Assessing the environmental effectiveness of the Spanish Marine Reserve Network using remote sensing

Healthy marine ecosystems provide a variety of ecosystem services crucial for human wellbeing. Effectively managed Marine Protected Areas (MPAs) are increasingly recognized to be an effective measure to protect endangered species and ensuring healthier ecosystems. This study assesses the environmental effectiveness of the Spanish Marine Reserve Network (MRN) with regard to: 1) water quality: chlorophyll-a concentrations (Chl-a), sea surface temperature (SST), and Salinity; and 2) protected species conservation: coverage and density of Posidonia oceanica (P. oceanica), using a Multiple-Paired-Before-After-Control-Impact (MPBACI) research design. Water quality and protected species indicators were compared before and after Marine Reserve (MR) designation, inside MRs and in different outer control areas of 1 km, 5 km, 10 km and equal-area buffers, for the whole MRN (Marine Reserve Network), by marine ecoregions and for some specific MRs. We used Copernicus Marine Monitoring Remote Sensing data to ascertain water quality values and validated their accuracy compared to in-situ data as well as the reliability of Chl-a concentration derived from Sentinel 2 (S2) images. Water quality results reveal significant differences in mean Chl-a and Chl-a range, mean Salinity and SST range between cases (inside MRs) and 5 km-buffer control areas for the entire MRN. Analyses by ecoregion showed no significant differences in water quality between cases and controls in the Western Mediterranean ecoregion or in the Azores, Canaries and Madeira ecoregion, whereas the Alboran Sea ecoregion MRs had higher mean Chl-a concentration and lower mean Salinity, mean SST and SST range than all controls. Results on P. oceanica beds showed a significant increase in P. oceanica density (almost twice) inside Tabarca MR (TBA) compared to outer control cases, but no significant changes in P. oceanica coverage. Validation of Remote Sensing (RS) data using in-situ measurements demonstrated significant differences for Chl-a concentration and no significant differences for SST and Salinity between both techniques. Chl-a concentration by S2 reveals statistically significant differences with in-situ data. Our findings suggest environmental effectiveness of the Spanish MRs and still limited sensitivity of open source medium resolution RS tools to assess MPA effectiveness.