Katherine R. O’Brien

Changes to chronic nitrogen loading from sewage discharges modify standing stocks of coastal phytoplankton

Nutrient delivery in subtropical coastal systems is predominantly via acute episodic high flow events. However, continuous nutrient discharges from point sources alter these natural fluctuations in nutrient delivery, and are therefore likely to lead to different ecosystem responses. The aim of this study was to assess how a reduction in chronic sewage nutrient inputs affected chlorophyll a (chl a) concentrations in a subtropical bay, in the context of seasonal fluctuations in riverine nutrient inflows. Reduced nutrient inputs from a large sewage treatment plant (STP) resulted in lower mean dissolved inorganic nitrogen and phytoplankton chl a concentrations during both the austral summer wet and winter dry season. This was measurable within 10 y of nutrient reductions and despite the confounding effects of nutrient inflow events. Our study demonstrates that reductions in STP inputs can have significant effects on phytoplankton biomass despite confounding factors over relatively short time frames.

How to Break the Cycle of Low Workforce Diversity: A Model for Change

Social justice concerns but also perceived business advantage are behind a widespread drive to increase workplace diversity. However, dominance in terms of ethnicity, gender or other aspects of diversity has been resistant to change in many sectors. The different factors which contribute to low diversity are often hotly contested and difficult to untangle. We propose that many of the barriers to change arise from self-reinforcing feedbacks between low group diversity and inclusivity. Using a dynamic model, we demonstrate how bias in employee appointment and departure can trap organizations in a state with much lower diversity than the applicant pool: a workforce diversity “poverty trap”. Our results also illustrate that if turnover rate is low, employee diversity takes a very long time to change, even in the absence of any bias. The predicted rate of change in workforce composition depends on the rate at which employees enter and leave the organization, and on three measures of inclusion: applicant diversity, appointment bias and departure bias. Quantifying these three inclusion measures is the basis of a new, practical framework to identify barriers and opportunities to increasing workforce diversity. Because we used a systems approach to investigate underlying feedback mechanisms rather than context-specific causes of low workforce diversity, our results are applicable across a wide range of settings.

Light history-dependent respiration explains the hysteresis in the daily ecosystem metabolism of seagrass

Oxygen flux between aquatic ecosystems and the water column is a measure of ecosystem metabolism. However, the oxygen flux varies during the day in a “hysteretic” pattern: there is higher net oxygen production at a given irradiance in the morning than in the afternoon. In this study, we investigated the mechanism responsible for the hysteresis in oxygen flux by measuring the daily pattern of oxygen flux, light, and temperature in a seagrass ecosystem (Zostera muelleri in Swansea Shoals, Australia) at three depths. We hypothesised that the oxygen flux pattern could be due to diel variations in either gross primary production or respiration in response to light history or temperature. Hysteresis in oxygen flux was clearly observed at all three depths. We compared this data to mathematical models, and found that the modification of ecosystem respiration by light history is the best explanation for the hysteresis in oxygen flux. Light history-dependent respiration might be due to diel variations in seagrass respiration or the dependence of bacterial production on dissolved organic carbon exudates. Our results indicate that the daily variation in respiration rate may be as important as the daily changes of photosynthetic characteristics in determining the metabolic status of aquatic ecosystems.

Seagrass Dynamics and Resilience

The vulnerability of seagrass ecosystems, and the services they provide, to damage and loss from anthropogenic stressors has led to a surge of interest in understanding their resilience. This chapter examines patterns of change in tropical and temperate Australian seagrasses to identify underlying causes of the observed patterns. It then relates seagrass dynamics to ecosystem resilience, and examines how resilience can be measured, managed and enhanced. Seagrasses in tropical waters show strong seasonal patterns in many places, with seagrass extent and cover increasing during the winter dry season and decreasing during the summer wet season. This seasonality is overlaid by a striking longer term trend of increase during El Nino periods and subsequent loss during wetter, stormier La Nina periods. Seasonality is less evident in temperate waters, where mapping of dynamics has generally been used to show longer term patterns, especially large-scale loss after decades of stability, sometimes with partial recovery. Changes in some places have been linear and in others strongly non-linear, possibly indicative of systems breaching a threshold or tipping point in levels of stressors such as pollutants. Resilience theory has become a powerful tool for understanding the dynamics of seagrass change. Seagrass resilience requires several key traits: genetic and species diversity, good water quality, connected ecosystems and continuous habitats, and balanced trophic interactions. These traits are integrated through ecological feedbacks. In Zostera muelleri meadows, for example, the capacity for seagrass to resist decline during pulses of poor water quality depends on its ability to: (1) efficiently remove excessive nutrients from the water, thereby limiting phytoplankton growth and improving water clarity, (2) suppress resuspension of sediment for improved water clarity, and (3) provide habitat for grazing animals that remove epiphytic algae. The increased understanding of resilience is shifting the focus of seagrass ecosystem management towards the management of stressors to optimise key feedbacks, and thus ultimately to enhance resilience. The chapter culminates in descriptions of practical management actions demonstrated to effectively enhance key traits and overall seagrass resilience.

Seagrasses in the South-East Australian Region—Distribution, Metabolism, and Morphology in Response to Hydrodynamic, Substrate, and Water Quality Stressors

This chapter describes the distribution of key seagrass species in the estuarine-nearshore coastal (ENC) continuum of the south-east region of Australia. We explore the potential influences of hydrodynamics (e.g. tidal currents, wave energy), estuary entrance dynamics (recruitment) and water quality, in addition to light, as primary stressors on seagrass processes and resilience. Despite primary controls exerted by light over seagrass distribution, there are significant areas of euphotic sediments in south-east region that are not colonised by seagrasses. In addition, seagrasses commonly display high degrees of inter-annual variability in coverage which cannot be explained solely by variations in light. We describe the main ecosystem types within the region, and demonstrate how the temporal and spatial gradients in hydrodynamic and water quality stressors (hence light climate), and the availability of suitable substrates for seagrass are controlled by the physical setting or geomorphology of the ecosystem. The opportunistic species Zostera muelleri is the most abundant species within the region, primarily occupying the highly dynamic estuarine niche. We provide a focus on Zostera muelleri to illustrate the direct positive/negative impacts of hydrodynamic, water quality and estuary entrance morphology stressors on seagrass metabolism and morphology across light gradients.

Seagrass Resistance to Light Deprivation: Implications for Resilience

Seagrass habitat is strongly constrained by light availability. Decline in benthic light due to anthropogenic activities (e.g. eutrophication, dredging and catchment modification) is a major threat to seagrass ecosystems, both within Australia and internationally. Even in pristine conditions, light available to seagrasses can be highly variable on timescales ranging from seconds to years. This chapter outlines the three primary mechanisms which enable seagrass to adapt to and/or resist temporary light deprivation: (1) consumption of accumulated carbon; (2) reduction in rates of growth and carbon loss; and (3) increased efficiency of radiation capture and usage. The capacity to withstand severe light deprivation ranges from only two weeks for small, colonising seagrass species such as Halophila ovalis , to beyond two years for large, persistent species such as Posidonia sinuosa. This “tolerance time” depends on the magnitude and timing of the light deprivation, current environmental conditions (e.g. temperature and sediment sulphides) as well as preceding conditions. This chapter proposes a simple conceptual model for seagrass resilience to temporary light reduction , combining both resistance (the capacity of seagrass to survive the light deprivation event), and the capacity to recover once the disturbance ends. Data is synthesized for several potential indicators of seagrass resistance to light reduction.

Comparing extraction rates of fossil fuel producers against global climate goals

Meeting global and national climate goals requires action and cooperation from a multitude of actors1,2. Current methods to define greenhouse gas emission targets for companies fail to acknowledge the unique influence of fossil fuel producers: combustion of reported fossil fuel reserves has the potential to push global warming above 2 °C by 2050, regardless of other efforts to mitigate climate change3. Here, we introduce a method to compare the extraction rates of individual fossil fuel producers against global climate targets, using two different approaches to quantify a burnable fossil fuel allowance (BFFA). BFFAs are calculated and compared with cumulative extraction since 2010 for the world’s ten largest investor-owned companies and ten largest state-owned entities (SOEs), for oil and for gas, which together account for the majority of global oil and gas reserves and production. The results are strongly influenced by how BFFAs are quantified; allocating based on reserves favours SOEs over investor-owned companies, while allocating based on production would require most reduction to come from SOEs. Future research could refine the BFFA to account for equity, cost-effectiveness and emissions intensity.Meeting emissions targets requires limiting use of fossil fuel reserves. For the largest investor- and state-owned producers allowable extraction varies dependent on the approach to calculate burnable fossil fuel allowance.