Co-occurring foundation species increase habitat heterogeneity across estuarine intertidal environments on the South Island of New Zealand

Estuaries are traditionally considered sedimentary 'bare' ecosystems, dominated by infauna that bury into sediments to avoid being eaten by fish or birds. However, estuaries can be converted to biogenic complex 'hard' habitats, like seagrass beds, seaweed patches or surface-deposits of live or dead shells. Furthermore, habitat heterogeneity is enhanced if these foundation species co-occur. Still, few studies have quantified abundances and co-occurrences of different types of foundation species along spatiotemporal stress gradients. We therefore quantified abundances of seagrasses (Zostera muelleri), seaweeds (Ulva spp., Gracilaria chilensis), surface deposited dead shells and densities of dominant and partly buried cockles (Austrovenus stutchburyi) in estuaries on the South Island of New Zealand. A total of 927 large-scale drone images, 1264 small-scale camera images, and 160 sediment-quadrats were collected from 32 common estuarine environments (fully crossed 5-factorial surveys with 2 latitudes x 2 sites x 2 intertidal elevations x 2 seasons x 2 intra-seasonal sampling months). Across the 32 environments, seagrass was most abundant (19-22 % cover, depending on sampling method), followed by shells (9-13 %) and seaweed (4 %). Scattered seaweed and shells were, despite their low cover, ubiquitous in the 32 environments, and seagrasses always co-occurred with shells and/or seaweed. The spatial gradients had a stronger influence on abundances of foundation species than temporal factors, that mainly affected seaweed and live cockles, with high (70 %), medium (50 %) and low (30 %) statistical agreement between analysis of drone vs. camera images for seaweed, shells and seagrass, respectively. Finally, correlation analysis revealed negative associations between seagrasses and both shells and seaweed, but with large variation between seasons. Our study highlights that foundation species rarely occur as single-species stands, and that the ecological impacts of scattered seaweeds and dead surface-deposited shells within seagrass beds should be studied in more detail. Our findings also underscore the critical role of spatiotemporal stressors in shaping estuarine ecosystems and highlight the importance of using supplementary sampling methods to inform management strategies for estuaries in the face of environmental change.