The diverse phytoplankton and zooplankton communities in the Gulf of Eilat-Aqaba exhibit changes in their spatial distribution and seasonal dynamics according to changing oceanographic conditions, including: sea water temperature, stability of the water column (stratification vs. mixing of the water column), and nutrient availability – storage materials (Lindell & Post, 1995; Farstey et al., 2002; Chernihovsky et al., 2018).
Approximately 138 species of phytoplankton have been documented in the Gulf of Eilat, with the main groups being: cyanobacteria (mainly Prochlorococcus spp., Synechococcus spp., and Trichodesmium spp., Lindell & Post, 1995), diatoms (80 species), dinoflagellates (57 species, Kimor & Golandsky, 1977; Winter et al., 1979; El-Sherbiny et al., 2019), and coccolithophores (52 species, Keuter et al., 2023). Species richness, biodiversity, and species composition change according to the seasonal dynamics of the water column and nutrient availability (see the section "Primary Productivity, Algae, and Storage Materials").
The zooplankton community in the Gulf of Eilat consists of 30 taxa, with copepods from the subclass Copepoda being the most dominant, with about 70 species documented and a population density ranging from 140 to 517 individuals per cubic meter (El-Sherbiny et al., 2019). A detailed list of other zooplanktonic organisms in the Gulf of Eilat (ciliates, mollusk veligers, tunicates, worms, etc.) is presented in Table 3 in Farstey et al. (2002) and Table I in El-Sherbiny et al. (2019).
Zooplankton biomass exhibits seasonal dynamics corresponding to their food availability, which is indirectly influenced by surface temperature changes in the Gulf of Eilat. During the summer, the upper water column is stratified, nutrient-poor, and more than 70% of the zooplanktonic organisms (greater than 100 micrometers) are found in the illuminated euphotic zone (up to 100 m depth). Toward the end of autumn and through winter, a gradual decrease in surface temperatures leads to vertical mixing of nutrients from deeper layers with the illuminated zone, encouraging increased primary productivity. Chlorophyll concentrations peak, and a rise in zooplankton biomass is also observed. At the onset of spring, zooplankton biomass reaches its peak, up to approximately 24 mg per cubic meter (Figure 1, Chernihovsky et al., 2018; Shaked & Genin, 2023). The depth of vertical mixing in the Gulf of Eilat is significantly deep (300-800 meters), and during this period, all algae and zooplankton are homogeneously distributed throughout the mixed layer (Lindell & Post, 1995; Kimor & Golandsky, 1977; Farstey et al., 2002; Keuter et al., 2023). Except for a few large zooplankton species (greater than 750 micrometers) such as euphausiids, isopods, amphipods, and copepods, which perform active vertical migration, their vertical distribution differs between day and night hours (Farstey et al., 2002; El-Sherbiny et al., 2019).
The ecosystem in the Gulf of Eilat includes many trophic levels and is mainly controlled by nutrient availability (bottom-up). In oligotrophic environments, zooplankton plays an important role as a link between primary production and higher trophic levels, impacting the entire marine food web and microbial loop (Levanon-Spanier et al., 1979; Sommer, 2000; Labiosa et al., 2003).
Future warming of sea temperatures and changes in nutrient availability may lead to changes in the composition and impact of plankton species. These changes could affect higher trophic levels and thereby influence marine biogeochemical cycles, particularly the carbon cycle (Frada et al., 2021).
Figure 1. Zooplankton biomass (mg per cubic meter) at depths of 0-100 meters from 2011 to 2022, divided into size fractions: 200-500 micrometers (blue), 500-1000 micrometers (red), and greater than 1000 micrometers (green). Taken from Shaked & Genin (2023).
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