长江口外颗粒有机碳及光合色素在不同溶氧水平下早期降解速率研究

有机质降解是低氧形成的重要原因.长江口外的低氧近几十年来日趋恶化,引起了大量学者关注,但现场有机质降解及相关的耗氧速率还鲜见报道.本研究通过人工控制体系溶氧(DO)水平,于2006年10月在曾经出现低氧的长江口外近海进行现场有机质降解培养实验,以模拟有机质在沉降进入近底层水后早期(9天)内的生物地球化学过程,估算有机碳降解速率和耗氧速率.培养实验表明,有机质在几天内即可出现明显的降解,并伴随体系pH的下降和细菌、病毒的增加.体系溶氧对有机碳(POC)降解速率影响较大,POC在DO＞95％条件下的降解速率是DO＜50％条件下的5倍,分别为1.5 mmol/(m3·d)和0.3 mmol/(m3·d).光合色素岩藻黄素(FUCO)和叶绿素a(Chla)含量在9天内呈指数下降,其中80％的FUCO在9天内发生了降解.指数方程能较好地实现对观测数据的拟合,结果表明Chl a的半衰期为3-4天.不同溶氧条件下的结果表明低溶氧条件(DO＜50％)下浮游植物更容易成为降解有机质的来源.根据POC降解速率估算得到长江口外溶氧消耗速率分别为82-91 mmol/(m2·d)(DO＞95％条件)和27-30 mmol/(m2·d)(DO＜50％条件),远高于深海氧通量和表层50 m南海的氧消耗通量;以此耗氧速率为基础,估算得到长江口外层化形成至发育出低氧的时间为50-150天.     

Twentieth century eutrophication of the St. Croix River (Minnesota–Wisconsin,USA) reconstructed from the sediments of its natural impoundment

Evaluation of land-use effects on coastal and marine ecosystems requires better understanding of the role of rivers in regulating mass transport from terrestrial to oceanic environments. Here we take advantage of the presence of a riverine lake to use paleoecological techniques to quantify impacts of logging, European-style agriculture, urbanization and continued terrestrial disturbance on mass transport and water quality in the northern drainage of the Mississippi River. Two 2-m sediment-cores recovered in 1999 from Lake St. Croix, a natural impoundment of the St. Croix River, were dated using ²¹⁰Pb and ¹³⁷Cs, and analyzed for historical changes (c. 1840–present) in sediment magnetic susceptibility, inorganic and organic matter content, biogenic silica, fossil pigments, and diatom microfossils. Inorganic sediment accumulation increased threefold between the mid-1800s and present, whereas clear signs of eutrophication were only evident after the mid-twentieth century when biogenic silica accumulation increased sixfold, diatom accumulation rates increased 20- to 50- fold, and the diatom community shifted from predominantly benthic species to assemblages composed mainly of planktonic taxa. Similarly, fossil pigment concentrations increased during the 1960s, and diatom-inferred total phosphorus (DI-TP) increased from ~30 μg TP l⁻¹ c. 1910 to ~60 μg l⁻¹ since 1990, similar to historical records since 1980. Together, these patterns demonstrate that initial land clearance did not result in substantive declines in water quality or nutrient mass transport, instead, substantial degradation of downstream environments was restricted to the latter half of the twentieth century. This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue.     

Collection of intact sediment cores with overlying water to study nitrogen- and oxygen-dynamics in regions with seasonal hypoxia

Settled particles of fresh, labile organic matter may be a significant source of oxygen demand and nutrient regeneration in seasonally-hypoxic regions caused by nutrient inputs into stratified coastal zones. Studying the dynamics of this material requires sediment sampling methods that include flocculent organic materials and overlying water (OLW) at or above the sediment–water interface (SWI). A new coring device (“HYPOX” corer) was evaluated for examining nitrogen- (N) and oxygen-dynamics at the SWI and OLW in the northern Gulf of Mexico (NGOMEX). The HYPOX corer consists of a “Coring Head” with a check-valve, a weighted “Drive Unit,” and a “Lander,” constructed from inexpensive components. The corer collected undisturbed sediment cores and OLW from sediments at NGOMEX sampling sites with underlying substrates ranging from sand to dense clay. The HYPOX corer could be deployed in weather conditions similar to those needed for a multi-bottle rosette water-sampling system with 20 L bottles. As an example of corer applicability to NGOMEX issues, NH₄⁺ cycling rates were examined at hypoxic and control sites by isotope dilution experiments. The objective was to determine if N-dynamics in OLW were different from those in the water column. “Ammonium demand,” as reflected by potential NH₄⁺ uptake rates, was higher in OLW than in waters collected from a meter or more above the bottom at both sites, but the pattern was more pronounced at the hypoxia site. By contrast, NH₄⁺ regeneration rates were low in all samples. These preliminary results suggest that heterotrophic activity and oxygen consumption in OLW in the hypoxic region may be regulated by the availability of NH₄⁺, or other reduced N compounds, rather than by the lack of sufficient labile organic carbon.     

Resistance to low oxygen in the Ponto–Caspian amphipod Pontogammarus robustoides varies among lentic habitats of its northern invaded range

The Ponto–Caspian amphipod Pontogammarus robustoides was introduced into Lithuanian inland waters more than 50 years ago and is now among the most successful local crustacean invaders. Existing as an oxyphilic species in its native range, in Lithuania it managed to establish in some lentic eutrophic waters facing long-term hypoxic conditions under winter ice cover, or shorter periods of hypoxia during the warm season. Recently, it has been observed to be further expanding in such waters. The aim of this study was to explore the possible divergence in anaerobic metabolism among introduced populations facing different selective pressures. A closed-bottle experiment was conducted using individuals from three Lithuanian lentic water bodies of different trophic status: mesotrophic, eutrophic and hypertrophic. Severe hypoxia was gradually reached, after which lactate dehydrogenase activity was significantly higher in amphipods from eutrophic and hypertrophic, than from mesotrophic environments, and was well correlated with trophic status of inhabited environments as assessed by chlorophyll a concentration. These findings suggest a physiological acclimation response to oxygen deficiency faced by the species in some environments in the northern invaded range, which may have a genetic background. Such resistance to oxygen deficiency may expand the environmental niche and promote species’ spread into previously unsuitable habitats, which may pose a threat to native species residing in refuges that are currently free of the invader.     

Microbial food web contributions to bottom water hypoxia in the northern Gulf of Mexico

Nutrients from the Mississippi/Atchafalaya Rivers greatly stimulate biological production in the ‘classical’ food web on the inner shelf of the northern Gulf of Mexico. Portions of this production, especially large diatoms and zooplankton fecal pellets, sink and decompose in the bottom water, consuming oxygen and contributing to the annual development of an extensive zone of bottom water hypoxia, typically >15,000 km² since 1993. The microbial food web is also active in the Mississippi River plume, but consists of small organisms that sink slowly. This ‘recycling’ food web has not been considered as a significant contributor to vertical flux and hypoxia. However, gelatinous zooplankton, especially pelagic appendicularians such as Oikopleura dioica, mediate the conversion of microbial web organisms to organic particles with high sinking rates. When pelagic appendicularians are abundant in coastal regions of the northern Gulf of Mexico, they stimulate the rapid vertical transfer of microbial web productivity in the surface layer, which is only 5–15 m thick in the coastal hypoxic region, to the sub-pycnocline layer that becomes hypoxic each summer. In this paper we present results from two studies examining the significance of this pathway. In both 2002 and 2004, we observed high production rates of appendicularians in coastal waters. Discarded gelatinous houses and fecal pellets from the appendicularian populations often provided more than 1 g m⁻² d⁻¹ of organic carbon for the establishment and maintenance of hypoxia in the northern Gulf of Mexico. This source of organic matter flux is especially important in regions far from the river plumes and during periods of low river discharge. Autotrophic elements of this food web are primarily supported by recycled inorganic nutrients originating in the Mississippi and Atchafalaya Rivers. Sources of dissolved organic matter (DOM) supporting the heterotrophic components of this microbial food web may include in situ production, the Mississippi/Atchafalaya Rivers, and Louisiana's coastal wetlands. If significant, the latter source provides a possible link between Louisiana's high rates of coastal land loss and the large hypoxic zone observed along the coast during summer. Both of the latter DOM sources are independent of phytoplankton production stimulated by inputs of riverine inorganic nutrients.     

2006年6月长江口低氧区及邻近水域浮游植物

根据2006年6月2-11日在长江12低氧区及邻近水域(26°-34°N,121°-126°E)27个站位的调查,对长江口低氧区及邻近水域的浮游植物群落结构特征进行了相关研究.经Utermohl方法初步分析,共发现浮游植物130种,隶属4门57属.甲藻和硅藻是2006年6月长江口低氧区及邻近水域的主要浮游植物门类,其优势物种是:具齿原甲藻Prorocentrum dentatum(=Prorocentrum donghaiense)、米氏凯伦藻Karenia mikimotoi、尖刺伪菱形藻Pseudo-nitzschia pungens、柔弱伪菱形藻Pseudo-nitzschia delicatissima、锥状施克里普藻Scrippsiella trochoidea和具槽帕拉藻Paralia sulcata.调查区浮游植物物种以广温、广布型为主.本次调查海域浮游植物的细胞丰度介于0.0026×105-37.37×105个/dm3.平均值为1.47×105个/dm3;甲藻占浮游植物细胞丰度的比例最大,细胞丰度介于0.0002×105-32.01×105个/dm3,平均值为1.39×105个/dm3;其次为硅藻,细胞丰度介于0.0001×105-18.72×105个/dm3,平均值为0.52×105个/dm3.具齿原甲藻和米氏凯伦藻所占丰度比例分别达到45.81%和26.44%,优势度分别为0.30和0.18.具齿原甲藻细胞丰度最高值出现在调查海域北部3号站-20m水层,为3.19×106个/dm3;米氏凯伦藻细胞丰度最高值出现在调查海域西南部29号站表层,为1.71×106个/dm3.浮游植物细胞丰度在水体中的垂直分布为表层最大,随着水深增加而逐渐降低.根据浮游植物的表层分布和断面分布可以发现,细胞丰度高值主要集中在调查区的西南部和北部.东南部浮游植物细胞丰度较低.同时调查海域东南部浮游植物多样性指数和均匀度指数较高,近岸低氧区和中部区域则较低.调查区水体层化现象明显,表层水和底层水之间氧的交换变弱,有机碎屑和浮游植物大量繁殖后沉降分解可能导致了底层低氧区的形成.     

Origins and temporal scales of hypoxia on the Louisiana shelf: Importance of benthic and sub-pycnocline water metabolism

Hypoxic-to-anoxic conditions (2–0 mg O₂ l^− 1) occur in the bottom waters of the northern Gulf of Mexico on the Louisiana shelf west of the Mississippi river delta during late spring and summer where the rate of oxygen consumption exceeds its rate of input from physical transport plus photosynthetic generation. Although consumption of oxygen in the water column primarily via oxic respiration is an important process, the loss of oxygen at and near the seafloor may also be an important sink contributing to seasonal low oxygen conditions in the relatively shallow overlying waters in this region. Associated with the flux of oxygen into the sediments is the flux of nutrients out of the sediments from the remineralization of sedimentary organic matter via a number of possible electron acceptors. The nutrients that are released from the sediment can potentially stimulate further primary production. This can lead to generation of oxygen in the water column and production of organic matter, much of which can be transported to the seafloor where it again becomes a sink for oxygen.A non-steady-state data driven numeric benthic–pelagic model was developed to investigate the role of sediment and water-column metabolism in the development of hypoxia on the Louisiana shelf. The model simulations bare out the importance of sediment oxygen demand as the primary sink for oxygen at the beginning and end of a hypoxic event on the shelf, but once hypoxia has developed, the sediments, now isolated from the oxygen-rich surface waters, are driven into a more anoxic mode, becoming more dependent on sulfate and metal reduction. As a result, the bottom water near the pycnocline becomes the major sink for oxygen.Model simulations also suggest that there is a delay of several weeks between metabolite production (especially ammonium) and its efflux from the sediments. Thus the maximum sediment ammonium export occurs in September and October in time to fuel autumnal phytoplankton production, thereby continuing a biogeochemical cycle that expands the temporal and spatial scales of hypoxia on the Louisiana shelf.     

Persistently declining oxygen levels in the interior waters of the eastern subarctic Pacific

Fifty years of measurements at Ocean Station Papa (OSP, 50°N, 145°W) show trends in the interior waters of the subarctic Pacific that are both impacted by short term (few years to bi-decadal) atmospheric or ocean circulation oscillations and by persistent climate trends. Between 1956 and 2006, waters below the ocean mixed layer to a depth of at least 1000 m have been warming and losing oxygen. On density surfaces found in the depth range 100-400 m (σ_θ = 26.3-27.0), the ocean is warming at 0.005-0.012 °C y⁻¹, whereas oxygen is declining at 0.39-0.70 μmol kg⁻¹ y⁻¹ or at an integrated rate of 123 mmol m⁻² y⁻¹ (decrease of 22% over 50 years). During this time, the hypoxic boundary (defined as 60 μmol O₂ kg⁻¹) has shoaled from ∼400 to 300 m. In the Alaska Gyre, the 26.2 isopycnal occasionally ventilates, whereas at OSP 26.0σ_θ has not been seen at the ocean surface since 1971 as the upper ocean continues to stratify. To interpret the 50 year record at OSP, the isopycnal transport of oxygenated waters within the interior of the subarctic Pacific is assessed by using a slightly modified “NO” parameter [Broecker, W., 1974. “NO” a conservative water-mass tracer. Earth and Planetary Science Letters 23, 100-107]. The highest nitrate-oxygen signature in interior waters of the North Pacific is found in the Bering Sea Gyre, Western Subarctic Gyre and East Kamchatka Current region as a consequence of winter mixing to the ∼26.6 isopycnal. By mixing with low NO waters found in the subtropics and Okhotsk Sea, this signature is diluted as waters flow eastward across the Pacific. Evidence of low NO waters flowing north from California is seen along the coasts of British Columbia and SE Alaska. Oxygen in the subsurface waters of the Alaskan Gyre was supplied ∼60% by subarctic and 40% by subtropical waters during WOCE surveys, whereas such estimates are shown to periodically vary by 20% at OSP. Other features discernable in the OSP data include periods of increased ventilation of deeper isopycnals on an ∼18 year cycle and strong, short term (few month) variability caused by passing mesoscale eddies. The potential impacts of declining oxygen on coastal ecosystems are discussed.     

Foraminiferal biofacies in the San José Calcareous Silt Member (Manzanilla Formation,Upper Miocene to Lower Pliocene) in the Manzanilla Bay area,north-east Trinidad,and their environmental significance

The Miocene–Pliocene Manzanilla Formation of northern Trinidad marks the arrival of the east-west flowing Orinoco River. Foraminifera were examined quantitatively in samples obtained at ∼5 m intervals from the lowest member of the formation, the San José Calcareous Silt Member, where exposed at eastern Manzanilla Bay and Point Noir. Recovery of Globorotalia merotumida indicates an age within the Globorotalia acostaensis Zone through Globorotalia margaritae evoluta Subzone, (N16–N19). Values of the information function (H) and the percentage of the total assemblage as planktonic foraminifera (%P), both traditionally used as palaeodepth indicators, were uncorrelated. The low values of H are indicative of freshwater influence. Cluster analysis revealed two major biofacies dominated by Pseudononion atlanticum and Hanzawaia carstensi respectively, for which similar values of mean H and mean %P suggest similar, inner to middle neritic palaeodepths, the P. atlanticum Biofacies indicating times of hypoxia. Entrained within these were rarer samples from a biofacies indicative of shallowing and increased freshwater input (Ammonia parkinsonia gr. Biofacies). An Amphistegina gibbosa Biofacies marked an interval in which water because sufficiently clear to support foraminifera symbiotic with algae, while a sample containing only Haplophragmoides wilberti was indicative of intertidal conditions. Some samples containing reddened foraminifera showed that deposition occasionally became very slow, but these were not limited to any one biofacies. The range of palaeodepths in which the San José Calcareous Silt Member was deposited is far less than in the underlying Brasso Formation, indicating that tectonic induction of transgressive–regressive cycles was slighter during San José times.     

Hypoxia by degrees: Establishing definitions for a changing ocean

The marked increase in occurrences of low oxygen events on continental shelves coupled with observed expansion of low oxygen regions of the ocean has drawn significant scientific and public attention. With this has come the need for the establishment of better definitions for widely used terms such as “hypoxia” and “dead zones”. Ocean chemists and physicists use concentration units such as ??molO₂/kg for reporting since these units are independent of temperature, salinity and pressure and are required for mass balances and for numerical models of ocean transport. Much of the reporting of dead zone occurrences is in volumetric concentration units of mlO₂/l or mgO₂/l for historical reasons. And direct measurements of the physiological state of marine animals require reporting of the partial pressure of oxygen (pO₂) in matm or kPa since this provides the thermodynamic driving force for molecular transfer through tissue. This necessarily incorporates temperature and salinity terms and thus accommodates changes driven by climate warming and the influence of the very large temperature range around the world where oxygen limiting values are reported. Here we examine the various definitions used and boundaries set and place them within a common framework. We examine the large scale ocean pO₂ fields required for pairing with pCO₂ data for examination of the combined impacts of ocean acidification and global warming. The term “dead zones”, which recently has received considerable attention in both the scientific literature and the press, usually describes shallow, coastal regions of low oxygen caused either by coastal eutrophication and organic matter decomposition or by upwelling of low oxygen waters. While we make clear that bathyal low oxygen waters should not be confused with shallow-water “dead zones”, as deep water species are well adapted, we show that those waters represent a global vast reservoir of low oxygen water which can readily be entrained in upwelling waters and contribute to coastal hypoxia around the world and may be characterized identically. We examine the potential for expansion of those water masses onto continental shelves worldwide, thereby crossing limits set for many not adapted species.