Vertical distribution patterns

Total live assemblage

The decrease in bottom-water oxygen concentrations at 140m following the SW monsoon was reflected in the vertical distribution of live macrofaunal Foraminifera in a number of ways. During the 2003 spring intermonsoon, the 140-m site was situated above the OMZ and bottom-water oxygen concentrations were 2.05mll-1. Despite this, the ALD5 was relatively shallow (0.57 cm). Only 11% of all live Foraminifera were found in sediment layers between 1 and 5 cm (Table 4.6 and Figure 4.6). Following the SW summer monsoon and the associated upward expansion of the OMZ to c.80m water depth, bottom-water oxygen concentrations at the 140-m site dropped to 0.11mll-1. The proportion of live Foraminifera found between 1 and 5 cm decreased to only 3.5 % and the ALD5 decreased to 0.31 cm. The seasonal increase in percentage of live Foraminifera found in the 0-1 cm layer was most apparent in the topmost layer (0-0.5 cm) where it rose from 60.4% to 89.4% after the SW monsoon. The 0.5-1 cm sediment layer actually showed a decline in percentage abundance from 28.7% to 7%.

The main foraminiferal groups

The three main foraminiferal groups, calcareous, monothalamous and other agglutinated forms, displayed different vertical distribution patterns and responses to the seasonal changes in bottom-water oxygen concentration (Table 4.6; Figure 4.6). Calcareous Foraminifera were present in all layers down to 5 cm depth during the spring intermonsoon. However, following the SW monsoon, calcareous forms were found only to a depth of 3 cm. In both seasons, most of the calcareous Foraminifera were found within the upper 1cm of the sediment, increasing from 89.2% of the total

live assemblage in the spring intermonsoon to 97.0% after the SW monsoon. The remaining 3% of live calcareous Foraminifera colonised sediment layers between 1 and 3 cm. The proportion of non-calcareous Foraminifera (monothalamous and other agglutinated) declined in all sediment layers following the SW monsoon. Other agglutinated taxa (multilocular agglutinated forms and ammodiscaceans) displayed a similar trend to calcareous forms. There was a reduction of the maximum colonisation depth from 3 cm during the intermonsoon to 2 cm during the monsoon – influenced seasons. Monothalamous forms maintained a relatively stable vertical distribution pattern between the two seasons, with live specimens found only within the 0-1cm sediment layer and only 1 specimen (of Saccamminid sp. 1) found below the upper 1cm sediment layer (1-2 cm layer, monsoon season).

Table 4.6 140-m site. Mean percentage abundance of live macrofaunal (>300µm) foraminiferal groups at 140m. a) All vertical layers analysed, b) Σ of vertical layers 0-1cm, 1-5cm, 0-5cm. Data are mean values of 4 replicates per season ± 95% Confidence Interval. Underlined data show a significant increase in percentage abundance from intermonsoon to monsoon (P < 0.05, student t-test assuming unequal variances). Grey boxes highlight layers where no live Foraminifera are present.

a)

140 m Mean % Total Live Mean %calcareous Mean %other agglutinated Mean %monothalamous

intermonsoon monsoon intermonsoon monsoon intermonsoon monsoon intermonsoon monsoon 0-0.5 60.4 ±14.3 89.4 ±12.2 55.2 ±11.3 86.7 ±11.9 1.2 ±0.7 0.6 ±0.3 4.0 ±2.5 2.1 ±0.6 0.5-1 28.7 ±15.8 7.0 ±7.0 24.3 ±13.9 6.9 ±7.2 2.8 ±1.4 0.0 ±0.0 1.6 ±1.2 0.2 ±0.3 1-2 8.7 ±2.3 1.2 ±1.6 8.3 ±2.1 1.0 ±1.4 0.4 ±0.6 0.4 ±0.2 0.0 ±0.0 0.2 ±0.2 2-3 1.5 ±0.6 0.4 ±0.6 1.2 ±0.5 0.4 ±0.6 0.3 ±0.4 0.0 ±0.0 0.0 ±0.0 0.0 ±0.0 3-4 0.3 ±0.4 0.0 ±0.0 0.3 ±0.4 0.0 ±0.0 0.0 ±0.0 0.0 ±0.0 0.0 ±0.0 0.0 ±0.0 4-5 0.5 ±1.5 0.0 ±0.0 0.5 ±1.5 0.0 ±0.0 0.0 ±0.0 0.0 ±0.0 0.0 ±0.0 0.0 ±0.0 b)

140 m Mean % Total Live Mean %calcareous Mean %other agglutinated Mean %monothalamous

intermonsoon monsoon intermonsoon monsoon intermonsoon monsoon intermonsoon monsoon 0-1 89.0 ±3.3 96.5 ±5.5 89.2 ±4.9 97.0 ±5.2 5.0 ±0.9 0.6 ±0.3 5.5 ±1.6 2.3 ±0.9 1-5 11.0 ±3.3 3.5 ±5.5 10.3 ±3.1 3.5 ±5.6 1.7 ±0.7 0.4 ±0.2 0.0 ±0.0 0.2 ±0.2 0-5 100 100 89.8 ±2.4 97.0 ±1.2 4.6 ±1.6 0.6 ±0.3 5.5 ±1.6 2.4 ±1.0

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Figure 4.6 140-m site. Vertical distribution of live macrofaunal (>300 µm) Foraminifera within the 0-5 cm layer during both seasons sampled: a) spring intermonsoon (April 2003), b) SW monsoon (October 2003). Data are mean values of total live individuals from 4 replicates (25.5 cm2) per season and are presented for major foraminiferal groups. 95% Confidence Intervals are shown. N = Mean live individuals (average of 4 replicate samples of 25.5 cm2). ALD5 = Average Living Depth in 0-5 cm. Mean percentage abundance of live individuals in the 0-1 cm layer is also shown. a) 140 m. Spring intermonsoon. 0-5 cm. b) 140 m. SW monsoon. 0-5 cm. ALD = 0.575 N = 189 0-1 cm = 89.0% 0 20 40 60 80 100 120 0-0.5 cm 0.5-1 cm 1-2 cm 2-3 cm 3-4 cm 4-5 cm 0 50 100 150 200 250 300 350 400 0-0.5 cm 0.5-1 cm 1-2 cm 2-3 cm 3-4 cm 4-5 cm Monothalamous Lagenina Other Agglutinated Buliminids Rotaliids Miliolids 0 50 100 150 200 250 300 350 400 450 4-5 cm 3-4 cm 2-3 cm 1-2 cm 0.5-1 cm 0-0.5 cm 0 50 100 150 200 250 300 350 400 450 N = 390 ALD = 0.31 0-1cm = 96.5% 5 0-0.5 cm 0.5-1 cm 1-2 cm 2-3 cm 3-4 cm 4-5 cm

Top-5-ranked species

The vertical distributions of the top-5-ranked species at 140 m during both seasons are shown in Figure 4.7. During the spring intermonsoon, the five most abundant species all displayed an Average Living Depth of < 1 (Figure 4.7). Three calcareous species, Uvigerina ex. gr. semiornata, Cancris auriculus and Cibicides sp. 1, displayed relatively shallow ALD5 (0.57, 0.66 and 0.45 cm respectively) and all three species displayed a peak in abundance in the upper 0-0.5 cm sediment layer. A few individuals of U. ex. gr. semiornata and C. auriculus were found in the 2 to 5 cm sediment layer. Possibly, this was a result of either contamination with surface layers during laboratory sampling procedure or redistribution by bioturbation, rather than the true living position of these individuals. The deep-infaunal species Globobulimina cf. G. pyrula and Reophax bilocularis displayed a deeper colonization of the sediment (ALD5 = 0.71 cm and 0.87 cm respectively) during the spring intermonsoon compared to the other most abundant species, with a peak in abundance within the 0.5-1 cm sediment layer. Following the SW monsoon, U. ex gr. semiornata and C. auriculus displayed a decrease in the Average Living Depth (ALD5 = 0.31, ALD5 = 0.29 cm respectively) following the SW monsoon and a substantially higher peak in abundance within the upper 0-0.5 cm sediment layer than seen during the spring intermonsoon season. In contrast, Globobulimina cf. G. pyrula displayed an increase in Average Living Depth following the SW monsoon (ALD5 = 1.21). Bolivina aff. dilatata was found between 0-2 cm sediment depth (ALD5 = 1.02) and Saidovina amygdalaeformis was found in the 0.5-1 cm sediment layer only (3 individuals).

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Figure 4.7 140-m site. Vertical distribution of the top-5-ranked live macrofaunal (>300µm) foraminiferal species in the 0-5 cm sediment layer a) spring intermonsoon (April 2003) b) SW monsoon (October 2003). Data are mean values of 4 replicates per season. 95% Confidence Intervals are shown. Average Living Depth in the 0-5 cm sediment layer (ALD5) is shown for each species.

4.3.2 The 300-m site