The effects of dissolved inorganic nutrients on thermal responses and recovery of Hawaiian corals

Abstract

My dissertation interest stems from a study in which I participated in 2017 which revisited the thermal thresholds of Hawaiian scleractinian corals first established by Drs. Paul Jokiel and Steven Coles in the 1970s (Jokiel & Coles 1977). This comparative analysis revealed not only an enhanced tolerance of corals to temperature stress but also prompted further inquiry into the role of environmental factors particularly nutrients. Conducted under conditions replicated from the original experiments, the 2017 study (Coles et al. 2018) demonstrated differences in coral survivorship, bleaching severity, and onset of bleaching. These findings suggest that corals have been acclimatizing or adapting to temperatures over the last 47 years, offering a hopeful perspective amidst the current crisis of global coral reef degradation. However, a notable difference in the levels of dissolved inorganic nutrients between the two periods, influenced by unique historical events in Kāne‘ohe Bay, triggered a need for a deeper investigation into how these factors affect the shifted temperature responses.The significance of coral reef ecosystems is often attributed to their limited spatial extent, they cover less than 1% of the ocean floor yet harbor tremendous biodiversity. However, the primary reason these small areas can act as biodiversity hotspots is the coral’s ability to create habitats under nutrient-poor conditions, an environmental scenario often referred to as a 'blue desert' (Dubinsky & Jokiel 1994). Nutrients, while essential for organisms to grow, survive, and reproduce, are considered detrimental to coral reef ecosystems when present in excessive amounts, disrupting the finely tuned balance of these ecosystems. Furthermore, over the past two decades, studies have found that excessive nutrients can synergistically increase the impacts of thermal stress and exacerbate coral bleaching, further underscoring the importance of maintaining stable nutrient levels, especially as ocean temperatures continue to rise due to climate change (Wooldridge 2009; Cunning & Baker 2013; Morris et al., 2019; Done & Wooldridge 2013; D’Angelo & Wiedenmann 2019; Rädecker et al., 2021). Yet, past studies have often focused on the extreme effects of nutrients and a fine-scale, naturally representative regime has not been thoroughly investigated, necessitating further research to better inform management strategies amid this urgent reef crisis. Therefore, my dissertation research seeks to elucidate the complex role of nutrients in scleractinian coral biology, specifically focusing on:I. Exploring changes in nutrient effects on thermal bleaching across time by comparing thermal responses of corals with simulated nutrient conditions from the 1970’s until present. II. Investigating the synergistic effects between the naturally occurring enrichment levels and thermal stress to determine threshold concentrations of ambiguous aspects of the nutrient and coral relationships under thermal stress. III. Testing the effects of mild nutrient enrichment of different nutrient types during postbleaching coral recovery. This research addresses critical aspects of reef stability that are often overshadowed by temperature stress, the impact of changing nutrient regimes in a warming climate. Clarifying the long-debated role of nutrients, which are essential yet potentially harmful, will contribute significantly to our understanding of coral biology and inform conservation efforts aimed at sustaining coral reef ecosystems in the face of ongoing environmental changes.