Abstract
The management and creation of Marine Protected Areas (MPAs) is currently under great focus, with international organisations aiming to protect 30% of our oceans by 2030. The success of MPAs depends on a nuanced understanding of local ecological dynamics and threats, which can significantly influence ecosystem balance. Herbivory can be a stressor for foundation species, namely kelp forests, contributing to their decline in several regions of the globe. However, the dynamics inherent to herbivory and MPA's implementation are still poorly understood. Here, the impact of protection status, depth, kelp species, and grazer type on herbivory (occurrence, rate, and grazer frequency) was assessed through a comprehensive experimental approach involving tethering experiments and faunal characterisation of macro-herbivores. The research was conducted in habitats off the central coast of Portugal: Peniche (PEN) and the MPA Berlengas Archipelago (MPA-BER). Our findings revealed that herbivory occurrence and rate are higher within the MPA, especially at greater depths. Instead of urchins, fish are the significant contributors to kelp consumption, showing a preference for the kelp S. polyschides. Results provide the first experimental evidence in the Atlantic region identifying fish as the dominant herbivores driving increased kelp biomass loss, a relationship potentially magnified by MPA implementation. Hence, protection status may not benefit all ecosystem components, enhancing the need for robust MPA management to balance trophic interactions and support biodiversity and ecosystem resilience.