Harmonic analysis of suspended particulate matter in the Menai Strait (UK)

An application of harmonic analysis is presented to estimate M2 and M4 tidal constituents of various suspended particulate matter (SPM) fractions in a marine ecosystem. The data were collected on two cruises in August and September 2004. In both sets of data there appeared to be clearly identifiable advection (i.e. semidiurnal increases in concentrations), resuspension (i.e. quarterdiurnal increases in concentrations), and flocculation (e.g. increases in median diameter at slack waters) signals. In most cases, the simulations on M2 and M4 components (estimated using harmonic analysis) explained a considerable proportion of variance (typical r2 is 0.4–0.65). However, some of the variables examined (e.g. median diameter in deeper layers) could not be modelled satisfactorily using the method chosen. In August, the amplitude of the M4 component was invariably lower than of the M2 component, thus indicating that advection signal dominated the overall dynamics observed. In September, however, this pattern was reversed for small and medium-sized particle diameters, thus indicating that resuspension for this fractions was prevalent. Another remarkable difference between the two data sets is the considerable difference between the estimated advection phases, which were, in general (and variable depending on the specific fraction), approximately 8 and 5 h after LW in August and September, respectively. These differences might have been partly related to the differences in the exact sampling locations, seasonal changes between the two cruises, local processes in the Menai Strait, and/or preceding meteorological conditions. The results are discussed in terms of their importance for the Menai Strait biota.