Ecohydrographic control on the community structure and vertical distribution of pelagic Chaetognatha in the Red Sea

Abstract

31The present study details the effects of basin-scale hydrographic characteristics of the Red Sea 32 on the macroecology of Chaetognatha, a major plankton component in the pelagic realm. The 33 hydrographic attributes and circulation of the Red Sea as a result of its limited connection with 34 the northern Indian Ocean make it a unique ecohydrographic region in the world ocean. Here, we 35 aimed to identify the prime determinants governing the community structure and vertical 36 distribution of the Cheatognatha in this ecologically significant world ocean basin. The intrusion 37 of Gulf of Aden Water influenced the Chaetognatha community composition in the south, 38 whereas the overturning circulation altered their vertical distribution in the north. The existence 39 of hypoxic waters (<100 µmol kg -1 ) at mid-depth also influenced their vertical distribution. The 40 detailed evaluation of the responses of the different life stages of Chaetognatha revealed an 41 increased susceptibility of adult individuals to hypoxic waters compared to immature stages. 42Higher oxygen demands of the adults for the egg and sperm production might have prevented 43 them from inhabiting the oxygen deficient mid-depth zones. The carbon and nitrogen content of 44 the Copepoda and Chaetognatha communities and the quantification of the predation impact of 45 Chaetognatha on Copepoda based on the feeding rate helped in corroborating the significant 46 trophic link between these two prey-predator taxa. The observed influences of physical and 47 chemical attributes on the distribution of Chaetognatha can be used as a model example for the 48 role of the hydrography on the zooplankton community of the Red Sea. 49 Zooplankton 51 52 53 54The phylum Chaetognatha embraces a major component of pelagic plankton in the global oceans 55 and their marginal seas (Nair 1978; Bohata and Koppelmann, 2013; Terazaki 2013; Kürten et al. 56 2014). As carnivores, chaetognaths are known to consume ~1 to 12 prey day -1 ranging from 57 oligotrophic to productive environments (Kimmerer 1984; Kehayias 2003) and therefore play a 58 pivotal role in the trophodynamics of marine ecosystems. Their sensitivity to the physical and 59 chemical environment leading to their association with specific water masses makes them 60 suitable bioindicators of ecohydrographic provinces (Nagai et al. 2006; Buchanan and Beckley 61 2016) and for regions with increased primary productivity and the underlying physical processes 62 (e.g. eddies, fronts, and upwelling) (Kusum et al. 2014; Lima 2014; Nair et al. 2015). 63 The Red Sea, geographically positioned between the Asian and African continents and 64 characterized by an area of 4.51 × 10 5 km 2 and a length of ~2000 km is a semi-enclosed inlet of 65 the Indian Ocean (Sofianos and Johns 2002). The high evaporation rates (>2 m yr -1 ), low 66 precipitation (0.05 -0.15 m yr -1 ) and negligible river discharges makes the Red Sea one of the 67 saltiest ocean basins with a salinity range of 36 to 41 (Miller 1964; Grasshoff...

Gelatinous Zooplankton in the Surface Layers of the Coastal Central Red Sea

“…In contrast, siphonophores showed no lagged correlation with environmental variables, which were generally weak. The weak relationship between siphonophore abundance and environmental properties is consistent with previous reports in the Red Sea for Chaetognatha (Cornils et al, 2007;Karati et al, 2019), and may result from their trophic dependence on copepods and other small heterotrophic organisms. The abundance of siphonophores may have also been affected by the input of allochthonous organic matter with dust deposition events (Jish Prakash et al, 2015;Rushdi et al, 2019), which can stimulate the growth of small zooplanktonic prey FIGURE 4 | A summary of (A) seasonal blooming dynamics of main groups gelatinous zooplankton taxa (i.e., tunicates, siphonophores, Ulmaridae) and relative dynamics of temperature ( • C) and chlorophyll a (µg L −1 ) found during the study period, and (B) rapid short-term fluctuations showed by Porpitidae and relative dynamics of temperature ( • C) over the study period.…”

“…However, the presence of dust or smaller planktonic organisms such as Synechococcus, Prochlorococcus, Eukariots and Bacteria, did not show any relationship in our multivariate or univariate exploratory analysis. Even though not captured in our study, these trophic dependences could have contributed to the uncoupling of this zooplanktonic component to environmental factors (e.g., temperature, salinity; Boero et al, 2008;Al-Aidaroos et al, 2016;Karati et al, 2019). Lastly, the occurrence of larger taxa (i.e., particularly Ulmaridae) appears difficult to explain with local factors, and may be dependent on processes occurring at larger spatial and temporal scales (Bastian et al, 2011;Canepa et al, 2013).…”

“…When combined, these data suggest that high temperature and oligotrophic conditions limit gelatinous zooplankton abundance in the Red Sea in summer. This may indicate a bottom-up control mechanism, in which gelatinous zooplankton occurrence depends on the occurrence of prey organisms (Cornils et al, 2007;Karati et al, 2019), and oligotrophic conditions limits gelatinous zooplankton abundance and biovolume to <8 ind m −3 and <10 3 -10 6 mm 3 m −3 , respectively.…”

“…Collectively, these studies documented gelatinous zooplankton in the Red Sea as including at least 26 species of Siphonophora, 29 species of Hydrozoa, 2 Scyphozoa species, and 24 species of Tunicata (Godeaux, 1987;Dowidar, 2003a,b;Zakaria, 2015). However, any argument supporting differences in abundance between studies is presumably limited due to insufficient understanding of temporal dynamics in these gelatinous zooplankton communities (Kheireddine et al, 2017;Karati et al, 2019).…”

Organic Carbon Export and Loss Rates in the Red Sea

“…The POC flux just below the productive layer ( ) was lower than those reported in other areas of the global ocean except for the eastern part of the Mediterranean Sea (Figure 10c). Recent studies showed that most of the biomass of zooplankton in the Red Sea is mainly found in the upper layer of the water column (0–100 m) due to the presence of low oxygenated waters below 100 m (Karati et al, 2019). The scenario of low abundance of mesozooplankton in Red Sea waters may explain these differences.…”