Net phytoplankton and zooplankton in the New York Bight, January 1976 to February 1978, with comments on the effects of wind, Gulf Stream eddies, and slope water intrusions

Results are given of monthly net phytoplankton and zooplankton sampling from a 10 m depth in shelf, slope, and Gulf Stream eddy water along a transect running southeastward from Ambrose Light, New York, in 1976, 1977, and early 1978. Plankton abundance and temperature at 10 m and sea surface salinity at each station are listed. The effects of atmospheric forcing and Gulf Stream eddies on plankton distribution and abundance arc discussed. The frequency of Gulf Stream eddy passage through the New York Bight corresponded with the frequency of tropical-subtropical net phytoplankton in the samples. Gulf Stream eddies injected tropical-subtropical zooplankton onto the shelf and removed shelfwater and its entrained zooplankton. Wind-induced offshore Ekman transport corresponded generally with the unusual timing of two net phytoplankton maxima. Midsummer net phytoplankton maxima were recorded following the passage of Hurricane Belle (August 1976) and a cold front (July 1977). Tropical-subtropical zooplankton which had been injected onto the outer shelf by Gulf Stream eddies were moved to the inner shelf by a wind-induced current moving up the Hudson Shelf Valley. (PDF file contains 47 pages.)

The distribution of larval fishes of the Charleston Gyre Region off the southeastern United States in winter shaped by mesoscale, cyclonic eddies

Serial, cyclonic, mesoscale eddies arise just north of the Charleston Bump, a topographical rise on the continental slope and Blake Plateau, and characterize the U.S. outer shelf and upper slope in the region of the Charleston Gyre. This region was transected during the winters of 2000, 2001, and 2002, and hydrographic data and larval fishes were collected. The hydrodynamics of the cyclonic eddies of the Charleston Gyre shape the distribution of larval fishes by mixing larvae from the outer continental shelf and the Gulf Stream and entraining them into the eddy circulation at the peripheral margins, the wrap-around filaments. Over all years and transects (those that intercepted eddies and those that did not), chlorophyll a concentrations, zooplankton displacement volumes, and larval fish concentrations were positively correlated. Chlorophyll a concentrations were highest in filaments that wrapped around eddies, and zooplankton displacement volumes were highest in the continental shelf–Gulf Stream–frontal mix. Overall, the concentration of all larval fishes declined from inshore to offshore with highest concentrations occurring over the outer shelf. Collections produced larvae from 91 fish families representing continental shelf and oceanic species. The larvae of shelf-spawned fishes—Atlantic Menhaden Brevoortia tyrannus, Round Herring Etrumeus teres, Spot Leiostomus xanthurus, and Atlantic Croaker Micropogonias undulatus—were most concentrated over the outer shelf and in the continental shelf–Gulf Stream–frontal mix. The larvae of ocean-spawned fishes—lanternfishes, bristlemouths, and lightfishes—were more evenly dispersed in low concentrations across the outer shelf and upper slope, the highest typically in the Gulf Stream and Sargasso Sea, except for lightfishes that were highest in the continental shelf–Gulf Stream–frontal mix. Detrended correspondence analysis rendered groups of larval fishes that corresponded with a gradient between the continental shelf and Gulf Stream and Sargasso Sea. Eddies propagate northeastward with a residence time on the outer shelf and upper slope of ∼1 month, the same duration as the larval period of most fishes. The pelagic habitat afforded by eddies and fronts of the Charleston Gyre region can be exploited as nursery areas for feeding and growth of larval fishes within the southeastern Atlantic continental shelf ecosystem of the U.S. Eddies, and the nursery habitat they provide, translocate larvae northeastward.

Mesoscale, cyclonic eddies as larval fish habitat along the southeast United States shelf: A Lagrangian description of the zooplankton community

The Charleston Gyre region is characterized by continuous series of cyclonic eddies that propagate northeastwards before decaying or coalescing with the Gulf Stream south of Cape Hatteras, NC, USA. Over 5 d, chlorophyll-a concentration, zooplankton displacement volume, and zooplankton composition and abundance changed as the eddy moved to the northeast. Surface chlorophyll-a concentration decreased, and zooplankton displacement remained unchanged as the eddy propagated. Zooplankton taxa known to be important dietary constituents of larval fish increased in concentration as the eddy propagated. The concurrent decrease in chlorophyll-a concentration and static zooplankton displacement volume can be explained by initial stimulation of chlorophyll-a concentration by upwelling and nutrient enrichment near the eddy core and to possible grazing as zooplankton with short generation times and large clutch sizes increased in concentration. The zooplankton community did not change significantly within the 5 d that the eddy was tracked, and there was no indication of succession. Mesoscale eddies of the region are dynamic habitats as eddies propagate northeastwards at varying speeds within monthly periods. The abundance of zooplankton important to the diets of larval fish indicates that the region can provide important pelagic nursery habitat for larval fish off the southeast coast of the United States. A month of feeding and growth is more than half the larval duration of most fish spawned over the continental shelf of the southeastern United States in winter.

Structural features of geostrophic circulation in open region of the South China Sea

On the basis of observation data of water temperature and salinity the mean seasonal geostrophic circulation in open region of the South China Sea (SCS) was computed by the dynamic method relative to the 800 decibar reference surface. The results of computation let go to following notices: In both main monsoons (winter and summer) there are two main geostrophic eddies: the anticlockwise eddy in the northern and northwestern part, and the clockwise eddy in the southern part of the SCS with corresponding divergent and convergent zones. The main frontal zones go along the middle latitudes of the sea from the southern continental shelf of Vietnam to the area west of Luzon Island. The strength and stability of the current in winter are higher than in summer. The Kuroshio has an enough strong branch intruding into the SCS through Bashi Strait in winter creating in the sea the water structure similar to that of the Northwest Pacific subtropical and tropical regions. In summer the Kuroshio water can intrude directly only into the area southwest of Taiwan.

The study of the sedimentation mechanism in channels under the impact of tidal currents

The purpose of this research is to study sedimentation mechanism by mathematical modeling in access channels which are affected by tidal currents. The most important factor for recognizing sedimentation process in every water environment is the flow pattern of that environment. It is noteworthy that the flow pattern is affected by the geometry and the shape of the environment as well as the type of existing affects in area. The area under the study in this thesis is located in Bushehr Gulf and the access channels (inner and outer). The study utilizes the hydrodynamic modeling with unstructured triangular and non-overlapping grids, using the finite volume, From method analysis in two scale sizes: large scale (200 m to 7.5km) and small scale (50m to 7.5km) in two different time durations of 15 days and 3.5 days to obtain the flow patterns. The 2D governing equations used in the model are the Depth-Averaged Shallow Water Equations. Turbulence Modeling is required to calculate the Eddy Viscosity Coefficient using the Smagorinsky Model with coefficient of 0.3. In addition to the flow modeling in two different scales and the use of the data of 3.5 day tidal current modeling have been considered to study the effects of the sediments equilibrium in the area and the channels. This model is capable of covering the area which is being settled and eroded and to identify the effects of tidal current of these processes. The required data of the above mentioned models such as current and sediments data have been obtained by the measurements in Bushehr Gulf and the access channels which was one of the PSO's (Port and Shipping Organization) project-titled, "The Sedimentation Modeling in Bushehr Port" in 1379. Hydrographic data have been obtained from Admiralty maps (2003) and Cartography Organization (1378, 1379). The results of the modeling includes: cross shore currents in northern and north western coasts of Bushehr Gulf during the neap tide and also the same current in northern and north eastern coasts of the Gulf during the spring tide. These currents wash and carry fine particles (silt, clay, and mud) from the coastal bed of which are generally made of mud and clay with some silts. In this regard, the role of sediments in the islands of this area and the islands made of depot of dredged sediments should not be ignored. The result of using 3.5 day modeling is that the cross channels currents leads to settlement places in inner and outer channels in tidal period. In neap tide the current enters the channel from upside bend of the two channels and outer channel. Then it crosses the channel oblique in some places of the outer channel. Also the oblique currents or even almost perpendicular current from up slope of inner channel between No. 15 and No. 18 buoys interact between the parallel currents in the channel and made secondary oblique currents which exit as a down-slope current in the channel and causes deposit of sediments as well as settling the suspended sediments carried by these currents. In addition in outer channel the speed of parallel currents in the bend of the channel which is naturally deeper increases. Therefore, it leads to erosion and suspension of sediments in this area. The speed of suspended sediments carried by this current which is parallel to the channel axis decreases when they pass through the shallower part of the channel where it is in the buoys No.7 and 8 to 5 and 6 are located. Therefore, the suspended sediment settles and because of this process these places will be even shallower. Furthermore, the passing of oblique upstream leads to settlement of the sediments in the up-slope and has an additional effect on the process of decreasing the depth of these locations. On the contrary, in the down-slope channel, as the results of sediments and current modeling indicates the speed of current increases and the currents make the particles of down-slope channel suspended and be carried away. Thus, in a vast area of downstream of both channels, the sediments have settled. At the end of the neap tide, the process along with circulations in this area produces eddies which causes sedimentation in the area. During spring some parts of this active location for sedimentation will enter both channels in a reverse process. The above mentioned processes and the places of sedimentation and erosion in inner and outer channels are validated by the sediments equilibrium modeling. This model will be able to estimate the suspended, bed load and the boundary layer thickness in each point of both channels and in the modeled area.