Chlorophyll in Water and Bottom Sediments as an Indicator of the Trophic State of Busse Lagoon (Sakhalin Island)

New associated data on the content of chlorophyll a and its derivatives in the water and bottom sediments of Busse Lagoon (island of Sakhalin) have been obtained. Chlorophyll a and chlorophyll degradation products (Chl a + Ph) in the water column are represented predominantly by the active form (61.4 ± 1.1%) and in bottom sediments by a product of its degradation (84.7 ± 2.5%). The connection between pigments in water and bottom sediments, as well as the abiotic conditions of primary production, has been revealed. For the first time—for a unique reservoir characterized by the overgrowth of aquatic vegetation, silt accumulation, and the regular death of aquatic organisms—the similarity of the average annual sedimentation rate in the lagoon with the ratio of pelagic and benthic pigments was shown. The prevailing importance of the biotic factor in the formation of trophic conditions has been established. Based on the average content of Chl a + Ph in water (4.1 ± 0.8 mg/dm3) and in bottom sediments (13.5 ± 4.0 μg/g DS), Busse Lagoon is a mesotrophic reservoir. The trophic state of the benthic zone in 2013–2021 is preserved, remaining oligotrophic in the shallow coastal zone and mesotrophic in the central part of the lagoon.