Margaret Tivey lectures on how conservative and non-conservative processes shape ocean chemistry, part of MIT's Marine Chemistry course (12.742, Fall 2006). The session examines the three main pathways by which fresh water and dissolved material enter the sea: river and estuarine mixing, subsurface groundwater discharge, and hydrothermal vent fluids at mid-ocean ridges. Tivey works through how salinity gradients and mixing curves reveal whether an element behaves conservatively, diluting in simple proportion to salinity, or non-conservatively, being added or removed through reactions, biological uptake, or precipitation. Estuarine mixing zones, coastal groundwater seepage, and vent fluid chemistry each get treated as case studies for detecting these departures from a straight mixing line. Running 84 minutes, the lecture is aimed at students who already have basic oceanography and chemistry background, and it stays close to data and graphs rather than broad narrative, building the analytical toolkit for interpreting real seawater composition.