Vertical dynamics of suspended particulate matter and chlorophyll-a in a well-mixed coastal turbid system

The particulate organic carbon (POC) content of suspended particulate matter (SPM) gradually increases along a horizontal gradient from high-turbidity to low-turbidity waters, reflecting a characteristic feature of SPM dynamics. Here, we investigate whether this gradual POC enrichment is also evident in the vertical in a turbid, shallow, well-mixed water column (MOW1), where variation in hydrodynamic forcing and phytoplankton production is likely to differ markedly from the cross-shore transects. Combining SPM, POC, and chlorophyll-a (Chla) data collected from surface and bottom layers at MOW1 during multiple tidal cycles between 2019 and 2022, we observe relatively higher POC and Chla content in surface waters, beginning in spring. During this season we observe a larger uncertainty in the Chla-SPM regression analysis, indicating a weaker association between the two, which contrasts winter conditions where the association is better. Monthly vertical profiles of SPM, constructed using optical backscattering sensor data, and Chla profiles from water sample data indicate that a significant fraction of phytoplankton is floc-attached and that free phytoplankton type predominates only during the growing period. Our findings suggest that, similar to the horizontal gradients in POC content of SPM, differential settling amongst various components of SPM seems to contribute to the compositional gradient within the vertical water column. Consequently, free phytoplankton cells may be able to prolong residence in the photic depths, particularly advantageous during the early stages of phytoplankton blooms when SPM concentrations remain high.