There are a number of sensors ...

There are a number of sensors available that can detect a suite of different parameters (and combinations thereof - pH, O2, Conductivity, Salinity, etc.). Further, many companies provide several telemetry options based on the frequency requirements of both the measurements and the site visits. Part of the decision would be looking at what level of accuracy you require for the data. For easy to use and relatively good accuracy for example, http://www.onsetcomp.com/products.