Temperature and salinity both affect the density of seawater.
Temperature decreases density while salinity increases it.
Solar heating and evaporative cooling affect temperature while fresh water input from rain and rivers and evaporation affect salinity. The chemical and biological parameters can have complex variability. Oxygen is often supersaturated in the surface as phytoplankton photosynthesize but can decrease dramatically subsurface as organic material is respired. Particle attenuation is the inverse of water clarity. Chlorophyll is one means of assessing phytoplankton biomass.