Why measure CO2 in water due ...

Why measure CO2 in water due to photosynthetic activity of plants ?

Plants consume CO2 from air not from water.

CO2 dissolves into water from air and may be released back into air when there is change in conditions - temperature, pressure, etc.

So why do you want to measure the CO2 in water in relation to photosynthetic activity of plants ?

I presume that 'plants' mean plants growing on land and not aquatic plants.

Aquatic plants and phytoplankton consume the dissolved CO2 but more CO2 from air dissolves into the water to replace the CO2 consumed. So how would measuring the CO2 from time to time help ?

Carbon footprint would refer to the Carbon from Antropogenic sources ( in fuel, fertilizer, etc.) and not CO2 naturally dissolved in water.