Anoxic chlorophyll maximum enhances local organic matter remineralization and nitrogen loss in Lake Tanganyika.

In marine and freshwater oxygen-deficient zones, the remineralization of sinking organic matter from the photic zone is central to driving nitrogen loss. Deep blooms of photosynthetic bacteria, which form the suboxic/anoxic chlorophyll maximum (ACM), widespread in aquatic ecosystems, may also contribute to the local input of organic matter. Yet, the influence of the ACM on nitrogen and carbon cycling remains poorly understood. Using a suite of stable isotope tracer experiments, we examined the transformation of nitrogen and carbon under an ACM (comprising of Chlorobiaceae and Synechococcales) and a non-ACM scenario in the anoxic zone of Lake Tanganyika. We find that the ACM hosts a tight coupling of photo/litho-autotrophic and heterotrophic processes. In particular, the ACM was a hotspot of organic matter remineralization that controlled an important supply of ammonium driving a nitrification-anammox coupling, and thereby played a key role in regulating nitrogen loss in the oxygen-deficient zone. Enigmatic blooms of phytoplankton in aquatic oxygen-deficient zones could exacerbate depletion of nitrogen. Here the authors perform stable isotope experiments on the oxygen-deficient waters of Lake Tanganyika in Africa, finding that blooms drive down fixed nitrogen and could expand as a result of climate change.