Outcome measurement in blue space and depression-related research: a systematic review and exploratory methodological evidence synthesis.

Blue-space exposure has been studied in relation to mental health, but studies have often labeled non-equivalent outcomes-including depression-focused symptoms, psychological distress, mental-health functioning, diagnostic disorders, and binary self-reports-as "depression." This systematic review examined how depression-related outcomes were operationalized in studies of adult blue-space exposure and whether the available evidence supported quantitative synthesis. The review was reported in accordance with the PRISMA 2020 reporting guideline. Six databases were searched for studies published between January 2000 and March 2026, and the protocol was prospectively registered in PROSPERO. Eight observational studies involving 35,797 adults were included. Blue-space exposure definitions, exposure contrasts, outcome constructs, covariate adjustment, and effect metrics varied substantially. One study using the 15-item Geriatric Depression Scale reported an inverse adjusted association between street-view blue space and geriatric depressive symptoms, but its exposure contrast, outcome scale, population, and setting were not directly comparable with those of the other studies. Because the available estimates could not be transparently expressed on a common effect scale, no meta-analysis was performed; findings were synthesized as structured study-level evidence. Current evidence does not establish whether associations between blue-space exposure and depression-related outcomes differ by outcome-measurement category. Future studies should prespecify the target construct, select measures aligned with the proposed mechanism and study population, and report sufficient domain- or item-level information to support transparent evidence synthesis.