Altered groundwater discharge and associated carbon fluxes in a wetland-drained coastal canal

Residential canal systems are becoming increasingly popular with the rising demand for absolute coastal waterfront properties. We hypothesize that canals alter groundwater-surface water connectivity and related carbon fluxes into coastal surface waters. Here, we quantified submarine groundwater discharge (SGD) in a residential canal system on Bribie Island (Australia) and associated carbon fluxes. SGD rates estimated from a radon (222Rn) mass balance model were 3.1 ± 1.5 cm d−1. These fluxes delivered 68 ± 44 and 70 ± 48 mmol m−2 d−1 of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) into the canal, respectively. Carbon dioxide (CO2) emissions to the atmosphere ranged from 15 to 28 mmol m−2 d−1. Multiple lines of evidence, including flux estimates and groundwater observations, converge to the conclusion that SGD was a major source of DOC and free CO2, but not carbonate alkalinity nor DIC to canal surface waters. In comparison to mangrove tidal creeks that often precede canals, the canal had (1) lower tidally-driven saline groundwater exchange rates but higher fresh groundwater discharge, (2) lower CO2 emissions to the atmosphere; and (3) acted as a driver rather than a buffer of local ocean acidification. These differences seem to be driven by the replacement of intertidal wetland vegetation with urban areas that prevent soil carbon accumulation and related biogeochemical processes around the canals. We suggest that decisions on canal construction should consider potential changes to groundwater-derived soil carbon losses and carbon cycling in receiving coastal waters. •Canals have altered SGD and derived carbon fluxes to surface water.•CO2 outgassing from canals differs from natural estuarine systems.•Canals may act as a driver of local ocean acidification.•Services provided by natural estuarine systems may not be provided by canal estates.