Stratification in acidified coastal floodplain drains

Abstract

Holocene sediments containing significant concentrations of iron pyrite and widespread throughout coastal lowlands in eastern and northern Australia. Drainage works and flood mitigation structures have augmented the accumulation and outflow of highly acidic soil-, surface- and groundwaters. These drainage waters have high concentrations of dissolved aluminium and iron. In this work we report physical, chemical and ecological changes within drainage channels caused by altered hydrology. Unusual stratification in an acidified drain during a dry period is described. Salt stratification in a reach of the drainage occurs due to the upstream migration of a salt wedge because of leakage of brackish water through floodgates. Drainwater transparency is uncharacteristically high because of aluminium-induced flocculation. This leads to increased penetration of solar radiation. Heating of the saline bottom layers then occurs, such that the temperature at the base of the drain can be more than 5.0 degrees Celsius higher than the surface. Similarly, pH often shows a stepped increased with depth in the salt-affected reach due to acid-neutralisation and displacement by the salt wedge. Dissolved oxygen maxima also occur at depth and appear to be related to high photosynthetic output of dissolved oxygen from submerged waterlilies (Nymphaea gigantea and N. caerulea ssp. zanzibarensis) are acid-tolerant and occur in high densities at acidified sites where more than 95% of their leaf cover is submerged. The dissolved oxygen profile resembles a positive heterograde curve which has only previously been reported in clear lakes and ponds densely colonised by benthic macrophytes. The stratification increases the risk of sub-lethal acid exposure and gas embolism in fish. It also encourages excessive waterlily growth, and may affect the establishment of other macrophytes. Increased waterlily biomass, associated with clear acidified water, accumulates nutrients which are subsequently released when the waterlilies die during floods. This causes catastrophic drops in dissolved oxygen concentration. A flood caused destratification of clear acidified water must be considered in the design of water quality sampling programs so that water quality differences that can occur with depth are not overlooked.