Sources and sinks of low molecular weight organic carbonyl compounds in seawater

Results from laboratory studies indicated that low molecular weight (LMW) carbonyl compounds, especially formaldehyde, acetaldehyde, acetone and glyoxal, can be formed in seawater by photochemical processes. Once formed, these compounds appear to be readily consumed by biota. These results suggest that concentrations of LMW carbonyl compounds should undergo diurnal variations in the illuminated layer of the sea. In support of this, diurnal fluctuations of LMW carbonyl concentrations were observed in humic-rich surface waters off the west coast of Florida over a three day sampling period using a shipboard HPLC system. Fluctuations in acetaldehyde were particularly strong and reproducible, with steady night-time concentrations of 2–3 nM and day-time concentrations reaching a maximum of 20–30 nM in the early afternoon. In contrast, diurnal fluctuations in formaldehyde were less distinct, ranging from 15 to 50 nM.The laboratory and field results are discussed in terms of biotic/abiotic sources and sinks of LMW carbonyl compounds in surface seawater. It is speculated that photooxidative cleavage of biologically refractory dissolved organic matter (DOM) in seawater to yield LMW organic fragments, such as carbonyl compounds, may be important in the breakdown and geochemical cycling of DOM in the ocean.     

Contemporary management issues confronting fisheries science

Stock collapses have occurred worldwide. The most frequently cited cause is over-fishing, suggesting that fisheries management has been ineffective in controlling exploitation rates. The progression of a fishery from an over-exploited to a collapsed state involves impairment of the reproductive capacity of the target species, i.e. recruitment over-fishing. In many cases, this occurs by reduction of the spawning stock biomass (SSB) through the systematic elimination of spawning components within a stock complex. While operational definitions of minimum levels of SSB have been developed, they have seldom been applied and never adopted in a Canadian groundfish management context. The answer to the question of how much is enough to perpetuate a stock under exploitation has been illusive. Serebryakov [J. Cons. Int. Explor. Mer, 47 (1990) 267] has advocated definition of critical levels of SSB based on survival rates (R/SSB). We review his method and discuss the utility of the approach. An alternative approach to the problem of estimating minimum SSB is through a fundamental revision of the traditional stock and recruitment relationship. Explicit theoretical SSB thresholds below which reproduction/recruitment is severely impaired based upon density-dependent mating success (or Allee effects) is considered a superior approach to the question of how much is enough because of its ecological grounding. However, the successful application of this approach will require re-definition of the space/time scales of the management unit. Finally, support is growing for the establishment of closed areas or “no-take zones” as an alternative approach to managing the problems of fishing a stock complex by enabling sub-populations to escape fishing. While the expected benefits of areas protected from fishing are numerous, clear demonstrations of benefits of such areas in marine temperate ecosystems are lacking. In fact, unintended negative consequences may result from such actions.     

Reduction of iodate in seawater during Arabian Sea shipboard incubations and in laboratory cultures of the marine bacterium Shewanella putrefaciens strain MR-4

Shipboard incubations from the US JGOFS cruise to the Arabian Sea (TN045) March, 1995 showed evidence of iodate reduction in 0.45 μ (Gelman Supor membrane) filtered seawater samples collected from intermediate depths (200–600 m) within the oxygen minimum zone (OMZ). Inorganic chemical reduction of iodate in these samples was ruled out as no free sulfide was measurable and concentrations of ammonia and nitrite were found to be less than 5 μM. To examine whether the reduction of iodate observed at sea could have been the result of bacterial metabolism, reduction of iodate (IO₃⁻) to iodide (I⁻) by Shewanella putrefaciens strain MR-4 was studied in artificial seawater using electrochemical methods. MR-4 is a ubiquitous marine bacterium which may be of considerable importance when considering redox zonation in the water column because it is a facultative anaerobe and may switch amongst a suite of electron acceptors to support metabolism. In all experiments MR-4 reduced all iodate to iodide. The rate of formation of [I⁻]in the culture followed pseudo-first order kinetics. This is the first report of the marine bacterial reduction of iodate where the concentrations of iodide and iodate were measured directly. Our results may help to explain the depth distribution of iodine speciation reported in productive waters like the Arabian Sea and for the first time couple iodine speciation with bacterial productivity in the ocean.     

Doubling the production and precision of the MBTH spectrophotometric assay for dissolved carbohydrates in seawater

Extensive use of the three step 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH) method for determining concentrations of both free and combined dissolved carbohydrate occurring in diverse marine environments has shown its suitability for studies of carbohydrate dynamics. Modification of the analytical sequence to extend over 2 working days to eliminate a confusing and error producing overlap of analytical operations allows a single analyst to determine 15 instead of 8 samples in the same 2-day period. This modification also permits an increase in the number of blank plus replicate determinations from 5 to 7 per sample so that a total of 105 reaction tubes can be assayed every 2 days compared to 40–50 for the original procedure. For a series of 7 Narragansett Bay samples, the standard deviation of the method serendipitously decreased by 50% (from ±5.3 to ±2.7 μg C l⁻¹) when the original procedure was compared with the modified procedure. Postponing the final analytical step for a day when the analyses can be completed with an autoprocessor will free the analyst for the processing of more samples, causing an additional doubling in productivity.     

Fish assemblages across the marine to low salinity transition zone of a temperate estuary

This 3-year study provides a large-scale perspective of fish assemblage structure across an ocean–estuarine ecotone, given range of salinity encountered (0.1–32) based on sampling at 12 stations along 40 km from the Mullica River (river), Great Bay (bay), and the adjacent inner continental shelf (ocean) in southern New Jersey. Otter trawl (4.9 m, 6 mm mesh) collections were dominated by young-of-the-year of most of the 49 species encountered. Species richness and abundance appeared greatest in the ocean, decreased (with an increase in inter-station variability) in the bay, and appeared to increase again towards the uppermost river stations. The same areas contained three non-discrete, but identifiable, fish assemblages based on Detrended Correspondence Analysis. Members of the Triglidae and Stromateidae characterized the ocean and bay, whereas representatives of the Percichthyidae and Ictaluridae characterized the river. Several species, including Anchoa mitchilli and Cynoscion regalis, exhibited a ubiquitous distribution across the sampling area. Further analyses with Canonical Correspondence Analysis identified salinity and geographic distance, among the variables examined, as the most important determinants in shaping the assemblages. Other contributors included habitat heterogeneity and water depth. In summary, these observations indicate that large-scale patterns in the structure of this estuarine fish assemblage are primarily a result of individual species' responses to dominate environmental gradients, as well as ontogenetic migrations, whereas smaller-scale patterns appear to be the result of habitat associations that are most likely driven by foraging, competition, and/or predator avoidance.     

The impact of sewage discharges at ocean outfalls on phytoplankton populations in waters surrounding the Hawaiian Islands

Primary production rates, chlorophyll a and nutrient concentrations, particulate carbon, nitrogen and phosphorus concentrations, Secchi depths and submarine light levels were measured at stations within, and near, the zone of mixing of the Sand Island and Mokapu Point ocean sewage outfalls near the island of Oahu in the Hawaiian Islands. Multiyear averages of several parameters indicated that the Sand Island outfall had no adverse impact on phytoplankton communities or water quality. The major effect of the Mokapu Point outfall was a reduction in photosynthetic rates, probably due to the rapid uptake of nutrients introduced into the mixed layer with the secondarily treated sewage. The Sand Island outfall, which discharged coarsely screened raw sewage within the thermocline, had less of an impact because the inorganic nutrient concentrations in the sewage were lower and because the sewage rarely rose into the mixed layer. The results indicate that, under appropriate conditions, ocean outfalls may be preferable to estuarine or freshwater outfalls and that secondary treatment is not always necessary to reduce the impacts of ocean outfalls to an acceptable level.     

Phytoplankton growth response to deep ocean water

Single species populations and natural populations of phytoplankton were grown in mixtures of surface sea water and deep ocean water. The yields of Phaeodactylum tricornutum and the natural species assemblages were positively correlated with the percentage of deep water in the mixture. Single species populations of Thalassiosira weissflogii and Amphidinum sp. showed yields which were positively correlated with per cent deep water at low deep water concentrations and negatively correlated at high deep water concentrations. Transition from the positive to the negative slope occurred at about 75% deep water for T. weissflogii and at about 25% deep water for Amphidinium. Populations were apparently limited by an inorganic nutrient, probably nitrogen, in the region of positive slope and by some other factor, probably organic, in the region of negative slope. The addition of EDTA increased the yields of T. weissflogii and Amphidinium in the presence of deep water and reduced lag times in the growth of the natural populations.     

Searching for sustainability in aquaculture: An investigation into the economic prospects for an integrated salmon–mussel production system

The pollution effects of cage aquaculture represent an external cost to society, and the challenge for environmental economists has been to estimate the magnitude of these costs and to suggest ways in which they can be mitigated or ‘internalised’. One possible mitigation strategy involves the development of integrated production systems based on polyculture, and this paper examines the financial viability of such a system that integrates the farming of salmon and mussels. The results demonstrate the commercial potential of an integrated salmon–mussel production system under present market conditions, but highlight the critical role played by future price trends.     

Bio-optical signatures and biogeochemistry from intense upwelling and relaxation in coastal California

The NSF-sponsored Coastal Ocean Processes Wind Events and Shelf Transport (WEST) experiment investigates the interplay between wind-driven transport and shelf productivity; while eastern boundary shelves are characterized by high productivity due to upward fluxes of nutrients into the euphotic zone, wind forcing also represents negative physical and biological controls via offshore transport and deep (light-limiting) mixing of primary producers. Although this interaction has been well documented for eastern boundary systems generally and for California specifically, one of the primary goals of WEST was to characterize more fully the interplay between positive and negative effects of wind stress, which result in the consistently elevated biological productivity in these shelf regions. During 3 month-long summer cruises (2000–2002) we observed extremes in upwelling/relaxation, using both in situ instrumentation and remotely sensed data. Relationships between optical and physical properties were examined, with emphasis on biogeochemical implications. During 2000, the WEST region was optically dominated by phytoplankton and covarying constituents. During 2001 and 2002, periods of more intense upwelling favorable winds, we observed a transition to optical properties dominated by detrital and inorganic materials. In all years, the continental shelf break provided a natural boundary between optically distinct shelf and open ocean waters. During 2002, we obtained discrete trace-metal measurements of particulate iron and aluminum; we develop a bio-optical proxy for acetic-acid leachable iron from backscatter and fluorescence, and demonstrate that particulate iron is not well correlated to traditional upwelling proxies such as macronutrients, temperature, and salinity. We conclude that the shelf break between ca. 100 and 200 m water depth serves as a natural break point between coastal and oceanic water masses in this region, and that the elevated biomass and productivity associated with this eastern boundary current regime is dominated by these iron rich, shallow shelf waters.