Scott W. Fowler

Role of large particles in the transport of elements and organic compounds through the oceanic water column

Abstract During the past decade data from a variety of sources have been obtained which show conclusively that the relatively rare, large particles sinking through the water column are responsible for the majority of the downward vertical mass flux in the sea. This finding has important implications for understanding the transfer, distribution and fate of elements and organic compounds in marine waters. The “large” (> 100 μ m) detrital particles responsible for vertical flux are primarily biogenic and range in size and composition from small, discrete fecal pellets and plankton hard parts to large, amorphous aggregates or “snow” which contain both organic and inorganic constituents. Depending on size, shape and density, these particles sink at rates ranging from 1000 m day −1 . Several methods have been developed for sampling these particles of which in situ sediment trapping has probably furnished the most comprehensive qualitative and quantitative information on the role large particles play in material transport. Flux studies have highlighted the importance of marine heterotrophs in packaging fine, suspended particulate matter into large rapidly sinking particles which accelerate the movement of incorporated materials to depth. Large particle production via biological packaging is not restricted to the euphotic zone but can occur at all depths and information is now accruing on rates of production of large particles in the water column. Chemical analyses of sedimenting particles collected in sediment traps and those sampled by other means have allowed quantifying vertical fluxes and residence times of elements, radionuclides and organic compounds (natural and anthropogenic) in various oceanic regimes. Pertinent studies dealing with the above aspects are reviewed and several areas for future research are suggested.

Critical review of selected heavy metal and chlorinated hydrocarbon concentrations in the marine environment

Abstract A critical review of existing concentrations of the potential pollutants Hg, Cd, Pb, PCBs and DDT in marine waters, sediments and biota of the global ocean indicates that the highest concentrations are usually found in the most densely populated and industrialized regions which are often located near major river estuaries. In general, present concentrations, particularly those in edible marine organisms, do not give rise for alarm; however, in some cases national and international concentration limits have been exceeded which has caused some concern for human health. In some ‘hot spots’ (e.g. Minamata Bay, Hudson-Raritan Estuary, Los Angeles Bight, Ems Estuary) where measures have been taken to eliminate the sources of contamination, a significant reduction in concentrations has occurred. Reliable temporal data are generally too sparse and have not been collected for a sufficient period to make accurate predictions about the environmental half-life of the contaminants; however, the widespread occurrence of persistent organochlorine residues of PCBs and DDT in remote areas far from known input sources suggests a long residence time in the ecosystem. Spatial data on a global scale are also limited; therefore, it is difficult to draw firm conclusions about long-term consequences of the shift in use of DDT and other chlorinated pesticides towards tropical areas and the southern hemisphere. However, the few reliable data available from these regions indicate concentrations of some organochlorine compounds in marine matrices that are as high or higher than those reported for the northern hemisphere. More data from a much wider area are needed in order to clarify this trend.

Dynamics of the downward flux of particles and carbon in the open northwestern Mediterranean Sea

Abstract Downward fluxes and the elemental composition of settling particles at 80, 200 and 1000m depth measured during an 18-month sediment trap experiment are presented, together with associated hydrographic and biological data, in order to assess the relative importance of these factors in controlling particle flux. Mass, carbon and nitrogen fluxes were generally higher during the first half of the year. Mass flux maxima occurred at different times of the year depending on trap depth, whereas the peak carbon flux was measured in late June. Most of the total carbon flux was composed of organic carbon. On an annual basis the total particle flux was 32.9, 40.6 and 8.1 g m −2 year −1 at 80, 200 and 1000 m depth, respectively. The organic carbon flux decreased from 4.8 and 5.0 g m −2 year −1 at 80 and 200 m, respectively to 1.2 g m −2 year −1 at 1000 m, and the corresponding nitrogen flux from 0.7 and 0.8 to 0.2 g m −2 year −1 . Organic carbon and nitrogen content of sinking particles decreased with depth, whereas the carbonate carbon content remained constant. The contribution of zooplankton feces to the carbon flux was highly variable throughout the year, ranging from 1.6 to 62%. Swimmers, including fish, that entered the traps in the upper 200 m were abundant and, on average, were twice the weight of the sinking particle flux. Pelagic production was the main source of particles during the late spring and mid-summer sedimentation pulses; however physical forcing, particularly the winter mixing of the water column, was partially responsible for the high sedimentation during the earlier part of the year. Wind gusts and concomitant vertical mixing appeared responsible for the carbon rich particle flux in summer. Separation of data for the mixed and stratified water column periods clearly indicated that most of the downward flux occured in mixed waterswhereas the organic carbon content was higher in particles sedimenting through stratified waters. The export of organic carbon from the euphotic zone (upper 200 m) represented only 5–7% of the estimated phytoplankton production in this region. Furthermore time-series measurements of POC flux averaged on an annual basis were only 14–42% of indirect estimates of carbon export or “new production” for this region. This discrepancy suggests that a good portion of the total downward carbon flux occurs through advection and/or diffusion of DOC.

Petroleum hydrocarbons and trace metals in nearshore Gulf sediments and biota before and after the 1991 war: An assessment of temporal and spatial trends

Abstract Immediately following the 1991 Gulf War, a survey was organized to determine the extent and degree of contamination by petroleum hydrocarbons and trace metals that entered the Gulf from the massive oil spill and oil field fires in Kuwait. Between June–October 1991 samples of nearshore sediments, bivalves and fish were collected from Kuwait, Saudi Arabia, Bahrain, UAE and Oman. Analyses revealed that the highest levels of contamination were along the heavily-impacted coast of Saudi Arabia between Ras Al Khafji and Ras Al Ghar, where concentrations of total petroleum hydrocarbons (expressed as Kuwait crude oil equivalents) ranged from 62–1400 μg g−1 dry wt in surface sediments, 570–2600 μg g−1 dry wt in clams and 9.6–31 μg g−1 dry wt in fish muscle. Gas chromatographic analyses indicated that much of the oil in the intertidal zone had substantially degraded within a few months of the spill. Concentrations of the oil-related metals Ni and V were slightly elevated in oil-contaminated sediments from Saudi Arabia but elsewhere in the Gulf were similar to levels measured in earlier years at those sites. This initial regional survey demonstrated that hydrocarbon contamination originating from the war-related pollution events was restricted to approximately 400 km from the source, that levels of combustion derived PAHs in the marine environment at that time (e.g. 1–450 ng g−1 dry wt for pyrene in sediments) were of the same order as those which have been measured in several coastal areas of the eastern United States and northern Europe, and that outside the immediate area of impact, petroleum hydrocarbon and trace metal levels in sediment and bivalves were generally as low as, or lower than, those concentrations measured at the same sites before the war.

Trace metals in mussels from the N.W. Mediterranean

Abstract A coastal survey in the northwestern Mediterranean region was initiated to measure existing levels of selected trace metals in mussels. For most metals the highest values were found in samples from port cities and areas in the vicinity of river discharge. Marked seasonal variation for many metals was evident; an overall increase in metal concentration in mussels from most stations during March 1974 was attributed to high precipitation and attendant run-off rather than to local pollution. Data comparison indicates that average metal levels in northwestern Mediterranean mussels do not differ markedly from those measured in similar species from different localities throughout the world.

Trace elements in zooplankton particulate products

IT has become increasingly evident that biogenous processes in oceanic surface layers play an important role in removing trace elements from sea water and transporting them to the sediments1,2. Plankton are strongly implicated in these processes and investigations of the chemistry of these organisms have demonstrated their ability to accumulate trace elements to relatively high levels3,4. Zooplankton metabolic activity can be expected to enhance the biogeochemical cycling of trace elements through the release of particulate matter3 such as faecal pellets, moulted exoskeletons and eggs. Furthermore, theoretical models5,6 on the vertical flux of particulates have emphasised the sinking of large particles, similar to those produced by zooplankton, as a mechanism for rapid transport of particulate matter and associated trace elements out of the mixed layer. Nevertheless, little, if any, data exist on (the trace element composition of these biogenic particles, and Boyle et al.7, interpreting their Pacific Ocean Cd profiles in terms of probable Cd regeneration from sinking biogenic debris, have stressed the need for information on the trace element composition of planktonic particulate matter. This note reports the concentrations of 18 trace elements in freshly released faecal pellets, moults and eggs from a representative planktonic crustacean. The high levels of many trace elements found in these biogenic products, relative to concentrations in the plankton which produce them, clearly indicate the importance of these particulates in oceanic trace element biogeochemical cycles.

Levels and trends of PCBs, chlorinated pesticides and petroleum hydrocarbons in mussels from the NW Mediterranean coast: comparison of concentrations in 1973/1974 and 1988/1989

A mussel watch survey was conducted in 1988-1989 along the Mediterranean coast of France and Italy, between Sete and Genoa, in order to assess contamination by organochlorine pesticides, PCBs and petroleum hydrocarbons. Results show concentrations of DDTs (average 130 ng g-1 dry wt.) and PCBs (average 527 ng g-1 dry wt.) higher in coastal areas receiving river discharges and close to large cities. Concentrations of total aliphatics (average 470 micrograms g-1 dry wt.) and PAHs (average 233 micrograms g-1 dry wt.) were generally higher also near large harbours, namely Marseille, Toulon and Genoa. PAHs in mussel samples from Genoa and Monaco were mostly of pyrolytic sources, likely forest fires and exhaust gases from cars, whereas PAHs in other stations were mainly of petrogenic sources. Comparison of chlorinated hydrocarbon concentrations measured in 1988-1989 with those measured in a similar survey carried out at the same sites in 1973-1974, shows that DDTs and PCBs decreased by a factor of approximately 5 in 15 years. This decrease is in agreement with the ban on DDT implemented in 1975 in western Europe and gradual cessation of PCBs production in the 1970s and 1980s. Nevertheless, the residues of these compounds measured in mussels confirm the well-known long persistence of DDTs and PCBs, which are still present in relatively high concentrations in a few areas.

Vertical transport of particulate-associated plutonium and americium in the upper water column of the Northeast Pacific

Concentrations of plutonium (Pu) and americium (Am) were determined in seawater, suspended particulate matter, sediment trap samples, and biogenic material collected at the VERTEX I site in the North Pacific off central California. From a vertical profile taken over the upper 1500m, the presence of sub-surface maxima of 239+240Pu and 241Am were identified between 100 to 750 m and 250 to 750 m, respectively. A large fraction (32%) of the filterable 239+240Pu in surface waters was associated with cells during a phytoplankton bloom; Pu: Am activity ratios in surface water and the suspended particles indicated that Pu was concentrated by the cells to a greater degree than Am. However, similar measurements beneath the surface layer showed an overall enrichment of Am over Pu on fine suspended particles with depth. Freshly produced zooplankton fecal pellets and large, fast sinking particles collected in PITS contained relatively high concentrations of Pu and Am. Both transuranic concentrations in trapped particles and transuranic flux tended to increase with depth down to 750 m, suggesting that their scavenging is in the upper water column. Am appeared to be scavenged by sinking biogenic particles to a greater extent than Pu. The results from in situ PIT measurements demonstrate for the first time the overall importance of the rarer, large biogenic particles in rapidly removing the transuranic elements from the euphotic zone and transporting them to depth; however, the exact role the particles play in the control of the observed transuranic levels in the surrounding seawater is unclear. Flux-derived residence times for Pu and Am (2 to 13 y and 1.5 to 4.5 y, respectively) in the upper mixed layers (< 250 m) at the VERTEX I site are of the same order as estimates based on delivery rate and mixed-layer transuranic inventory data from the Mediterrenean.

Factors affecting methyl and inorganic mercury dynamics in mussels and shrimp

Radiotracer experiments were designed to study the effect of certain factors on the accumulation and loss of methyl and inorganic mercury in mussels (Mytilus galloprovincialis) and benthic shrimp (Lysmata seticaudata). Methyl mercury was accumulated from both food and water to a greater degree than inorganic mercury by both species. There was a tendency for small mussels to concentrate more mercury than larger individuals, but the reason for this difference remains unclear. A trend towards greater mercury accumulation at higher temperatures was noted for mussels, but the temperature effect was relatively small over a 10 Co range between 8° and 19°C. Methyl mercury residues were eliminated by both species more slowly than those of the inorganic form. Loss from mussels was somewhat more rapid at higher temperatures; however, as in the case of accumulation, the effect of temperature was relatively small. Loss rates for small mussels were not significantly different from those for large individuals. Enhanced methyl mercury elimination was noted for the group of mussels maintained in their natural environment. The more rapid metal turnover in these individuals compared with mussels held in the laboratory was attributed to differences in the availability of natural particulate food matter and, hence, subsequent growth of the animals in the two experimental systems. The observed differences underscore the need for caution in predicting in situ flux of metals such as mercury in certain species based solely on data derived from laboratory experiments.

Vertical fluxes of polycyclic aromatic hydrocarbons and organochlorine compounds in the western Alboran Sea (southwestern Mediterranean)

Time-series samples of sinking particles were collected in the Alboran Sea with sediment traps moored at three water column depths (250, 500 and 750 m) between February and May 1992. The samples were analyzed for individual polychlorobiphenyl congeners (i.e. PCBs IUPAC no.: 28, 52, 101, 118, 153, 138 and 180), chlorinated pesticides (DDE and Lindane) and individual polycyclic aromatic hydrocarbons (PAHs; 3–6 aromatic rings). Average concentrations of PCB and total PAHs were 78.9 ng g− (range: 27.3–212) as Clophen A40 equivalents and 1176 ng g− (range: 104–2780), respectively. The 1,1-dichloro-2,2-bis(p-chlorophenyl)ethene (4,4′-DDE) and 1,2,3,4,5,6-hexachlorocyclohexane (γ-HCH) were the only chlorinated pesticides identified throughout the sampling period, with respective concentrations of 6.6 ng g− (range 1.1–15) and 16.8 ng g− (range 5.4–48). Corresponding fluxes of PCBs (as Clophen A40 equivalents) and PAHs were 15.1 ng m−2 d−1 (range: 6.9–24.9) and 225 ng m−2 d−1 (range: 15–625). Contaminant fluxes were analyzed with other collective fluxes obtained from the same samples (i.e. fecal pellets, mass and organic carbon); while higher molecular weight PAHs (MW > 228 daltons) exhibited a strong correlation with the mass fluxes (r2 = 90.7%, p < 0.001, n = 18), the lower molecular weight compounds (MW < 202 daltons) covaried with organic carbon and fecal pellet fluxes (r2 = 76.4%, p < 0.001, n = 18). The ΣPCB fluxes covaried with organic carbon, and fecal pellet fluxes and depth (r2 = 97.6%, p < 0.001, n = 9). The 4,4′-DDE only covaried with fluxes of organic carbon (r2 = 85.87%, p < 0.001) and γ-HCH with the fecal pellets fluxes and depth (r2 = 85.2% and p = 0.001, n = 9). Comparison with similar data from the northwestern Mediterranean and other oceans suggests that the Alboran Sea is a relatively clean environment primarily due to its relative remoteness from input terrigenous sources.