diff --git a/backend/migrations/Version20260408000000.php b/backend/migrations/Version20260408000000.php
new file mode 100644
index 0000000..eede796
--- /dev/null
+++ b/backend/migrations/Version20260408000000.php
@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260408000000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add source_observation_id to impact_score for nearest-observation scoring';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE impact_score ADD COLUMN source_observation_id UUID NULL');
+        $this->addSql('ALTER TABLE impact_score ADD CONSTRAINT fk_impact_score_observation
+            FOREIGN KEY (source_observation_id) REFERENCES sargassum_observation(id) ON DELETE SET NULL');
+        $this->addSql('CREATE INDEX idx_impact_score_source_obs ON impact_score (source_observation_id)');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('DROP INDEX idx_impact_score_source_obs');
+        $this->addSql('ALTER TABLE impact_score DROP CONSTRAINT fk_impact_score_observation');
+        $this->addSql('ALTER TABLE impact_score DROP COLUMN source_observation_id');
+    }
+}
diff --git a/backend/src/Entity/ImpactScore.php b/backend/src/Entity/ImpactScore.php
index cfca589..ebdf634 100644
--- a/backend/src/Entity/ImpactScore.php
+++ b/backend/src/Entity/ImpactScore.php
@@ -31,6 +31,11 @@ class ImpactScore
     #[ORM\JoinColumn(nullable: false)]
     private CoastalPoint $coastalPoint;
 
+    /** Observation source ayant produit ce score — permet de trier par distance observation→spot. */
+    #[ORM\ManyToOne]
+    #[ORM\JoinColumn(nullable: true, onDelete: 'SET NULL')]
+    private ?SargassumObservation $sourceObservation = null;
+
     #[ORM\Column]
     private \DateTimeImmutable $timestamp;
 
@@ -80,4 +85,6 @@ class ImpactScore
     public function setTrend(?string $trend): static { $this->trend = $trend; return $this; }
     public function getEtaHours(): ?int { return $this->etaHours; }
     public function setEtaHours(?int $etaHours): static { $this->etaHours = $etaHours; return $this; }
+    public function getSourceObservation(): ?SargassumObservation { return $this->sourceObservation; }
+    public function setSourceObservation(?SargassumObservation $obs): static { $this->sourceObservation = $obs; return $this; }
 }
diff --git a/backend/src/Service/Forecast/ImpactScoreService.php b/backend/src/Service/Forecast/ImpactScoreService.php
index d6c6ee4..d26645b 100644
--- a/backend/src/Service/Forecast/ImpactScoreService.php
+++ b/backend/src/Service/Forecast/ImpactScoreService.php
@@ -77,7 +77,7 @@ class ImpactScoreService
                 }
 
                 $score = $this->computeScore($spot, $obsId, $forecasts);
-                $this->persistScore($spot, $score);
+                $this->persistScore($spot, $score, $observation);
                 $this->invalidateCache($spotId);
 
                 // Alerte push si score passe en "high"
@@ -264,28 +264,11 @@ class ImpactScoreService
             : null;
     }
 
-    /** @param array{score: int, level: string, distance: float|null, density: float, trend: string, etaHours: int|null} $data */
-    private function persistScore(CoastalPoint $spot, array $data): void
+    /**
+     * @param array{score: int, level: string, distance: float|null, density: float, trend: string, etaHours: int|null} $data
+     */
+    private function persistScore(CoastalPoint $spot, array $data, SargassumObservation $observation): void
     {
-        // "L'observation la plus proche gagne" : ne pas écraser un score avec une distance
-        // plus grande que le score actuel, sauf si celui-ci a plus de 6h (stale).
-        if ($data['distance'] !== null) {
-            $current = $this->connection->fetchAssociative(
-                'SELECT distance_to_nearest_sargassum AS dist, timestamp
-                 FROM impact_score WHERE coastal_point_id = :id ORDER BY timestamp DESC LIMIT 1',
-                ['id' => (string) $spot->getId()]
-            );
-            if ($current !== false && $current['dist'] !== null) {
-                $ageSeconds = (new \DateTimeImmutable())->getTimestamp()
-                            - (new \DateTimeImmutable($current['timestamp']))->getTimestamp();
-                $isStale    = $ageSeconds > 21_600; // 6h
-                $isCloser   = $data['distance'] < (float) $current['dist'];
-                if (!$isStale && !$isCloser) {
-                    return; // garder le score de l'observation plus proche
-                }
-            }
-        }
-
         $s = new ImpactScore();
         $s->setCoastalPoint($spot);
         $s->setScore($data['score']);
@@ -294,6 +277,7 @@ class ImpactScoreService
         $s->setDensityEstimate($data['density']);
         $s->setTrend($data['trend']);
         $s->setEtaHours($data['etaHours']);
+        $s->setSourceObservation($observation);
 
         $this->em->persist($s);
     }
@@ -301,12 +285,25 @@ class ImpactScoreService
     /** @return array{value: int, level: string, distance: float|null, density: float|null, trend: string|null, etaHours: int|null, computedAt: string}|null */
     private function loadLatestScore(string $spotId): ?array
     {
+        // Parmi tous les scores récents (7j), prendre celui dont l'observation source
+        // est la plus proche du spot — déterministe, sans garde arbitraire.
+        // Les anciens scores sans source_observation_id sont conservés en fallback.
         $row = $this->connection->fetchAssociative(
-            'SELECT score, level, distance_to_nearest_sargassum AS distance,
-                    density_estimate AS density, trend, eta_hours, timestamp AS computed_at
-             FROM impact_score
-             WHERE coastal_point_id = :spotId
-             ORDER BY timestamp DESC LIMIT 1',
+            'SELECT ist.score, ist.level,
+                    ist.distance_to_nearest_sargassum AS distance,
+                    ist.density_estimate AS density, ist.trend, ist.eta_hours,
+                    ist.timestamp AS computed_at
+             FROM impact_score ist
+             LEFT JOIN sargassum_observation o ON o.id = ist.source_observation_id
+             JOIN coastal_point cp             ON cp.id = :spotId
+             WHERE ist.coastal_point_id = :spotId
+               AND (o.detected_at IS NULL OR o.detected_at > NOW() - INTERVAL \'7 days\')
+             ORDER BY
+                 CASE WHEN ist.source_observation_id IS NOT NULL
+                      THEN ST_Distance(o.geometry::geography, cp.geometry::geography)
+                      ELSE 999999999.0 END ASC,
+                 ist.timestamp DESC
+             LIMIT 1',
             ['spotId' => $spotId]
         );