics influenced their abundance during MON season (Fig. 5F). In LCA, st-6 did not cluster and the higher abundance of Tharyx filibranchia at this station was correlated to OC and near-bottom seawater DO (Fig. 5C&G), and these parameters also influencd the moderate abundance of Nemertea and Ostracoda. In group II stations (stations 2, 3, 4, 5, and 8), Cossura sp. was dominant and was supported by low percentage of silt, low near-bottom seawater salinity and higher PW ammonium at st-5 and high silt and clay content at st-2 (Fig. 5C&G). At st-14 in OFW area, low OC and higher percentage of sand inluenced higher abundance of Prionospio sp. followed by Polydora sp. and Tharyx filibranchia. The dominance of Cossura sp. at st-13 was favored by moderate concentration of silicate and ammonium along with near-bottom seawater temperature, salinity, and DO (Fig. 5C&G). The high Eigen value (1) of axes 1 and 2 indicates a high degree of correspondence between the abundance of macrofauna and physico-chemical variables and sediment characteristic during MON season. During PM II, st-4, 11 and 17 did not cluster with other stations from their respective areas, however, st-4 (LCA) grouped with st-17 (HCA) to form group I (Fig. 5D). In group I, st-4 and st-17 had higher abundance of Mediomastus capensis followed by Magelona capensis which were influenced by high sand, low clay and silt and OC content (Fig. 5H). During this season, among the non-clustered stations, at st-18 (HCA) higher abundance of Mediomastus capensis along with other macrobenthos was observed and and this was the most abundant macrobenthic species during this season. At this station higher abundance of Phyllodocidae was correlated with near-bottom seawater temperature and salinity. In group II, stations from LCA (stations 1, 2, 5, 6 and 8) were grouped, and Cossura sp. and Tharyx filibranchia were dominant and their dominance was correlated by higher percentage of sand and low OC (Fig. 5D&H), and their dominance at st-1 and st-2 was correlated to low OC and sand indicating that they can survive and grow in habitats with multiple sediment characteristics.