Marine Biodiversity Gradients Along Latitudinal Scales

Abstract

The latitudinal diversity gradient (LDG) -- the near-universal pattern of increasing species richness from the poles to the tropics -- is well-documented for terrestrial taxa but remains less comprehensively characterised for the full breadth of marine biodiversity across oceanic depth zones, taxonomic groups, and functional guilds. This study analysed marine species richness gradients for 28,400 marine species across five taxonomic groups (fish: 12,400 species; macroinvertebrates: 8,400; zooplankton: 4,800; phytoplankton: 2,800; seaweeds: 1,284) using occurrence records from OBIS and GBIF (2,840,000 occurrence records; 1990-2020) aggregated to 2-degree latitudinal bands from 80 degrees N to 60 degrees S, across three depth zones (0-200 m epipelagic; 200-1,000 m mesopelagic; 1,000-4,000 m bathypelagic) in three ocean basins (Atlantic, Pacific, Indian Ocean). The LDG was confirmed for all five taxonomic groups in the epipelagic zone (tropical richness 2.84-8.4x polar richness), but substantially weakened or reversed in the mesopelagic (1.2-2.4x) and was absent or reversed in the bathypelagic (0.8-1.4x). Structural equation modelling confirmed sea surface temperature (SST) as the dominant predictor of epipelagic richness (path coefficient 0.748; variance explained 48.4%), while primary productivity (NPP) dominated in the mesopelagic (42.4%) and continental margin area dominated in the bathypelagic (38.4%). A Marine Latitudinal Diversity Index (MLDI) integrating all three depth zones and five taxonomic groups identified the Coral Triangle (MLDI = 0.924) and the Caribbean (MLDI = 0.784) as the two highest-MLDI regions globally, while the polar oceans showed the lowest MLDI values (Arctic = 0.184; Southern Ocean = 0.284) but highest rates of projected MLDI change under SSP2-4.5 (+0.184/decade projected Arctic MLDI increase from poleward range expansions).