Distribution Pattern of Macrobenthic Organisms In A Tropical Monsoon Inuenced Port, New Mangalore, India

Study was carried out to understand the impact of natural and anthropogenic impacts on the macrobenthic organisms at the New Mangalore port, inuenced by south-west monsoon located along the south-west coast of India. Soft bottom macrobenthos are the group of highly diverse benthic invertebrates in the coastal regions. The spatio-temporal variation in their abundance and diversity was observed along with the variations in the water column and sediment characteristics. Among the 61 taxa of macrobenthos reported, 41 belonged to the polychaetes. The organic carbon levels of >2% supported higher abundance of deposit feeders. A correlation in the abundance of polychaetes and sediment characteristics along with dissolved oxygen and pore water nutrients was observed. A shift in the community of macrobenthic species in response to environmental parameters was observed with the change in the season. During post-monsoon I, Prionospio sp., was dominant , whereas during pre-monsoon the amphipod, Ampelisca sp. was dominant. The abundance of opportunistic polychaetes, Prionospio sp., Cossura sp., and Tharyx libranchia varied with the seasons indicating a change in the habitat characteristics during different seasons. The occurrence and dominance of macrobenthic speceis was inuenced by the physical processes mainly governed by the exchange of seawater between the port and the Arabian sea. The stations located in the high circulation area showed higher seasonal variation in the macrobenthic community indicating pivotal role of local hydrodynamics on macrobenthic organisms. The occurrence of opportunistic species inside the port demonstrate the role of anthropogenic stress in structuring the macrobenthic community. Elucidated spatio-temporal variation in macrobenthic taxa at New Mangalore port. 61 taxa of macrobenthos were reported, and polychaetes dominated with 41 taxa. Shift in the dominance of macrobenthic species was observed with change in seasons. Occurrence of opportunistic species validated anthropogenic stress within the port. Perturbations due to local hydrodynamics in port inuenced macrobenthic community.

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).

Sediment texture
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 signi cantly 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.

Organic carbon
A signi cant 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).
Chlorophyll-a A signi cant 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 nearbottom 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 nearbottom 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).

Nutrients
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.

Macrobenthic community
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 signi cantly between the stations and seasons ( Fig. 3; Spplementray Table I 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.  Table I (B)). The abundance of Isopod and Tharyx libranchia 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 libranchia, 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 (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 signi cant 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).  (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 in uenced 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 libranchia 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 ).

Relationship between environmental variables and macrobenthic community
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 libranchia in uenced 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 libranchia 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 in uenced 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 in uenced their abundance during MON season (Fig. 5F). In LCA, st-6 did not cluster and the higher abundance of Tharyx libranchia at this station was correlated to OC and near-bottom seawater DO (Fig. 5C&G), and these parameters also in uencd 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 libranchia. 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 in uenced 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 libranchia 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.

Discussion
The macrobenthic community of the New Mangalore Port exhibited signi cant spatio-temporal variation in their distribution. The scenario during different seasons with respect to macrobenthic abundance, biomass and community structure along with sediment characteristics and environmental parameters is discussed in the following sections.
Scenario during Post-monsoon I (PM I) During PM I, the total abundance of macrobenthos was lower when compared to other seasons. The polychaete, Prionospio sp. was the most abundant species, speci cally at st-15 where the habitat was hypoxic (0.9 mg.L − 1 ), indicating their tolerance to low oxygen conditions. The occurrence of this species in high abundance along with few other macrobenthos in low oxygen concentration (0.1 mg.L − 1 ) has also been reported by Levin et al. (2009). The Prionospio sp. is a deposit feeder and the dominance of sand during this season when compared to other seasons and this could be a reason for the higher abundance. The Cossura sp. and Polydora sp. which are also deposit feeders were dominant indicating they can primarily feed on the sediment organic carbon. Earlier studies have indicated that higher organic carbon possibly caused by depletion in oxygen cand lead to decline in the species diversity, abundance and biomass (Jorgensen 1977; Revsbech and Jorgensen 1986; Snelgrove and Butman 1994; Hyland et al. 2005; Musale and Desai 2011). The sediment organic carbon during this season was moderate and abundance of detritivores was higher which are able to feed and proliferate on moderate to high organic carbon.
The detritivorous macroebnthic organaisms have an ability to survive in sediments with moderate organic carbon (Ansari et al. 2014). The higher abundance of macrobenthos was in HCA followed by LCA and OFW indicated distinct spatial distribution in different areas within the port. It can be noted that the tidal exchange of seawater between the port and the open sea was maximum in HCA, and this will result in healty bottom water condition owing to higher exchange of seawater and could be atttibuted to higher abundance and biomass of macrobenthos in this area. The deposit feeders, Prionospio sp. followed by Polydora sp. and Cossura sp. were dominant, and in HCA the percentage of clay was higher than LCA and OFW, and the average OC was 2.2% and such conditions are preferred by deposit feeding polychaetes. Musale et al. (2015) reported that in the Visakhapatnam port, Prionospio sp. was abundant in the outer harbor region which is a semi-polluted environment with moderate OC and high percentage of sand and these conditions were similar to that in the HCA, indicating the preference of Prionospio sp. to sandy-silt habitat with OC ranging between 2-3%. Prionospio sp. are capable of constructing tubes in which they hide from adverse conditions and also protect them from the predators (Moritz 2012), and such a habit can be attributed to their higher abundance in HCA. It is apparent that the distribution of this species is dependent on its ability to thrive in semi-polluted environment and it can be a good indicator of benthic habitat. In LCA the moderate abundance of macrobenthic organisms was observed with ve dominant macrobenthic forms and the polychaete, Cossura sp. contributed ~ 50% to the total abundance, however, the biomass was minimum. In this area, the sediment was dominated by silt, and had moderate organic carbon. Considering the properties of sediment dynamics, it has been suggested that high silt-clay fraction in the sediment contains more food particles (Sanders 1960 During PreM season, the macrobenthic abundance was dominated by the benthic amphipod, Ampelisca sp. (48% to the total), a surface deposit feeder followed by the Pelecypods (10%), and the abundance of Pelecypods was higher in LCA. Increase in tube building amphipods favor sedimentation of ne particles (Mills 1967) and this also stimulates the recruitment of other benthic species (Glemarec et al. 1986; Dauvin and Bellan-Santini 1990). Moreover, amphipod tubes may make it di cult for bivalve larvae to settle, recruit and burrow due to the reduced ow velocity amongst the tubes (Hunt 2005)  The polychaete, Magelona capensis also contributed signi cantly to the total macrobenthic abundance during this season, and indicated its preference to silty-clay sediment and moderate OC. This species is capable of living as benthic lter feeder and deposit feeder, indicating its preference to ne sand to muddy sediments with moderate organic carbon. The amphipod, Ampelisca sp. has been termed as an opportunistic species, and reported in high abundance in hydrodynamically unstable areas of the Brazilian shelf (Santos and Pires-Vanin 2004) and behaves both as lter-feeding and deposit feeding organism (Paganelli et al. 2012). In the New Mangalore Port, HCA is physically more unstable compared to the other two areas (LCA and OFW) and at station 18 highly diverse groups of macrobenthos (polychaete and non-polychaetes) were reported during this season and also during other seasons. It is possible that at this port the east-west split restricts the tidal ow to a relatively small amount of water moving swiftly through the entrance, creating hydrodynamically unstable environment which may be suitable for Ampelisca sp. A method was developed by Grifoll et al. (2010) to assess the water degradation risk owing to port activities considering the characteristics and mature of pollutants along with the hydrography, and they suggested that regions with low replenishment are accompanied by higher water quality degradation risk. They also indicated that this is associated with tides and currents which depends on the geography and structure of the port on which the ow of water between the port and the open sea is depended which will determine the dilution of water within the port. Barnard (1970), reported that the sediments in the channel are highly diverse and strongly affect the benthic population. A study carried out at Visakhapatnam port also reported dominance of crustaceans in the outer harbour region which is less polluted when compared to inner harbor during PreM season in the silty habitats (Musale et al. 2015). The stress-tolerant macrobenthic organism, Cossura sp. was abundant in OFW followed by gastropods and pelecypods (12%). The Cossura sp. might have survived in this area over a long duration and might have enhanced their growth in anthropogenically disturbed area and can be termed as an indicator species. In OFW, the silt and sediment chlorophyll-a were considerably higher with low OC and such conditions favor suspension feeders (pelecypods and gastropods). In general higher abundance of suspension feeders may reduce the concentration of suspended particles which will enhance the growth of benthic algae (Wall et al. 2008), which could be useful as food for the pelecypods and gastropods. High densities of bivalves were reported in stable marine salinities but their abundance reduced considerably with lowering of salinity (Pillay and Perissinotto 2008) and this can be attributed to occurrence of pelecypods in the high saline bottom water during PreM and decrease in their abundance during monsoon.

Scenario during Monsoon
During monsoon, the abundance of macrobenthic organisms signi cantly varied within different areas of the port (Two-way ANOVA; P < 0.001). As observed in other seasons, during monsoon also higher abundance of macrobenthic organisms was reported in HCA and lower in OFW. The near-bottom seawater DO concentration indicated hypoxic concitions, and most of the organisms reported were deposit feeders which had an ability to inhabit polluted hypoxic conditions. It has been reported that the severity of hypoxia affects the benthic organisms (Diaz and Rosenberg 1995) and as the severity of hypoxia increases towards anoxia, sensitive species die-off decreasing the macrobenthic diversity of the affected area ( In HCA, the effect of major environmental parameters was moderate and OC was also low (1.9%) bene tting the opportunistic species (Cossura sp., Prionospio sp. Polydora sp.). Among these, Cossura sp. is a well-known opportunistic species (Sivadas et al. 2010) and also the most dominant species in OFW area.

Scenario during Post-monsoon II (PM II)
During PM II the abundance of macrobenthos was maximum and Mediomastus capensis, Tharyx libranchia and Cossura sp. contributed nearly 60% to the total abundance, while about 50% of the sediment was silty-clay and OC was ~ 3% and such conditions contributed to their higher abundance. The M. capensis, and the species belonging to Genus Mediomastus, which subsurface deposit feeders are predominantly found in silty and clayey sediments (Stewar et al. 2002). The New Mangalore Port basin was developed on a sandy beach, however, the major portion of the channel is located on the silty bed (Dattatri et al. 1997;Parvathy et al. 2014). Even though the littoral drift from the adjacent beaches is negligible, considerable amount of siltation occurs in the approach channel and port basin during southwest monsoon season as a result of deposition of suspended sediment due to coastal and tidal currents, and nearly 80% siltation has been observed during monsoon when the wave activity is at its maximum (Parvathy et al. 2014). This structural pattern is ideal for most of the macrobenthic organisms. The Mediomastus sp. an opportunistic species capable of inhabiting turbulent environment with moderate organic carbon. In this season, Mediomastus capensis was dominant at st-18 (station located at the port mouth) which has been considered as a disturbing location owing to its location in the HCA and the average OC was low (1.9%). Rivero et al. (2005) reported that, the mouth of the Mar del Plata Harbor (Argentina) is characterized by high environmental energy and Mediomastus sp. was dominant and these species can ourish in habitat with moderate amount of OC (Pearson and Rosenberg 1978). Basically, some surface deposit feeders (Tharyx libranchia) and sub-surface deposit feeder (Mediomastus sp.) are not capable of thriving in low oxygen conditions, however they appeared to be tolerating stressful conditions (low DO concentrations) during MON. The abundance of Mediomastus capensis and Tharyx libranchia during PM II indicate the return of normoxic conditions in the benthic environment at this port, which indicates to the seasonal variation in the macrobenthic community. The dominant species were Mediomastus capensis, Tharyx libranchia and Cossura sp., which are deposit feeders capable of feeding on freshly settled organic carbon and the subsurface deposit feeders (SSDF) capable of feeding on aged organic matter in the sediment.
The maximum abundance in HCA was contributed by Mediomastus capensis, Magelona capensis and Sternaspis scutata, which are subsurface deposit feeders and burrowers and their feeding activity usually occurs below the surface (Jumars et al. 2015). Surface deposit feeders and burrowers are commonly observed in high abundance when the sediment has ne grains and high organic matter (Mendez 2013) as observed in HCA. The low OC in HCA compared to LCA can be attributed to its utilization by the deposit feeders. Souza et al. (2013), emphasized that Magelonids are mainly non-selective surface deposit feeders, but some species exhibit alternate feeding mode (suspension feeding) (Fauchald and Jumars 1979), and such alteration in feeding strategies may support Magelonids to inhabit both muddy and sandy sediments (Rouse and Pleijel 2001).
It was observed that at oil berth (station 10) in OFW, the macrobenthic organisms were not reported or absent during all the seasons except during PM II. It has been reported that in Bahrain where the industrial growth focuses on the segment of oil re ning, aluminum and petrochemical industries they generate e uents which contain hydrocarbons (Sridhar 2015), and these e uents can cause the issues of macrobenthic groupings which involve altering the community structure, greater numbers of species, and decrease in the normal biodiversity. This can be attributed to lower abundance of opportunistic polychaetes in OFW compared to LCA and HCA.
A comparative account of macrobenthos during different seasons As discussed in the previous sections, the community composition and abundance of macrobenthos varied with the seasons, and indicated a shift in the community structure and also the dominance of macrobenthic species within the port area. Such a shift in the community can be attributed to the signi cant variations in the sediment characteristics mainly the texture, organic carbon and the sedeiment chlorophyll-a. The higher bottom water column chlorophyll-a will contribute to higher chlorophyll-a in the sur cial sediments. Jenness and Duineveld (1985) indicated that tidal currents strongly affect the distribution of the algae (phytoplankton) at the sediment-water interface, including other suspended organic material and detritus which also follows similar pattern, and the algae are deposited on the sediment surface during periods of slack tidal currents, providing food source for epibenthic fauna and other surface deposit feeders. They also pointed out that this high level of chlorophyll-a was alternatively buried in the sediment to a depth of 5 cms and then resuspended. The port construction also determines the current pattern which in uences the water stagnation and ushing in the regions of breakwater or land lls (Grancharova and Grancharov 2013). If waste water is own into the region where the circulation or ushing is low in the port, it can deteriorate the habitat owing to increase in phytoplankton as a resultant of eutrophication (Champ 2003). This points out the importance of water column chlorophyll-a in structuring benthic diversity and community structure. Higher bottom water chlorophyll-a during PreM supported higher abundance of suspension feeders and also surface deposit feeders, and similar pattern was also observed during MON season. Several studies have indicated poor water quality due to anthropogenic activities in major ports of India and attributed this to the weak ushing (Sawant et al. 2007;Musale et al. 2015;Rajaneesh et al. 2015). The HCA area, which has been considered as hydrodynamically active had maximum abundance of macrobenthic organisms during PreM and MON season contributed by deposit feeders and suspension feeders. During PM II, maximum abundance of macrobenthos was observed speci cally in the HCA followed by LCA. During this season the sediment chlorophyll-a was maximum in the HCA followed by LCA and similar trend was also observed for the near-bottom seawater chlorophyll-a and all the dominant organisms reported were deposit feeders. Thus it can be stated that the in ux of food from the water column into the sediment may be sustaining the higher abundance of benthic epifauna. The CCA also showed correlation between the abundance of macrobenthic speceis and environmental variables and sediment texture, suggesting optimum or favorable conditions for the macrobenthic organisms to survive and proliferate during PM II. The low percentage of organic carbon can be attributed to its utilization by deposit feeders in this area owing to their higher abundance.

Conclusion
This is the rst report on the distribution of macrobenthic organismsin the New Mangalore Port, a tropical monsoon-in uenced environment. Seasonal variation in the abundance and biomass of macrobenthic organisms was observed, and it varied with different areas within the port owing to the variation in the sediment characteristics, environmental variables and perturbations in the region. The variation in the hydrodynamics conditions with the seasons had signi cant impact on the occurrence and distribution of macrobenthic organisms. The occurrence of pollution indicator species such as Prionospio sp., Cossura sp. and Tharyx libranchia in the port area pointed that the area is anthropogenically disturbed. The dominance of amphipod, Ampelisca sp. in the channel connecting port to the open sea can be attributed to its dual mode of feeding and preference to hydrodynamically unstable conditions. The baseline information on the benthic biodiversity generated in this study will be helpful for future research and to compare the potential impact at the port environment.