Three-dimensional coupling between size-fractionated chlorophyll-a, POC and physical processes in the Taiwan Strait in summer

Abstract

Coastal water masses are inherently diverse, often with multiple water sources of different origins. Knowledge of characteristic salinity and temperature fingerprints can be very useful to differentiate between component water masses. For example, summertime river plumes carry relatively warm and less saline water into coastal waters, along with nutrients, clastic and organic sediments, and terrestrial-sourced biogenic particles. In an upwelling regime, relatively colder water carries marine-sourced nutrients and particles in water masses from deeper depths offshore. These two regimes are subject to biological-physical coupling that affects their size-differentiated particulate chlorophyll-a and organic carbon contents, and we show here how both regimes can be identified in these terms in the Taiwan Strait in summer. A close examination shows that microphytoplankton is the major contributor to chlorophyll-a concentrations in both regimes at all depths, and picophytoplankton is a minor contributor in the subsurface cold-water regime. The contribution from nanophytoplankton is relatively insignificant. Shipboard hydrographic profiling and water sampling was conducted over 28 h at a fixed location off the mouth of the Minjiang River in the northern Taiwan Strait, where the river plume and upwelling regimes overlap. Water samples were filtered onboard using a nested filtration system to separate suspended particles into >153, 63–153, 10–63, and 0.7–10 μm size classes. Our findings show that in the surface nepheloid layer, the river plume accounted for the physical process affecting large (>10 μm) terrestrial-sourced non-phytoplankton POC (organic debris), with an average C/N ratio of 92.2. The Upwelling regime accounted for the physical process affecting marine-sourced POC <10 μm containing fluorescence. In the intermediate nepheloid layer, which coincided with the subsurface chlorophyll maximum layer, most large (>10 μm) POC that contained fluorescence were either upwelling- or offshore-sourced, with an average C/N ratio of 8.5. In the benthic nepheloid layer, terrestrial- and marine-sourced POC containing fluorescence of all sizes were associated with river plume and upwelling regimes. Large (>63 μm) non-phytoplankton organic particles were also present, with an average C/N ratio of 10.2. Both spatial and temporal datasets point to strong three-dimensional coupling between biogenic particles and physical processes. The river plume and upwelling regimes further differentiate size-fractionated chlorophyll-a and POC.