The effect of long‐term aerial exposure on intertidal mudflat erodibility

• Published in: Earth surface processes and landforms Vol. 45; no. 14; pp. 3623 - 3638
• Main Authors: Nguyen, Hieu M., Bryan, Karin R., Pilditch, Conrad A.
• Format: Journal Article
• Language: English
• Published: Bognor Regis Wiley Subscription Services, Inc 01-11-2020
• Subjects: Air exposure; Biofilms; Biomass; Chlorophyll; Chlorophyll a; Duration; erosion rate; Erosion rates; erosion threshold; Evaporation; Exposure; Intertidal environment; Intertidal zone; intertidal zones; long‐term exposure; Low tide; low‐lying areas; Moisture content; Mud flats; Neap tides; Sea level rise; Sediment; sediment stability; Sediment transport; Sediments; Stability; Stabilizing; the Firth of Thames; Water content; Water levels
• ISSN: 0197-9337; 1096-9837
• DOI: 10.1002/esp.4990

Intertidal zones by definition are exposed to air at low tide, and the exposure duration can be weeks (e.g. during neap tides) depending on water level and bed elevation. Here we investigated the effect of varying exposure duration (6 h to 10 days) on intertidal mudflat erosion (measured using the EROMES device), where the effects of water content and biofilm biomass (using chlorophyll‐a content as a proxy, Chl‐a μg g−1) were taken into account. Sediments were collected between spring and summer (in October 2018, January 2019 and February 2019) from an intertidal site in the Firth of Thames, New Zealand. Longer exposure duration resulted in more stable sediments [higher erosion threshold (Ƭcr, N m−2) and lower erosion rate (ER, g m−2 s−1)]. After 10 days, exposure increased Ƭcr by 1.7 to 4.4 times and decreased ER by 11.6 to 21.5 times compared with 6 h of exposure. Chl‐a and water content changed with exposure duration and were significantly correlated with changes in Ƭcr and ER. The stability of sediments after two re‐submersion periods following exposure was also examined and showed that the stabilizing effect of exposure persisted even though water content had increased to non‐exposure levels. Re‐submersion was associated with an increase in Chl‐a content, which likely counteracted the destabilizing influence of increased water content. A site‐specific model, which included the interplay between evaporation and biofilm biomass, was developed to predict water content as a function of exposure duration. The modelled water content (WMod.) explained 98% of the observed variation in water content (WObs.). These results highlight how the exposure period can cause subtle changes to erosion regimes of sediments. An understanding of these effects (e.g. in sediment transport modelling) is critical to predicting the resilience of intertidal zones into the future, when sea‐level rise is believed to exacerbate erosion in low‐lying areas. © 2020 John Wiley & Sons, Ltd.
Longer exposure duration resulted in more stable sediments [higher erosion threshold (Ƭcr, N m−2) and lower erosion rate (ER, g m−2 s−1)]. Chl‐a and and water content changed with exposure duration. The stability of sediments after re∐submersion periods following exposure showed that the stabilizing effect of exposure persisted even though water content had increased to non∐exposure levels. A site∐specific model, which included the interplay between evaporation and biofilm biomass, was developed to predict water content as a function of exposure duration.