Giant kelp-associated variation in coastal seawater chemistry across contrasting sites in Chile and Tasmania

BACKGROUND AND AIMS: Widespread shifts in seawater chemistry are occurring across spatial and temporal scales, with important consequences for coastal ecosystems. Giant kelp (Macrocystis pyrifera) forests elevate seawater pH and dissolved oxygen (DO) through photosynthesis, potentially providing short-term refugia from ocean acidification and deoxygenation. However, whether these effects persist across contrasting environmental settings remains unclear. Here, we assess how biological and oceanographic conditions regulate giant kelp-mediated modification of seawater chemistry across multiple sites. METHODOLOGY: Hourly measurements of seawater pH, DO, and temperature were collected during spring-summer 2022-2023 using paired deployments inside and outside giant kelp forests at one site in central Chile and four sites in Tasmania, Australia. The influence of giant kelp density was also evaluated at two sites in southern Chile and three sites in Tasmania. An upwelling index was calculated for the central Chile site to assess the influence of regional oceanographic forcing on kelp-associated seawater chemistry patterns. KEY FINDINGS: Hourly pH and DO were higher inside giant kelp forests than outside at the central Chile site and at one Tasmanian site. At these locations, stronger daytime pH-DO relationships indicated that photosynthetic carbon uptake exceeded nighttime respiration, generating a net positive metabolic signal. In Tasmania, giant kelp density was positively associated with hourly pH and DO, whereas no such relationship was detected in southern Chile. At the central Chile site, kelp-associated effects intensified during a strong upwelling event, reducing the severity of low pH and DO conditions. CONCLUSIONS: Giant kelp forests can locally buffer short-term fluctuations in seawater pH and DO, but this capacity is highly site dependent and influenced by giant kelp density and environmental conditions. Overall, our findings suggest that continued giant kelp forests loss of in Tasmania may reduce their potential to provide short-term refugia, while in Chile, the strength of kelp-mediated seawater chemistry modification is likely to remain strongly influenced by variability in upwelling and freshwater inputs.