Netherlands Journal of Sea Research最新文献

One- and two-dimensional fractal dimensions of in situ flocs were determined from the dependence of the are and perimeter of projection of the flocs on their greatest length, using an underwater camera and image-analysis system. Values for the fractal dimensions of flocs in the Elbe estuary and the North Sea varied among the samples: D₁ in the range of 1.03 to 1.14 and D₂ in the range of 1.41 to 1.81. D₂ as obtained here represents the three-dimensional fractal dimension of flocs, which has been obtained in previous studies from the dependence of settling velocity or porosity of flocs on their characteristic length. Comparisons of environmental factors showed no relationships between the fractal dimension and fluid shear. Salinity effects on the fractal dimension were probably also insignificant in our samples. The relationship of D₁ with the concentration of suspended matter was fairly convincing and suggests that the flocs had a rougher edge of projection at high than at low concentrations.

The development and decline of a phytoplankton spring bloom dominated by the prymnesiophyte Phaeocystis pouchetii were studied in Balsfjord, northern Norway between 30 March and 27 May 1992. At a fixed station, the concentration and composition of suspended particulate matter was monitored and compared to the particulate matter collected in sediment traps at six different depths. Direct sedimentation of phytoplankton contributed a minor fraction to particle flux and was confined to a few diatom genera. No evidence was found for pronounced aggregation of Phaeocystis colonies during bloom decline or direct sedimentation of either Phaeocystis colonies or single cells. Particle flux was dominated by faecal-pellet sedimentation during most of the study period, suggesting zooplankton grazing to be a main loss factor. Despite an abrupt decrease in faecal-pellet sedimentation after the decline of the bloom, particulate-carbon sedimentation rates remained high. High post-bloom sedimentation rates were characterized by elevated C/N and C/Chl a ratios of largely amorphous sedimented material. Post-bloom sedimentation coincided with a decrease in transparent exopolymeric particles (TEP) in the surface layer, suggesting that this change resulted from aggregation and sedimentation of carbon-rich exopolymeric material accumulated in the surface layer in the course of the bloom. While organic-carbon accumulation indicates the significance of disintegration of Phaeocystis colonies, post-bloom mucilage sedimentation could be secondary pathway for the vertical flux of Phaeocystis-derived organic matter.

To assess relationships between the life cycle of the seagrass Zostera noltii and light conditions in its habitat, the seasonal dynamics of a seagrass-dominated community on a tidal flat off Terschelling were studied. The main components of this community were seagrass, periphyton and the periphyton grazing mudsnail Hydrobia ulvae. Total biomass of the seagrass stand showed a unimodal curve with a maximum of more than 110 gADW·m⁻² in August and a minimum of less than 10 gADW·m⁻² in January. Chlorophyll density of periphyton on the seagrass leaves followed a more or less similar pattern, ranging from 0.4 μg chlorophyll·cm⁻² in May to more than 3 μg chlorophyll·cm⁻² at the end of August. Periphyton biomass was, however, already maximal in May with almost 1.6 mgADW·cm⁻² and subsequently decreased to less than 0.6 mgADW·cm⁻² in August. The total weight of H. ulvae was more or less stable, varying between more than 150 and less than 400 gDW·m⁻², although significant changes were observed within size classes.

Light is assumed to be the primary limiting factor for seagrass distribution in the Wadden Sea. The light conditions of seagrass in the study area were influenced by periphyton and mudsnails. Leaf growth rates and biomass development appeared to be related with light conditions in the seagrass habitat. Shading caused by periphyton during the growing season was estimated at 10 to 90% of incident light, resulting in a reduction of about 2 to 80% of the yearly period during which the light compensation point (LCP) of the seagrass is exceeded. The mudsnails were found to be theoretically able to ingest daily 25 to 100% of the standing stock of periphyton and microphytobenthos. It is concluded that shading by periphyton and grazing by mudsnails play an important role in the seasonal biomass development and survival of Z. noltii in the seagrass-dominated community on a tidal flat off Terschelling.

An analysis of particle settling behaviour was carried out in one of the University of Hamburg's mesocosm carousel flumes, using a flocculated suspension from the Elbe Estuary. After fourteen days of stirring with a simulated tidal-current oscillation to a maximum of 24 cm·s⁻¹, the material in suspension was allowed to settle for an extended period. Results obtained from image analysis of silhouette photos taken in situ, and from total SPM and particle size analysis of the inorganic constituent grains of water samples collected, showed different stages in the settling of the flocculated material. Inorganic-rich flocs began to settle out approximately one minute after the paddles in the flume had been stopped. This initial settling period was followed by an increase in the modal diameter and concentration of organic-rich flocs to a maximum after five minutes. The increase in floc diameter preceded a rapid clearing of the water column during which the shape of the settling flocs changed from roughly spherical to elongate stringers. Fifty percent of the material in suspension at 1 m below the surface had settled by eleven minutes after stirring had stopped and by one hour the water column appeared to have reached an equilibrium concentration of small flocs representing only ten percent of the original concentration. The implications of this rapid flocculation on results obtained from in situ settling columns are discussed.

Long-term (>6 months) current measurements from five moorings in the Iceland Basin have been analysed for the mean currents and the structure of the variable current components. The time-averaged flow at all five moorings had a strong baroclinic character. The mean circulation in the upper layers with relatively warm Sub-Polar Mode Water appears to have a general north-eastward direction with maximum mean velocities of 6 to 7 cm·s⁻¹. In the bottom layer south of Iceland, where the cold Iceland-Scotland Overflow Water flows westwards along the topography in a Deep Northern Boundary Current, mean velocities of the order of 10 to 20 cm·s⁻¹ have been observed. Over the deep slope of the Hatton Bank, water enters the Iceland Basin in a branch of the Deep Northern Boundary Current which has a cyclonic rotation sense in the Iceland Basin. The variable part of the current has been analysed by means of principal-component analysis. The current variations in the central Iceland Basin appear to have a mainly barotropic character while variations in the baroclinic flow of Iceland-Scotland Overflow Water contributed 10% or less to the total energy of the variable deep flow. Over the slope of the Hatton Bank the variable currents had a mainly baroclinic character with shear in both current speed and direction. Comparison of the geostrophic velocity with the mean Eulerian velocity has revealed that the σ_θ=27.725 kg·m⁻³ surface can be used adequately as level of non-motion for the geostrophic modelling of the flow along the Iceland and Hatton slopes. The mean westward geostrophic transport of ISOW south of Iceland relative to this reference surface amounted to 3.5 Sv, in agreement with existing independent estimates.