Copepods and other zooplankton components were identified following Giesbrecht (1892), Williamson (1967), Heron & Bradford-Grieve (1995) and Conway et al. (2003). The three counts of total zooplankton at different depths and all seasons were treated statistically to determine the standard error and standard deviation of these counts. The surface water temperature varied seasonally

from a winter minimum of 22.8 °C to a summer maximum of 30.5 °C. The vertical thermal profile showed clear stratification in summer and slight differences during other seasons, whereas the vertical thermal difference within the epipelagic zone was small (Figure 2). Dissolved oxygen was relatively high in the surface water (6.6–7 mg l− 1) as well as within the epipelagic zone (5.3–7.8 mg l− 1), with some stratification during summer, autumn and winter, and distinct stratification in spring Apitolisib (Figure 3). In our study, maximum dissolved oxygen in INK 128 in vitro spring coincided with the highest content of chlorophyll a within the depth range of 50–75 m, supporting the role of phytoplankton photosynthesis in the oxygenation of the water column. The phytoplankton biomass in the epipelagic zone exhibited low as well as moderate values over

the year, whereas concentrations of chl a fluctuated between 0.04 μg l− 1 at 100 m in spring and 1.12 μg l− 1 at 75 m, also in spring. The surface water was usually poor in phytoplankton, whereas the vertical profile displayed

slight variations during summer, autumn and winter, and displayed a clear subsurface chlorophyll high in spring ( Figure 4). The epipelagic zooplankton off Sharm El-Sheikh was composed Thymidylate synthase mainly of copepods, which constituted seasonally 78.6–93.2% of the total zooplankton with a mean of 86.5%. The molluscan larvae (gastropods and bivalves) were second in order of abundance, making up 2.6–15.2% with a mean of 7.6%, followed by appendicularians (1.4–3.7%, mean: 2.4%) and chaetognaths (0.7–1.6%, mean: 1.1%). Cnidarians demonstrated a comparatively small relative abundance (0.2–1.4%) in the total zooplankton. The contributions of the main groups to the total zooplankton during the present study (Table 1) were roughly similar to those reported in another study (ElSherbiny et al. 2007), but are more or less different from those found in the northern Gulf of Aqaba (Cornils et al. 2005). The zooplankton density during the present study showed relatively wide seasonal variations in the water column (∼ 1.1 × 103 − ∼ 5 × 103 organisms m− 3), with a conspicuously high density (4952 and 4445 organisms m− 3) within the surface layer (0–25 m) in summer and the 25–50 m depth range in spring. The standard error and standard deviation of total zooplankton density are given in Table 2. The vertical profile demonstrated decreasing zooplankton density with depth during all seasons, particularly in the deep layer from 50 to 100 m (Figure 5).