Complex sentence: It was surveyed for diatoms in 1984 and 1999, the changes being consistent with the pH of the lake having increased however diatoms from sediment cores indicate a decrease in tarn pH from 6.3 to 5.8.
Split into simpler sentences: It was surveyed for diatoms in 1984 and 1999, the changes being consistent with the pH of the lake having increased. However, diatoms from sediment cores indicate a decrease in tarn pH from 6.3 to 5.8.