Complex sentence: Biodiversity is higher at the Barents Sea site than the upper Kandalaksha, mostly due to long periods of ice cover and lower salinity, but the biomass of algae and invertebrates is much higher.
Split into simpler sentences: Biodiversity is higher at the Barents Sea site than the upper Kandalaksha, mostly due to long periods of ice cover and lower salinity at the southern location. But the biomass of algae and invertebrates is much higher.