The observed high values of sediment Corg on the slope, and low values in the basin region, were in agreement with previous studies of the western Indian margin. According to Rao & Veerayya (2000), diverse topographic features on the slope and the associated hydrodynamic processes play an important role in the enrichment of Corg. DO values ranged from 0.08 to 2.3 ml·l−1. The lowest DO value was measured at the upper slope station. Based on DO concentrations, the OMZ was found to extend from a water depth of 102–1001 m in the study area, with the core of the OMZ located at 525 m, where the lowest DO value was recorded.
Macrofaunal biomass increased from 34 to 102 m, then showed an abrupt decline with the lowest value measured at 1001 m, before increasing again from 1001 m to 2546 m. Previous studies from the shelf region (Harkantra et al. 1980; Parulekar et al. 1982; Harkantra 2004) report higher biomass values at the shelf stations. The differences noted may be due to the varied sampling methods including different types of gear, water depth and the sampling season. Moreover, all the previous studies sampled at shallower depths (<200 m). Although the biomass in the present study was low, the values increased on the shelf from the shallower to the deeper regions, a pattern not reported in earlier studies (Kurian 1971; Parulekar & Dwivedi 1974; Ansari et al. 1977; Jayaraj et al. 2007, 2008b).
The high benthic biomass at shallower depths in the shelf region may reflect higher food availability in the form of Corg and Chl-a. The supply of food to the sub-tidal benthic environment depends on the proximity to the shore and water depth (Levinton 1982). Sediment Chl-a at 34 m and 102 m was moderately high. Furthermore, biomass was higher at 102 m compared to shallower regions, due to the presence of Decapoda Natantia and Arenicola sp. Although the highest Corg was recorded in the lower part (1001 m) of the OMZ, biomass was lower at this depth and was related to faunal abundance. Comparatively high biomass was observed at 525 m, which was not reflected in abundance. This was due to the presence of the large-sized species P. pinnata. There was an increase in biomass thereafter to the lower boundary of the OMZ towards the deeper region.
Macrofaunal abundance and composition
Macrofaunal abundance increased from the shallow to the deeper region of the shelf, decreased to its lowest value in the lower part of the OMZ and then increased towards the deeper region. Macrofaunal abundance in the western Indian OMZ core (525 m) and the lower part of the OMZ were extremely low compared to all other OMZ margins studied to date, except for the Pakistan margin located on the northeastern side of the Arabian Sea (Table 6). In the present study, low faunal abundance was observed on the slope at depths where DO concentrations were 0.08–0.28 ml·l−1, although measurements of Chl-a and Corg were high. Levin et al. (2009) suggested that oxygen thresholds for macrofaunal abundance exist at ∼0.11–0.13 ml−1 when the other conditions are favorable. It is possible that oxygen thresholds in our study area are higher than on the Pakistan margin.
Table 6. Comparison of macrofaunal abundance (ind·m−2) in the western and eastern continental margin of the Arabian Sea.
|Water depth (m)||DO (ml·l−1)||Western India (present study)||Pakistan (Hughes et al. 2009)||Pakistan (Levin et al. 2009)||Oman (Levin et al. 2000)|
|34||0.69||971|| || || |
|48||0.56||2907|| || || |
|102||0.45||3722|| || || |
|140||2.12|| ||14,894|| || |
| ||0.11|| ||10,464|| || |
|300||0.11|| ||323|| || |
|400||0.13|| || || ||12,362|
|525||0.08||424|| || || |
|700||0.16|| || ||0||16,283|
|753||0.119|| || ||414|| |
|803||0.122|| || ||1656|| |
| ||0.137|| || ||414|| |
|850||0.2|| || || ||19,193|
|853||0.125|| || ||2622|| |
| ||0.14|| || ||414|| |
|903||0.128|| || ||2967|| |
| ||0.147|| || ||6762|| |
|940||0.13|| ||5222|| || |
| ||0.17|| ||3380|| || |
|953||0.134|| || ||4544|| |
| ||0.156|| || ||2898|| |
|1001||0.28||453|| || || |
| ||0.27|| || || ||5818|
|1003||0.146|| || ||3726|| |
| ||0.174|| || ||4692|| |
|1053||0.164|| || ||2208|| |
| ||0.199|| || ||690|| |
|1200||0.35|| ||1003|| || |
|1250||0.52|| || || ||2485|
|1524||1.35||707|| || || |
|1850||1.72|| ||8531|| || |
|2001||2.3||1244|| || || |
|2546||2.3||1188|| || || |
Among the polychaetes, Spionidae were highly dominant within the OMZ core, whereas Cirratulidae were the predominant taxon in other regions. Spionidae and Cirratulidae were also the dominant families within the OMZ on the upper slope (400–700 m) off Oman (Levin et al. 2000) and at a shallow shelf station (140 m), the lower OMZ boundary (1200 m) and below the OMZ (1850 m) on the Pakistan margin (Hughes et al. 2009). In the present study, cossurids were mainly restricted to the lower part of the OMZ (1001 m), although they were also present on the lower slope which is located just beneath the OMZ. This polychaete group was abundant in the OMZ core (100–200 m) off Central Chile (Gallardo et al. 2004). Because these polychaetes are deposit-feeders (both surface and sub-surface), their predominance may reflect food availability (e.g. sediment Corg and Chl-a) within the OMZ region. Paraonidae were abundant at sites above and below the OMZ regions as well as the upper shelf and the basin. Similarly, paranoids were abundant at 1850 m on the Pakistan margin (Hughes et al. 2009).
The Cossuridae and Spionidae were important taxa where oxygen concentrations were lowest. Cossuridae are common in many bathyal OMZs, including the Pakistan margin where they are present at depths of 940–1200 m, both areas of high Corg. Furthermore, the global bathyal data on benthic faunal abundance and biomass indicate a reduction in density at the OMZ core (Rosenberg et al. 1983; Mullins et al. 1985; Wishner et al. 1995) and an increase at the OMZ boundaries (Levin 2003).
Influence of habitat heterogeneity on macrofaunal community structure
The MDS ordination based on the macrofaunal community clustered the sites into three groups representing three different bathymetric provinces. Group 1 comprised stations of the shelf region, where opportunistic species such as Aricidea sp., Prionospio pinnata, Mediomastus sp. and Ancistrosyllis constricta were dominant. These polychaete species were most abundant at a depth of 102 m, where the DO concentration was low (<0.5 ml·l−1). For most of these species, this tolerance of stressful conditions such as the low DO (<0.5 ml·l−1) has been observed in the OMZs of the Oman margin and off central Chile (Levin et al. 2000; Gallardo et al. 2004).
Group 2 was restricted to the slope region (1001–1524 m), where the SSDF species Cossura sp. and Cirratulidae sp. 2 predominated within the macrofaunal community. Although 1524 m was outside the OMZ, it was close to the OMZ boundary and the measured DO was not as high, nor as stable as the values measured at deeper stations.
Group 3, the most diverse group, was dominated by different taxa, including Tanaidacea, Levinsenia sp. 2 and Cirratulidae sp. 2. This group was confined to the two deepest (basin) sites, of which the 2546 m region had a higher value of Corg (excluding those measured at the OMZ stations). The nature and variability of the organic matter supplied to the deep-sea floor influence the structure and function of the communities (Grassle & Morse-Porteous 1987). Tanaidacea are not very diverse in shallow waters but are well represented in the deep sea (Dojiri & Sieg 1997; Pavithran et al. 2007), where they are one of the most abundant taxa. Among the crustaceans, Isopoda and Amphipoda are also particularly abundant and diverse in the deep sea, where they are among the most typical members of the benthic communities (Sanders et al. 1965; Brandt et al. 2007). The diversity of the Group 3 assemblage in the present study was comparable to that of ‘normal’ deep-sea habitats.
The OMZ core fauna did not cluster with any group. This was due to the higher abundance of P. pinnata (Fig. 5), which is known to tolerate low oxygen concentrations (Gallardo et al. 2004). The dominance of P. pinnata has also been reported within the OMZ off Concepecion (Palma et al. 2005). In addition, one particular morphological adaptation of this species, an expanded branchial structure, has been observed only in the OMZ settings, specifically at the lowest level of oxygen concentration on the upper slope (Fig. 9) and not in any other habitat included in the present study. Some other species were restricted to specific areas. Mediomastus sp., Arenicola sp., Levinsenia sp. 1, Ancistrosyllis constricta were confined to the shelf, Cirratulidae sp. 2 to the slope and Lumbrinereis sp. 2, Polyopthalmus sp., Cirratulidae sp. 3 to the basin sites (Table 3). Of the non-polychaete taxa, oligochaetes were found only on the shelf and thus were also restricted to the upper portion of the OMZ. Oligochaet have been reported from the low-oxygen zone in a basin off Peru with partially laminated sediments (Levin et al. 2002). During the present study, macrofaunal diversity was generally higher at shelf and basin sites than on the slope, reflecting the lower concentration of oxygen in the slope region.
Figure 9. Photograph of anterior part of Prionospio pinnata with well-developed branchia observed in the upper slope OMZ.
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The bathymetric distribution of polychaete feeding types is directly related to the amount of organic matter available in the sediments, with SDF strongly dominating the slope regions. Moreover, the proportion of carnivores in the shelf and basin regions increased with depth, proportional to the decline in the deposit-feeding component. The positive relationship between Corg and deposit feeders, and the increase of carnivores where deposit feeders were less abundant, has been observed at the Oman and Crete margins (Levin et al. 2000; Tselepides et al. 2000).
The result of the present study suggests that macrobenthic community structure on the western Indian margin is not determined by a single factor, but instead is influenced by a combination of environmental factors. Discussing animal-sediment relationships, Snelgrove & Butman (1994) concluded that the complexity of soft-sediment communities may defy any simple paradigm with regard to any single factor controlling their settlement and colonization. The distribution of annelids within OMZs worldwide has been reviewed by Levin (2003), who has suggested the different patterns of community structure are due to changes of hydrodynamic, bathymetric or geochemical factors rather than dissolved oxygen alone. Among the biological parameters, abundance and biomass positively correlated with sand and correlated negatively with silt. This is because the sand percentage was higher in shallow water, where higher faunal abundance and biomass were also higher. Similarly, P. pinnata, Cossura sp. and Ancistrosyllis sp. were also observed in sandy sediment at low oxygen concentrations on the western Indian shelf (Jayaraj et al. 2008b). The increase of Chl-a, in both water and sediment, was related to the enhanced phytoplankton production in the study area.