Two long-term environmental monitoring plastic optical fibre (POF) sensors are described, using light intensity modulation. An evanescent field biofouling sensor was developed and characterised with sensitivity ±0.007 refractive index units or 0.5\% below n=1.4 and ±0.002 refractive index units or 0.15\% above n=1.4. A tapered POF was developed and tested as a strain sensor, which demonstrated a significant proportion of excellent linearity and exhibited good agreement with measured strain values. The POF strain sensor appeared to be well-behaved in that it showed little data scatter and was able to monitor strains up to 1.4\% without failure. These two applications clearly demonstrate the potential POF sensors have for cheap, on-line continuous environmental monitoring.