Abstract
Dissolved oxygen concentrations have fluctuated naturally over geological history. In recent years, a decrease in estuarine oxygen levels has been directly related to the process of anthropogenic eutrophication. The condition of low dissolved oxygen brought on by eutrophication is called hypoxia and is generally considered undesirable in terms of environmental water quality. However, the incidence and impact of hypoxia on New Zealand's estuarine ecosystems is largely unknown.
In order to identify the environmental risks posed by hypoxia and to assist in estuarine management, this thesis looked at the factors which influence hypoxia formation and persistence, and to investigate the temporal and spatial variability of dissolved oxygen concentrations in a New Zealand estuarine habitat. It also aimed to provide baseline information regarding hypoxic effects upon a dominant estuarine mysid species (Tenagomysis novae-zealandiae) and to experimentally determine the behaviour of T. novae-zealandiae during depressed oxygen concentrations.
Investigation of dissolved oxygen concentrations in the shallow Kaikorai Estuary revealed hypoxia to be a relatively common condition, with the lowest oxygen concentrations occurring just after dawn. During the study period there was a clear hypoxic event from mid March to early April2005, as a result of increased water temperatures during settled weather, respiration of aquatic vegetation and the lack of freshwater and tidal mixing. This would appear to be the first report of hypoxic and near anoxic conditions in a South Island estuarine habitat.
T. novae-zealandiae abundance or distribution in the Kaikorai embayment was not related to severely depressed oxygen concentrations (< 0.5 - 4.0 mg O2/L). T. novae-zealandiae densities were influenced more by abiotic and biotic combinations of strong light avoidance, close aquatic vegetation association, low flow rates and season. Peak abundances of T. novae-zealandiae in the Kaikorai embayment were 3489 individuals per m3, which is higher than previously recorded for this species. The estuarine abundance of T. novae-zealandiae increased over the study period with a peak in late March 2005.
Laboratory experiments demonstrated T. novae-zealandiae was tolerant of hypoxia (4.0 O2/L) and severe hypoxia (1.5 mg O2/L) but very sensitive to anoxia (< 0.5 mg O2/L). Complete mortality occurred within 22 min when oxygen levels dropped to 0.5 mg O2/L, but the LC so value for T. novae-zealandiae was found to be 1.1 1 mg/L O2. Clear changes in T. novae-zealandiae activity and behaviour occurred as dissolved oxygen levels decreased. Reduced metabolic output through inactivity appears to be a strategy of T. novae-zealandiae to tolerate hypoxia and severe hypoxia. However, this behaviour was unsuccessful during anoxia, as all test organisms demonstrated escape responses and erratic swimming behaviours immediately prior to any loss of equilibrium and mortality.
The behavioural and physiological responses of T. novae-zealandiae to hypoxia, suggest that this species is conditioned to tolerate and survive moderately-low dissolved oxygen levels in the range of 1.5 - 4.0 mg O2/L. Therefore, hypoxic conditions in the Kaikorai Estuary appear only to pose a moderate environmental risk to T. novae-zealandiae.