Abundance, size distribution, and stable carbon and nitrogen isotopic compositions of marine organic matter isolated by tangential-flow ultrafiltration

Tangential-flow ultrafiltration was used to isolate particulate and high-molecular-weight dissolved material from seawater collected at various depths and geographic regions of the Pacific and Atlantic Oceans. Ultrafiltration proved to be a relatively fast and efficient method for the isolation of hundreds of milligrams of material. Optical and electron microscopy of the isolated materials revealed that relatively fragile materials were recovered intact. Depth-weighted results of the size distribution of organic matter in seawater indicated that ˜ 75% of marine organic carbon was low-molecular-weight (LMW) dissolved organic carbon (< 1 nm), ˜ 24% was high-molecular-weight (HMW) dissolved organic carbon (1–100 nm), and ˜ 1% was particulate organic carbon (> 100 nm). The distribution of carbon in surface water was shifted to greater relative abundances of larger size fractions, suggesting a diagenetic sequence from macromolecular material to small refractory molecules. The average C:N ratios of particulate organic matter (POM) and HMW dissolved organic matter (DOM) were 7.7 and 16.7, respectively. Differences in C:N ratios between POM and HMW DOM were large and invariant with depth and geographic region, indicating that the aggregation of HMW DOM to form POM must be of minor significance to overall carbon dynamics. The stable carbon isotope composition (δ¹³C) of POM averaged −22.7%. in surface water and −25.2%. in subsurface water. Several possible explanations for the observed isotopic shift with depth were explored, but we were unable to discern the cause. The δ¹³C of HMW DOM samples was relatively constant and averaged −21.7%., indicating a predominantly marine origin for this material. The δ¹⁵N values of POM were highly variable (5.8–15.4%.), and the availability of nitrate in surface waters appeared to be the major factor influencing δ¹⁵N values in the equatorial Pacific. In the upwelling region nitrate concentrations were relatively high and δ¹⁵N values of POM were low, whereas to the north and south of the upwelling nitrate concentrations were low and δ¹⁵N values were high. The δ¹⁵N values of HMW DOM reflected the same trends observed in the POM fraction and provided the first such evidence for biological cycling of dissolved organic nitrogen (DON). Using the observed δ¹⁵N values and an estimate of meridional advection velocity, we estimated a turnover time of 0.3 to 0.5% day⁻¹ for HMW DON. These results suggest a major role for DON in the upper ocean nitrogen cycle.     

Interannual and regional variability in distribution and ecology of juvenile pollock and their prey in frontal structures of the Bering Sea

The distribution, size, length-specific weight, growth, and feeding of age-0 walleye pollock (Theragra chalcogramma) were examined along with their prey distribution patterns in two contrasting transects over a 4-year period (1994–1997) in relation to biophysical properties of frontal regions around the Pribilof Islands, Bering Sea. There were significant interannual differences in catch of age-0 pollock, but transect and habitat differences (inshore vs. front vs. offshore) were not significant for either catch or size of pollock. There were significant variations in length-specific weight and growth of pollock, but the trends were inconsistent. Copepods dominated the zooplankton biomass in all habitats and years; there were no consistent differences in the densities of the dominant zooplankton taxa among the habitats. There were, however, strong habitat and transect differences in juvenile pollock diet, particularly for the larger and presumably rarer prey taxa (euphausiids, chaetognaths, fish). We did not find any evidence that occupying a particular habitat was beneficial to young pollock, although other factors (e.g. bioenergetic advantage and predation refuge) that we did not examine here could have been more variable and critical to pollock survival. In a physically dynamic system such as the Pribilof Islands, age-0 pollock may need to continuously search for optimal conditions of high prey availability and low predation pressure.     

Reproducibility of spiked-sediment bioassays using the marine benthic amphipod, Corophium volutator

Corophium volutator (Pallas; Amphipoda: Corophiidae) is an ecologically important representative of the infauna of marine sediments and is widely distributed along the north-east American and the west European coasts. Methods for conducting short-term toxicity tests with lindane-spiked sediment using this species were developed. Corophium volutator were used in 10-d lindane-spiked sediment tests conducted on seven occasions throughout the year, to investigate the repeatability of test methods and to quantify differences in sensitivity for lindane due to seasonal field sampling. The 10-d LC₅₀ values found for lindane ranged from 0.78 to 1.49 μg/g dry weight (dw), a relatively small range. Higher values were found in the months of April 1994 and April 1995 (1.46 and 1.49 μg/g, respectively). Positive controls conducted with cadmium (aqueous phase) also revealed lower sensitivity of amphipods in those months. The results however, show that a factor of two difference in LC₅₀ values among tests conducted in different seasons was within the range of expected variability. A study on the size-structure of the wild amphipod population indicated that animals of all size-classes were available throughout the whole year except for the months of April and May. Experiments conducted with organisms of different length-classes revealed that differences in sensitivity to lindane were not size-related. The present study also supports the use of field-collected organisms of ecological relevance, such as C. volutator, in ecotoxicological studies.     

One-dimensional modelling of individual waves and wave-induced longshore currents in the surf zone

A probabilistic model ( -model) was developed to describe the propagation and transformation of individual waves (wave by wave approach). The individual waves shoal until an empirical criterion for breaking is satisfied. Wave height decay after breaking is modelled by using an energy dissipation method. Wave-induced set-up and set-down and breaking-associated longshore currents are also modelled. Laboratory and field data were used to calibrate and verify the model. The model was calibrated by adjusting the wave breaking coefficient (as a function of local wave steepness and bottom slope) to obtain optimum agreement between measured and computed wave height. Four tests carried out in the large Delta flume of Delft Hydraulics were considered. Generally, the measured H_1/3-wave heights are reasonably well represented by the model in all zones from deep water to the shallow surf zone. The fraction of breaking waves was reasonably well represented by the model in the upsloping zones of the bottom profile. Verification of the model results with respect to wave-induced longshore current velocities was not extensive, because of a lack of data. In case of a barred profile the measured longshore velocities showed a relatively uniform distribution in the (trough) zone between the bar crest and the shoreline, which could to some extent be modelled by including space-averaging of the radiation force gradient, horizontal mixing and longshore water surface gradients related to variations in set-up. In case of a monotonically upsloping profile the cross-shore distribution of the longshore current velocities is reasonably well represented.     

Seabed geotechnical parameters from electrical conductivity measurements

Electrical conductivity measurements provide useful geotechnical information, particularly porosity data, and are well suited for use in conjunction with acoustic profiling techniques. A proposed conductivity measurement system for surveying the seabed is characterized by a towed inductive multifrequency probe operating underwater. It is integrated with a shipboard data acquisition and processing system.     

The influence of acoustic emissions for underwater data transmission on the behaviour of harbour porpoises (Phocoena phocoena) in a floating pen

To prevent grounding of ships and collisions between ships in shallow coastal waters, an underwater data collection and communication network is currently under development: Acoustic Communication network for Monitoring of underwater Environment in coastal areas (ACME). Marine mammals might be affected by ACME sounds since they use sounds of similar frequencies (around 12 kHz) for communication, orientation, and prey location. If marine mammals tend to avoid the vicinity of the transmitters, they may be kept away from ecologically important areas by ACME sounds. One marine mammal species that may be affected in the North Sea is the harbour porpoise. Therefore, as part of an environmental impact assessment program, two captive harbour porpoises were subjected to four sounds, three of which may be used in the underwater acoustic data communication network. The effect of each sound was judged by comparing the animals' positions and respiration rates during a test period with those during a baseline period. Each of the four sounds could be made a deterrent by increasing the amplitude of the sound. The porpoises reacted by swimming away from the sounds and by slightly, but significantly, increasing their respiration rate. From the sound pressure level distribution in the pen, and the distribution of the animals during test sessions, discomfort sound level thresholds were determined for each sound. In combination with information on sound propagation in the areas where the communication system may be deployed, the extent of the 'discomfort zone' can be estimated for several source levels (SLs). The discomfort zone is defined as the area around a sound source that harbour porpoises are expected to avoid. Based on these results, SLs can be selected that have an acceptable effect on harbour porpoises in particular areas. The discomfort zone of a communication sound depends on the selected sound, the selected SL, and the propagation characteristics of the area in which the sound system is operational. In shallow, winding coastal water courses, with sandbanks, etc., the type of habitat in which the ACME sounds will be produced, propagation loss cannot be accurately estimated by using a simple propagation model, but should be measured on site. The SL of the communication system should be adapted to each area (taking into account bounding conditions created by narrow channels, sound propagation variability due to environmental factors, and the importance of an area to the affected species). The discomfort zone should not prevent harbour porpoises from spending sufficient time in ecologically important areas (for instance feeding areas), or routes towards these areas.     

Mechanisms for marine regime shifts: Can we use lakes as microcosms for oceans?

Although striking regime shifts have been observed in marine systems, mechanistic explanations for these phenomena remain scarce. Here, we review studies of regime shifts in lakes and compare them to what has been found in marine systems to explore the extent to which lakes can be viewed as microcosms for studying essential ecosystem feedbacks in marine systems. We conclude that despite obvious differences, which have been discussed by several authors, lakes are in fact quite similar to oceans in many aspects. In particular, similar mechanisms may be involved in causing alternative attractors (and thus a tendency to show occasional regime shifts) in both systems. A striking similarity exists between the mechanisms that may have caused massive marine anoxic events in the distant past, and the shift of many deep lakes to a mode of anoxic phosphorus recycling. Biotic interactions are relatively poorly understood in the oceans. However, based on what we know from lakes, we expect competitive interactions and diffuse multi-species interactions to be a common cause of alternative attractors in marine communities. In addition, overexploitation traps are an obvious cause of multiple attractors to be expected in the oceans. These should not be limited to direct effects of human overexploitation. Shifts between overexploited and under-exploited states may happen at various trophic levels. Finally, we conclude from observed patterns and theoretical results that implications of alternative attractors will be quite different in benthic and open ocean systems. Benthic regime shifts might happen easily but can be relatively local. By contrast, open ocean shifts might not arise so easily but tend to be impressive in magnitude and scale when they happen.     

Do bottom-intensified diurnal tidal currents shape the alignment of carbonate mounds in the NE Atlantic?

Measurements of near-seabed currents at different carbonate mound locations have demonstrated the presence and influence of bottom-magnified diurnal-period tidal motions. These bottom-trapped waves occur at a depth where the product of local vertical density stratification and seabed slope is a maximum. The seabed currents are magnified significantly if the diurnal forcing period is resonant with the combination of stratification and seabed slope. At the Belgica mounds (eastern Porcupine Sea Bight), there is a correlation between the cross-slope alignment of individual carbonate mounds and the direction of the major axis of the largest (diurnal) tides. The pattern suggests that the enhanced tidal currents play a major role in the shape of developing mounds over a long time period. A similar relation appears to hold at the Logachev mounds (SE Rockall Bank), although less clear because tides are not amplified to the same degree. At other mound locations where enhanced diurnal currents are not present, a more irregular distribution is observed. This suggests that the diurnal currents may be important only at certain carbonate mound locations, and that at these locations a more distinctive alignment of mound structures is produced.