International Journal of Marine Biology and Research

The composition and abundances of populations of planktonic ostracods have been analysed from a day and a night series of stratified samples collected from the Gulf of Oman during the Northeast monsoon to a maximum depth of 2000 m. The profiles show that the ostracods were strongly affected by the intense Oxygen Minimum Zone (OMZ), which at the time extended down through much of the water column from 80–1000 m. Comparisons with similar profile data at similar latitude in the Northeast Atlantic demonstrate the overwhelming impact of the OMZ on the distributions of the planktonic ostracods. Forty-two species were identified from the Gulf of Oman, far fewer than the 61 species collected in the Northeast Atlantic. The ostracods are detritivores and probably play an important role in modifying midwater fluxes of organic material.

Keywords: Myodocopa; Oceanic; Mesozooplankton; OMZ; Zoogeography; Halocyprida; Cypridina;

The quantitative and qualitative bathymetric distributions of halocyprid ostracod assemblages have been well studied in the North Atlantic (Angel, 2010), but not in other oceans. The sampling procedures adopted in the Gulf of Oman were identical to those used for the Atlantic surveys (Angel, 1969, 1979; Angel and Fasham, 1973, 1974, 1975; Angel et al. 2007; Fasham and Angel, 1975), so direct comparisons can be made between the profiles observed. The environment conditions in the Gulf of Oman are markedly different from those in the North Atlantic. In Arabian Sea the seasonal cycle of the monsoon winds results in extreme seasonality in the physical, chemical and biological environment. During the Southwest Monsoon (August-September), the winds blow alongshore persistently in excess of 30 knots (Naqvi et al., 2003) and this results in intensive upwelling both at a series of centres along the Arabian Peninsula in the lea of peninsulas and also further offshore along the axis of the Findlater jet (Boos and Emanuel, 2009). During active upwelling sea surface temperatures at the inshore upwelling Centres Sea can be lower than 15°C, and the upwelled water, originating from below the thermocline, introduces nutrient-rich water into the wind-mixed layer (Morrison et al. 1998). These high concentrations of nutrients stimulate extremely high levels of primary production (Rixen and Ittekkot, 2005, Naqvi et al., 2010), which results in sharp increases in organic fluxes. The bacterial decomposition of these high organic fluxes rapidly exhausts the dissolved oxygen concentrations in the water at subthermocline depths. The exhaustion of dissolved oxygen is unusually rapidly because the oxygen concentrations are already low because the subthermocline water mass is Indian Ocean Deep Water is ‘old’ and already has a depleted oxygen content. This Indian Ocean Deep Water originates as North Atlantic deep water entering the Indian Ocean around the Cape of Good Hope, and so has not been at the surface where its dissolved oxygen content would have been replenished for many months (Morrison et al., 1999). Thus during the South-west monsoon an intense oxygen minimum zone (OMZ) develops.

Conversely during the Northeast Monsoon (December to February) the winds are light and are blow in a direction that is unfavourable for coastal upwelling. Sea surface temperatures warm and the ecosystem in the wind-mixed become oligotrophic. Evaporation rates are high and keep the sea surface temperatures relatively cool. The neighbouring land is very arid and in the absence of rainfall and freshwater run-off sea surface salinities increase, so the density of the surface waters increases so that they sink (Naqvi, et al., 2003). This sinking of the surface waters lowers the base of the oxygen minimum zone (OMZ) to around 1000 m (Morrison et al., 1999).

Prior to 1990 the little that was known about the deep pelagic fauna of the Arabian Sea was mainly based on the transits of the early exploratory oceanographic voyages, such as by the Tiefsee and the Dana. The existence of the OMZ was only recognized during the John Murray Expedition (1933–1934), and report series of this expedition includes a short paper on the handful of planktonic ostracod species collected (Cannon, 1940). Further data on the ostracods was published by George (1969) in a preliminary analysis of the plankton samples collected during the International Indian Ocean Expedition (1959–1965) (IIOE). The IIOE was a multi-vessel expedition during which the biological sampling was extensive geographically and seasonally, but was mostly restricted to sampling the upper 200 m. George’s (1969) summarised data from 1223 standardised samples collected in vertical hauls of the Indian Ocean Standard Net (0.32mm mesh) from 200-0m. George reported that in the North Arabian Sea abundances of halocyprid ostracods peaked during the Northeast monsoon. Leveau (1967) also reported on a collection of halocypids from the Arabian Sea, and there have been a few follow-up studies of the halocyprids (James, 1975, George and Nair 1980, Nair and Madhupratap, 1984). Information about the region’s planktonic ostracod assemblages remained sparse until Drapun and Smith (2012) published their excellent monograph summarising the halocyprid results from the 1995 JGOFS Indian Ocean Experiment (Smith and Madhuptratap, 2005). Their sampling using a MOC1 sampler was limited to the upper 1000m, but their geographical coverage was extensive.

In 1994 a study of the cross-slope relationships between the biological populations in the euphotic zone and the oxygen minimum zone off the coast of Oman was undertaken during the Southwest Monsoon (Herring et al., 1998). Detailed observations were made on the vertical distributions of micronektonic species at three cross-shelf stations close to 19°N 59°E, and the sampling was supplemented with profiles of backscatter intensity from an Acoustic Doppler Current Profiler (ADCP) (Roe et al., 1996). This samplng revealed exceptionally large, but species-poor populations of micronekton (fish, decapods and euphausiids). Many of the micronektonic species were undertaking diel migrations moving from daytime depths within the OMZ up into the epipelagic zone to feed at night (Herring et al., 1998). At the time there were large swarms of the predatory swimming crab Charybdis smithii (Van Couwelaar, et al., 1997); the crabs were entirely restricted to the upper well-oxygenated waters in the wind-mixed layer. The ADCP surveys revealed strong acoustic backscatter from within the OZM during the day, but at night it became totally devoid of sound-scatterers. All of the back scatterers had migrated up into the wind-mixed layer above 70 m (see figure 13 in Herring et al., 1998). Unfortunately during this cruise, heavy seas and technical problems with the nets prevented complete sampling of mesozooplankton profiles

In 1997 during the “Scheherazade” cruise aboard the RRS Charles Darwin (Herring et al, 1999), another attempt was made to characterise the mesoplanktonic populations and biological processes during the post-upwelling regime of the Northeast Monsoon in the Gulf of Oman. The cruise had the two main aims -

1.To describe quantitatively and qualitatively the structure of the pelagic assemblages.
2.To assess the impact of the OMZ has on the pelagic assemblages and their diel vertical migrations.

In this paper we report on the data gathered for the halocyprid ostracods, on how the changes in total ostracod abundances are related to the gradients of the physico-chemical parameters, and how the species composition of the assemblages change with depth. Comparisons are made between similar bathymetric profiles of planktonic ostracods from comparable latitude in the Northeast Atlantic, which demonstrates the severity of the restrictions the OMZ poses on the ostracod populations.

The observations were made at Discovery station 54001 (24° 12’ N, 58° 40’ E) (Figure 1) between 23rd and 25th February 1997. The total water column was systematically sampled both by day and by night to a depth of 1000 m using the multiple rectangular midwater trawl (RMT1+8) (Roe and Shale, 1979; Roe et al., 1980). The profiles were extended deeper to 1000–1400 m by day and to 1400–2000 m by night. Studies in the Atlantic had demonstrated that diel vertical migrations by the planktonic ostracods are restricted to the upper 1000 m (Angel et al., 1982), so it was assumed that the data from below 1000m will not have changed either quantitatively or qualitatively during the day/ night cycle. The volumes of water filtered during each tow were estimated from flow meter data, and the physical and chemical characteristics of the water column were profiled nearby using a Conductivity, Temperature Depth probe (CTD) fitted with an oxygen sensor and a fluorometer.

The sampling protocol was to subdivide the upper 200m of the water column into 50m depth horizons and from 200–1200 m into 100m horizons, and below 1200 m into 200 m horizons. Most tows were of one hour duration, during which approximately 2500 m3 of water would have been filtered. However, since the daytime sample in the upper 150m were so massive, the durations of the night time tows were reduced to 30 minutes. Below 1200 m the tow durations were increased to two hours.

The samples were initially preserved in buffered 5% seawater formalin for 24 h, before being transferred into Steadman’s preserving fluid (0.5% propylene phenoxetol, 4.5% propylene glycol, 5%formalin seawater solution) for long term storage. Several months later the samples were transferred to the Natural History Museum, London, where prior to sorting the Steadman’s preserving fluid was replaced with 80% industrial methylated ethanol.

The halocyprids were generally sorted from the complete samples under a stereoscopic microscope. After the night sample from 150-100m yielded over 19,000 individuals, the remaining shallow samples were subsampled using a Folsom splitter. Identifications were made using a Wild M5 stereomicroscope at x50 magnification. Each species was separated into males, females and juvenile instars, before being counted and measured.

The Physico-chemical Environment

Table 1 summarises the profile data for the mean temperature, salinity and oxygen concentrations within each of the sampling horizons derived from the CTD, and the profiles are plotted in Figure 2. The isothermal wind mixed layer extended down to 70–80 m. Below 100 m water temperature declined almost linearly with depth to 2000 m. Salinities similarly declined apart from a small salinity maximum between 200 and 400 m, the result of the outflow of Persian Gulf Water. Oxygen concentrations were high within the wind-mixed layer, but diminished rapidly from 121.8 μM at 78 m to only 4.7 μM at 102 m. At 100-150 m the mean oxygen concentration was only 3.2 μM, and decreased even further to a minimum of < 2 μM between 362–566 m. Below 600m the oxygen concentrations increased very gradually with increasing depth to 10 μM at a depth of 1100 m. The oxygen concentrations then rose slightly more quickly to reach 84.9 μM at a depth of 2000 m. Chlorophyll concentrations were maximum (2.0 mg.l^-1) at 14 m, and declined to 0.12 mg.l^-1 at 70 m, and were negligible at deeper depths.

The total numbers of ostracods sorted and analysed from the day and night series were 19545 and 39761 respectively. The lower daytime abundances were principally the result of the subsampling. The planktonic ostracods included both halocyprid and myodocopid species. In the upper 200m the ostracod assemblage was dominated by a Cypridina sp. – a myodocopid which is poorly known taxonomically. This species forms dense swarms in the region, which generate impressive displays of bioluminescence. The ostracod abundances expressed as numbers per1000 m³ of water filtered are listed in Table 2, and the profiles are plotted in figure 3 together with the oxygen concentrations. In the upper 200 m, halocyprid abundances were high 3–4 m^-3 and if the Cypridina data are included they are 4-5 m^-3.

Their abundances remained exceptionally high down through the top of the thermocline and the oxycline to depths of 200 m where the oxygen concentrations had fallen to 2.5–3.3 μM. Below 200 m ostracod densities declined abruptly to almost zero, and remained exceptionally low to below 1000 m, but then their adundances began to increase very gradually; it was not until the oxygen concentrations had risen to >20 μM that the halocyprid densities became comparable to those in the North Atlantic. Estimates of total ostracod abundance per m2 in the upper 1000m were respectively 799 by day and 611 by night – reasonably similar. Excluding the data for Cypridina, the estimates become even closer - 456 and 512 respectively.

Table 2 lists the estimated percentages of the total halocyprid population in the upper 1000 m occurring within each sampling stratum. These profiles suggest that there was a slight upward migration at night - the median depth of the halocyprid population in the upper 1000 m rose from 108m by day to 92 m at night. There was a small increase in numbers at 300–400 m at night, suggesting some species may have been undertaking a reverse migration, moving deeper into the OMZ at night.

The majority of the specimens were identified; the proportion of unidentifiable specimens (mostly juveniles) increased with depth, but never exceeded 5%of the total catch. The total number of halocyprid species was 41 plus the single myodocopid species (see Table 3). Tables 4-6 list the abundances of females, males and juveniles of each species, expressed as numbers per 1000 m³. Only 16 species were collected by day in the upper 800m and 15 by night. Whereas at 1000-1400m immediately below the OMZ, 22 species were identified; of which ten had not been encountered in the upper 1000 m. At night at 1400–2000 m 28 species were identified, of which only ten had been collected from above 1000 m.

In the North Atlantic several series of replicated samples collected at several depths (Angel 1984, Angel, et al. 1982; Angel, unpublished data) showed that at 600m or deeper that the overall abundances of halocyprid populations differed by < 25% between samples irrespective of the time of day, and the species composition and the rank order of species abundance remained the same. This leads to the conclusion that diel vertical migration by ostracods is almost entirely restricted to the upper 600m. So in the Gulf of Oman, where no evidence of spatial heterogeneity resulting from mesoscale eddies was observed, we feel justified in assuming that the single samples are providing representative data on the species composition, rank order of abundance, and bathymetric profiles of the individual species.

The mesh size of the RMT1 sampler is 0.32 mm and the minimum carapace length of the ostracods it retains is about 0.5 mm. Thus adults of all the halocyprid species will have been fully retained, with the exception of the males of Archiconchoecia striata. For those species with adult carapace lengths >2mm, the last three juvenile instars (A-1 to A-3) will have been retained, but these juveniles stage of the smaller will have been extruded through the mesh. This compromises the accuracy of the overall estimates of halocyprid abundances. The accuracy would potentially be increasd by using a finer meshed sampler (e.g. Drapun and Smith, 2012), However since the early juvenile instars cannot be identified there would not have been any improvement in the precision of the individual species data. In addition the time taken to process the total samples would have been substantially increased so that even more sub-sampling would have been needed.

The abundant populations in the upper 100 m in the Gulf of Oman were dominated by small species (< 2 mm) namely Euconchoecia omanensis, Porroecia porrecta, Pseudoconchoecia concentrica, and Cypridina sp. By night, 95% of the ostracod sampled consisted of species that brood their embryos within the carapace (i.e. the species of Euconchoecia and Cypridina). This is in stark contrast to the halocyprid populations in the North Atlantic where the brooding species are either absent or rare. Just below the oxycline at 100-200m, these species were partially replaced by Proceroecia procera, Discoconchoecia elegans, Metaconchoecia acuta and Conchoecetta giesbrechti. These are species must be able to tolerate prolonged exposure to very low oxygen concentrations. Below 200 m, both sample sizes and the numbers of species caught declined sharply. At 200-400 m, within the salinity maximum associated with the outflow of Persian Gulf Water, the small ostracod population was dominated by Conchoecetta giesbrechti and Metaconchoecia inflata. Metaconchoecia inflata was the only species, to occur almost exclusively below 150 m, and so seemed permanently to occupy water almost devoid of