The middle to late Pleistocene geomagnetic field recorded in fine-grained sediments from Summer Lake, Oregon, and Double Hot Springs, Nevada, U.S.A.

A 400,000 year record of the paleomagnetic field has been acquired from 22 meters of middle to late Pleistocene fine-grained sediments from Summer Lake in south-central Oregon and Double Hot Springs in northwestern Nevada. The stratigraphy is based on 55 tephra layers, nine of which have been correlated with tephra layers from other localities on the basis of their distinct major- and trace-element geochemistry and their distinct petrography. The paleomagnetic samples carry a strong and stable magnetization that does not appear to have been affected by the inclination error commonly associated with the magnetization of sediments. The samples have accurately recorded the declination and inclination of the geomagnetic field at or near the time of deposition except for errors arising from rotations of discrete blocks of sediment predominantly about vertical axes. Errors introduced by this type of rotation were corrected by using paleomagnetic directions associated with correlated tephra layers. The Summer Lake paleomagnetic record suggests that secular variations occurred throughout the middle and late Pleistocene often maintaining the same waveform through several oscillations. The amplitudes of these variations were similar to those of Holocene variations, and the periods ranged from 15,000 years to greater than 100,000 years.     

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.     

Intraspecific Interactions and Risk of Predation for Dyspanopeus sayi (Decapoda: Xanthidae) Living on Polychaete (Filograna implexa,Serpulidae) Colonies*

Abstract. Along the Atlantic coast of the United States, the mud crab Dyspanopeus sayi frequently occurs in crevices and depressions within aggregated calcareous tubes (termed “heads”) of the serpulid polychaete Filograna implexa. In New Jersey, U.S.A., waters, crab number was significantly correlated with the logarithm of worm tube colony volume. Laboratory experiments suggested that crabs compete for shelter within worm colonies. Additional laboratory experiments indicated that colony occupancy greatly decreased the risk of predation by blue crabs (Callinectes sapidus), and field tethering experiments also demonstrated that colony occupancy significantly decreased predation risk. Thus, the consequence of holding space on worm tube heads is a greatly increased probability of survival.     

Hydrocarbon effects on fouling assemblages: the importance of taxonomic differences, seasonal, and tidal variation

Effects of hydrocarbon-contaminated substrata on recruitment of three species of fouling organisms were studied along the Louisiana gulf coast. Clay tiles (232 cm2) were exposed to crude oil, 10% water soluble fraction of crude oil, or 25 g/kg artificial seawater, and placed out at two locations, in two seasons, and at two tidal levels in an estuary near Port Fourchon, Louisiana. Bryozoan (Membranipora savartii) recruitment was significantly reduced in all experiments on crude oil-exposed tiles. However, oysters (Crassostrea virginica) and barnacles (Balanus eburneus) exhibited recruitment facilitation, and oysters grew to larger size, on crude oil-exposed tiles in 1-4 of the five experiments. When oyster larvae were exposed to the same treatments in the laboratory, settlement was, however, significantly depressed on crude oil-exposed tiles as compared with controls, although oyster size was larger on crude-oil exposed tiles. Recruitment on tiles exposed to the water soluble fraction of crude oil was similar to control tiles in nearly all experiments for all taxa. We suggest naturally occurring biofilms (which hydrocarbons facilitate) may promote or inhibit recruitment, depending on the taxon, because hydrocarbons facilitated recruitment only in field experiments, not in lab experiments without biofilms. However, stronger currents in the field experiments may have more rapidly diluted hydrocarbons, and hydrocarbon effects were not large in comparison with natural seasonal and tidal variation in recruitment.     

Clear‐sky closure studies of lower tropospheric aerosol and water vapor during ACE‐2 using airborne sunphotometer, airborne in‐situ, space‐borne, and ground‐based measurements

We report on clear‐sky column closure experiments (CLEARCOLUMN) performed in the Canary Islands during the second Aerosol Characterization Experiment (ACE‐2) in June/July 1997. We present CLEARCOLUMN results obtained by combining airborne sunphotometer and in‐situ (optical particle counter, nephelometer, and absorption photometer) measurements taken aboard the Pelican aircraft, space‐borne NOAA/AVHRR data and ground‐based lidar and sunphotometer measurements. During both days discussed here, vertical profiles flown in cloud‐free air masses revealed 3 distinctly different layers: a marine boundary layer (MBL) with varying pollution levels, an elevated dust layer, and a very clean layer between the MBL and the dust layer. A key result of this study is the achievement of closure between extinction or layer aerosol optical depth (AOD) computed from continuous in‐situ aerosol size‐distributions and composition and those measured with the airborne sunphotometer. In the dust, the agreement in layer AOD (λ=380–1060 nm) is 3–8%. In the MBL there is a tendency for the in‐situ results to be slightly lower than the sunphotometer measurements (10–17% at λ=525 nm), but these differences are within the combined error bars of the measurements and computations.