Tracing the quarter-diurnal signatures of nutrients and dissolved organic matter to evaluate their nonconservative behaviors in coastal seawaters

In June 2009, six consecutive observations with duration times of 13 h or 26 h were performed at the sea surface of outer Hangzhou Bay in the East China Sea. Physical (current direction and velocity), hydrological (salinity, suspended particulate matter (SPM) turbidity, total volume and mean size of suspended flocs), and chemical tracer (nutrients, chromophoric dissolved organic matter, and dissolved organic nitrogen) data were collected. Harmonic analysis that included diurnal, semidiurnal, and quarter-diurnal constituents was conducted on time series data for all of the parameters measured. Spectral analysis was performed to determine whether the three harmonics could characterize the variations observed in the data. The amplitude ratio of quarter-diurnal harmonic to the sum of diurnal and semidiurnal harmonics was expected to quantify the nonconservative extent of the chemical tracers. This ratio generally was higher in turbid seawater, whereas the lowest ratio occurred at a station where the seawater displayed zero turbidity throughout the experiment. The quarter-diurnal phase lags between SPM and chemical tracers were believed to provide information about how SPM and chemicals interact. How field processes such as resuspension and flocculation/disaggregation affected the kinetics of the chemical tracers also was explored by plotting the quarter-diurnal phase spectra along the 32 size classes obtained from Laser In Situ Scattering and Transmissometry measurements. This study provided an interesting and potentially useful technique to estimate the nonconservative behaviors of biogenic elements and to study the relationships between the fields of physical oceanography and biogeochemistry in estuarine and coastal seawaters.