Abstract
Abiotic stressors in the water column are a well-documented driver of kelp forest declines in temperate coastal ecosystems worldwide, yet the effect of these stressors on microbial communities within kelp forests is not well understood. Heterotrophic microbes in the water column underpin biogeochemical cycling in the global ocean, metabolizing dissolved organic carbon (DOC) and reintroducing it into the marine foodweb. This study focused on the effects of abiotic stressors on kelp forest microbial communities, with particular regard to hydrogen peroxide (H2O2), a well-documented driver of oxidative stress. The aim of this thesis was to examine the dynamics of H2O2 in a coastal ecosystem; the effects on kelp forest microbial communities; and to determine other abiotic drivers of heterotrophic bacterial production in coastal waters. H2O2 is formed through both photochemical degradation of dissolved organic matter in the water column and biotic processes. H2O2 dynamics within a coastal kelp forest in Otago, New Zealand were highly variable, with no clear seasonal trends or depth stratification as seen in many other studies on oceanic H2O2 dynamics. Statistical modelling revealed that H2O2 variation was driven primarily by changes to the light environment, as well as salinity. Changes to H2O2 concentrations observed in this study are likely a reflection of the highly variable abiotic environment within this kelp forest ecosystem. Characterising drivers of H2O2 dynamics in coastal waters is important for understanding changes to the abiotic stress environment and how this may affect organisms within these ecosystems. Seasonal changes in H2O2 concentrations and heterotrophic bacterial production rates within macroalgal habitat appear to be driven by habitat-forming primary producers. Through a combination of direct photosynthetic release of H2O2 and exudation of DOC into the water column, the macroalgae Macrocystis pyrifera and Ulva spp. directly modulate H2O2 concentrations in situ. Photochemical degradation of DOC in the water column is a key abiotic source of H2O2 formation, and DOC release by algae is likely driving higher rates of H2O2 production within algal habitat. Despite documented inhibition of heterotrophic bacterial production in response to H2O2 exposure, bacterial production rates were coupled with H2O2 and DOC dynamics, indicating that DOC provision by algae is driving bacterial production in waters entrained within algal habitat, ameliorating negative effects of H2O2 exposure. Through DOC release, macroalgae also appear to be driving microbial community structure, diversity, and carbon uptake strategies, with genetically distinct microbial communities observed in Ulva spp. and M. pyrifera habitat compared to pelagic habitat, and higher uptake rates of some carbon substrates within macroalgal habitat, compared to pelagic habitat. Kelp forest microbial communities were observed to be remarkably resilient to H2O2 and temperature stress. The bacterial production rates of M. pyrifera biofilm and planktonic communities did not significantly decrease under increasing H2O2 stress, and biofilm production rates were highly variable. Increased temperature stress resulted in higher bacterial production rates in planktonic communities regardless of H2O2 concentrations, while in biofilm communities, no temperature effect was observed. Increasing H2O2 initially correlated with increased bacterial production rates in biofilm communities across temperature regimes, but at a higher concentration inhibited bacterial production, indicating that a threshold H2O2 level had been passed for these communities. The mechanisms that control these changes to bacterial communities remain unclear, with numerous opportunities for future research. Optimal heterotrophic bacterial production rates occurred within a narrow window in the light environment in a fjord system. Regardless of whether bacterial communities were sampled from open waters or within a macroalgal canopy, statistical modelling revealed that increasing light levels resulted in increases in bacterial production rates up to a level of ~200 µmol photons m-2 s-1, and declining rates of bacterial production beyond this threshold. This indicates that increasing light levels stimulates high levels of primary production, culminating in the release of DOC into the water column, and stimulating bacterial production, but at very high light levels bacterial production is inhibited by environmental parameters. This thesis addresses some of the unknown interactions between kelp forests and their associated bacterial communities, characterising some of the key drivers of bacterial production in kelp forest habitats. Findings from this work have important implications in regard to future changes to kelp forest systems, and highlight the need for further research to better understand mechanisms of change to kelp forest microbial communities.