Scott Doney teaches this session of MIT's 12.742 Marine Chemistry (Fall 2006), covering how oceanographers quantify the fluxes and rates behind biogeochemical cycling. The lecture centers on geochemical signatures left by organic matter remineralization in seawater and explains how scientists use chemical tracers, rather than direct biological sampling, to infer respiration rates and nutrient cycling in the ocean. Doney works through the logic connecting measured tracer distributions to underlying biological processes, showing how nutrient and oxygen data can be turned into quantitative estimates of organic matter breakdown. The seventy-one minute session assumes familiarity with earlier course material on ocean chemistry and is aimed at students building toward independent research in marine biogeochemistry. It is one lecture from a selective set of recorded sessions from the course, released through MIT OpenCourseWare.