The response of estuarine bivalve Austrovenus stutchburyi to hypoxic multiple-stressor environments

Abstract

Eutrophication-induced hypoxia is amongst the most widespread anthropogenically deleterious environmental issue occurring globally in coastal marine environments. Along with habitat loss, overfishing and harmful algal blooms, hypoxia contributes to a suite of major stressors on marine organisms. These cause stress to aerobic organisms, changes to biogeochemical processes and loss of biodiversity and ecosystem function, particularly in estuaries.

The response of a species to a single stressor is often very different when compared to exposure in an environment with multiple stressors. The focus of the current thesis is on the sublethal and lethal effects of hypoxia, warming and nutrient enrichment on the estuarine bivalve Austrovenus stutchburyi. Hypoxia and warming rarely occur in isolation as stressors that shape a species or community response. Midday low tides have been identified as a driver of mortality events for bivalves globally as a combined result of thermal stress and desiccation. Laboratory experiments were used to test the effects of simulated midday low tides and chronic NH3 exposure on activity, respiration and survival of A. stutchburyi experiencing oxygen depletion.

A long-term decline in oxygen solubility associated with warming was identified for Coastal Otago. Monitoring of bottom-water dissolved oxygen (DO) in a shallow microtidal estuary on the South East Coast of New Zealand was undertaken to bridge observational and experimental work. This monitoring revealed diel- and seasonal DO cycling. Modelling indicated spatial variability within Waitati Inlet. Land-based influence from freshwater and tidal forcing was driving low DO in Waitati channel, while PAR and wind speed were key drivers of biological respiration and wind-induced mixing in Warrington channel which has Ulva spp., present in it. Daily DO minima occurred primarily early morning at low tide as a likely consequence of respiration by primary producers and the tidal ebb carrying water depleted of oxygen.

Siphon activity and respiration rates were enhanced under hypoxia (20% saturation) compared to normoxia in 22 °C and 25 °C water conditions, while survivorship decreased with exposure to increasingly stressful environments. Experimental findings indicated the likely existence of a positive feedback loop whereby A. stutchburyi, stressed by low oxygen, produce more ammonia waste, increasing risk of mortality and subsequent NH3 concentrations. A lethal emersion thermal threshold was surpassed at 33 °C in 20% and 100% DO, suggesting extreme thermal events such as heatwaves could have substantial detrimental implications for bivalve populations and subsequent ecosystem functions.

The findings of the present thesis are relevant to understanding the likely biological consequences of observed patterns of warming in the coastal marine environment and the decline in oxygen solubility observed in Otago. The increased biological oxygen demand due to warming may lead to more frequent hypoxic events and degradation of estuarine environments.