Testing long-term trends in turbidity in the Menai Strait, North Wales

This paper examines two data sets, separated by 33 years, for evidence of long-term changes in turbidity in the Menai Strait, North Wales. The rationale for this examination is that there is evidence for a steady deterioration in water clarity in a 27-year time series (1962–1988) of Secchi depth measurements. The data sets compared here were collected in 1963 and 1964, and in 1996, and comprise measurements of sediment concentrations, both organic and inorganic, determined by traditional sampling methods. In addition, the 1996 data set includes daily measurements of total and inorganic suspended sediment load determined by a recording colour sensor, calibrated using the samples from that year. In each of the years 1963, 1964 and 1996, the suspended sediment load is observed to change with an annual cycle; annual mean suspended sediment concentrations are determined by fitting a curve to the data in each year. The result is that there has been a small increase in the suspended sediment load in the Strait between 1963 and 1996, but that this increase is not statistically significant at the 95% confidence level. The concentrations in 1996 are also significantly lower than those predicted by extrapolating the data from the 27-year time series. It is proposed that the turbidity in the Menai Strait is responding to long-term variations in wind forcing: the mean wind strength over the British Isles increased during the period 1960–1990, but then decreased during the 1990s. If this interpretation is correct, it would imply that the trends observed in the Menai Strait would have been widespread in similarly shallow water throughout north-west Europe.