Possible implications of sea level changes for species migration through the Suez Canal

The Suez Canal connects the Mediterranean Sea (MS) and the Red Sea (RS), two marginal seas with contrasting thermohaline properties. Since opening, the canal has become a major conduit for biological invasions, with roughly 500 Red Sea species recorded in the Mediterranean, while reverse (MS to RS) migration has appeared minimal. Previous explanations emphasized a dominant northward current, environmental filtering, and species’ adaptive traits. However, the implications of the canal’s hydrodynamics, its deepening and widening over time, and regional to global sea-level (SL) variability for migration direction and rates have not been rigorously assessed. This study aims to quantify long-term variations in flow transport through the Suez Canal and relate them to seasonal and decadal SL differences between Port Suez and Port Said to evaluate how these dynamics may facilitate or hinder species migration. Focusing on organisms that drift or swim through the canal, we combine 3D hydrodynamic modeling with reconstructed SL records to produce the first continuous transport record for 1923–2016 and interpret its ecological implications.

Prior studies documented extensive Lessepsian (RS to MS) invasions and very limited reverse migration, attributing asymmetry to persistent northward flow, environmental differences, and species’ ecological traits. Earlier hydrodynamic studies of the canal used simplified models or short observational windows, lacking climatic context. The accelerated invasion since the early 1980s has been linked to Mediterranean warming and reduced physical barriers due to canal enlargement. Broader literature reports that MS sea level exhibits strong decadal variability influenced by North Atlantic Oscillation and changes in deep-water formation, while RS and northern Indian Ocean sea levels are modulated by monsoon-driven and Indo-Pacific climate variability. These climatic controls suggest that SL differences across the canal (SLD) could modulate flow seasonality and magnitude, thereby influencing migration opportunities.

Sea level reconstructions: Port Suez SL was reconstructed using Gulf of Aden (GOA) tide-gauge data (Aden, Djibouti), satellite absolute dynamical topography (ADT) over GOA and the northern Red Sea (1992–2016), and sporadic historical SL observations at Port Suez. Assumptions validated by satellite era data: (i) long-term SL trends at Port Suez resemble those at the GOA; (ii) the seasonal SL cycle in the RS is robustly monsoon-driven, allowing use of a mean seasonal anomaly for reconstruction. Annual means were built (1879–2016) from in situ and ADT records; gaps (e.g., 1894–1916, 1970–1992) were linearly interpolated. Monthly values were derived by superimposing annual means with a weighted mean seasonal anomaly computed from 41 years of Port Suez measurements (1920s–1960s and 1980–1986). Port Said SL (1923–2016) was assembled by interweaving Port Said tide-gauge data (1923–1947), Alexandria tide-gauge data (1948–2006; highly correlated and with stable offset relative to Port Said), and ADT (2007–2016). Sea-level differences (SLD = SL_RS − SL_MS) were placed on a common datum using previously published mean SLD for 1980–1986 (11.9 cm) and validated against historical SLD estimates for earlier periods. Hydrodynamic modeling: The Suez Canal flow was simulated using MITgcm in hydrostatic, Boussinesq mode with linear free surface. Domain spans Port Suez to Port Said; horizontal resolution 20 m (cross-canal) × 100 m (along-canal), 12 vertical layers of 2 m, 10 s timestep. Bathymetry for different construction stages (1920, 1956, 1980, 2001, 2010, 2015) was reconstructed from satellite imagery, Suez Canal Authority dimensions, and historical Great Bitter Lake (GBL) surveys; side slopes set to 0.25, trapezoidal cross-sections; GBL center depth matched main-channel depth per stage. Boundary and surface forcing: Sponge layers at both ends relaxed temperature, salinity, and SL to climatologies; GBL salinity relaxed to historical values (1923–1955 seasonal 50.5–53.5 to 44.5–47 psu; post-1980 constant 44.5 psu as upper bound). MS boundary temperature and salinity from SeaDataNet; Gulf of Suez boundary from 1999–2000 observations. Atmospheric fluxes (evaporation, heat fluxes, wind stress) computed in situ using NCEP-based climatologies and bulk formulas; meridional winds varied linearly along canal, based on historical Port Said and Port Suez records. Simulation suites: (1) Realistic R-series (R1920, R1956, R1980, R2001, R2010, R2015) included stage-specific bathymetry, climatological seasonal SL anomalies at both ends with long-term SLD offsets, GBL salinity, and atmospheric forcing; used to quantify seasonal transports per construction stage. (2) SL-series (SL1920, SL1980, SL2010) were barotropic experiments forced solely by SL at both ends (constant T,S), to isolate SLD control on transport. (3) SLMATRIX: 96 steady experiments spanning six bathymetries × 16 SLD values (−0.25 to +0.50 m in 0.05 m steps) to derive transport–SLD relationships and enable linear interpolation for small SLD perturbations. (4) RMATRIX: 24 realistic experiments (four per bathymetry) representing the range of seasonal SLD patterns for six time blocks (1923–1955, 1956–1979, 1980–2000, 2001–2009, 2010–2014, 2015–2016). Reconstruction of monthly transports (1923–2016) used observed monthly SLD to interpolate between RMATRIX solutions month-by-month; method validated against R-series output. Model–data comparisons and validations included matching reconstructed SLD to historical estimates, satellite–gauge consistency checks (GOA annual-mean SL differences < 0.35 cm), and assessment that SL-series captures SLD dominance while noting winter northward overestimation relative to realistic runs.

• Seasonal flow reversal is driven primarily by the sea-level difference (SLD) between RS and MS: winter SLD > 0 yields northward flow (RS→MS); summer SLD < 0 yields southward flow (MS→RS). This seasonality persists across all construction stages.
• Transport magnitudes increased with canal deepening/widening, but long-term trends in SLD modulated the relative strength and duration of the two seasonal regimes.
• Northward transport increased markedly after major expansions: early 1980s and in 2015 with the New Suez Canal. Annual peak northward transport reached ~1070 m³ s⁻¹ in 2016 (about an order of magnitude higher than in the 1920s).
• Southward (summer) transport peaked at ~300 m³ s⁻¹ in 2001 (vs. ~10 m³ s⁻¹ in the 1920s), then declined despite further expansions (2010, 2015).
• Seasonal duration of southward flow expanded from ~Aug–Sep in the 1920s to ~Jun–Oct by 2000, then contracted to ~Jul–Sep by 2015.
• Reconstructed SLD declined from ~20.6 cm in the 1920s to ~5 cm around 2001, then rose to ~15 cm by 2016.
• 1923–1980: High-to-moderate SLD, dominated by northward transport (seasonal duration 9–11 months). Expansion in the 1950s modestly increased southward season and tripled annual average transport relative to earlier decades, but northward regime remained dominant.
• 1980–2000: Canal cross-section nearly tripled, increasing transport in both directions. Opposing SL trends—Eastern Mediterranean Transition (EMT) affecting MS SL (drop 1987–1993 then rise) and cooling-driven SL decline in the northern Indian Ocean/RS—reduced SLD overall (with a transient rise 1986–1993), enhancing southward transport and extending its duration to Jun–Oct by 2000.
• 2001–2016: Accelerated SL rise in the northern Indian Ocean increased SLD, halving southward transport and shortening its season to ~3 months, even as canal capacity increased.
• Modeled along-canal currents can reach ~0.7 m s⁻¹, representing a strong upstream barrier and enhanced downstream dispersal.
• Ecological implication: Physical barrier to southward migration has been substantially reduced since the early 1980s, making MS→RS migration seasonally feasible; since 2001, feasibility decreased but remains higher than pre-1980 conditions. If current SL trends persist, seasonal southward flow may cease by ~2040.

By reconstructing a century-scale transport record and linking it to SLD variability, the study demonstrates that seasonal and decadal sea-level changes on either side of the Suez Canal strongly control flow direction, magnitude, and duration. This directly addresses the hypothesis that canal dynamics influence asymmetry and timing of species migration. When SLD is high (favoring northward flow), a prolonged window enhances RS→MS dispersal, aligning with observed sustained invasions. During periods of reduced SLD (mid-1980s to early 2000s), increased southward transport and longer summer duration likely opened a viable window for MS→RS migration by drift or weak swimming, reducing the previously strong dynamic barrier. Despite these physical opportunities, reported MS→RS establishment remains sparse, suggesting additional ecological filters (environmental tolerances, community interactions, propagule pressure, and detection/reporting biases). Since the early 2000s, strengthened SLD from rapid northern Indian Ocean SL rise has curtailed the southward season, indicating a narrowing opportunity for reverse migration. The modeling implicates Indo-Pacific climate variability (EMT, monsoon and wind-driven heat redistribution, radiative forcing) as key drivers of SLD and thus biotic connectivity, highlighting a climate–invasion linkage.

The study provides the first continuous reconstruction (1923–2016) of flow transport through the Suez Canal, showing that sea-level differences between the Red Sea and Mediterranean dominate canal flow seasonality and trends, while canal expansions amplify transport magnitudes. Findings support a long-standing northward invasion regime until the 1980s, followed by enhanced bidirectional potential since the early 1980s due to canal enlargement, Indian Ocean cooling, and the Eastern Mediterranean Transition, which reduced SLD and strengthened/lengthened the summer southward flow. After 2001, accelerated sea-level rise in the northern Indian Ocean increased SLD, diminishing the southward window despite further canal expansion. Projections suggest that if current SL trends continue, seasonal southward flow could cease by ~2040, effectively closing the MS→RS migration window. Future work should integrate ecological filters (life histories, environmental tolerances, community dynamics) with physical connectivity and improve direct observations to better predict invasion risks under ongoing climate change.

• Sparse direct current measurements within the canal; transport largely inferred from modeling and reconstructed SLD.
• Port Suez sea-level record is fragmentary; reconstructions rely on GOA tide gauges and satellite ADT with assumptions about trend similarity and fixed seasonal cycle.
• Portions of the Port Suez record (e.g., 1980–1993) are approximated, limiting detailed interpretation.
• Bathymetry assembled indirectly from multiple sources; GBL salinity implemented via relaxation with simplifying assumptions (e.g., constant post-1980 upper-bound value), not explicitly simulating salt-bed fluxes.
• RMATRIX/SLMATRIX interpolation methods, while validated, overestimate winter northward transport relative to fully realistic runs.
• Focus restricted to species that drift or swim; does not address introductions via ballast water or other vectors.
• Ecological establishment factors (temperature/salinity tolerances, biotic interactions, community structure, propagule pressure, detection lags) were not modeled and may govern realized invasions.