Complex sentence: Because of the rapids, the water in Pyhäjärvi is warmer and richer in ozone than that in the northern lake, Näsijärvi, which results in the life in this lake is richer, even though the water is more polluted.
Split into simpler sentences: Because of the rapids, the water in Pyhäjärvi is warmer and richer in ozone than that in the northern lake Näsijärvi. As an effect, the life in this lake is richer, but the water is also more polluted.