Plankton response to El Niño 1997–1998 and La Niña 1999 in the southern region of the California Current

The IMECOCAL Program began in 1997, with the objective of sampling plankton systematically in the Mexican region of the California Current. We present results of chlorophyll a concentrations and zooplankton displacement volumes for the eight cruises from September 1997 to October 1999. The abundance of 22 zooplankton groups was also analyzed for the first four cruises. The response of plankton to the 1997–1998 El Niño was atypical. From September 1997 to January 1998, chlorophyll a and zooplankton volume were at typical values (median integrated chlorophyll was 27 mg/m² and zooplankton 100 ml/1000 m³ in 9801/02). After the peak of El Niño, the system shifted to cooler conditions. Integrated chlorophyll gradually increased to a median of 77 mg/m² in April 1999. In contrast, zooplankton volumes decreased from October 1998 onward, despite favorable phytoplankton availability in 1999. Zooplankton structure was dominated by copepods and chaetognaths through the ENSO cycle, but interannual changes were evident. In the fall of 1997 there was a higher proportion of copepods, chaetognaths, and other minor groups, while the fall of 1998 zooplankton was richer in salps and ostracods. Historical data from previous Baja California CalCOFI cruises indicated that zooplankton volumes measured during the IMECOCAL cruises were above the long-term mean for the period 1951–1984. This suggests a differential response of plankton to the El Niño of 1997–1998 compared to the El Niño of 1957–1959. Regional differences in zooplankton volumes were also found, with central Baja California having 41% higher biomass than northern Baja California. Volumes from both regions were larger than those recorded by CalCOFI off southern California during 1997–1998, but the situation was reversed in 1999. The higher biomasses in the 1997–1998 El Niño can be attributed to high abundance of salps, which showed an affinity with warm, saline water.     

Plankton biomass and larval fish abundance prior to and during the El Niño period of 1997-1998 along the central Pacific coast of México

The temporal and spatial distributions of zooplankton biomass and larval fish recorded during 27 months (December 1995-December 1998) off the Pacific coast of central México are analyzed. A total of 316 samples were obtained by surface (from 40-68 to 0 m) oblique hauls at 12 sampling sites using a Bongo net. Two well-defined periods were observed: a pre-ENSO period (December 1995-march 1997) and an ENSO event (July 1997-September 1998) characterized by impoverishment of the pelagic habitat. The highest biomass concentrations occurred at coastal stations during the pre-ENSO period. During the El Niño period no spatial patterns were found in coastal waters. The months with highest biomass were those in which the lowest sea surface temperature (SST) occurred (January-May), and this pattern was also observed during the ENSO period. A typical, although attenuated, seasonal environmental pattern with enhanced phytoplankton (diatoms and dinoflagellates) was prevalent during the El Niño event in nearshore waters. During the El Niño period the phytoplankton was mainly small diatoms (microphytoplankton), while dinoflagellates were practically absent. The most parsimonious generalized linear models explaining spatial and temporal distribution of larval fish species included the ENSO index (MEI), upwelling index (UI) and distance to the coast. The environmental variability defined on an interannual time-scale by the ENSO event and the seasonal hydroclimatic pattern defined by the UI (intra-annual-scale) controlled the ecosystem productivity patterns. The small-scale distribution patterns (defined by a cross-shore gradient) of plankton were related to the hydroclimatic seasonality and modulated by interannual anomalies.     

西沙群岛珊瑚礁海域浮游动物群落结构初步分析*

为了解西沙群岛珊瑚礁海域浮游动物的群落特征, 于2015年5月末至7月初在西沙群岛8个岛礁(七连屿、永兴岛和东岛3个岛屿以及浪花礁、盘石屿、玉琢礁、华光礁和北礁5个环礁)进行了浮游动物采样, 分析了浮游动物的多样性、丰度和群落结构特征, 并比较了5个环礁潟湖内和向海礁坡区浮游动物群落组成的差异。调查海域共鉴定浮游动物180种(包括浮游幼虫13个类群), 其中桡足类最多, 达83种, 其次是水母类(38种)、浮游软体类(14种)、毛颚类(11种)和浮游被囊类(7种); 浮游动物平均丰度为256.4±117.8个·m^-3, 桡足类占总丰度的51.08%, 其次是浮游幼虫(16.30%)、浮游被囊类(13.22%)和毛颚类(7.70%)。浮游动物种数、多样性和均匀度指数、丰度在岛屿和环礁之间以及环礁的潟湖区和向海礁坡区均存在差异; 浮游动物多样性和丰度在东岛、玉琢礁和华光礁较高, 而在七连屿和北礁较低; 5个环礁向海礁坡区的浮游动物多样性和丰度皆高于潟湖区的。多元统计分析结果表明调查岛礁的浮游动物可划分两个聚群(相似水平85%); 岛屿近岸及环礁的潟湖群落(Ⅰ)和岛屿远岸及环礁的向海礁坡群落(Ⅱ); 两个聚群浮游动物组成差异较显著(r=0.832, p<0.001); 前者的种数、多样性指数、总丰度和主要浮游动物类群如桡足类、毛颚类和浮游幼虫的丰度显著低于后者。环礁潟湖区和向海礁坡区的空间异质性和生态环境差异可能是导致浮游动物群落结构呈现不同特征的主要因素。     

The Western Boundary of the Equatorial Pacific Upwelling: Some Consequences of Climatic Variability on Hydrological and Planktonic Properties

The longitude of the western limit of the equatorial Pacific upwelling is a key parameter for studies of carbon budget and pelagic fisheries variability. Although it is well defined at the surface on the equator by a salinity front and a sharp variation of the partial pressure of CO₂, data from two equatorial cruises make it clear that this hydrological limit does not necessarily coincide with the boundary of the nitrate and chlorophyll enriched area. In January-February 1991 during a non-El Niño period, when trade winds and the South Equatorial current (SEC) were favorable to upwelling, the two limits were at the same longitude. Conversely, in September-October 1994 during El Niño conditions, when the equatorial upwelling had stopped, the nitrate and chlorophyll enriched zone was found a few degrees of longitude east of the hydrological boundary (5.5° at the surface and 2.5° for the 50 m upper layer), whereas no such offset was observed for zooplankton biomass. A simple model, based on the HNLC (High Nutrient - Low Chlorophyll) ecosystem functioning, was initialized with nitrate uptake measurements and estimates of upwelling break duration. The model results support the hypothesis that zonal separation of the limits arises from biological processes (i.e. nitrate uptake and phytoplankton grazing) achieved during that upwelling break.     

Biogeochemistry of surface sediments off Concepción (∼36°S), Chile: El Niño vs. non-El Niño conditions

We compared the signals of several water column properties (upwelling intensity, sea level anomaly, temperature, nutrients, dissolved oxygen, chlorophyll-a, and surface sediments) of the continental shelf off Concepción (36°S) during the 1997-1998 El Niño with those of a normal year (2002-2003). We found that the primary hydrographic effect of El Niño 1997-1998 was a reduction in the input of nutrient-rich, oxygen-poor Equatorial Subsurface Water over the shelf. This affected the biology of the water column, as evidenced by the reduced phytoplankton biomass. Surface sediment properties (biogenic opal, organic carbon, bulk δ¹⁵N) observed during El Niño 1997-1998 reflected a reduced export production and the sediments failed to show the water column seasonality that occurs under normal conditions. In addition, weakened denitrification and/or upper water column fertilization could be inferred from the sedimentary δ¹⁵N. Although diminished, export production was preserved in the surface sediments, revealing less degraded organic matter in the upwelling period of the El Niño year than in the normal year. We suggest that the fresher organic material on the seafloor was probably associated with a severe reduction in the polychaete Parapronospio pinnata, which is considered to be the most important metazoan remineralizer of organic carbon at the sediment-water interface in the study area.     

Changes in the cetacean assemblage of a coastal upwelling ecosystem during El Niño 1997–98 and La Niña 1999

We report results of ecosystem studies in Monterey Bay, California, during the summer upwelling periods, 1996–99, including impacts of El Niño 1997–98 and La Niña 1999. Random-systematic line-transect surveys of marine mammals were conducted monthly from August to November 1996, and from May to November 1997–99. CTDs and zooplankton net tows were conducted opportunistically, and at 10 predetermined locations. Hydroacoustic backscatter was measured continuously while underway to estimate prevalence of zooplankton, with emphasis on euphausiids, a key trophic link between primary production and higher trophic level consumers.The occurrences of several of the California Current’s most common cetaceans varied among years. The assemblage of odontocetes became more diverse during the El Niño with a temporary influx of warm-water species. Densities of cold-temperate Dall’s porpoise, Phocoenoides dalli, were greatest before the onset of El Niño, whereas warm-temperate common dolphins, Delphinus spp., were present only during the warm-water period associated with El Niño. Rorqual densities decreased in August 1997 as euphausiid backscatter was reduced. In 1998, as euphausiid backscatter slowly increased, rorqual densities increased sharply to the greatest observed values. Euphausiid backscatter further increased in 1999, whereas rorqual densities were similar to those observed during 1998. We hypothesize that a dramatic reduction in zooplankton biomass offshore during El Niño 1997–98 led to the concentration of rorquals in the remaining productive coastal upwelling areas, including Monterey Bay. These patterns exemplify short-term responses of cetaceans to large-scale changes in oceanic conditions.     

Zooplankton and the oceanography of the eastern tropical Pacific: A review

We review the spatial and temporal patterns of zooplankton in the eastern tropical Pacific Ocean and relationships with oceanographic factors that affect zooplankton distribution, abundance and trophic relationships. Large-scale spatial patterns of some zooplankton groups show broad coincidence with surface water masses, circulation, and upwelling regions, in agreement with an ecological and dynamic partitioning of the pelagic ecosystem. The papers reviewed and a new compilation of zooplankton volume data at large-scale show that abundance patterns of zooplankton biomass have their highest values in the upwelling regions, including the Gulf of Tehuantepec, the Costa Rica Dome, the equatorial cold tongue, and the coast of Peru.Some of the first studies of zooplankton vertical distribution were done in this region, and a general review of the topic is presented. The possible physiological implications of vertical migration in zooplankton and the main hypotheses are described, with remarks on the importance of the oxygen minimum zone (OMZ) as a barrier to both the vertical distribution and migration of zooplankton in the region. Recent results, using multiple-net gear, show that vertical distribution is more complex than previously thought. There are some well-adapted species that do live and migrate within the OMZ.Temporal patterns are reviewed and summarized with historical data. Seasonal variations in zooplankton biomass follow productivity cycles in upwelling areas. No zooplankton time series exist to resolve ENSO effects in oceanic regions, but some El Niño events have had effects in the Peru Current ecosystem. Multidecadal periods of up to 50 years show a shift from a warm sardine regime with a low zooplankton biomass to a cool anchovy regime in the eastern Pacific with higher zooplankton biomasses. However, zooplankton volume off Peru has remained at low values since the 1972 El Niño, a trend opposite to that of anchoveta biomass since 1984.Studies of trophic relations emphasize the difference in the productivity cycle in the eastern tropical Pacific compared to temperate or polar ecosystems, with no particular peaks in the stocks of either zooplankton or phytoplankton. Productivity is more dependent on local events like coastal upwelling or water circulation, especially in the equatorial countercurrent and around the equatorial cool-tongue. Micrograzers are very important in the tropics as are predatory mesozooplankton. Up to 70% of the daily primary productivity is consumed by microzooplankton, which thus regulates the phytoplankton stocks. Micrograzers are an important link between primary producers, including bacteria, and mesozooplankton, constituting up to 80% of mesozooplankton food. Oceanography affects zooplankton trophic relationships through spatial–temporal effects on primary productivity and on the distributions of metabolic factors, food organisms, and predators. This paper is part of a comprehensive review of the oceanography of the eastern tropical Pacific.     

Abundance, distribution and patch formation of zooplankton

The goal of studies described here was to determine the responses of zooplankton taxa to phytoplankton patches which develop in and near intrusions of cold, nutrient-rich Gulf Stream water. To achieve this goal we determined the horizontal and vertical distributions of abundant mesozooplankton taxa on the south-eastern continental shelf of the USA between 29°30′ and 31°N. The study period was from June 23 to August 16, 1981. Highest concentrations of zooplankton usually occurred in and near patches of phytoplankton. Increased phytoplankton appeared to trigger the formation of patches of the calanoid copepod Temora turbinata and the cyclopoid copepods Oithona spp. and Oncaea spp. The patches of zooplankton had greater alongshore than cross-shelf dimensions. T. turbinata responded rapidly to increased concentrations of phytoplankton by reproducing and aggregating in and above intruded waters. Oithonidae which were often, but not always, abundant in phytoplankton patches eventually attained high concentrations over most of the middle and part of the inner shelf. Their concentration and that of Oncaeidae increased steadily. Oncaeidae were not abundant in recently upwelled waters, as was T. turbinata but reached high concentrations in older intrusions when the abundance of T. turbinata remained level or decreased slowly. Both cyclopoid taxa are thought to reproduce slowly (egg sacs) compared to T. turbinata. Another taxon, the doliolids, became abundant far more rapidly in intruded waters (by asexual reproduction) than did the other three taxa. Doliolids were the most opportunistic intrusion zooplankton form. They do not regularly occur in low abundance on the shelf, as do the three copepod taxa, but develop in pulses in regions where T. turbinata and Oncaea are not abundant. Of the four taxa studied the abundance of doliolids increased and decreased most rapidly, whereas Oithona and Oncaea increased slowly and did not decrease during the study period. T. turbinata and Oncaea were most abundant at 60% of all stations in the intruding water. Doliolids and Oithona on the other hand, were mostly in the thermocline and intrusion. Whereas phytoplankton patches, which developed in intrusions, were physically induced (PAFFENHÖFER and LEE, 1988), patches of zooplankton were biologically induced.     

Upper-ocean thermal structure and heat content off the US West Coast during the 1997-1998 El Niño event based on AXBT and satellite altimetry data

During the 1997/1998 El Niño event, extensive oceanic temperature profiles were taken off the coast of California in January and February 1998 using Airborne Expendable Bathythermographs (AXBTs). These AXBT measurements are compared with altimetry-based upper-ocean temperature estimates using TOPEX and ERS satellite altimetry data. The altimetry-based temperature estimates are well correlated with the AXBT data, in particular when combining the two satellite data sets together to form a blended altimeter temperature estimate. Both the AXBT and altimetry data show that the nearshore coastal El Niño signal differed from that further offshore. The AXBT data show that near shore, the warm anomalies extended to much greater depths and had greater amplitude. A time series of the satellite-derived layer-averaged temperatures, averaged separately over the nearshore and offshore halves of the AXBT analysis domain, also shows a larger El Niño signal in the nearshore half. The role of local atmospheric forcing of the coastal oceanic temperature anomalies is analyzed using NCEP reanalysis and coastal upwelling data sets. The forcing terms include Ekman pumping, radiation, surface heat fluxes, precipitation, and alongshore wind stresses that drive coastal upwelling (expressed as a coastal downwelling index, CDI). The temperature forcing from all of the terms except the CDI anomalies are small. The CDI anomalies can explain most of the slowly varying temperature changes that occur near the coast during a two-year period spanning the El Niño event, as well as some of the larger amplitude, rapid (monthly) warming episodes that appear to be part of the El Niño signal. Several distinct rapid warming episodes, however, are not correlated with the CDI anomalies, and therefore we conclude that the nearshore El Niño signal originates from a combination of both a remote oceanic pathway and local atmospheric forcing.     

Influence of ENSO variability on sinking-particle fluxes in the northeastern equatorial Pacific

A time-series sediment trap was operated from July 2003 to July 2008 at a station located in the 10°N thermocline ridge of the northeastern equatorial Pacific (10°30′N, 131°20′W), with the aim of understanding variations in natural background sinking-particle flux and the influence on such fluxes of ENSO (El Niño-Southern Oscillation). Each one of weak El Niño, moderate El Niño and moderate La Niña were observed during the monitoring period. During non-ENSO periods, total mass fluxes varied from 4.1 to 36.9 mg m⁻² d⁻¹, with a distinct seasonal variation, ranging from an average flux of 14.0 mg m⁻² d⁻¹ in the warm season (June-November) to 25.3 mg m⁻² d⁻¹ in the cold season (December-May). This seasonal fluctuation was characterized by a distinct difference in CaCO₃ flux between the two seasons. The enhanced particle fluxes during the cold season are attributed to the supply of nutrient-enriched subsurface water by wind-driven vertical mixing, supported by a simultaneous reduction in sea surface temperature and enhanced trade winds. The weak El Niño event occurred in the monitoring period had no recognizable effect on particle fluxes in the study area, but the moderate El Niño event was accompanied by a significant reduction in particle fluxes to 60% of the average background value in the warm season. In contrast, particle fluxes during the moderate La Niña increased to a maximum value of 129.9 mg m⁻² d⁻¹, almost three times the average background value. Organic carbon and biogenic silica fluxes were most sensitive to the El Niño and La Niña conditions. The observed variations of particle fluxes are synchronized with those of chlorophyll-a, suggesting primary productivity for the main cause of flux change. The present data indicate that marked seasonal variability in background fluxes commonly exceeds the variability associated with ENSO and post-ENSO signals, which should be taken into account when evaluating the influence of ENSO on sinking particle fluxes in the 10°N thermocline ridge area.     

Biological and chemical consequences of the 1997–1998 El Niño in central California waters

The physical, chemical and biological perturbations in central California waters associated with the strong 1997–1998 El Niño are described and explained on the basis of time series collected from ships, moorings, tide gauges and satellites. The evolution of El Niño off California closely followed the pattern observed in the tropical Pacific. In June 1997 an anomalous influx of warm southerly waters, with weak signatures on coastal sea level and thermocline depth, marked the onset of El Niño in central California. The timing was consistent with propagation from the tropics via the equatorial and coastal wave-guide. By late 1997, the classical stratified ocean condition with a deep thermocline, high sea level, and warm sea surface temperature (SST) commonly associated with El Niño dominated the coastal zone. During the first half of 1998 the core of the California Current, which is normally detected several hundred kilometers from shore as a river of low salinity, low nutrient water, was hugging the coast. High nutrient, productive waters that occur in a north–south band from the coast to approximately 200 km offshore during cool years disappeared during El Niño. The nitrate in surface waters was less than 20% of normal and new production was reduced by close to 70%. The La Niña recovery phase began in the fall of 1998 when SSTs dropped below normal, and ocean productivity rebounded to higher than normal levels. The reduction in coastal California primary productivity associated with El Niño was estimated to be 50 million metric tons of carbon (5×10¹³ g C). This reduction certainly had deleterious effects on zooplankton, fish, and marine mammals. The 1992–1993 El Niño was more moderate than the 1997–1998 event, but because its duration was longer, its overall chemical and biological impact may have been comparable. How strongly the ecosystem responds to El Niño appears related to the longer-term background climatic state of the Pacific Ocean. The 1982–1983 and 1992–1993 El Niños occurred during the warm phase of the Pacific Decadal Oscillation (PDO). The PDO may have changed sign during the 1997–1998 El Niño, resulting in weaker ecological effects than would otherwise have been predicted based on the strength of the temperature anomaly.     

Variations in nutrients, carbon and other hydrographic parameters related to the 1976/77 and 1988/89 regime shifts in the sub-arctic Northeast Pacific

For time series at Station P (50°N, 145°W) and stations along Line P, long term changes in eight oceanographic and chemical parameters (sea surface temperature (SST), salinity, oxygen, phosphate (PO₄), silicate (SiO₄), nitrate (NO₃), dissolved inorganic carbon (DIC), and apparent oxygen utilisation (AOU)) were influenced by climate regime shifts with a “step change” of anomalies for nutrients and carbon in the sub-arctic Pacific during the 1976/77 and 1988/89 regime shifts. The presence of regime shifts in the data in the late 1970s and the late 1980s was supported by the statistical test of [Rodionov, S.N., 2004. A sequential algorithm for testing climate regime shifts. Geophysical Research Letters 31, L09204, doi:10.1029/2004GL019448], based on the Student t-test. The response of nutrients and carbon to the regime shifts was more intensive in 1976/77 than in 1988/89. Salinity, PO₄, SiO₄, NO₃, oxygen and DIC showed positive anomalies during 1950-1975 and negative ones during 1976-1995. The effect of La Niña on nutrients and carbon was larger than that of El Niño. Strong La Niña events (e.g. 1988/89) caused a sudden increase in nutrients and carbon. Two regime shifts (1976/77 and 1988/89) occurred just after two strong La Niña events in 1976/77 and 1988/89. At Station P, upwelling of nutrient-poor subsurface water tended to decrease the nutrients at the surface.     

Predation by jellyfish on large and emergent zooplankton: Implications for benthic–pelagic coupling

Stable carbon isotopes were used to determine the contribution of emergent demersal zooplankton to the diet of the scyphozoan jellyfish Catostylus mosaicus at Smiths Lake, New South Wales, Australia. A preliminary study in 2004 indicated that there was no difference in the δ¹³C of ectodermal tissue and mesoglea of the medusae. In 2005, medusae and zooplankton present during the day and night were sampled and isotopic signatures were modelled using IsoSource. Modelling indicated that: (1) mollusc veligers and copepods sampled during the day contributed <13% of the carbon to the jellyfish; (2) copepods sampled at night contributed up to 25%; and (3) the large, emergent decapod Lucifer sp. contributed 88–94%. We hypothesised, therefore, that medusae derive most of their carbon from emergent species of zooplankton. In 2006, sampling done in 2005 was repeated three times over a period of 4 weeks to measure short-term temporal variation in isotopic signatures of medusae and zooplankton, and emergent demersal zooplankton was specifically sampled using emergence traps. Short-term temporal variation in isotopic signatures was observed for some taxa, however, actual variations were small (<1.5‰) and the values of medusae and zooplankton remained consistent relative to each other. IsoSource modelling revealed that mysid shrimp and emergent copepods together contributed 79–100% of the carbon to the jellyfish, and that the maximum possible contribution of daytime copepods and molluscs was only 22%. Jellyfish apparently derive most of their carbon from emergent zooplankton and by capturing small numbers of relatively large taxa, such as Lucifer sp. or mysid shrimp. Small but abundantly captured zooplankton (such as mollusc veligers) contribute only minor amounts of carbon. Jellyfish have a major role in the transfer of carbon between benthic and pelagic food webs in coastal systems.     

广东省南澳岛东部海域浮游动物群落结构及其影响因素

广东省南澳海域是粤东重要的海产养殖基地, 分析该海域浮游动物群落结构特征对评估其生态环境质量具有重要意义。文章根据2014年9月(秋季)、12月(冬季)、2015年4月(春季)和2016年7月(夏季)在南澳岛东部海域的浮游动物调查, 分析该海域浮游动物的群落结构特征, 探讨环境因素对其时空分布的影响。共鉴定浮游动物206种(包括浮游幼虫), 桡足类种数最多, 达94种; 远岸海域浮游动物的种数高于近岸海域。浮游动物丰度和生物量的季节变化明显, 夏、秋季高于冬、春季; 浮游动物丰度和生物量的分布趋势较一致, 夏季高值区主要出现在近岸, 秋季由近岸向远岸海域递增。浮游动物不同类群和优势种的丰度也存在季节变化, 桡足类是调查期间丰度较高的类群, 秋季水母类和海樽类丰度明显增加; 优势种后圆真浮萤(Euconchoecia maimai)和针刺真浮萤(Euconchoecia aculeata)在夏季丰度高, 小齿海樽(Doliolum denticulatum)在秋季占绝对优势。温度、盐度和浮游植物生物量是影响南澳岛东部海域浮游动物时空变化的主要环境因子, 说明该海域浮游动物群落特征受海流、水团和养殖活动的综合影响。     

Seasonal variation of the neustonic zooplankton community in southern waters of Korea

We investigated the seasonal variation of the neustonic zooplankton community in the surface layer (above 30 cm in depth) of 16 stations in southern waters of Korea from April 2002 to April 2003. In the neustonic community, copepods accounted for 59% of total organisms, as the most predominant group, while other dominant groups were invertebrate larvae, cladocerans, tunicates and fish eggs. Pontellid copepods, a major group of neustonic zooplankton, showed a marked seasonal variation and displayed a significant positive correlation with temperature (P < 0.05). Fish eggs appeared from spring to early summer in the inshore waters while tunicates and oceanic copepods (Eucalanus spp. and Euchaeta spp.) were dominant in the offshore waters during high temperature periods. This suggests that the distribution pattern of the neustonic zooplankton is seasonally affected by the hydrological conditions of in- and offshore waters. The abundance of zooplankton was higher in the surface layer (< 15 cm) than in the subsurface layer (15–30 cm), and was also higher at night than during the daytime. Of the dominant copepod families, the density of acartiid and pontellid copepods was higher in the surface layer than the subsurface layer. The density of acartiid and corycaeid copepods was significantly higher during daytime than nighttime. This indicates that the surface layer in temperate regions may provide a favorable habitat for neustonic copepods despite the dynamic environmental fluctuations.     

Zooplankton distribution and cross-shelf transfer of carbon in an area of complex mesoscale circulation in the northern California Current

We conducted a research cruise in late summer (July–August) 2000 to study the effect of mesoscale circulation features on zooplankton distributions in the coastal upwelling ecosystem of the northern California Current. Our study area was in a region of complex coastline and bottom topography between Newport, Oregon (44.7°N), and Crescent City, California (41.9°N). Winds were generally strong and equatorward for >6 weeks prior to the cruise, resulting in the upwelling of cold, nutrient-rich water along the coast and an alongshore upwelling jet. In the northern part of the study area, the jet followed the bottom topography, creating a broad, retentive area nearshore over a submarine shelf bank (Heceta Bank, 44–44.4°N). In the south, a meander of the jet extended seaward off of Cape Blanco (42.8°N), resulting in the displacement of coastal water and the associated coastal taxa to >100 km off the continental shelf. Zooplankton biomass was high both over the submarine bank and offshore in the meander of the upwelling jet. We used velocities and standing stocks of plankton in the upper 100 m to estimate that 1×10⁶ m³ of water, containing an average zooplankton biomass of ～20 mg carbon m^?3, was transported seaward across the 2000-m isobath in the meandering jet each second. That flux equated to offshore transport of >900 metric tons of carbon each day, and 4–5×10⁴ tons over the 6–8 week lifetime of the circulation feature. Thus, mesoscale circulation can create disparate regions in which zooplankton populations are retained over the shelf and biomass can accumulate or, alternatively, in which high biomass is advected offshore to the oligotrophic deep sea.     

A study on zooplankton distribution patterns and indicator species in Kuroshio upstream area and adjacent East China Sea

On the basis of the data of zooplankton biomass and three major taxa—— Copepoda, Chaetognatha andSiphonophora of May-June 1986, July-August and December 1987, the distributional patterns and the indicator species of zooplankton in the Kuroshio and adjacent waters of the East China Sea are preliminarily studied. The results are as follows:The horizontal distribution of zooplankton biomass and the abundance of copepods, chaetognaths and siphonophores arecurred in the continent area northwest of Taiwan and the south-centre section of the East China Sea continent, which are the mix front of different waters. Zooplankton in the water area inside of Ryukyu Islands presented low abundance and high diversity. There are clear seasonal variations in zooplankton biomass and abundance in the study area. The strength or weakness of different water masses and fronts is the basic reason for the variations of zooplankton biomass and abundance.The species composition of zooplankton in the study area is complex and var     

The Newport line off Oregon - Studies in the North East Pacific

The Newport Hydrographic (NH) Line along 44.65°N off central Oregon was sampled seasonally during two epochs: 1961-1971 through the TENOC program and 1997-2003 through the GLOBEC Northeast Pacific Long Term Observations Program (LTOP); some observations are available for 2004 and 2005. During TENOC, the line extended 305 km offshore to 128°W, with stations 18 km apart over the continental shelf and 36 km offshore. During LTOP, the line was shorter (to 126°W) with closer station spacing over the continental shelf (9 km apart) and slope (18 km apart). LTOP cruises included biochemical sampling and underway current measurements. During both TENOC and LTOP, the seasonal cycle is very strong (accounting for >50% of the variance in surface layer properties), with rapid transitions in spring and fall. The summer regime is subject to coastal upwelling driven by southward winds, equatorward surface currents, and advection of low-salinity waters from the Columbia River. The winter regime off Newport is subject to coastal downwelling and poleward surface currents driven by northeastward winds. Comparison between TENOC and LTOP summer regimes shows the near-surface layer (0-100 m) at most locations is significantly warmer and fresher during LTOP than TENOC, and steric heights over the continental margin are significantly higher. Comparison of LTOP and TENOC winters shows that average differences at most locations were not statistically significant, but that the variance of steric height and shelf-break temperatures was significantly higher during LTOP than TENOC. Interannual variability of climate indices is also stronger during LTOP, which included a rare Subarctic invasion in 2002 as well as the strong 1997-1998 El Niño. During both TENOC and LTOP, interannual variability of steric height is closely related to the El Niño/La Niña cycle. Nutrient concentrations and nitrate-to-phosphate ratios of upwelling-source waters vary inversely with halocline temperature. Both reflect alongshore advection by coastal currents: southward currents bring cool, nitrate-rich waters in summer (especially during the Subarctic invasion), and northward currents bring relatively warm, nitrate-poor waters to the NH line in winter (especially during El Niño). Seasonal and interannual variations in the nutrient level of upwelling-source water are reflected in time series of vertically-integrated chlorophyll over the LTOP survey region (about 150 km by 300 km). Seasonal variations in chlorophyll and currents are congruent with seasonal variations in copepod biomass and diversity. We were not successful in establishing a clear connection between chlorophyll levels and interannual variations in copepod biomass or diversity, nor in explaining the large decrease in the survival rate of coho salmon between TENOC (6%) and LTOP (3%).