基于ADCP回声的黑潮-亲潮混合区浮游动物昼夜垂直迁移研究

利用深海潜标所搭载的声学多普勒流速剖面仪(acoustic doppler current profiler,ADCP)得到的后向散射强度Sv,研究了黑潮-亲潮混合区浮游动物的垂向分布、其昼夜垂直迁移(diel vertical migration;DVM)的基本特征、多时间尺度变化及对反气旋式中尺度暖涡的响应.结果表...     

南海北部声散射季节变化

声散射是重要的声学现象,海洋水体产生的高频声散射信号既可用于开展多种目的的声学海洋学研究,也可能对水下声学设备产生干扰,而海洋水体背景声散射具有显著的时空变异特征,因此针对特定海区开展声散射时变观测具有重要意义。本文利用在南海北部布放的锚系系统所搭载的声学多普勒流速剖面仪,获取了覆盖4个季节的累计约80 d的声散射数据,数据包括75 kHz和300 kHz两个频段,观测水深几乎覆盖了从海面到约600 m水深的整个水体。结果表明,水体在垂向上分布着上散射层和深散射层2个主要散射层。上散射层分布深度在冬夏较浅,位于约100 m以浅,在春秋较深,位于约200 m以浅;深散射层分布深度同样为冬季最浅,位于约300 m以深,但夏季则最深,位于约400 m以深。因此,两散射层的距离在夏季最远,在春秋最近。2个散射层的声散射强度（Sv）同样具有明显的季节变化,上散射层散射强度夏秋较强而春冬较弱,深散射层则正好相反。     

2012年夏季楚科奇海浮游动物垂直迁移及上升流对其生物量的影响

The diel vertical migration(DVM) of zooplankton and the influence of upwelling on zooplankton biomass were examined using water column data of current velocity and mean volume backscattering strength(MVBS)collected by moored acoustic Doppler current profilers(ADCPs) deployed in the southeastern Chukchi Sea during the 5th Chinese National Arctic Research Expedition(CHINARE) in summer 2012, combined with the satellite observational data such as sea surface temperature(SST), wind, and chlorophyll a(Chl a). Hourly acoustic data were continuously collected for 49-d in the mooring site. Spectral analysis indicated that there were different migrating patterns of zooplankton, even though precisely classifying the zooplankton taxa was not available. The prevailing 24-h cycle corresponded to the normal DVM with zooplankton swimming upwards at sunrise and returning to deep waters at sunset. There was a clear DVM in the upper 17 m of the water column during the period with distinct day-night cycles, and no active DVM throughout the water column when the sun above the horizon(polar day), suggesting that light intensity was the trigger for DVM. Also there was a second migrating pattern with 12-h cycle. The upwelling event occurring in the northwest of Alaskan coastal area had important influence on zooplankton biomass at the mooring site. During the upwelling, the SST close to the mooring site dropped significantly from maximal 6.35°C to minimal 1.31°C within five days. Simultaneously, there was a rapid increase in the MVBS and Chl a level, suggesting the aggregation of zooplankton related to upwelling.     

南黄海梭形纽鳃樽昼夜垂直移动的初步研究

本文对2006年12月、2007年5月和6月在南黄海两个连续站S1-4和S3-3采集的梭形纽鳃樽的昼夜垂直移动进行了研究。结果表明, 在不同季节不同海域该种昼夜垂直移动行为略有差异。在S1-4站位, 12月份梭形纽鳃樽复体从未到达底层(50—70m), 在0—50m的水层中表现出不规则的昼夜垂直移动。从中午(12:00)到傍晚(18:00)主要分布在表层, 之后移动到较深的水层(10—30m), 而单体由于数量较少而未进行昼夜垂直移动分析; 5、6月份, 该种单体和复体都出现了明显的反向昼夜垂直移动, 即白天迁移到表层, 夜间向底层迁移, 5月份尤为明显。在S3-3站位, 12月份和5月份梭形纽鳃樽的单体和复体几乎在整个水层都有分布。12月份梭形纽鳃樽复体在夜间有向下移动的趋势, 在06:00时到达最底层, 白天主要分布在20m以上的水层中。5月份梭形纽鳃樽单体和复体昼夜垂直移动不明显, 但主要分布在10—20m的水层中。     

Diel Vertical Migration of the Marine Cladoceran Podon Leuckarti: Variations with Reproductive Stage

Diel vertical migration (DVM) of the marine cladoceran Podon leuckarti was investigated with reference to their reproductive stages and size. Males and females, except for gamogenic females with an advanced resting egg (GA), aggregated at the near-bottom layer or at the surface during the day and dispersed into the water column at night. Both the near-bottom aggregation and the surface aggregation during daytime are suggested to be behavior that reduces the predation risk from visual predators. However, GA aggregated in the near-bottom layer during daytime and avoided the surface layer. The near-bottom aggregation might be more effective behavior for GAs to reduce risk of visual predation than the surface aggregation because of the conspicuous resting egg they carried. These results show that carrying an advanced resting egg influenced the DVM of P. leuckarti.     

The diel verticalmigration of sound scatterers observed by an acoustic Doppler current profiler in the Luzon Strait from July 2009 to April 2011

Acoustic Doppler current profiler （ADCP） receives echoes from sound scatterers, then their speed is calcu- lated by the Doppler effect. In the open ocean, most of these backscatterers are from the plankton. The sound scatterers descend down to depth at around dawn, their mean speed is 2.9 cm/s, then they ascend up to the surface layer at around dusk with a mean speed of 2.1 cm/s, in the Luzon Strait. The descending speed is faster, which suggests that this zooplankton population may accelerate its downward migration under the action of the gravity. The vertical distribution of a mean volume backscattering strength （MVB- S） in the nighttime has two peaks, which locate near the upper and lower boundary layers of halocline, respectively. However, the backscatterers only aggregate near the surface layer in the daytime. The diel ver- tical migration （DVM） of sound scatterers has several characteristic patterns, it is stronger in summer, but weaker in winter, and the maximum peak occurs in September. The DVM occurrence is synchronous with the seawater temperature increasing at around dawn and dusk, it may affect the ocean mixing and water stratification,     

Distribution depth of the transforming stage larvae of myctophid fishes in the subtropical–tropical waters of the western North Pacific

We describe the day–night vertical distribution patterns of 18 species or types of myctophid fish larvae at the transforming stage based on discrete depth sampling from the surface down to 1000-m depth in the subtropical–tropical western North Pacific. A total of 551 transforming stage larvae were collected at the 19 sampling stations. Except for the Diaphus species and Notolychnus valdiviae, all of the transforming stage larvae (including genera Benthosema, Bolinichthys, Centrobranchus, Ceratoscopelus, Diogenichthys, Hygophum, Lampanyctus, Lobianchia, Myctophum, Symbolophorus, and Triphoturus) were collected in the lower mesopelagic zone from 600- to 900-m depth during both day and night, showing no diel vertical migration (DVM). On the contrary, the Diaphus species and N. valdiviae larvae undergo DVM during the transforming stage, occurring below 200-m layer during the daytime and migrating up to the upper 150-m layer at night, i.e., they show earlier adaptation to juvenile–adult behaviors. Most myctophid fish larvae are known to undertake substantial ontogenetic vertical migration (OVM) from the epipelagic to mesopelagic zones during their early life stage. Although considerable sampling effort was carried out in this study, transforming larvae, except for the above two migratory ones, were not collected in the epipelagic and upper mesopelagic zones, strongly suggesting that their sinking speed would be high. It would be advantageous for survival to spend their highly vulnerable transforming stage in the lower mesopelagic zone, where predation pressures are lower and physical conditions are more stable than in the upper layers.     

基于多波束测深仪的西太平洋声散射层测量

为了研究西太平洋声散射层的垂向分布特征和日变化规律,分析了多波束测深系统的水体影像数据。观测结果表明,西太平洋存在着两个声散射层,一个声散射层位于0～200 m,另一个声散射层位于500～700 m,两个声散射层散射强度具有明显的日变化特征,上层散射层的散射强度呈现白天弱,夜晚强的特征,而下层散射层的散射强度日变化规律与上层相反,并且发现深散射层的厚度也存在日变化特征;分析了此种方法的优缺点,对以后声散射层的观测分析提供了新的思路。此外,利用同时下放的声速仪(SVP)的温度和深度数据对下放式声学多普勒流速剖面仪(LADCP)的观测结果进行了修正,得到了更为精确的声散射层垂向位置分布。     

Seasonal variation of diel vertical migration of zooplankton from ADCP backscatter time series data in the Lazarev Sea,Antarctica

Ten-month time series of mean volume backscattering strength (MVBS) and vertical velocity obtained from three moored acoustic Doppler current profilers (ADCPs) deployed from February until December 2005 at 64°S, 66.5°S and 69°S along the Greenwich Meridian were used to analyse the diel vertical zooplankton migration (DVM) and its seasonality and regional variability in the Lazarev Sea. The estimated MVBS exhibited distinct patterns of DVM at all three mooring sites. Between February and October, the timing of the DVM and the residence time of zooplankton at depth were clearly governed by the day–night rhythm. Mean daily cycles of the ADCP-derived vertical velocity were calculated for successive months and showed maximum ascent and descent velocities of 16 and –15 mm s^?1. However, a change of the MVBS pattern occurred in late spring/early austral summer (October/November), when the zooplankton communities ceased their synchronous vertical migration at all three mooring sites. Elevated MVBS values were then concentrated in the uppermost layers (<50 m) at 66.5°S. This period coincided with the decay of sea ice coverage at 64°S and 66.5°S between early November and mid-December. Elevated chlorophyll concentrations, which were measured at the end of the deployment, extended from 67°S to 65°S and indicated a phytoplankton bloom in the upper 50 m. Thus, we propose that the increased food supply associated with an ice edge bloom caused the zooplankton communities to cease their DVM in favour of feeding.     

南海东北部陆坡区中上层浮游动物的垂直分布*

根据2016年3月和9月南海东北部陆坡区浮游动物垂直分层采样, 比较浮游动物种类组成、丰度和生物量的垂直分布和季节变化, 探讨影响其差异的原因。文中共鉴定浮游动物225种, 其中3月和9月分别出现150种和169种; 桡足类种数达132种, 其次是毛颚类和管水母类, 各18种; 浮游动物种数在50~100m水层最高, 一般随水深增加而减少。浮游动物丰度和生物量主要集中在0~100m, 二者在100m以深水层随深度增加而降低, 而水柱生物量在100~1000m占总水柱生物量的60%以上。浮游动物优势种季节和垂直变化明显, 3月近海种如普通波水蚤(Undinula vulgaris)和微刺哲水蚤(Canthocalanus pauper)等在100m以上水层丰度较高; 9月外海种如达氏筛哲水蚤(Cosmocalanus darwinii)和黄角光水蚤(Lucicutia flavicornis)等在100m以浅水层相对于3月丰度增加; 隆线似哲水蚤(Calanoides carinatus)在3月丰度和平均体长高于9月, 并且体长较大者主要分布在深层。浮游动物可分0~100m、100~400m和400~1000m三个群落, 因不同水层种类组成和丰度差异引起。浮游动物丰度和生物量的垂直变化与温度、叶绿素a质量浓度等因子呈显著正相关。南海东北部陆坡浮游动物季节和垂直变化受季风、沿岸流和中尺度涡的影响。     

The diel vertical migration of the sound-scattering layer in the Yellow Sea Bottom Cold Water of the southeastern Yellow Sea：focus on its relationship with a temperature structure

Using the hydroacoustic method with a 200 kHz scientific echo sounding system,the diel vertical migration(DVM) of the sound-scattering layer(SSL) in the Yellow Sea Bottom Cold Water(YSBCW) of the southeastern Yellow Sea was studied in April(spring) and August(summer) of 2010 and 2011.For each survey,13–27 hours of acoustic data were continuously collected at a stationary station.The acoustic volume scattering strength(Sv) data were analyzed with temperature profile data.In the spring of both 2010 and 2011,the SSL clearly showed the vertical migration throughout the entire water column,moving from the surface layer at night to near the bottom during the day.Conductivity,temperature,and depth data indicated that the entire water column was well mixed with low temperature of about 8 C.However,the SSL showed different patterns in the summers of 2010 and 2011.In the summer of 2010(28 C at the surface),the SSL migrated to near the bottom during the day,but there were two SSLs above and below the thermocline at depth of 10–30 m at night.In the summer of 2011(20 C at the surface),the SSL extended throughout the entire water column at night,possibly owing to an abrupt change in sea weather conditions caused by the passage of a Typhoon Muifa over the study area.It was concluded that the DVM patterns in summer in the YSBCW area may be greatly influenced by a strengthened or weakened thermocline.     

Nitrogen sources for new production in the NE Arabian Sea

New productivity measurements using the ¹⁵N tracer technique were conducted in the north-eastern (NE) Arabian Sea during six expeditions from 2003 to 2007, mostly in winter. Our results indicate that the NE Arabian Sea has a potential for higher new productivity during blooms. Nitrate uptake by plankton is the highest during late winter. New productivity and f-ratios in the NE Arabian Sea are mainly controlled by hydrodynamic and meteorological parameters such as wind strength, sea surface temperature (SST), mixed layer depth (MLD) and mixed layer nitrate. Deepening of the mixed layer supplies nitrate from below, which supports the observed nitrogen uptake. Higher f-ratios during blooms indicate the strong coupling between surface layers and sub-surface layers. Deepening of mixed layer below 100 m (from its inter-monsoon value between 30 and 40 m) transferred often more than 100 mmol N–NO₃ m^? 2 into the surface layers from below. The observed winter blooms in the region are supported by such input and are sustained for more than a month. Higher new productivity has been found in late winter, whereas transport of nitrate is maximum in early winter. In general, new production varies progressively during winter. Diurnal cycling of the mixed layer could be the reason for the under utilization of entrained nitrate during early winter. New productivity values and wind strength show significant differences during Feb–Mar 03 and Feb–Mar 04. These differences indicate that the winter cooling and parameters related the biological productivity also vary inter-annually. However, the difference between the new productivity values between Feb–Mar 03 and Feb–Mar 04 is much lower than the difference between Jan 03 and Feb–Mar 03. The results suggest that amplitude of seasonal variation is higher than the inter-annual variation in the region. During spring, Fickian diffusive fluxes of nitrate into the surface layer range from 0.51 to 1.38 mmol N–NO₃ m^? 2 day^? 1, and can account for 67% and 78% of the observed nitrogen uptake in the coastal and open ocean regions, respectively. We document the intra-seasonal and inter-annual variations in new productivity during winter and identify sources of nitrate which support the observed productivity during spring.     

Diel vertical migration of the copepod Calanus sinicus before and during formation of the Yellow Sea Cold Bottom Water in the Yellow Sea

To understand the effects of the Yellow Sea Cold Bottom Water(YSCBW)on the diel vertical migration(DVM)of the copepod Calanus sinicus,we surveyed vertical distribution of C.sinicus at a fixed station in the Yellow Sea before(spring)and during(summer)formation of the YSCBW.Cold water(<10 C)was observed in the bottom layer when the water column was thermally stratified in summer,but the water column was thermally well-mixed in spring 2010.Samples were collected from five different layers at 3-h intervals using an opening-closing net.Adult females(1–155 ind./m3)showed a clear normal DVM pattern throughout the entire water column in spring,whereas adult males did not migrate.DVM of copepodite V(CV)individuals was not clear,but the maximum abundance of CI–CIV occurred consistently in the upper 10–20 m layer,where there was a high concentration of chlorophyll-a(Chl-a)(0.49–1.19μg/L).In summer,weak DVM was limited to cold waters beneath the thermocline for adult females(<30 ind./m3),but not for adult males.The maximum abundance of CI–CIV also occurred consistently in the subsurface layer(20–40 m)together with high concentrations of Chl-a(0.81–2.36μg/L).CV individuals(1–272 ind./m3)moved slightly upward nocturnally to the near-surface layer(10–20 m),where the average temperature was 25.74 C,but they were not found in the surface layer(0–10 m;28.31 C).These results indicate that the existence of the YSBCW affected food availability at depth and the vertical temperature distribution,leading to variation in the amplitude and shape of stage-specific vertical distributions(CI to adults)in C.sinicus before and during the formation of cold waters in the Yellow Sea during the study period.     

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.     

Downward carbon transport by diel vertical migration of the copepods Metridia pacifica and Metridia okhotensis in the Oyashio region of the western subarctic Pacific Ocean

Seasonal change in the downward carbon transport due to respiration and mortality through diel vertical migration (DVM) of the calanoid copepods Metridia pacifica and Metridia okhotensis was estimated in the Oyashio region, western subarctic Pacific during six cruises from June 2001 to June 2002. M. pacifica (C4, C5 and adult females) was an active migratory species throughout the year though its DVM amplitude varied among seasons and stages. The mean distribution depths of adult females during the daytime were positively related with the illumination level in the water column, being shallowest in April and deepest in January. M. okhotensis generally showed less-extensive migrations than M. pacifica. Therefore, together with their lower abundance, this species is considered to be a less-important mechanism of downward transport of carbon except for April when their DVM was more active and descended deeper than M. pacifica, which remained in the upper 150 m even during the daytime. The mean migrating biomass of the two Metridia species was 558 mg C m⁻² d⁻¹ and was high during summer to winter (263–1676 mg C m⁻² d⁻¹) and low during spring (59–63 mg C m⁻² d⁻¹). Total downward flux through DVM fluctuated between 1.0 and 20.0 mg C m⁻² d⁻¹ with an annual mean of 8.0 mg C m⁻² d⁻¹. Contribution of the respiratory flux was greater than the mortality flux and accounted for 64–98% of total migratory flux throughout the year except for January when contribution of both fluxes was equal. Overall the annual carbon transport by DVM of Metridia spp. was estimated as 3.0 g C m⁻² year⁻¹, corresponding to 15% of the annual total POC flux at 150 m at the study site, suggesting that DVM is a significant process for carbon export in the subarctic region as well as that in tropical and subtropical oceanic regions. Since DVM in M. pacifica is more active during the non-bloom season when the gravitational flux of particulate matter is low, this species plays an important role in driving the biological pump in the subarctic Pacific during summer to winter.     

The seasonal cycle of surface chlorophyll in the Peruvian upwelling system: A modelling study

The seasonal variability of surface chlorophyll in the northern Humboldt Current System is studied using satellite data, in situ observations and model simulations. The data show that surface chlorophyll concentration is highest in austral summer and decreases during austral winter, in phase opposition with coastal upwelling intensity. A regional model coupling ocean dynamics and biogeochemical cycles is used to investigate the processes which control this apparently paradoxical seasonal cycle. Model results suggest that the seasonal variability of the mixed layer depth is the main controlling factor of the seasonality. In winter, the mixed layer deepening reduces the surface chlorophyll accumulation because of a dilution effect and light limitation. In summer, biomass concentrates near the surface in the shallow mixed layer and nitrate limitation occurs, resulting in a biomass decrease in the middle of summer. Intense blooms occur during the spring restratification period, when winter light limitation relaxes, and during the fall destratification period, when the surface layer is supplied with new nutrients. Model sensitivity experiments show that the seasonal variations in insolation and surface temperature have little impact on the surface chlorophyll variability.     

Ideal shocks in 2‐layer flow Part I: Under a rigid lid

Previous work on the classical problem of shocks in a 2‐layer density‐stratified fluid used either a parameterized momentum exchange or an assumed Bernoulli loss. We propose a new theory based on a set of viscous model equations. We define an idealized shock in two‐layer density stratified flow under a rigid lid as a jump or drop of the interface in which (1) the force balance remains nearly hydrostatic in the shock, (2) there is no exchange of momentum between the two layers except by pressure forces on the sloping interface, and (3) dissipative processes can be treated with a constant viscosity. We proceed in two steps. First, we derive a necessary condition for shock existence based on a requirement for wave steepening. Second, we formulate and solve a set of viscous model equations. Some results are the following: Shocks require strong layer asymmetry; one layer must be much faster and/or shallower than the other layer. The linearized equations describing the shock tails provide boundary conditions and a proof of shock uniqueness. It is possible to derive an analytical solution for weak shocks if the steepening condition is met. The weak shock solutions provide closed form expressions for the Bernoulli loss in each layer. Bernoulli losses are strongly concentrated in the expanding layer as the relative layer depth change is much larger in that layer. Bernoulli losses are independent of layer viscosity. A sudden cessation of shock existence is found for strong shocks when the possible end state migrates into the supercritical regime. Surprisingly, the new ideal shock theory compares well with a 2‐D, time‐dependent shallow water model (SWM) with a flux formulation, but with no viscous formulation. Both the Bernoulli drop and shock cessation condition agree quantitatively.     

Diel vertical migration of medusae in the open Southern Adriatic Sea over a short time period (July 2003)

Diel vertical migration (DVM) of medusae was investigated at a fixed station in the oligotrophic Southern Adriatic Sea at several depths during summer (July) 2003. We hypothesized that medusan DVM is considerably influenced by environmental variables such as hydrographic features, light intensities, and potential prey densities. We used short-term repetitive sampling as an approach to detail these relationships. Of the 26 species collected, the highest abundance was in the layer between the thermocline (15 m) and 100 m depth, where Rhopalonema velatum predominated, reaching the maximum count of 93 individuals per 10 m³. Seven species were observed over a wide depth range: Solmissus albescens (15–1200 m), R. velatum (0–800 m), Persa incolorata (50–1200 m), Octophialucium funerarium (200–1200 m), Arctapodema australis (200–1200 m), Amphinema rubra (100–800 m), and Rhabdoon singulare (15–600). According to the medusan weighted mean depth (WMD) calculations, the longest DVMs were noted for the deep-sea species S. albescens , O. funerarium , and A. australis . The shallowest species, Aglaura hemistoma , was primarily non-migratory. Certain medusan assemblages were associated consistently with a particular depth layer characterized by a particular light intensity. The interplay of environmental factors and trophic relationships explains some of the features of medusan migratory patterns. These findings thus contribute to understanding the variables that determine patterns of medusan vertical migratory behavior.     

Phytoplankton community structure in the Arabian Sea during and after the SW monsoon, 1994

Diatoms, dinoflagellates, coccolithophores, nanoflagellates, picophytoplankton and procaryote algae (Synechococcus spp. and prochlorophytes) were quantified by microscopy and flow cytometry, and their biomass determined, at 12 stations along a 1600 km transect across the Arabian Sea at the end of the SW monsoon in September, and during the inter-monsoon period of November/December 1994. The transect spanned contrasting oceanic conditions that varied from seasonally eutrophic, upwelling waters through mesotrophic, downwelling waters to permanently oligotrophic, stratified waters. The overall diversity of diatoms, dinoflagellates and coccolithophores along the transect was not significantly different between the SW monsoon and inter-monsoon. However, diatoms showed greatest diversity during the SW monsoon and coccolithophores were most diverse during the inter-monsoon. Integrated phytoplankton standing stocks during the SW monsoon ranged from 3 to 9 g C m^-2 in the upwelling eutrophic waters, from 3 to 5 g C m^-2 in downwelling waters, and from 1 to 2 g C m^-2 in oligotrophic waters. Similar phytoplankton standing stocks were found in oligotrophic waters during the inter-monsoon, but were ca. 40% lower compared to the SW monsoon in the more physically dynamic waters. Phytoplankton abundance and biomass was dominated by procaryote taxa. Synechococcus spp. were abundant (often >10⁸ cells l^-1) during both the SW monsoon and inter-monsoon, where the nitrate concentration was ⩾0.1 μ mol l^-1, and often dominated the phytoplankton standing stocks. Prochlorophytes were restricted to oligotrophic stratified waters during the SW monsoon period but were found at all stations along the transect during the inter-monsoon, dominating the phytoplankton standing stocks (>40%) in the oligotrophic region during this period. Of the nano- and micro-phytoplankton, only diatoms contributed significantly to phytoplankton standing stocks, and then only in near-shore upwelling waters during the SW monsoon. There were significant changes in the temporal composition of the phytoplankton community. In nearshore waters a mixed community of diatoms and Synechococcus spp. dominated during the SW monsoon. This gave way to a community dominated by Synechococcus spp. in the intermonsoon. In the downwelling zone, a Synechococcus spp. dominated community was replaced by a mixed procaryote community of Synechococcus spp. and prochlorophytes. In the oligotrophic stratified waters, the mix of procaryote algae was replaced by one dominated by prochlorophytes alone.     

Field measurements of fairweather bottom boundary layer processes and sediment suspension on the Louisiana inner continental shelf

Field measurements of bottom boundary layer and sediment-transport processes were made on the Louisiana inner continental shelf in spring 1992 at a depth of 15.5 m, and in spring and summer 1993 at a depth of 20.5 m. Two different wave–current boundary layer/sediment-transport models were applied to the measured near-bed flows. In addition, the log-profile method was applied to estimate hydraulic roughness and bed stress. Consistent with the results of others, our measurements show that near-bed flows were very weak under non-storm conditions. Bed stresses were typically too low to resuspend bed sediments. However, the advection of high-turbidity layers or plumes past the instrumentation apparently caused a sustained period of high suspended sediment concentration throughout the log layer in spring 1993. In the absence of wave activity or high suspended sediment concentrations, boundary layer profiles showed the bed to have been hydraulically very smooth with

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cm. However, wave agitation, combined with increased suspended sediment concentration caused hydraulically rough conditions with

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cm.