Thermocline characteristics and phytoplankton dynamics in Agulhas Bank waters

Eastern and central Agulhas Bank waters are characterized by strong, shallow thermoclines and well developed subsurface maxima of phytoplankton biomass. The thermoclines are advectively maintained and as such are dynamically stable, but they do exhibit short-term variations in absolute temperature gradients. There are corresponding variations in subsurface phytoplankton biomass maxima. The water-column phytoplankton production maxima are generally situated in the biomass maxima because these are sufficiently shallow to ensure adequate light. Self-shading becomes a limiting factor with the development of high phytoplankton biomasses. Western Agulhas Bank waters are characterized by deeper thermoclines and less intense subsurface maxima of phytoplankton biomass. Due to the depth of the thermoclines the biomass maxima are generally situated below the depth of the 1-per-cent surface light intensity and, consequently, phytoplankton production is limited. Possible mechanisms for the maintenance of these deep chlorophyll maxima are discussed.     

Post-Palaeozoic stratigraphy of the eastern Agulhas Bank,South African continental margin

The Agulhas Bank, which forms the continental margin on the southern tip of Africa, consists of a thick Meso-Cainozoic sedimentary sequence (up to 6.2 km) resting on and behind pre-Mesozoic continental acoustic basement. The stratigraphy of this sequence is outlined and its history and facies variations mentioned where they are known. Refraction seismic velocity and bottom sample data indicate a basic three-fold subdivision of the Mesozoic sequence, which can be correlated with the onshore succession in the Algoa Basin. It is separated by a major hiatus from the Cainozoic sediments, which consist of a Palaeogene and Neogene sequence subdivided by another well-defined level of erosion. Various formations within the Cainozoic are defined and named. An outline of the bathymetry of the eastern Agulhas Bank is also given.     

Anchovy spawning in relation to the biomass and the replenishment rate of their copepod prey on the western Agulhas Bank

Anchovy biomass and copepod standing stocks and growth rates on the Agulhas Bank were compared during the peak spawning period (November) in 1988 and 1989. In 1988, copepod biomass over the western Agulhas Bank was low (1,0 g dry mass·m^?2) relative to anchovy biomass there (14,7 g dry mass·m^?2). In November 1989 in the same area, fish biomass was much lower (5,7 g dry mass·m^?2), following a recruitment failure, and copepod biomass was higher (2,4 g dry mass·m^?2), possibly as a result of lesser predation by anchovy. By contrast, the eastern Agulhas Bank had a larger biomass of copepods (4–6 g dry mass·^?2) and a lower biomass of anchovy during both years. Knowing, from laboratory studies, that a prey biomass of 0,78 g·m^?2 is required for fish to obtain their daily maintenance ration, it is suggested that spawning on the western Agulhas Bank was food-limited in 1988. Copepods on the western Bank may be replaced by local growth or transport from the eastern Bank. Growth rates of copepods on the western Bank were 10–50 per cent of maximum in 1988, but total production (c. 100 mg dry mass·m^?2·day^?1) was low, primarily because biomass was low and less than the rate of consumption by anchovy (243 mg copepod dry mass·m^?2·day^?1). On the eastern Bank, copepod production exceeded anchovy consumption and it is concluded that the flux of copepod biomass onto the western Bank may be as important as local growth in replenishing copepod stocks there. Feeding conditions for anchovy on the western Agulhas Bank are often marginal compared to the situation on the eastern Bank, and it is suggested that the selection of the western Bank as the major spawning area is related more to the success of transport and survival of eggs and larvae on the West Coast recruiting grounds than to feeding conditions per se.     

九龙江口生物可利用磷的行为与入海通量

于1995年3、5月在九龙江口进行两个航次的现场调查,研究不同形态磷的河口行为,并估算生物可利用磷的入海通量。结果表明,在S＜20区域内,溶解活性磷（DRP）的行为受控于“缓冲机制”,其含量在8～10μg／dm3水平上下波动。颗粒磷中MgCl2。可提取磷（MgCl2-P）含量随盐度增大而增大,并与叶绿素a以及悬浮物中Fe、Mn、Al含量呈正相关关系。NaOH／NaCl可提取磷（NaOH／NaCl－P）随盐度增大按指数关系下降,其含量与悬浮物中Fe、Mn、Al含量呈正相关关系。HCl可提取磷（HCl－P）在河口区的行为表现为保守混合。九龙江口生物可利用磷入海通量约517t／a,其中潜在生物可利用磷（PBAP）的贡献为溶解活性磷（DRP）的3倍。     

A 450-kyr record of hydrological conditions on the western Agulhas Bank Slope, south of Africa

The Agulhas Bank region, south of Africa, is an oceanographically important and complex area. The leakage of warm saline Indian Ocean water into the South Atlantic around the southern tip of Africa is a crucial factor in the global thermohaline circulation. Foraminiferal assemblage, stable isotope and sedimentological data from the top 10 m of core MD962080, recovered from the western Agulhas Bank Slope, are used to indicate changes in water mass circulation in the southeastern South Atlantic for the last 450 kyr. Sedimentological and planktonic foraminiferal data give clear signals of cold water intrusions. The benthic stable isotope record provides the stratigraphic framework and indicates that the last four climatic cycles are represented (i.e. down to marine isotope stage (MIS) 12). The planktonic foraminiferal assemblages bear a clear transitional to subantarctic character with Globorotalia inflata and Neogloboquadrina pachyderma (dextral) being the dominant taxa. Input of cold, subantarctic waters into the region by means of leakage through the Subtropical Convergence, as part of Agulhas ring shedding, and a general cooling of surface waters is suggested by increased occurrence of the subantarctic assemblage during glacial periods. Variable input of Indian Ocean waters via the Agulhas Current is indicated by the presence of tropical/subtropical planktonic foraminiferal species Globoquadrina dutertrei, Globigerinoides ruber (alba) and Globorotalia menardii with maximum leakage occurring at glacial terminations. The continuous presence of G. menardii throughout the core suggests that the exchange of water from the South Indian Ocean to the South Atlantic Ocean was never entirely obstructed in the last 450 kyr. The benthic carbon isotope record and sediment textural data reflect a change in bottom water masses over the core location from North Atlantic Deep Water to Upper Southern Component Water. Planktonic foraminiferal assemblages and sediment composition indicate a profound change in surface water conditions over the core site approximately 200–250 kyr BP, during MIS 7, from mixed subantarctic and transitional water masses to overall warmer surface water conditions.     

Biology of the redspotted tonguesole Cynoglossus zanzibarensis (Pleuronectiformes: Cynoglossidae) on the Agulhas Bank,South Africa

The biology of the redspotted tonguesole Cynoglossus zanzibarensis, a common African cynoglossid inhabiting the Agulhas Bank, South Africa, is described. Growth studies based on sectioned sagittal otoliths revealed that C. zanzibarensis is relatively fast-growing and long-lived, attaining ages >8 years. Growth in length was rapid in immature fish, fish attaining 56% of their maximum size within their first year. By sexual maturity, fish had attained 28% of their maximum age and 68% of their maximum length. Total length-at-age was best described by the Von Bertalanffy growth model with combined-sex growth described as L_t = 354.78(1?e^{?0.43 (t+1.17)}) mm TL. Sexually dimorphic growth patterns were evident, females attaining larger lengths, but at a slower growth rate than males. Despite the similar mean size of adult fish, the trawl-sampled adult population was dominated by females, with a sex ratio of 1 male:2.4 females. Female C. zanzibarensis mature in their second year of life (275 mm TL), after which they spawn small, pelagic eggs throughout the year. Approximations of the rates of total, natural and fishing mortality were estimated to be 0.62, 0.48 and 0.14 year^?1 respectively.     

Phytoplankton community characteristics in the coastal waters of the southeastern Arabian Sea Phytoplankton community characteristics in the coastal waters of the southeastern Arabian Sea

Remote sensing applications are important in the fisheries sector and efforts were on to improve the predic-tions of potential fishing zones using ocean color. The present study was aimed to investigat...     

环渤海湿地盐地碱蓬对碳、氮、磷吸收通量的研究

主要调查环渤海湿地的盐地碱蓬对碳、氮、磷的吸收通量。调查表明,环渤海湿地的盐地碱蓬面积为7 397 hm2。2009年7月、10月和11月盐地碱蓬总生物量(干重)分别为2.104×104t、2.615×104t和1.523×104t。7月、10月和11月,盐地碱蓬对碳的吸收通量分别为6 577 t、8 538 t和5 713 t;对氮的吸收通量分别为201.2 t、278.6 t、177.7 t;对磷的吸收通量分别为17.00 t、32.40 t、16.60 t;盐地碱蓬对碳、氮、磷的最大吸收通量均为10月。     

A transition in weather over the Agulhas Current

Aircraft observations were used to investigate contrasting weather conditions over the northern edge of the warm Agulhas Current south of George, South Africa, during June 1989. A sea surface temperature front of 7°C over 10 km was observed near 35°S, 23°E. Surface-sensible and latent heat fluxes increased by a factor of five on the warm side. A deepening of the moist unstable layer was indicated by increased equivalent potential temperatures over the warm side. Most significantly, convectively generated turbulence, as inferred from variances in air pressure, increased exponentially with sea surface temperature and surface layer winds doubled south of 35°S. The sea state, initially smooth over the continental shelf, became rough seawards of the northern edge of the Agulhas Current. Numerical model simulations were performed to assess air-sea interactions. Model-simulated thermodynamic variables responded in close agreement with observations, but the increase in cross-frontal winds was not fully resolved. The aircraft observations provide evidence that the Agulhas Current affects the large-scale pressure field and may enhance the rain-bearing capacity of transient weather systems.     

Export stoichiometry and migrant-mediated flux of phosphorus in the North Pacific Subtropical Gyre

Export processes play a major role in regulating global marine primary production by reducing the efficiency of nutrient cycling and turnover in surface waters. Most studies of euphotic zone export focus on passive fluxes, that is, sinking particles. However, active transport, the vertical transfer of material by migrating zooplankton, can also be an important component of carbon (C) and nitrogen (N) removal from the surface ocean. Here we demonstrate that active transport is an especially important mechanism for phosphorus (P) removal from the euphotic zone at Station ALOHA (Hawaii Ocean Time-series program; 22°45′N, 158°W), a P-stressed site in the North Pacific Subtropical Gyre. Migrant excretions in this region are P-rich (C₅₁:N₁₂:P₁) relative to sinking particles (C₂₅₀:N₃₁:P₁), and migrant-mediated P fluxes are almost equal in magnitude (82%) to P fluxes from sediment traps. Migrant zooplankton biomass and therefore the importance of this P removal pathway relative to sinking fluxes has increased significantly over the past 12 years, suggesting that active transport may be a major driving force for enhanced P-limitation of biological production in the NPSG. We further assess the C:N:P composition of zooplankton size fractions at Station ALOHA (C₈₈:N₁₈:P₁, on average) and discuss migrant-mediated P export in light of the balance between zooplankton and suspended particle stoichiometries. We conclude that, because active transport is such a large component of the total P flux and significantly impacts ecosystem stoichiometry, export processes involving migrant zooplankton must be included in large-scale efforts to understand biogeochemical cycles.     

珠江口及邻近水域鱼类群落结构研究

根据1986～1987年珠江口及邻近水域底拖网鱼类采样资料,运用系统聚类与排序等多元分析方法和群落多样性指数,研究该水域鱼类群落结构及其与环境因素的关系,分析结果把该水域鱼类划分为淡水、河口和沿岸3个群落,分别位于莲花山,虎门河道、珠江河口伶仃洋和珠江口浅海水域,各群落结构组成呈沿盐度梯度而成带分布的格局.淡水群落结构简单稳定,沿岸群落结构比较复杂,河口群落受水动力过程和生物活动影响最大,群落结构的季节变化较为显着.     

Macrobenthic community structure in the Changjiang Estuary and its adjacent waters in summer

On the basis of data collected in the summer of 2006 from 27 sampling stations in the Changjiang Estuary and its adjacent waters, the ecological characteristics of macrobenthos and the relationship between the macrobenthos and the environmental factors were studied using hierarchical cluster and non-metric multidimensional scaling (MDS).The biomass, abundance, Shannon-Wiener's and Margalef's indices of the macrobenthos were presented. The results showed that a total of 253 macrobenthic species were found in the research region, and most of them belong to mollusks and polychaetes. The dominant species were Cossurella dimorpha, Eocylichna cylindrella, Episiphon kiaochowwanense, Nassarius semiplicatus, Ocstergrenia variabilis and Sternaspis scutata. The average abundance of the macrobenthos was (313.15±233.4) ind./m2, and the average biomass was (15.2±11.2)g/m2.The distribution patterns of the abundance and biomass of the macrobenthos were similar. The abundance and biomass in the area close to the estuary were lower than those from the area more distant to the estuary; the central part of the research region had higher abundance and biomass than other parts of the research region. In accordance with the results, four macrobenthic communities with distinct spatial differences were identified. The low abundance and biomass in the area close to the estuary should be caused by the high sedimentation rate. The statistical analysis indicated that the depth is the most important factor affecting the distribution of macrobenthos.     

Routes of Agulhas rings in the southeastern Cape Basin

Using weekly sea surface height data, Agulhas rings from the period October 1992 to December 2006 are detected and tracked, from their formation dates and throughout the Cape Basin. While 102 of them formed at the Agulhas Current retroflection, their subsequent subdivisions and junctions led to 199 trajectories. The rings geographical probability of presence shows two maxima. One, related to numerous ring passages, lies in the submarine bight formed by the Erica seamount, the Schmitt-Ott seamount, and the northeastern tip of the Agulhas Ridge. The other one, to be ascribed to topographic blocking of the eddies, is southeast of the latter obstacle. On the basis of topographic effects three routes for Agulhas rings are distinguished, a Northern route for rings that enter the south-Atlantic northeast of the Erica seamount, a Central one for those passing westward between this seamount and the tip of the Agulhas Ridge, and a Southern one farther south. Despite its bathymetric obstacles, the central route is the dominant one, both in terms of percentage of eddy crossings at its definition segment, and in terms of conveyed volume transport. Specific behaviours of rings along each route are described, referring to observations in previous studies. Some rings from the Northern route interact with the flow regime of the South African continental slope. The southernmost trajectories of the Central route are thought to settle the location of the climatological Subtropical Front in that region. The rings of the Southern route experience important core property alteration as they transit through the subantarctic domain.     

Suspended matter transport in coral reef waters of the Abrolhos Bank, Brazil

The coral reef system of the Abrolhos Bank, Brazil, is located between 10 and 65 km off the coast. Suspended particulate matter between the coast and the inner arc of the reef was mainly composed of kaolinite clay and reworked fossil carbonate fragments, resuspended from nearshore shoals. Strong permanent and tidal alongshore currents in the nearshore channel together with the geomorphological configuration of the inner arc formed an efficient hydrodynamic and topographic barrier to offshore transport of land-derived material. Most of the material was being transported alongshore. Suspended particulate matter between the inner and outer arcs was dominated by carbonate shells, coral fragments, and needle-shaped biogenic opal, and the reef system was in large part being dominated by the oligotrophic waters of the Brazil Current.     

Phytoplankton community characteristics in the coastal waters of the southeastern Arabian Sea

Remote sensing applications are important in the fisheries sector and efforts were on to improve the predictions of potential fishing zones using ocean color. The present study was aimed to investigate the phytoplankton dynamics and their absorption properties in the coastal waters of the southeastern Arabian Sea in different seasons during the year 2010 to 2011. The region exhibited 73 genera of phytoplankton from 19 orders and 41 families. The numerical abundance of phytoplankton varied from 14.235×103 to 55.075×106 cells/L. Centric diatoms dominated in the region and the largest family identified was Thalassiosiraceae with main genera as Skeletonema spp., Planktionella spp. and Thalassiosira spp. Annual variations in abundance of phytoplankton showed a typical one-peak cycle, with the highest recorded during premonsoon season and the lowest during monsoon season. The species diversity index of phytoplankton exhibited low diversity during monsoon season. Phytoplankton with pigments Chlorophyll a, Chlorophyll b, Chlorophyll c, peridinin, diadinoxanthin, fucoxanthin, β-carotene and phycoerythrobilin dominated in these waters. The knowledge on phytoplankton dynamics in coastal waters of the southeastern Arabian Sea forms a key parameter in bio-optical models of pigments and productivity and for the interpretation of remotely sensed ocean color data.     

夏季台湾浅滩周边水域悬浮物质浊度和叶绿素a荧光分布特征

2004年7~8月在台湾浅滩周边水域进行了两个断面的水体悬浮物浊度、叶绿素a荧光调查,通过对该区域夏季悬浮物质浊度、叶绿素a荧光的分布变化特征研究,结合水文相关要素分析,初步揭示了该区域海洋沉积和再悬浮过程对悬浮体浊度、叶绿素a荧光分布的影响.悬浮物质浊度在近岸与陆架外缘主要受再悬浮作用影响,而浅滩区再悬浮作用不明显;在陆架外缘区浮游植物的聚集作用对浊度有一定的贡献作用,而近岸区则不明显.     

Study on fish community structure in the Zhujiang Estuary and its adjacent waters

I~IOXMoreattentionhasbenpaidtothefishcornlnwhtiesintheestuaryandcoastalwaters,andanemphasishashogivenontheuseofvariousdiversityindicesandmultivariateanalysismethanethat~onthaisindicesinthestudyofthecharacteristicsOffishcornmwhties(Zhuetal.,1994;Zhuet...     

A preliminary study of age and growth of the monkfish Lophius upsicephalus (Pisces: Lophiidae) on the Agulhas Bank,South Africa

The sagittal otoliths of Lophius upsicephalus, although extremely variable in shape, were found to be the only structures suitable for age estimation. The annuli are multi-ringed zones separated by "wider than normal" opaque zones. They are difficult to interpret, especially those of fish older than nine years. Periodicity of annulus formation could not be determined by standard methods. By inference, annulus formation could be related to feeding habits and reproductive seasonality. The growth in length (sexes combined) is best described by

Length-at-age = 733,7 (1 – e^{?0,1054 (t + 1,879)})mm.

There is a significant difference in growth by mass between the sexes. The respective equations describing growth by mass (in g) for males and females are:

Weight-at-age = 8616 (1 – e^{?0,1054 (t + 1,879)})^2,805 and

Weight-at-age = 9499 (1 – e^{?0,1054 (t + 1,879)})^2,890.