Driving Kelp Forest Restoration Success by Optimising Substrate and pH in Green Gravel Production

ABSTRACT Kelp forests are foundational marine ecosystems that support biodiversity and structure coastal communities yet are declining globally due to anthropogenic stressors and climate change, thereby driving the need for scalable restoration tools. The effects of settlement substrate and seawater pH on the early performance of the warm‐temperate kelp Laminaria ochroleuca were evaluated within a green gravel restoration framework. Two substrate types differing in mineralogy, carbonate (limestone) versus silicate (granite), were compared alongside two pH treatments. Kelp recruits' growth and tensile strength were assessed on limestone versus granite, whereas growth and photosynthetic performance were evaluated under two seawater pH levels (7.6 and 8.3). Limestone increased tensile strength by 50% and blade length by ~15% relative to granite, confirming the critical role of substrate for secure attachment in high‐energy reefs. A slightly alkaline medium (pH 8.3) enhanced these effects by increasing the maximum quantum yield of photosystem II (F v /F m ) and further promoting blade elongation, whereas other parameters (α = initial slope of the light‐response curve and rETR max = relative photosynthetic electron transport chain) remained unchanged across treatments. The results challenge the assumption that mild acidification universally benefits kelp through added CO 2 ; instead, they indicate that the energetic costs of acid stress can outweigh carbon advantages during the nursery phase. By demonstrating a clear synergy between limestone substrate and slightly alkaline seawater, the study provides a simple, low‐cost solution for hatcheries aiming to mass‐produce robust plantlets of L. ochroleuca for reef‐scale restoration. These findings provide actionable guidance for practitioners, directly supporting the conservation and recovery of kelp forests and the ecosystem services they underpin in the face of accelerating environmental change.