We examined the dynamics of the carbonate system in a complex mixing scheme with enhanced biological consumption modulated by both a river plume and summer coastal upwelling in a large shelf system, the northern South China Sea (NSCS) shelf. The plume waters originated from a large flooding upstream the Pearl River, and extended from the mouth of the Pearl River estuary to the middle shelf and were characterized by low dissolved inorganic carbon (DIC) and total alkalinity (TAlk), and a high aragonite saturation state (Ωarag). In contrast, the upwelled water occupying the nearshore area was distinguished by high DIC and TAlk and a low Ωarag. While the dynamics of the carbonate system were largely shaped by physical mixing through plume and upwelling processes between the plume water, the offshore subsurface water and the offshore surface water, biological consumption of DIC was observable in both the river plume and the coastal upwelling areas and contributed to the elevated Ωarag during their pathway. Correlations between salinity normalized TAlk and DIC indicated that organic carbon production rather than biocalcification exclusively induced the DIC removal. By using a three end‐member mixing model, we estimated the net community productivity in the plume water and the upwelled water to be 36 ± 19 mmol C m−2 d−1 and 23 ± 26 mmol C m−2 d−1, respectively. With the combination of stoichiometric relationship analysis of the carbonate system and applying the three end‐member mixing model, we successfully differentiated semiquantitatively the biologically mediated DIC variations from its overall mixing control. We also attempted to link this natural process to the carbonate saturation on the NSCS shelf, contending that at present natural factors associated with the river plume and the coastal upwelling largely modulate the dynamics of the carbonate system on the NSCS shelf, whereas anthropogenic stressors such as ocean acidification currently play a relatively minor role.

The physiological response to individual and combined stressors of elevated temperature and pCO2 were measured over a 24-day period in four Pacific corals and their respective symbionts (Acropora millepora/Symbiodinium C21a, Pocillopora damicornis/Symbiodinium C1c-d-t, Montipora monasteriata/Symbiodinium C15, and Turbinaria reniformis/Symbiodinium trenchii). Multivariate analyses indicated that elevated temperature played a greater role in altering physiological response, with the greatest degree of change occurring within M. monasteriata and T. reniformis. Algal cellular volume, protein, and lipid content all increased for M. monasteriata. Likewise, S. trenchii volume and protein content in T. reniformis also increased with temperature. Despite decreases in maximal photochemical efficiency, few changes in biochemical composition (i.e. lipids, proteins, and carbohydrates) or cellular volume occurred at high temperature in the two thermally sensitive symbionts C21a and C1c-d-t. Intracellular carbonic anhydrase transcript abundance increased with temperature in A. millepora but not in P. damicornis, possibly reflecting differences in host mitigated carbon supply during thermal stress. Importantly, our results show that the host and symbiont response to climate change differs considerably across species and that greater physiological plasticity in response to elevated temperature may be an important strategy distinguishing thermally tolerant vs. thermally sensitive species.

Abstract. The East China Sea (ECS) and the South China Sea (SCS) are two major marginal seas of the North Pacific with distinct seasonal variations of primary productivity. Based upon field observations covering both the ECS and the northern SCS (NSCS) during December 2008-January 2009, we examined southward long-range transport of nutrients from the ECS to the northeastern SCS (NESCS) carried by the China Coastal Current (CCC) driven by the prevailing northeast monsoon in wintertime. These escaped nutrients from the ECS shelf, where primary production (PP) was limited in winter, might however refuel the PP on the NESCS shelf at lower latitude, where the water temperature remained favorable, but river-sourced nutrients were limited. By combining the field observation of nitrate+nitrite (NO 3 +NO 2 , DIN) with our best estimate of volume transport of the CCC, we derived a first-order estimate for DIN flux of 1430 ± 1024 mol s −1 . Under the assumption that DIN was the limiting nutrient, such southward DIN transport would have stimulated 8.84 ± 6.33 × 10 11 gC of new production (NP), accounting for 33-74 % of the NP or 14-22 % of PP in winter on the NESCS shelf shallower than 100 m.

Thraustochydrids has been known for their ubiquitous distribution in the ocean. However, a few efforts have been made to investigate their ecology. In this study, we have applied molecular method, acriflavine direct detection, and classical oceanographic methods to investigate the abundance and diversity of thraustochytrids in the North Pacific subtropical gyre. Our results revealed interesting temporal and spatial variations of their population. Out of three seasons (spring, summer, and fall), cruise Hawaii Ocean Time-series (HOT)-216 during November 2009 obtained the highest abundance of thraustochytrids ranging from 1,890 (Station S1C1, 45 m) to 630,000 (Station S2C12, 100 m) cells L(-1) of seawater, which accounted for a 0.79 to 281.0 % biomass ratio to that of bacteria in terms of gram carbon per liter. A patchy distribution of these organisms was widely observed in the water column and they were somehow related to the maximum chlorophyll layers. A total of 25 operational taxonomic units (OTUs) from cruise HOT-216 formed four phylogroups in the specific labyrinthulomycetes 18S rRNA-based phylogenetic tree, with the largest group of 20 OTUs fell into the Aplanochytrium cluster and the others aligned with uncultured clones or none, thus appeared to be undescribed. This study indicates the presence of new thraustochytrids lineages and their quantitative importance in the marine water column.