Data transcribed from OP
df <- data.frame(CTD = c(1, 1, 1, 2, 2),
Depth = c(17.78, 3.89, 1.44, 13.23, 1.34),
Temperature = c(8.28, 8.31, 8.49, 8.25, 8.31),
Salinity = c(31.9, 31.39, 30.45, 32.61, 29.11),
Silicate = c (17.19643, 19.51786, 24.07143, 14.78571, 27.64286))