**2. New water quality sensors are helping to democratize access to data**

A variety of water quality sensors are commonly used for both in situ continuous monitoring as well as sample-based testing. Off-the-shelf sensors are readily available to measure pH, dissolved oxygen (DO), conductivity (often used to measure salinity), turbidity, temperature, chlorine, various dissolved ions (including fluoride, ammonia, silver, nitrate, and nitrite that are often by-products of fertilizers), and total organic carbon (TOC). Water quality assessments include the identification of coloured dissolved organic matter (CDOM), Secchi disk depth (SDD), turbidity, total suspended sediments (TSS), water temperature (WT), total phosphorus (TP), sea surface salinity (SSS), dissolved oxygen (DO), biochemical oxygen demand (BOD), and chemical oxygen demand (COD). State-of-the-art fluorimeter solutions are not ideally designed for in situ long-term use. This is primarily because biofouling and baseline drift attenuate and confuse the signal [13]. At present, there are no viable *in-situ* electronic sensors for monitoring the microbial contamination of drinking water for example.

This chapter briefly outlines the rationale behind and development of five innovative sensors for water quality parameters—for monitoring catchment (watershed) level to the sea and addressing a range of current and emerging challenges in terms of target analytes.

Those innovations briefly addressed in this chapter are as follows:

