Variable Hydrographic Conditions Research Articles

Hydrographic control on carbon isotope fractionation in coccolithophores in the North Atlantic during the Mid-Pleistocene

The carbon isotopic fractionation during photosynthesis (Ɛp) from sedimentary alkenone biomarkers produced by coccolithophores is a widely used proxy for concentrations of past dissolved CO2 in seawater (CO2[aq]). Currently, Ɛp records covering the last 1 Myr exist only for low-latitudes regions, which are characterized by stratified waters and nutrient (NO3− and PO4−) limitation. Higher latitudes are affected by more variable hydrographic conditions which are expected to produce larger changes in factors which are important for Ɛp, such as light, temperature, CO2 and growth rate. Understanding these processes at high latitudes is important in order to derive correct estimates of past CO2 concentrations. Here we present new Ɛp and alkenone-based SST records and a review of previously published paleoceanographic multi-proxy datasets, from sites across a latitudinal transect in the North Atlantic (61°37°N) covering the interval between ∼800 and 400 ka. During this period, the subpolar hydrographic fronts shifted latitudinally, following orbital and sub-orbital climate variability, leading to large zonal and meridional environmental gradients in the North Atlantic. We observe that Ɛp and climate-state relationships (depicted by benthic δ18O) are similar across different latitudes. Ɛp is lower at mid-latitude regions (Sites U1385 and U1313), due to a higher coccolithophore growth rate during colder intervals. In the high-latitude sites, a longitudinal gradient is observed, with higher Ɛp during glacial intervals eastwards (Site 982) compared to interglacials. The opposite is observed on the sites located westwards (Sites U1314 and 984), where Ɛp is higher during interglacials compared to glacials. We suggest that the depressed interglacial Ɛp values at Site 982 is the result of the uninterrupted northward flow of warm Atlantic waters towards the east, which sustained high coccolithophore productivity and growth rates. Combining high-latitude planktonic foraminifera species and calcium carbonate records, we reconstructed the effect of non-CO2 factors on Ɛp.

Seasonal influence of intrusion from the Kuroshio Current on microplankton biomass and community structure in the northern Satsunan area, western Japan

Seasonal variations in hydrographic conditions, nutrients, and microplankton abundance and biomass were investigated inside and outside Kagoshima Bay in the northern Satsunan area. The area is a nursery ground for the migrating fish. During the mixing season from November to April, highly saline water (>34.5) originating from a branch of the Kuroshio Current, the western boundary current of the North Pacific Subtropical Gyre, intruded from the surface layer into the bay causing extrusion of coastal bottom water. This intrusion generated vertical mixing between surface water and nutrient-rich bottom water. Consequently, chlorophyll a concentrations increased (>1 μg l−1) around the bay mouth. During the stratified season from May to August, the coastal surface water was extruded due to intrusion of saline water from the bottom layer. High chlorophyll a concentrations (>1 μg l−1) were also observed in the surface layer in July during the rainy season. Such differences in hydrographic conditions between mixing and stratified seasons were due to vertical distribution of highly saline water from outside the bay, which seasonally migrated between surface and subsurface layers. Relatively distinct redundancy analysis plots and boxplots generated from a generalized linear model showed that microplankton community structure differed between mixing and stratified seasons. The mixing and stratified seasons were characterized by high diatom abundance and low microplankton abundance, respectively, suggesting that vertical mixing increases nutrient supply and consequently increases diatom abundance. Thus, microzooplankton (mainly naked ciliates and athecate dinoflagellates) abundance was related to the abundance of prey organisms, such as diatoms, thecate dinoflagellates and cryptophyte-like flagellates, rather than hydrographic conditions. Thus, saline water intrusion from a branch of the Kuroshio Current likely governs hydrographic conditions in Kagoshima Bay and consequently affects temporal variations in abundance and taxonomic composition of phytoplankton and microzooplankton communities.

Anomalous hydrographic conditions off the northwestern coast of the Baja California Peninsula during 2013–2016

A succession of interannual warm events occurred in the Pacific during the 2013–2016 period, when positive thermal anomalies were detected. A broad region with abnormally warm waters in the northeastern Pacific Ocean was observed in 2013, which by 2015 had reached waters off Baja California (Mexico). In addition to the ongoing warming effect, early evidence of an El Niño event was detected in 2015. This paper analyzes hydrographic data measured at Todos Santos Bay and environs, as well as satellite-derived sea-level data, to characterize changes in the hydrographic conditions during the 2013–2016 period. The highest positive temperature anomalies (~5 ºC) were observed in September 2015, with a steep decline to neutral conditions in spring 2016. Records for the bay showed slight variations in hydrographic conditions during spring, highlighting the importance of local over remote forcings during this time of year. In autumn, by contrast, interannual signals prevailed over local processes. Satellite-derived sea-level data revealed an alongshore propagation of remote signals, identified as shallow coastal poleward flows.

Biogeochemistry of dimethylsulfoniopropionate, dimethylsulfide and acrylic acid in the Yellow Sea and the Bohai Sea during autumn

Environmental context Dimethylsulfide (DMS) is a climatically important biogenic trace gas that is emitted from oceans. This research focuses on the spatiotemporal distributions of DMS and its related compounds, i.e. dimethylsulfoniopropionate (DMSP) and acrylic acid (AA), and the influencing factors in the Yellow Sea and the Bohai Sea during autumn. In addition, the sea-to-air flux of DMS, kinetic responses of DMSP consumption as well as DMS and AA production are also investigated. This study is helpful in understanding the marine sulfur cycle in marginal seas in China. Abstract The biogeochemistry of dimethylsulfoniopropionate (DMSP), dimethylsulfide (DMS) and acrylic acid (AA) in the Yellow Sea (YS) and the Bohai Sea (BS) was investigated in November 2013. The concentrations (and ranges) of total DMSP (DMSPt), dissolved DMSP (DMSPd), DMS and AA in surface waters were 30.71 (1.07–122.50), 6.60 (0.85–35.67), 1.48 (0.53–5.32) and 42.2 (13.8–352.8) nmol L–1 respectively. The concentrations of DMSPd and AA were positively correlated with chlorophyll-a levels, which suggests that phytoplankton biomass has an important function in controlling DMSPd and AA distributions. Furthermore, DMS and AA concentrations revealed significant positive relationships with DMSPd concentrations. The average ratios of AA/(DMSP+AA) and DMS/AA were 53.98 and 7.62% respectively. The vertical profiles of DMSP, DMS and AA were characterised by high concentrations that mostly occur near the surface. Even under highly variable hydrographic conditions, a positive relationship was observed between DMSPt and chlorophyll-a concentrations. The rates of DMSPd consumption, as well as DMS and AA production, significantly varied with marine environments. The sea-to-air fluxes of DMS from the YS and the BS to the atmosphere were estimated to be in the range of 3.01 to 6.91μmol m–2day–1.

Effects of hydrographic conditions of ponds on juvenile fish assemblages in the Kakum mangrove system, Ghana

The importance of mangrove ecosystems as nursery grounds for fishes is well established and documented. This paper reports on the possible role of hydrographic dynamics in the selection and utilization of tropical mangrove ponds as nursery habitats for juvenile fishes of ecological and commercial importance. The study was conducted in the Kakum River Estuary mangrove ecosystem near Elmina in the Central Region of Ghana during the peak of the monsoon season. Fish assemblages were sampled using a cast net and a pole seine. Aspects of hydrographic conditions of the ponds were studied. A total of 265 fish specimens, belonging to 18 species and 12 families were sampled. The commonest fish was the blackchinned tilapia Sarotherodon melanotheron, a typical brackish water fish in West Africa, which accounted for 66.4% of the total fish sampled. Ten of the 18 species belonging to 7 families, namely Elops lacerta, Eucinostomus melanopterus, Porogobius schlegelii, Gobionellus occidentalis, Mugil bananensis, Liza falcipinnis, Epinephelus sp., Serranus accraensis, Penaeus notialis and Callinectes amnicola were of marine origin. Spatial and temporal variations in the prevailing hydrographic conditions appeared to influence the fish species composition, density and size classes to varying degrees. Changes in salinity, dissolved oxygen, pH, conductivity and pond size correlated significantly with abundance of juvenile fish. Major fluctuations in fish species composition occurred on a weekly basis with changing pond depth and volume, indicating the dynamic nature of mangroves ponds that may serve the ecological needs of different species over time. Smaller juvenile fishes seemed better adapted to high variations in hydrographic conditions compared to larger juveniles. Higher fish densities and lower species diversity were encountered when the ponds were shallow compared to deep ponds. The utilization of mangrove tidal ponds as nurseries by juvenile fish may therefore be influenced primarily by the salinity and pond size.

Seasonal and spatial distribution of mesozooplankton in a tropical estuary, Nha Phu, South Central Viet Nam

This study provides a description of mesozooplankton (holo- and meroplankton) abundance, biomass and diversity patterns inside and outside a tropical estuary (Nha Phu Estuary, Khanh Hoa, Viet Nam). In total 185 zooplankton species have been recorded during the study period (2009–2010), copepods contribute with the largest share of species (more than 100), Tunicata with 20, Cnidaria with 17 and Chaetognatha with 9 species. At the most species rich site the number of zooplankton species varies between 55 and 123. The number of species and the annual variation in numbers declines towards the head of the estuary (14–37 species). In contrast, the highest numbers of individuals occur in the inner part of NPE. Calanoids that are the most abundant group of the copepods occur in densities up to 28.2 ind. L−1 (Aug. 9). At ‘Outer NPE’ and ‘Outside NPE’ the maximum density of calanoids is 5.8 and 10.7 ind. L−1, respectively. The declining diversity of zooplankton towards the head of the estuary is also supported by various indices (Shannon’s index, Margalef’s index). A cluster analysis on similarity of species supports a clustering of the inner NPE sites vs the other sites. There is a general separation between the dominant copepod species in the inner (Bestiola sp., Acartia pacifica, Pseudodiaptomus incisus) and outer (Paracalanus gracilis, Acrocalanus gibber, Subeucalanus subcrassus, Oithona rigida, Corycaeus andrewsi, Oithona plumifera) part of the estuary though a few species are common in both areas (Paracalanus crassirostris and Euterpina acutifrons). The zooplankton community at the inner NPE is subjected to more variable hydrographic conditions (salinity in particular) than the communities at the other sites where more stable conditions prevail. A short residence time in the inner part of the estuary due to the tide is supposed to impede a strong horizontal structuring of the zooplankton community.

Temporal and spatial variations of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in the East China Sea and the Yellow Sea

Temporal and spatial distributions of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) were determined in the East China Sea and the Yellow Sea during June–July, 2006 and January–February, 2007. The concentrations of DMS and total DMSP in surface water in the study area were 5.64 (1.79–12.24) and 28.25 (13.98–44.93) nmol L −1 in summer, and were 1.79 (1.02–3.51) and 11.01 (6.90–17.98) nmol L −1 in winter, respectively. The distributions of DMS and DMSP in the study area were obviously influenced by the Yangtze River effluent and the Kuroshio water. Even under highly variable hydrographic conditions, a significant relationship was observed between DMS and chlorophyll a concentrations in summer as well as in winter, suggesting that phytoplankton biomass might play an important role in controlling DMS distribution in the study area. The summer ratios of DMS/chlorophyll a and DMSP/chlorophyll a were approximately twofold higher than winter values, corresponding with the temporal variation in phytoplankton community structure between summer and winter. The sea-to-air fluxes of DMS were estimated to be 5.32 and 11.92 μmol m −2 d −1 using the equations of Liss and Merlivat (1986) and Wanninkhof (1992), respectively.

Short-term hydrographic variability in a stratified Arctic fjord

Abstract Fjords in the Arctic often have a more complex circulation pattern than the classical two-dimensional estuarine circulation. This is due to the effects of the Earth's rotation on stratified waters in wide fjords. Observations from a semi-enclosed fjord basin, Van Mijenfjorden on Spitsbergen, show that the hydrography and circulation vary considerably on short timescales (hours) in the summer season. The depth and distribution of the low salinity upper water layer respond quickly to changes in the wind field. The Coriolis effect has an essential impact on the circulation, inducing eddy-like flow patterns and strong cross-fjord gradients. Within the upper layer, the lowest salinity values and highest temperatures were found on the northern side of the fjord in calm wind periods. When the wind was strong from the west, the cross-fjord gradients were reversed. Internal wave activity contributes to large vertical displacement of water below the upper layer. Knowledge of such strongly variable hydrographic conditions in fjords are important for sampling strategy and interpretation of data, for instance of primary production and sedimentation processes, and for the understanding of fjords as depositional systems.

The abundance of juvenile yellowtail (Seriola quinqueradiata) near the Kuroshio: the roles of drifting seaweed and regional hydrography

AbstractWe assess the effect of drifting seaweed (Sargassum sp.) biomass, geography and hydrography on juvenile yellowtail (Seriola quinqueradiata) abundance variation off the southeast coast of Japan, near the Kuroshio Current. The amount of drifting seaweed mats progressively increased northeastward into the cooler, coastal waters. Frontal structure indexed using a station‐to‐station ΔSST did not explain spatial variation in the seaweed mat distribution, although the western extent of the Kuroshio Current appeared to act as a boundary. Juvenile yellowtail constituted 51–62% of the fish collected in association with drifting seaweed mats in April 1996 and 1997 and 29% in June 1996. The abundance of juvenile yellowtail was positively correlated with seaweed biomass. The geographic distribution of juvenile yellowtail associated with drifting mats varied among sampling periods, being more southwesterly in April and more northeasterly in June. Simple multiple regression models based on seaweed biomass and geographic distribution (latitude) explained between 35% and 43% of the variation in juvenile yellowtail abundance in spring. Associations with spatial and temporal variations in hydrographic conditions did not contribute to explained variation in a meaningful manner. The results presented here indicate that, off the southeast coast of Japan during April, yellowtail juveniles are likely to be most abundant when seaweed biomass is high, occur offshore, and are bounded by the western extent of the Kuroshio Current near the 19–20°C SST isotherm.

Effects of decadal climate change on zooplankton over the last 50 years in the western subarctic North Pacific

AbstractDecadal‐ to multi‐decadal variations have been reported in many regional ecosystems in the North Pacific, resulting in an increasing demand to elucidate the link between long‐term climatic forcing and marine ecosystems. We detected phenological and quantitative changes in the copepod community in response to the decadal climatic variation in the western subarctic North Pacific by analyzing the extensive zooplankton collection taken since the 1950s, the Odate Collection. Copepod species were classified into five seasonal groups depending on the timing of the annual peak in abundance. The abundance of the spring community gradually increased for the period 1960–2002. The spring–summer community also showed an increasing trend in May, but a decadal oscillation pattern of quasi‐30‐year cycles in July. Phenological changes coincided with the climate regime shift in the mid‐1970s, indicated by the Pacific decadal oscillation index (PDO). After the regime shift, the timing of the peak abundance was delayed one month, from March–April to April–May, in the spring community, whereas it peaked earlier, from June–July to May–June, in the spring–summer community, resulting in an overlap of the high productivity period for the two communities in May. Wintertime cooling, followed by rapid summertime warming, was considered to be responsible for delayed initiation and early termination of the productive season after the mid‐1970s. Another phenological shift, quite different from the previous decade, was observed in the mid‐1990s, when warm winters followed by cool summers lengthened the productive season. The results suggest that climatic forcing with different decadal cycles may operate independently during winter–spring and spring–summer to create seasonal and interannual variations in hydrographic conditions; thus, combinations of these seasonal processes may determine the annual biological productivity.

Holocene variability of bottom water hydrography on the Kara Sea shelf (Siberia) depicted in multiple single-valve analyses of stable isotopes in ostracods

Ostracods secrete their valve calcite within a few hours or days, therefore, its isotopic composition records ambient environmental conditions of only a short time span. Hydrographic changes between the calcification of individuals lead to a corresponding range (max.–min.) in the isotope values when measuring several (≥5) single valves from a specific sediment sample. Analyses of living (stained) ostracods from the Kara Sea sediment surface revealed high ranges of >2‰ of δ 18O and δ 13C at low absolute levels ( δ 18O: <3‰, δ 13C: <−3‰) near the river estuaries of Ob and Yenisei and low ranges of ∼1‰ at higher absolute levels ( δ 18O: 2–5.4‰, δ 13C: −3‰ to −1.5‰) on the shelf and in submarine paleo-river channels. Comparison with a hydrographic data base and isotope measurements of bottom water samples shows that the average and the span of the ostracod-based isotope ranges closely mirror the long-term means and variabilities (standard deviation) of bottom water temperature and salinity. The bottom hydrography in the southern part of the Kara Sea shows strong response to the river discharge and its extreme seasonal and interannual variability. Less variable hydrographic conditions are indicative for deeper shelf areas to the north, but also for areas near the river estuaries along submarine paleo-river channels, which act as corridors for southward flowing cold and saline bottom water. Isotope analyses on up to five single ostracod valves per sample in the lower section (8–7 cal. ka BP) of a sediment core north of Yenisei estuary revealed δ 18O and δ 13C values which on average are lower by 0.6‰ in both, δ 18O and δ 13C, than in the upper core section (<5 cal. ka BP). The isotope shifts illustrate the decreasing influence of isotopically light river water at the bottom as a result of the southward retreat of the Yenisei river mouth from the coring site due to global sea level rise. However, the ranges (max.–min.) in the single-valve δ 18O and δ 13C data of the individual core samples are similar in the upper and in the lower core section, although a higher hydrographic variability is expected prior to 7 cal. ka BP due to river proximity. This lack of variability indicates the southward flow of cold, saline water along a submarine paleo-river channel, formerly existing at the core location. Despite shallowing of the site due to sediment filling of the channel and isostatic uplift of the area, the hydrographic variability at the core location remained low during the Late Holocene, because the shallowing proceeded synchronously with the retreat of the river mouth due to the global sea level rise.

Spatial Distribution of Fish Larvae in a Bay of the Gulf of California (June and November 1997)

Bahía Concepción is one of the largest coastal bodies of water on the peninsular side of the Gulf of California, which is characterized by great fish species diversity. Spatial distributions of fish larvae in Bahí a Concepcioó n during June and November 1997 were analyzed; these months were representative of the extreme hydrographic conditions during an annual cycle in the Gulf. Zooplankton samples (333-mm conical net) and conductivity, temperature, and depth data were obtained at each sampling station. The Bray-Curtis dissimilarity index defined three groups of stations in June (mouth, central, and interior) and two in November (mouth and central-interior), which vary in species composition and dominance. In June, Gerreidae (Eucinostomus gracilis) and Clupeidae (Opisthonema sp.) larvae were the dominant species in the bay mouth; Sciaenidae type 1, Clupeidae (Harengula thrissina), and Pomacentridae (Stegastes rectifraenum) larvae were the dominant species in the central bay; and Gerreidae (E. dowii) larvae in the bay interior. The differentiation of three groups is associated with variations in hydrographic conditions recorded from the mouth to the bay interior, coinciding with a well-defined thermocline throughout the bay as a result of weak winds prevailing in the central Gulf region. In November, Mullidae and Clupeidae (Etrumeus teres) larvae were the dominant taxa in the bay mouth, and Gobiidae (Ilypnus gilberti) and Blenniidae (Hypsoblennius gentilis) larvae dominated in the central and interior bay. The similarity of the larval composition of the central and interior bay is associated with a straight spatial gradient of temperature and salinity and homogeneity in the water column; this condition was caused by strong winds and tides that affect the region in late fall. In addition, the presence of mesopelagic species (e.g., Vinciguerria lucetia) in the bay interior during November indicates a clear influence of the Gulf waters in the bay at that time, possibly as a result of intensive mixing.

Ichthyoplankton distribution off the Peninsula de Mejillones, Chile (23° S, 71° W), under variable hydrographic conditions during the austral summer and winter of the 1997 El Niño

This paper identifies the larval fish community distribution, emphasizing non-commercial species, in the coastal zone and the adjacent oceanic area off the Peninsula de Mejillones (23° S, 71° W) during the 1997 El Nino. We performed one survey in the austral summer (January) and one in the austral winter (July). In January, cold, upwelled water dominated the area off the Peninsula de Mejillones with a cold-water filament extending 60 nm offshore. In July, warm Subtropical Surface Waters affected the area. We identified a total of 56 taxa, which included taxa from different adult habitats (coastal, epipelagic, epi-mesopelagic and mesopelagic). The most abundant species were Engraulis ringens (18% and 21%), Bathylagus sp. (4% and 5%), Diogenichthys laternatus(12% and 39%), Lampanyctus parvicauda (9% and 2%), Triphoturus mexicanus (14% and 3%) and Vinciguerria spp. (21% and 23%), in January and July, respectively. A multidimensional scaling (MDS) ordination and similitude dendograms revealed changes in the larval fish community structure influenced by hydrographic conditions. The rank correlations between the assemblage similarity matrix and the physical variables matrix indicate the importance of the distance from shore and production processes (represented by chlorophyll-a) on the assemblage distributions. Off the Mejillones' coast, there is a complex spatial and temporal hydrographic system where ichthyoplankton of coastal and oceanic fish species coexist. Despite both short-term and longer-term disruptive oceanographic processes (e.g., mesoscale filaments, El Nino events), it seems that the numerically dominant species persist in the area due to their various behavioral strategies and physiological adaptations.

Uncertainty reduction of estimated geostrophic volume transports with altimeter observations east of Taiwan

The common geostrophic estimation of ocean current velocity uses only water temperature and conductivity profiles. The geostrophic volume transport of a western boundary current, like the Taiwan Current (Kuroshio east of Taiwan), between the coast and its eastern boundary can be easily estimated based on hydrographic survey data. But the eastern boundary of the Taiwan Current is very uncertain due to extremely variable hydrographic conditions. This uncertainty is strongly correlated with the propagating mesoscale eddies originating from the interior of the western North Pacific Ocean. The uncertainty of estimated transport can be greatly reduced if eddy distribution is considered when determining the integration boundaries with the assistance of satellite altimeter measurements. Eight hydrographic surveys east of Taiwan between November 1992 and June 1996 are demonstrated in this study. The average geostrophic transport of the Taiwan Current with a reference set to 1000 dbar at 22°N between the east coast of Taiwan and 124°E is 22.9 ±14.2 Sv and changes to 22.1 ± 8.3 Sv, the uncertainty of which is nearly halved after taking account of the eddy distribution. The estimation uncertainty is insensitive to vertical displacements of the reference level within the depth range between 800 and 2000 dbar.

Developing Baltic cod recruitment models. II. Incorporation of environmental variability and species interaction

We investigate whether a process-oriented approach based on the results of field, laboratory, and modelling studies can be used to develop a stock–environment–recruitment model for Central Baltic cod (Gadus morhua). Based on exploratory statistical analysis, significant variables influencing survival of early life stages and varying systematically among spawning sites were incorporated into stock–recruitment models, first for major cod spawning sites and then combined for the entire Central Baltic. Variables identified included potential egg production by the spawning stock, abiotic conditions affecting survival of eggs, predation by clupeids on eggs, larval transport, and cannibalism. Results showed that recruitment in the most important spawning area, the Bornholm Basin, during 1976–1995 was related to egg production; however, other factors affecting survival of the eggs (oxygen conditions, predation) were also significant and when incorporated explained 69% of the variation in 0-group recruitment. In other spawning areas, variable hydrographic conditions did not allow for regular successful egg development. Hence, relatively simple models proved sufficient to predict recruitment of 0-group cod in these areas, suggesting that key biotic and abiotic processes can be successfully incorporated into recruitment models.

LATE QUATERNARY NORTH ATLANTIC PALEOCEANOGRAPHIC RECORDS AND STABLE ISOTOPIC VARIABILITY IN FOUR PLANKTONIC FORAMINIFERAL SPECIES

The distribution of planktonic foraminiferal species in two cores, ENAM 30 and ENAM 94-13, taken within the framework of the European North Atlantic Margin (ENAM) project from water depths of around 1200 m, south-west of the Faeroe Islands, were investigated. The oxygen isotope record of the polar Neogloboquadrina pachyderma sinistral includes Oxygen Isotope Stages (OIS) 4 to OIS 1 in core ENAM 30, whereas that of core ENAM 94-13 precedes the OIS 6/7 boundary. The age of the sediments is based on AMS 14C measurements. The micropaleontological record suggests much more variable hydrographic conditions during OIS 7 to OIS 3, compared to the more stable OIS 2 and OIS 1. Trends of dramatic warming through glacial-interglacial boundaries are followed by a general trend of decreasing temperatures towards the end of interglacial stages. This implies that colder episodes could also develop in the near future of the Holocene at a sub-Milankovitch time scale, after the end of the recent warming phase. The oxygen and carbon isotopic signal of the sub-polar species N. pachyderma dextral, Turborotalita quinqueloba and Globigerina bulloides, from deglacial, interglacial and interstadial sediments, were analyzed for comparison with the signal of N. pachyderma sinistral. Our investigation shows that the low relative abundance of the polar morphotype of N. pachyderma in sediments from warmer climates can yield an isotopic signal that does not correspond to the prevailing climatic conditions of the area at the time slice studied, probably for not being the most representative species of the sample. This fact is further supported by the older AMS 14C age of N. pachyderma, compared to the ages of the sub-polar species from the same level. The signals of the three sub-polar species, on the other hand, show a more coherent trend in the intervals investigated and may reflect better the actual environmental conditions. Warm climatic episodes within Oxygen Isotope Stage 6, indicated by the high relative abundance of sub-polar species, were not apparent in the oxygen isotope record of N. pachyderma sinistral, while the lower values of the isotopic records of the sub-polar species accurately reflect these warmer intervals.

Biological and physical controls on dissolved dimethylsulfide over the north-eastern continental shelf of New Zealand

Data presented in this paper are part of an extensive investigation of the physics of cross-shelf water mass exchange in the north-east of New Zealand and its effect on biological processes. Levels of dissolved dimethylsulfide (DMS) were quantified in relation to physical processes and phytoplankton biomass. Measurements were made at three main sites over the north-east continental shelf of New Zealand's North Island during a current-driven upwelling event in late spring 1996 (October) and an oceanic surface water intrusion event in summer 1997 (January). DMS concentrations in the euphotic zone ranged between 0.4 and 12.9nmoldm−3. Integrated water column DMS concentrations ranged from 33 to 173μmolm−2 in late spring during the higher biomass (15–62 Chl-a mgm−2) month of October, and from 25 to 38μmolm−2 in summer during the generally lower biomass (16–42 Chl-a mgm−2) month of January. We observed high levels of DMS in the surface waters at an Inner Shelf site in association with a Noctiluca scintillans bloom which is likely to have enhanced lysis of DMSP-producing algal cells during phagotrophy. Integrated DMS concentrations increased three-fold at a Mid Shelf site over a period of a week in conjunction with a doubling of algal biomass. A high correlation (r2=0.911, significant <0.001) of integrated DMS and chlorophyll-a concentrations for compiled data from all stations indicated that chlorophyll-a biomass may be a reasonable predictor of DMS in this region, even under highly variable hydrographic conditions. Integrated bacterial production was inversely correlated to DMS production, indicating active bacterial consumption of DMS and/or its precursor.

Abundance of Selected Fish Species in Relation to Temperature and Salinity Patterns in the Sagavanirktot Delta, Alaska, Following Construction of the Endicott Causeway

Hydrography data from 1985 to 1993 in the vicinity of the Endicott Causeway near Prudhoe Bay, Alaska, were analyzed to determine if the trend in yearly post-causeway construction temperature and salinity was toward colder and more saline conditions. No significant (P&lt;=0.05) relationship was found for temperature; however, salinity levels significantly decreased with time following construction. Intra-year variations in hydrographic conditions appeared to be due to coast-wide meteorological and oceanographic events rather than causeway-induced. We analyzed abundance indices, as log e, transformed catch-per-unit-effort (CPUE), of four marine species (arctic cod, Boreogadus saida; fourhorn sculpin, Myoxocephalus quadricornis; arctic flounder, Pleuronectes glacialis; saffron cod, Eleginus gracilis); two freshwater species (round whitefish, Prosopium cylindraceum; arctic grayling, Thymallus arcticus); and the anadromous rainbow smelt, Osmerus mordax, to determine whether there were increases in the abundance of marine species and declines in anadromous and freshwater species. We developed the "best-fit" regression models, based on the criterion of maximized regression F-ratio (i.e., minimized regression p-values), using year, temperature, salinity, and all possible interactions. Results suggested that all species exhibited changes in abundance levels over the period of record. In particular, arctic flounder and rainbow smelt both showed sharp increases in abundance five years after construction. For all species, the observed changes in abundance levels appears to be due to naturally occurring events in the species life history or changes in coast-wide meteorological conditions rather than to causeway-induced alterations in local hydrographic conditions.

Density and biomass of the meiofauna of the upper continental slope in two regions of the Gulf of Mexico

A comparative study of the density and biomass of the meiofauna distributed on the upper continental slope of the western and southern Gulf of Mexico was conducted on board the R/V Justo Sierra. A total of 48 samples were obtained with the aid of a Gomex box corer, at depths ranging from 196 to 540 m off the coast of Tamaulipas and Yucatán. Both regions have contrasting hydrographic and sedimentary conditions; the former is of terrigenous nature, while the latter is carbonaceous. The highest density and biomass values (mean = 1,829,216 ± 489,005 ind/m2 and 0.304 f 0.082 gC/m2, respectively) were recorded off Tamaulipas. These parameters were tive to six times lower in order of magnitude in the Yucatán slope. Meiofauna biomass values were significantly correlated with density (r2 = 0.99). Significant differences in density [Fc(1,46) = 38.51, P &lt; 0.005] and biomass [Fc( 1,46) = 37.72, P &lt; 0.005] were recognized between both areas studied. No clear spatial or bathymetric patterns were observed in the meiofaunal components identified in Tamaulipas. This pattern was attributed to the prevailing variable hydrographic conditions and the different organic carbon sources present in the westem region of the Gulf of Mexico. In contrast, the depth and particulate organic carbon of the Yucatán slope exert a significant influence on meiofaunal density and biomass. In this region, predation by larger size classes seems to control the density. The density and biomass values recorded in both regions of the gulf studied fall within the range of previous meiofauna reports from the northern Gulf of Mexico and similar slope habitats of the world oceans.

A comparison of the amphiura filiformis-associations North-East of the Dogger Bank and of the German Bight

Very few benthos surveys and even fewer detailed investigations have been carried out in the central North Sea. Therefore the macrozoobenthos of a muddy area north-east of the Dogger Bank was intensively investigated. Samples were taken at two stations in 1983 every two months. Species composition, abundance, biomass and seasonal changes of the Amphiura filiformis-association were determined. In comparison to the same type of association in the German Bight ( Salzwedel et al., 1985), the area north-east of the Dogger Bank has higher species numbers, three times the abundance and twice the biomass. The investigated area is located at the transition from the southern to the central North Sea. Seasonal variations in hydrographic conditions are much lower than in the German Bight. The fauna consists of species occurring either south or north of the Dogger Bank. First results of a mapping of the benthos from the central North Sea (ICES North Sea Benthos Survey) are showing that the fauna corresponds more with the association south of the Dogger Bank.