Progress in Oceanography最新文献

Heterotrophic prokaryotes (HPs) represent the largest fraction of living biomass in the ocean. Comprehensively understanding the spatio-temporal variability of their controlling factors remains a challenge in microbial oceanography, especially in little explored low latitude regions such as the Red Sea, one of the hottest and saltiest basins on Earth. In this study, we assessed the vertical (5–1000 m) and latitudinal (16°-27° N) variations in HPs and their bottom-up (resource availability) and top-down controls (protistan grazing and viral lysis) at eight stations along the Red Sea, in three cruises carried out between 2017 and 2019. The decrease in HPs abundances with depth was less pronounced than that of heterotrophic nanoflagellates (HNFs) and viruses. We found that inorganic nutrient and dissolved organic carbon (DOC) concentrations do not vary significantly from north to south, thus suggesting a similar bottom-up control on HPs abundances along the latitudinal gradient. We found significant southward increase in the HP:HNF ratio (r = 0.56, p < 0.0001, n = 140), suggesting that HNFs have a lower impact on their HPs prey in the southern Red Sea. The preference of HNFs for larger HPs cells with depth was found only in the spring cruise. Viral abundances do not show any marked latitudinal gradient but show a significant positive relationship with HPs abundances in the water column in all seasons. The higher linear regression slope found in summer suggests that viruses are more important for HPs mortality in the warmer months. This study strengthens the importance of top-down controls in maintaining lower HPs stocks in the Red Sea and suggests that both latitudinal and seasonal variations have minor but measurable roles.

Despite their small spatial extent, coastal upwelling systems are an important source of oceanic nitrous oxide (N₂O) to the atmosphere. To date, hot-spot N₂O emissions have been reported for low oxygen waters of the eastern boundary upwelling systems at their tropical latitudes, but there is a limited number of studies in their “oxygenated” temperate latitudes. This is the first study of the N₂O cycle in the NW Iberian Upwelling system, where we investigated the seasonality of the N₂O concentrations and their emissions to the atmosphere, along with the spatial differences in this coastal region in response to the upwelling. Monthly observations were collected from February 2017 to July 2018, in two hydrographic sections within the Ría of Vigo and Ría of A Coruña, two coastal embayments with contrasting response to the upwelling of the Eastern North Atlantic Central Water (ENACW) in the region. N₂O concentrations ranged between 8.56 to 12.53 nmol kg⁻¹ (94–121 % of saturation) in the shelf, and from 8.62 to 17.60 nmol kg⁻¹ (94–203 % of saturation) inside the rías, with the highest N₂O concentration at the bottom, which increase as the upwelling progress from April to October. The air-sea fluxes of N₂O varied between −1.6 to 3.26 µmol m⁻² d⁻¹ in the shelf and −1.53 to 7.00 µmol m⁻² d⁻¹ inside the rías. Local differences on the ventilation and remineralization pattern drives the seasonality of N₂O and differences between Ria of Vigo and Ria of A Coruña, being the higher values of N₂O concentrations and air-sea fluxes registered in the inner Ria of Vigo. Our study reports the N₂O emissions of an upwelling system in a temperate latitude, where the upwelling waters are central waters relatively well ventilated in terms of oxygen content, behaving as a moderate low net source of N₂O to the atmosphere compared to tropical upwelling latitudes, characterised by a lower oxygen content.

We investigated the plankton community structure and biomass during the post-bloom season in the Oyashio region of the western subarctic Pacific, including pico-, nano-, microplankton and mesozooplankton. We found that the nitrate, phosphate and silicic acid concentrations remained high at >4.2 μM, >0.77 μM and >7.1 μM, respectively, in the euphotic layer at almost all sampling stations, but that the chlorophyll a concentrations were low (<3 µg Chl. a l⁻¹). These findings indicate high nutrient and low chlorophyll (HNLC)-like conditions. In the phytoplankton community, pennate diatoms, the larger subpopulation of pico-sized eukaryotic phytoplankton, and nano-flagellates substantially contributed to the low biomass of the chain-forming centric diatoms that mainly comprised the spring phytoplankton bloom. The microzooplankton biomass was 2.7–4.4 fold greater than the phytoplankton biomass in the surface layer. Naked ciliates substantially contributed to the microzooplankton community (40–87 %). The naked ciliate growth rates during our in situ bottle incubation experiments were significantly greater than the maximum growth rates as calculated from cell volume and water temperature. The mesozooplankton biomass was mainly composed of krill and copepods and was 5.9–9.3 fold higher than the microzooplankton biomass. This inverted biomass pyramid with relatively low microzooplankton and high mesozooplankton biomass may be explained by the high production and growth rates of the microzooplankton. The ratio of phytoplankton growth (µ, d⁻¹) to grazing mortality (m, d⁻¹) by microzooplankton were relatively low at 0.26–0.44 m/µ in our dilution experiments. These low values indicate that microzooplankton grazing does not regulate phytoplankton growth and suggests that microzooplankton feed on an alternative nutritional source, such as heterotrophic prey items, or mixotrophy to fulfill their growth needs. Additional research is needed during the post-bloom period to further evaluate the mechanisms that sustain microzooplankton dominance and production in the Oyashio region under the HNLC-like conditions, especially for naked ciliates.

In summer, the Madden-Julian Oscillation (MJO) greatly influences the intraseasonal variability of the South China Sea (SCS). Previous studies have revealed MJO effects on surface chlorophyll (Chl) concentration, but the impact of the MJO on the ecosystem's structure and functionality remains unexplored. Here, we investigated the marine ecosystem response to the MJO by analyzing phytoplankton pigment data collected in cruises from 2010 to 2014. The results indicated the strong influence of the MJO on the structure of phytoplankton size classes (PSCs) in the upper 50 m of the SCS basin. To further explore the ecosystem's response to MJO, we utilized a well-calibrated physical-biogeochemical model (ROMS-CoSiNE) of the SCS to conduct numerical experiments with and without MJO forcings. Our model demonstrated that MJO-induced deep mixing and upwelling increased nutrients in the upper layer, increasing the Chl concentration with a higher proportion of nanophytoplankton (15 %) and a lower proportion of picophytoplankton (−20 %). Moreover, The MJO-forced model experiment exhibited a substantial enhancement in primary production (56 %) and export production (23 %), resulting in a notable decrease in the e-ratio. This reduction in the e-ratio cannot be attributed to changes in PSCs but can be explained by the time lag between primary and export production. This lag was prolonged by the physical processes of upwelling and mixing, which slows down the particle sinking. Our results emphasize the important role of MJO in regulating the ecosystem at intraseasonal scale, thus improving our comprehension of the nonsteady dynamics of ecosystems in the SCS.

Net sea-air CO₂ fluxes (FCO₂) in the Drake Passage (DP) were studied at a climatological scale (1999–2019) using observations from the Surface Ocean CO₂ Atlas (SOCAT) database. Based on the monthly climatological position of the main circumpolar fronts of the DP (the Subantarctic Front (SAF), the Polar Front (PF) and the Southern Antarctic Circumpolar Current Front (SACCF)) and the thermal and nonthermal contributions to FCO₂, we present a regional subdivision into different regimes that provide new insights into the processes controlling these fluxes. Our results indicate that the region in the north of SAF (R1) behaves as an annual CO₂ sink (-1.3 ± 1.0 mmol m⁻² d⁻¹); this sink weakens between SAF-PF (R2) and PF-SACCF (R3) and the region south of SACCF (R4) acts as an annual CO₂ source (2.2 ± 3.3 mmol m⁻² d⁻¹). The annual mean CO₂ uptake in DP is 1.3 ± 15.5 Tg C yr^-1. Analysis of thermal (TE) and nonthermal (nonTE) effects on seasonal sea surface CO₂ partial pressure (pCO₂^sw) variability indicates that DP is mainly dominated by nonTE. Results emphasize that carbon fluxes are driven by mesoscale and submesoscale processes north of the PF and by the upwelling of Upper Circumpolar Deep Waters in the Antarctic boundary of the DP, while seasonal patterns are mostly modulated by local factors such as nutrient availability, biological activity and ice cover.

Operational analysis and forecast products of shelf sea biogeochemistry often lack reliable information on uncertainty. This is problematic, as good quality uncertainty information is both requested by the product end-users and essential for data assimilation. To address this problem we developed a quality-assessed ensemble representation of many leading sources of uncertainty in a coupled marine physical-biogeochemical model of the North-West European Shelf. Based on these ensembles we have estimated the uncertainty of several marine ecosystem health indicators (MEHIs), acting as proxies for biological productivity, phytoplankton community structure, trophic fluxes, deoxygenation, acidification and carbon export. We have also evaluated how observable these MEHIs are from the most widely available observations of total chlorophyll (mostly from the surface), highlighting those MEHIs and locations that need to be better monitored. Our results show that the most uncertain and the least observable MEHI is the phytoplankton community composition, highlighting the value of its observations (and their assimilation) particularly in the UK regional waters. We demonstrate that daily operational estimates of the other MEHIs, produced by the Met Office, are fairly well constrained. We also quantify how much MEHI uncertainties are reduced when one substantially coarsens the MEHI spatial and temporal resolution, as in the global and/or climate applications.

Sea ice microalgae are an important source of energy for the polar marine food web, representing the primary carbon source prior to pelagic phytoplankton blooms. Here we investigate community dynamics of sea ice microalgal communities in land-fast sea ice across six different fjords in high-Arctic Svalbard, Norway, during Spring (April – May). We found that light (0.1 – 23% incoming PAR / 0.1 – 193 μmol photons m^-2s^-1) played a central role in determining community composition, with more diverse assemblages observed in sites with more light transmitted to the bottom ice community. In April, microalgal assemblages were similar when under-ice light transmittance was similar, independent of geographical location, however this light-derived separation of community structure was not evident in May. At all sites, assemblages were dominated by pennate diatoms, with the most abundant taxon being Nitzschia frigida. However, with increasing under-ice light transmittance, we saw an increase in the relative abundance of Dinophyceae, Navicula spp. and Thalassiosira spp.. A positive relationship between light and δ¹³C enrichment and C:N ratios in the ice algal biomass demonstrated the effect of light on the biochemical composition of ice algae. Light did not correlate with cell abundance or chlorophyll a concentration. With anticipated changes to Arctic sea ice extent and snow cover as a result of climate change, we will see shifts in the light transmitted to the bottom ice community. These shifts, whether caused by reduced light transmittance from increased snow cover or increased light transmittance from thinning ice, snow depth or increased rainfall, will likely alter sea ice microalgal community composition, which in turn, may influence the success of secondary production and biogeochemical cycling in polar waters.

Primary production in the northern Adriatic (NAd) reaches its yearly peak in the winter with high-intensity variations from year to year. According to the hypothesis, the intensity of local winter primary production, controlled by the degree of the spreading of Po River waters across the NAd, reflects on the annual secondary production of the ongoing year. The hypothesis is evaluated here based on the new data set and extends from 2018 to 2020, referring additionally to 2017 data which are already published. Data collected in 2017 and 2020 support the hypothesis, pointing to the large organic outputs after highly productive winters. Despite the lack of seasonal data for 2018 and 2019, large annual production was deducted by large abundances of the allochthonous gelatinous zooplankton species – Mnemiopsis leidyi. Numerical models show that in 2018–2020 the NAd was mostly “separated” from the rest of the Adriatic Sea by a northern branch of a large cyclonic gyre with high salinity water (from central Adriatic and/or Kvarner Bay) entering the NAd along the eastern (Istrian) coast. Such a circulation system could favour the spreading of the Po River waters across the NAd, inducing high primary production in winter, at the beginning of the yearly pelagic cycle, with a subsequent retention/accumulation of organic matter produced in the following months in the area. Using climate projections of temperature and salinity and the associated circulation and following the observed biological relations, a prediction of the organic matter production in the NAd can be obtained. With increased horizontal density gradients in future winters, an intensification of transversal motions across the NAd is expected. Thus, the retention of the Po waters with higher winter production in the NAd may be predicted. Following the hypothesis, a higher annual organic production and a probable higher occurrence of gelatinous plankton in the east of the NAd are expected.

The invasive European green crab (Carcinus maenas) was first detected on the US west coast around 1989 and has expanded its range northward from central California to southern Alaska. The eastern Salish Sea was initially thought to be protected from invasion by the dominant seaward surface current in the Strait of Juan de Fuca (SJdF). However, this “oceanographic barrier” has been breached as established green crab populations have been detected in the eastern Salish Sea in recent years. Here we carried out particle-tracking simulations to understand possible natural pathways of green crab larvae invading the eastern Salish Sea. Both diel vertical migration and temperature-dependent mortality were considered in these simulations. Our results suggest that green crab larvae from the outer coast (outside the Salish Sea) and Sooke Basin (in SJdF) could be carried into the eastern Salish Sea in a narrow time window during the later cold season (esp. in March) when frequent flow reversals in SJdF occur and the seasonally rising water temperature becomes relatively favorable for green crab larvae. The major pathway for larvae to reach the eastern Salish Sea is along the southern coast of SJdF. The probability of live larvae reaching the eastern Salish Sea is highly sensitive to water temperature. Sensitivity simulations indicate that a temperature increase of 0.5–1 °C would double or quadruple the probability of successful arrival in the eastern Salish Sea. This suggests that invading green crabs might have taken advantage of the mild winter conditions in recent warm years. Our results also suggest that the warming climate in the near future may facilitate green crab larval exchange across the Salish Sea.