Phytoplankton and Bacterial Responses to Monsoon-Driven Water Masses Mixing in the Kuroshio Off the East Coast of Taiwan

Abstract

Biogeochemical responses to mixing were examined in two cruise surveys along a transect across the Kuroshio Current (KC) in May and July 2020. Two stations located at the South China Sea (SCS)–KC mixing and the KC waters were chosen for the diel study. In the euphotic zone (~100 m depth), the average values of nitrate (0.97–1.62 μM), chlorophyll-a (Chl-a, 0.36–0.40 mg/m3), and primary production (PP; 3.46 ± 1.37 mgC/m3/day) of the mixing water station (MWS) of the two cruises were several folds higher than those of the KC station (KCS; nitrate, 0.03–0.10 μM; Chl-a, 0.14–0.24 mg/m3; and PP, 0.91 ± 0.47 mgC/m3/day). In the July cruise, the maximal bacterial production (BP) at the MWS (3.31 mgC/m3/day) was 82% higher in comparison with that of the KCS (1.82 mgC/m3/day); and the readings of Chl-a showed no trend with BP in the oligotrophic KCS, but a positive relationship was found among these measurements at the mesotrophic MWS. This implies that the trophic status of the system might affect phytoplankton–bacteria interactions. The backward-trajectory analyses conducted by an observation-validated three-dimensional model identified that the prevailing southwest monsoon drove a northeastward “intrusion” of the SCS waters in July 2020, resulted in mixing between SCS and Kuroshio (KC) waters off the east coast of southern Taiwan. For the first time, this study demonstrates that the high biological biomass and activities that occur in the KC are induced by the northward intrusion of the SCS waters.

Effects of the Coastal Uplift on the Kuroshio Ecosystem, Eastern Taiwan, the Western Boundary Current of the North Pacific Ocean

“…The positive linear correlation of the onshore uplift area and Q 250 obtained from the six of our observations can be plausibly attributed to the aforementioned dynamics. Additionally, this coastal uplift was also found in the upstream and downstream of the KTV1 transect along the Kuroshio-influenced regions (Supplementary Figure S1; Lai et al, 2021). It suggests that this coastal uplift may occur year-round in a broad-scale region on the onshore side of Kuroshio, eastern Taiwan.…”

“…Growth of heterotrophic bacteria may be limited by substrate supply (e.g., organic carbon) and/or temperature in the shelf region and in the Kuroshio (Jiao and Yang, 2002;Shiah et al, 2003;Lai et al, 2021). As stated above, heterotrophic bacteria were significantly related to autotrophic picoplankton.…”

“…During the uplift, the cold and nutrient-rich deep waters shoal to shallow water and enhance phytoplankton growth (Rochford, 1984;Roughan and Middleton, 2002). The shoaled nutrient-rich waters may be important for biological production and plankton dynamics (e.g., picoplankton composition) in oligotrophic ecosystems, such as the Kuroshio (Kobari et al, 2020;Lai et al, 2021). Using temperature modulated dilution experiments, Liu et al (2020) have demonstrated that nutrient-dependent temperature sensitivities of picophytoplankton in the subtropical Kuroshio, one of the warmest regions of the global ocean (Wu et al, 2012); It suggests the importance of nutrient availability in evaluating the response of primary productivity to the projected ocean warming, especially in the oligotrophic ocean.…”

Viral shunt in tropical oligotrophic ocean

“…1 ). Take carbon as an example, the ocean has an inventory of dissolved organic carbon (DOC) (~660 gigaton of C) roughly equal to that of atmospheric CO 2 ( 1 , 2 ). Most oceanic DOC is derived from autochthonous food web processes, e.g., algal exudation, metazoan excretion, bacteriovory, and viral lysis ( 3 ).…”

“…Most oceanic DOC is derived from autochthonous food web processes, e.g., algal exudation, metazoan excretion, bacteriovory, and viral lysis ( 3 ). Heterotrophic bacteria are the key microorganisms responsible for DOC decomposition and constitute a critical component in the “microbial loop” ( 4 – 7 ), which emphasizes the connections between DOC, bacteria, protozoans, and the traditional grazing (phytoplankton to zooplankton to fishes) food chain ( 2 ).…”