Cynthia H. Pilskaln

The nitrogen isotope biogeochemistry of sinking particles from the margin of the Eastern North Pacific

The nitrogen isotopic composition of time-series sediment trap samples, dissolved NO~ , and surficial sediments was determined in three regions along the margin of the eastern North Pacific: Monterey Bay, San Pedro Basin, and the Gulf of California (Carmen and Guaymas Basins). Complex physical regimes are present in all three areas, and each is influenced seasonally by coastal upwelling. Nevertheless, sediment trap material evidently records the isotopic composition of new nitrogen sources, since average d15N is generally indistinguishable from d15N values for subsurface NO~ . Surficial sediments are also very similar to the average d15N value of the sediment traps, being within 1&. This di⁄erence in d15N between trap material and sediment is much less than the previously observed 4& di⁄erence for the deep sea. Better organic matter preservation at our margin sites is a likely explanation, which may be due to either low bottom O 2 concentrations or higher organic matter input to the sediments. All sites have d15N for sub-euphotic zone NO~ (8—10&) substantially elevated from the oceanic average (4.5—5&). This isotopic enrichment is a result of denitrification in suboxic subsurface waters (Gulf of California) or northward transport of denitrification influenced water (Monterey Bay and San Pedro Basin). Our results therefore suggest that downcore d15N data, depending on site location, would record the intensity of denitrification and the transport of its isotopic signature along the California margin. Temporal variations in d15N for the sediment traps do appear to respond to upwelling or convective injections of NO~ to surface waters as

Upward transport of oceanic nitrate by migrating diatom mats

The oligotrophic gyres of the open sea are home to a flora that includes the largest known phytoplankton. These rare species migrate as solitary cells or aggregations (mats) between deep nutrient pools (below 80–100 m) and the surface. This migration contributes to new production because of the concomitant upward transport of nitrate. But just how significant this contribution is remains uncertain because of the difficulty of making quantitative measurements of these rare cells. Here we report remote video observations of a previously undersampled class of diatom (Rhizosolenia) mats throughout the upper 150 m of the central North Pacific Ocean. These mats are virtually invisible to divers, and their presence increases the calculated phytoplankton-mediated nitrate transport into the surface ocean by up to a factor of eight. Cruise averages indicate that Rhizosolenia mats transport 18–97 µmol N m−2 d−1; however, this value reached 171 μmol N m−2 d−1 at individual stations, a value equivalent to 59% of the export production. Although considerable temporal and spatial variability occurs, this means of upward nutrient transport appears to be an important source of new nitrogen to the surface ocean, and may contribute to other regional elemental cycles as well.

Biogenic matter diagenesis on the sea floor: A comparison between two continental margin transects

Benthic chamber measurements of the reactants and products involved with biogenic matter diagenesis (oxygen, ammonium, nitrate, silicate, phosphate, TCOP, alkalinity) were used to define fluxes of these solutes into and out of the sediments off southern and central California. Onshore to offshore transects indicate many similarities in benthic fluxes between these regions. The pattern of benthic organic carbon oxidation as a function of water depth, combined with published sediment trap records, suggest that the supply of organic carbon from vertical rain can just meet the sedimentary carbon oxidation + burial demand for the central California region between the depths 100-3500 m. However, there is not enough organic carbon raining through the upper water column to support its oxidation and burial in the basins off southern California. Lateral transport and focusing of refractory carbon within these basins is proposed to account for the carbon buried. The organic carbon burial efficiency is greater off southern California (40-60%) compared to central California (2-20%), even though carbon rain rates are comparable. Oxygen uptake rates are not sensitive to bottom water oxygen concentrations nor to the bulk wt. % organic carbon in surficial sediments. Nitrate uptake rates are well defined by the depth of oxygen penetration into the sediments and the overlying water column nitrate concentration. Nitrate uptake accounts for about 50% of the total denitrification taking place in shelf sediments and denitrification (0. l-l .O mmolN/m*d) occurs throughout the entire study region. The ratio of carbon oxidized to opal dissolved on the sea floor is constant (0.8 t 0.2) through a wide range of depths, supporting the hypothesis that opal dissolution kinetics may be dominated by a highly reactive phase. Sea floor carbonate dissolution is negligible within the oxygen minimum zone and reaches maximal rates

Distribution and transport of suspended particulate matter in Monterey Canyon, California

From August 1993 to August 1994, six moorings that measure current, temperature, salinity, and water clarity were deployed along the axis of Monterey Canyon to study the circulation and transport of water and suspended particulate matter through the canyon system. The moorings occupied three sites that are morphologically different: a narrow transverse section (axis width 900 m) at 1450 m water depth, a wide transverse section at 2837 m, and a third site in the fan valley axis farther offshore at 3223 m that recorded for 3 yr. In addition, CTD/transmissometer casts were conducted within and near the Monterey Canyon during four cruises. Our data show a mainly biogenic, surface turbid layer, a limited intermediate nepheloid layer, and a bottom nepheloid layer. There is a consistent presence of a turbid layer within the canyon at a water depth of about 1500 m. Tidal flow dominates at all sites, but currents above the canyon rim and within the canyon appear to belong to two distinct dynamic systems. Bottom intensification of currents plays an important role in raising the near-bottom shear stress high enough that bottom sediments are often, if not always, resuspended. Mean flow pattern suggests a convergence zone between the narrow and wide site: the near-bed (100 m above bottom where the lowest current meter was located) mean transport is down-canyon at the 1450-m site, while the near-bottom transport at the 2837-m site is up-canyon, at a smaller magnitude. Transport at the 3223-m site is dominantly NNW, cross-canyon, with periods of up-canyon flow over 3 yr. A very high-turbidity event was recorded 100 m above the canyon bottom at the narrow site. The event started very abruptly and lasted more than a week. This event was not detected at either of the deeper sites. A canyon head flushing event is likely the cause.

Pathways between Primary Production and Fisheries Yields of Large Marine Ecosystems

The shift in marine resource management from a compartmentalized approach of dealing with resources on a species basis to an approach based on management of spatially defined ecosystems requires an accurate accounting of energy flow. The flow of energy from primary production through the food web will ultimately limit upper trophic-level fishery yields. In this work, we examine the relationship between yield and several metrics including net primary production, chlorophyll concentration, particle-export ratio, and the ratio of secondary to primary production. We also evaluate the relationship between yield and two additional rate measures that describe the export of energy from the pelagic food web, particle export flux and mesozooplankton productivity. We found primary production is a poor predictor of global fishery yields for a sample of 52 large marine ecosystems. However, chlorophyll concentration, particle-export ratio, and the ratio of secondary to primary production were positively associated with yields. The latter two measures provide greater mechanistic insight into factors controlling fishery production than chlorophyll concentration alone. Particle export flux and mesozooplankton productivity were also significantly related to yield on a global basis. Collectively, our analyses suggest that factors related to the export of energy from pelagic food webs are critical to defining patterns of fishery yields. Such trophic patterns are associated with temperature and latitude and hence greater yields are associated with colder, high latitude ecosystems.

Seasonal record for alkenones in sedimentary particles from the Gulf of Maine

Abstract C37–39 alkenones and C36 alkenoates, biomarkers of haptophyte origin, were measured in a 10-month sediment trap times series from the Wilkinson Basin in the Gulf of Maine (GOM). Highest biomarker flux to the seabed was observed in summertime, the period when surface waters are stratified and a persistent, subsurface chlorophyll maximum (SCM) exists within the upper thermocline and at the base of the euphotic zone throughout the GOM. Comparison of biomarker content and composition of sediment trap particles and underlying surface sediments indicates significant loss (>50%) of signal due to the impact of early diagenesis. Despite such loss, however, C37 alkenone unsaturation patterns (U37K′) are not altered. Estimates of algal growth temperature made from analysis of U37K′ in these sedimentary materials correspond with water temperature measured at the SCM, identifying this biological oceanographic feature as a key site of alkenone export production to the GOM sediment record. Given the common occurrence of SCM in surface waters of the world ocean, particularly the expansive oligotrophic regions of the subtropical to temperate ocean, export of alkenones produced within such features is a potentially widespread biological oceanographic phenomenon which shapes the sediment record for these biomarkers.

Siliceous microfossil distribution in the surficial sediments of Lake Baikal

Examination of surficial sediments at 16 stations shows minor, but consistent differences in the numbers and kinds of siliceous microfossils deposited in different regions of Lake Baikal. There is a general north-south decreasing trend in total microfossil abundance on a weight basis. Endemic plankton diatom species are the most abundant component of assemblages at all stations. Chrysophyte cysts are present at all stations, but most forms are more abundant at northern stations. Non-endemic plankton diatom species are most abundant at southern stations. Small numbers of benthic diatoms and sponge spicules are found in all samples. Although low numbers are present in offshore sediments, the benthic diatom flora is very diverse. Principal components analysis confirms primary north-south abundance trends and suggests further differentiation by station location and depth.

Does the fall phytoplankton bloom control recruitment of Georges Bank haddock, Melanogrammus aeglefinus, through parental condition?

In 2003, the Georges Bank stock of haddock (Melanogrammus aeglefinus) experienced the largest recruitment event recorded during its assessed history. Several hypotheses have been advanced to explain recruitment variability in this much-scrutinized stock, including variability in the retention of eggs and larvae on Georges Bank, the timing of haddock spawning, and variability in the spring bloom, which influences larval growth and survival. Although these processes may contribute to the formation of successful year classes, none of the factors associated with these previous hypotheses provides an adequate explanation of the 2003 recruitment event. We analyzed data on the dynamics of the fall phytoplankton bloom the year prior to spawning and show it to be highly correlated with subsequent recruitment. We suggest that the fall bloom affects recruitment through enhanced condition of adults and by increasing the quantity and quality of their reproductive output, which in turn leads to a higher probability of ...

Siliceous microfossil succession in the recent history of two basins in Lake Baikal, Siberia

As part of the international cooperative Baikal Drilling Project, siliceous microfossil assemblage succession was analyzed in two short (∼ 30-cm) sediment cores from Lake Baikal. One core was recovered from the north basin (Core 324, 55°15′N, 109°30′E), a second from between the central and southern basins (Core 316, 52°28′N, 106°5′E). The northern core had higher amounts of biogenic silica (40 g SiO2 per 100 g dry weight sediment) compared to the southern core, and increased deposition in the more recent sediments. Weight percent biogenic silica was lower in the southern core, ranging from approximately 20–30 g SiO2 per 100 g dry weight sediment throughout the entire core. Trends in absolute microfossil abundance mirror those of biogenic silica, with generally greater abundance in the northern core (86–275×106 microfossils g−1 dry sediment) compared to the southern core (94–163×106 microfossils g−1 dry sediment).Cluster analyses using relative abundance of the dominant diatom and chrysophyte taxa revealed four zones of microfossil succession in each core. Microfossil assemblage succession in the north basin may be reflecting shifts in nutrient supply and cycling driven by climatic changes. The most recent sediments in the northern basin (Zone 1,c. 1890s–1991 A.D.) were characterized by an increased abundance ofAulacoseira baicalensis andAulacoseira ‘spore’. Zone 3 (c. 1630s–1830s A.D.) was dominated by the endemicCyclotella spp. and reduced abundance of theAulacoseira spp. Zone 3 corresponds approximately to the Little Ice Age, a cooler climatic period. The microfossil assemblages between Zones 1 and 3 (Zone 2,c. 1830s–1890s A.D.) and below Zone 3 (Zone 4,c. 830s–1430s A.D.) are similar to one another suggesting they represent transitional intervals between warm and cold periods. Southern basin sediments record similar changes in the endemic taxa. However, the increased abundance of non-endemic planktonic taxa (e.g.Stephanodiscus binderanus, Synedra acus, Cyclostephanos dubius) during two periods in recent history (post World War II and late 1700s) suggests evidence for anthropogenic induced changes in southern Lake Baikal.

Temporal variability in sediment fluxes in the San Pedro Basin, southern California bight

A 6 month sediment trap study (January–July 1988) conducted in San Pedro Basin off southern California in which bi-weekly particulate samples were obtained, indicates that significant, high frequency temporal variability exists in the flux of particulate matter to the sea floor at this site. During this period, three intervals of high fluxes were observed for both lithogenic and biogenic sediment components, which can be correlated to local climate and oceanographic events. The highest total fluxes occurred in winter (late January—early February), with up to 70% of the flux being lithogenic. The high lithogenic flux during this time of year coincides with the time of highest rainfall, and increased run-off probably contributed to the high detrital fluxes. Resuspension and basinward advection of shelf sediments may have also contributed to the winter peak in lithogenic flux. High C/N ratios and low δ13Corg values suggest the input of continentally-derived organic carbon to San Pedro Basin during the winter flux peak. Two other periods of high flux occur during April-early May and June. The April-early May flux peak is a function of both a spring bloom in early April and the onset of upwelling in late April. Both hydrographic regimes result in high biogenic fluxes (calcium carbonate, biogenic silica and organic carbon). The June increase in flux is associated with a second phase of upwelling. The average organic carbon flux at 500 m water depth during the study period was 0.032 g C m−2 day−1, which is less than 25% of the expected new production. Utilizing the empirical relationship developed by Suess [(1980) Nature, 228, 260–263], for estimating organic carbon flux as a function of primary productivity and water depth, we predict an average organic carbon flux of 0.032 g C m−2 day−1 at our trap depth (500 m) for this region. The excellent match between observed and predicted organic carbon fluxes suggests that our sediment trap sampling provides a reliable measure of changes in overlying surface water productivity. Similarly, total sediment flux estimates are in good agreement with sea-floor sediment accumulation rates, particularly when remineralization of biogenic material is taken into account.