Compromised Refugia: Latitudinal Gradients Mimic Depth and Constrain Subtropical Reef Habitats in the Nilotic Basin

Upper-mesophotic rocky reefs (25-30 m depth) have been proposed as climate refugia in the ultra-oligotrophic Eastern Mediterranean for fish communities in warming shallow waters. A pronounced environmental gradient runs from north to south along the southeastern Levant, driven by sediment and nutrient discharge downstream of the receding Nile Delta. We hypothesized that higher surface productivity near the Nile Delta would support an algae-dominated epibenthic food web and richer fish assemblages at the southern sites. We surveyed epibenthic cover, habitat structure, and fish assemblages at five sites spanning ~160 km of coastline within a shared 25-30 m depth band between 2017 and 2022. Contrary to this hypothesis, our findings reveal a distinct coastal ecocline, a benthic shift from algae- to bryozoan-dominated epibenthos that closely mirrored the depth-related shift observed at the northern sites (between 10 m and 45 m). This benthic shift was coupled with reduced abundance of herbivorous and micro-invertivorous fish and with significantly lower fish species richness and diversity at the southern sites, despite their higher structural complexity. Redundancy analysis showed that depth, latitude, visibility, and season together explained 55.5% of the variance in epibenthic community composition, while an analogous spatial model explained 20% of the variance in fish abundance. This benthic shift, associated with gradients in depth, latitude, sedimentation, and light attenuation, appears to be a major architect of fish community structure in this region, rather than structural complexity. This implies that the southern Levant may act not as a corridor but as a restrictive filter, trapping species between warming surface waters and a deep-sea-like habitat that lacks the conditions required to support higher trophic levels; the capacity of upper-mesophotic reefs to serve as thermal refugia is not uniform across space.