Consequences of kelp forest community composition shifts for ecosystem function

Kelp forests underpin the productivity and structure of temperate marine ecosystems, yet these habitats are increasingly threatened by climate-driven stressors, sedimentation, and shifts in species composition. Because the vast majority of kelp production enters food webs through grazing and detrital pathways, understanding how changes in macroalgal communities alter trophic interactions and ecosystem function is essential. The present thesis combined field-based isotopic analyses with controlled laboratory experiments to investigate how kelp community composition influences the feeding dynamics of a key New Zealand kelp forest grazer, the sea urchin Evechinus chloroticus, and one of its major predators, the spiny rock lobster Jasus edwardsii, and how these processes regulate the flow of kelp derived organic matter.Stable isotope analysis of Evechinus muscle tissue and stomach contents across the strong environmental gradient of Doubtful Sound, Fiordland revealed clear spatial patterns in resource use linked to shifts in macroalgal communities. Outer and mid-fjord sites dominated by Ecklonia radiata supported isotopically enriched urchins characteristic of productive kelp-based food webs, whereas freshwater-influenced inner and mid-fjord sites showed depleted δ¹⁵N values and increased reliance on estuarine species such as Ulva sp. Mixing models confirmed these patterns, demonstrating reduced contribution of E. radiata to sea urchin diets in freshwater-influenced areas, replaced by greater reliance on freshwater layer macroalgae. Despite being present in macroalgal tissues, phlorotannins were undetectable in urchin stomach contents, suggesting substantial degradation during digestion and highlighting a key role for Evechinus as a biochemical mediator that converts chemically defended kelp tissue into phenolic-depleted particulate organic matter, enhancing its accessibility to the detrital food web.Controlled grazing experiments quantified Evechinus feeding preferences among four ecologically relevant brown macroalgae, where feeding preference was inversely correlated with total phenol concentration (TPC). The low-phenolic laminarians Macrocystis pyrifera and the invasive Undaria pinnatifida were highly preferred and similarly palatable, being consumed at a mean rate of 3.00 g ± SE = 0.45 g per day and 2.98 ± 0.38 g per day, respectively. Ecklonia radiata was moderately grazed at nearly half that rate (1.54 g ± 0.26 g per day), and the high-phenolic fucoid Carpophyllum flexuosum was barely consumed (0.69 g ± 0.15 g per day). Elevated TPC in C. flexuosum during the second trial coincided with reduced grazing, suggesting chemical defence plays a role in feeding deterrence. However, limited intraspecific variation in TPC prevented isolating the independent effects of phenolic content from species identity and nutritional or morphological traits, indicating that feeding preferences in nature likely arise from a combined influence of chemical defence, nutritional quality, and availability. These findings highlight how shifts from laminarian to fucoid- or Undaria-dominated assemblages may alter both grazing pressure and the quality and timing of macroalgal detritus entering the food web.No-choice predation experiments with Jasus edwardsii showed that urchin size overwhelmingly determined predation probability, with lobsters consuming urchins almost exclusively M. pyrifera vs C. flexuosum) had no detectable effect on predation of small urchins in the size ranges tested, although long-term fucoid diets severely reduced gonad indices in larger individuals (> 80 mm TD), suggesting reduced prey quality or value in fucoid-dominated assemblages. Choice experiments showed no statistically significant lobster preference between urchins of contrasting diet histories (M. pyrifera vs Cystophora retroflexa) at the relatively small sample size used, though trends toward greater consumption of Macrocystis-fed urchins (chosen 62.5% of the time) warrants further study with increased statistical power. Collectively, these results indicate that kelp forest degradation or shifts toward high-TPC fucoids may not strongly directly alter predation on juvenile urchins but could reduce prey value for predators, weakening trophic support to higher levels. Overall, the present thesis demonstrates that changes in kelp community composition influence grazer feeding behavior and the flow of macroalgal derived organic matter to multiple trophic levels.