Physiological performance and adaptive capacity of kelp (Laminariales) in a changing environment

Kelp ‘forests’ form the foundation of many temperate reef ecosystems. As ecosystem engineers, kelp modify abiotic conditions, create habitat for other species and support highly diverse communities. Climate change represents a threat to kelp ecosystem function, stability and biodiversity and on the east and west coasts of Australia recent warming events have caused significant damage to kelp forests. The south east coast of Australia is warming at approximately four times the global average and the continuation of this warming will likely have further negative impacts on the health and distribution of kelp forests and the ecosystems they create. Climate change is a multifactor stressor but multifactor studies to determine the impact of climate change on biota are rare. This thesis examines the physiological response and adaptive potential of ecosystem engineering kelps Macrocystis pyrifera & Ecklonia radiata under relevant multifactor climate change scenarios in south eastern Australia. I assessed a range of physiological parameters to understand the multivariate response in these kelp to higher temperature, reduced nitrates and higher light, incorporating growth rates, photophysiology (as derived from PAM fluorometry and pigment concentrations), nutrient profiles (concentration of C and N, C:N ratios and isotopic signatures of δ13C and δ15N), and nucleic acid levels (RNA and DNA concentrations and RNA:DNA ratios). This approach enabled a holistic evaluation of kelp performance to climate change and also, a test of the applicability of the Growth Rate Hypothesis (GRH) in predicting future kelp distribution. For temperate seaweed, the GRH predicts that selection will favour rapid growth in high latitudes where light is limiting, hence greater nutrient requirements. In this context, it was anticipated that longer periods of warm, oligotrophic East Australian Current (EAC) waters acting simultaneously with a reduction in kelp canopy (increased light) will interact to negatively impact kelp physiological performance, growth and survival. Chapter 2 examined the effects of temperature, nitrate and light on the growth and physiological performance of Macrocystis pyrifera from Tasmania. High temperatures led to down regulation of photosystem II (PSII) as well as photosystem impairment when combined with low light. High light photoinhibition occurred in temperatures above 12 °C. These deleterious effects were characterised by excessive tissue necrosis and mortality. RNA concentrations were associated with stressful conditions but were decoupled from growth, showing no support for the GRH in this species. As expected, optimum growth occurred at lower temperatures but unexpectedly at low nitrates, perhaps reflecting an adaptive response to the typically low ambient nutrient levels that occur on the east coast of Tasmania. Chapter 3 investigated the influence of temperature, nitrate and light on the widely distributed kelp Ecklonia radiata from two different locations (bioregions). Temperature drove significant variation in PSII metrics overall although optimum PSII performance occurred at low temperatures for Tasmanian E. radiata only, whilst light and nitrate had few significant main or interaction effects. Growth was driven by temperature (Tasmanian) or light (NSW). This chapter highlighted the importance of considering latitudinal variation in responses to climate change but also showed a lack of support for the GRH in this species. Chapter 4 examined the adaptive potential of Tasmanian Ecklonia radiata to climate change across haploid and diploid life-cycle stages. There was strong family-level variation in growth, reproduction and photosystem traits in gametophytes and porophytes, indicating the potential for adaptive responses to climate change. Furthermore, there were significant genotype x environment interactions for some traits indicating families will respond differently to changes in temperature and light. The high adaptive potential suggested by this study could enhance the resilience of E. radiata to climate change. Overall, this thesis indicates that increasing temperatures will continue to impact the performance and distribution of kelp, particularly in Tasmania where there were strong effects of high temperature on both E. radiata and M. pyrifera. This thesis also highlights the importance of multifactor studies in determining additive effects of climate stressors on kelp and the complex nature of physiological responses to these stressors. Finally, the high familylevel variation of fitness traits in the key early life-history stages highlighted adaptive potential in E. radiata to climate change.