Environmental changes and the responses of the ecosystems of the Yellow Sea during 1976–2000

The changes in the environmental features of the Yellow Sea during the last 25 years of the 20th century were studied using a set of seasonally monitored data along a transect (at 36°N) maintained by the State Oceanic Administration of China. The data included the ocean temperature (T), salinity (S) and biogenic elements, such as dissolved oxygen (DO), phosphorus (PO₄-P), silicon (SiO₃-Si) and dissolved inorganic nitrogen (DIN).The seasonal (summer and winter) values and the annual mean of these elements showed significant changes during the monitored period. Time series of T, S, DIN and N:P ratios exhibited positive trends, while those of DO, P and Si exhibited negative trends. During this period, the annual mean of T and DIN in the Yellow Sea increased by 1.7 °C and 2.95 μmol L⁻¹, respectively, while those of DO, P and Si decreased by 59.1, 0.1 and 3.93 μmol L⁻¹, respectively. In the 1980s, particularly in between 1985 and 1989, concentrations of P and Si dropped to near the ecological threshold for growth of diatoms. The N:P ratio increased from 4 in 1984 to over 16 in 2000. The climate trend coefficients, R_xt, for these time series are all above 0.43 with significance levels of 95%, except for salinity. The increases in T were consistent with the recent climate warming in northern China and the adjacent seas, i.e. the Bohai Sea and the East China Sea. The reduction of DO was probably attributable to the increase in T and decrease in primary production in these regions. The positive trend of DIN was mainly attributable to precipitation and partly to Changjiang River discharge. The negative concentration trends of P and Si were due to the decreases in their concentrations in seawater that flowed to the Yellow Sea from the Bohai Sea. As a result, N:P ratios greatly increased in the seawater of the Yellow Sea.Moreover, some important responses of the Yellow Sea ecosystems to the changes in physical variables and chemical biogenic elements were obviously displayed. These responses include strengthening nutrient limitation, decreasing chlorophyll a, primary production and phytoplankton abundance, succession of dominant phytoplankton species from diatoms to non-diatoms, changes in fish community structure and species diversity.     

Changes in the shelf macrobenthic community over large temporal and spatial scales in the Bohai Sea, China

Over the past 20 years, the Bohai Sea has been subjected to a considerable human impact through over-fishing and pollution. Together with the influence of the Yellow River cut-off, the ecosystem experienced a dramatic change. In order to integrate available information to detect any change in macrobenthic community structure and diversity over space and time, data collected during the 1980s and the 1990s from 3 regions of the Bohai Sea (Laizhou Bay, 16 stations, 37–38°N, 119–120.5°E; central Bohai Sea, 25 stations, 38–39°N, 119–121°E; eastern Bohai Bay, 12 stations, 38–39°N, 118.5–119°E) were reanalyzed in a comparative way by means of a variety of statistical techniques. A considerable change in community structure between the 1980s and the 1990s and over the geographical regions at both the species and family level were revealed. After 10 years, there was a considerable increase in abundance of small polychaetes, bivalves and crustaceans but decreased number of echinoderms. Once abundant in Laizhou Bay in the 1980s, a large echinoderm Echinocardium cordatum and a small mussel Musculista senhousia almost disappeared from the surveying area in the 1990s. Coupled with the increased abundance was the increased species richness in general whereas evenness was getting lower in central Bohai Sea and Bohai Bay but increased in Laizhou Bay. K-dominance plot showed the same trend as evenness J′. After 10 years, the macrobenthic diversity in the Bohai Sea as a whole was slightly reduced and a diversity ranking of central Bohai Sea > Laizhou Bay > eastern Bohai Bay over space was also suggested. Sediment granulometry and organic content were the two major agents behind the observed changes.     

Distribution of phytoplankton in the southern Black Sea in summer 1996, spring and autumn 1998

The species composition, abundance, and biomass of micro- (>15 μm) and nano- (<15 μm) phytoplankton were studied along the southern Black Sea during June–July 1996 and March–April and September 1998. A total of 150 species were identified, 50% of them being dinoflagellates. The average total phytoplankton abundance changed from 77×10³ cells l⁻¹ in spring to 110×10³ cells l⁻¹ in autumn and biomass from 250 μg l⁻¹ in summer to 1370 μg l⁻¹ in spring. Based on the extensive sampling grid from June–July 1996, phytoplankton seemed to have a rather homogeneous biomass distribution in the southern Black Sea. In all periods, the coccolithophorid Emiliania huxleyi was the most abundant species, its contribution to the total abundance ranging from 73% in autumn to 43% in spring. However, in terms of biomass, diatoms made up the bulk of phytoplankton in spring (97%, majority being Proboscia alata) and autumn (73%, majority being Pseudosolenia calcar-avis), and dinoflagellates in summer (74%, Gymnodinium sp.). There was a remarkable similarity in the dominant species between the western and eastern regions of the southern Black Sea, indicating transport of phytoplankton within the basin.     

Diatom stratigraphy and long-term dissolved silica concentrations in the Baltic Sea

In many parts of the world coastal waters with anthropogenic eutrophication have experienced a gradual depletion of dissolved silica (DSi) stocks. This could put pressure on spring bloom diatom populations, e.g. by limiting the intensity of blooms or by causing shifts in species composition. In addition, eutrophication driven enhanced diatom growth is responsible for the redistribution of DSi from the water phase to the sediments, and changes in the growth conditions may be reflected in the sediment diatom stratigraphy.To test for changes in diatom communities we have analyzed four sediment cores from the Baltic Sea covering approximately the last 100 years. The sediment cores originate from the western Gulf of Finland, the Kattegat, the Baltic Proper and the Gulf of Riga. Three out of the four cores reveal only minor changes in composition of diatom assemblages, while the Gulf of Riga core contains major changes, occurring after the second World War. This area is set apart from the other Baltic Sea basins by a high frequency of low after spring bloom DSi concentrations (< 2 µmol L^− 1) during a relatively well defined time period from 1991–1998. In 1991 to 1993 a rapid decline of DSi spring concentrations and winter stocks (down to 5 µmol L^− 1) in the Gulf was preceded by exceptionally intense diatom spring blooms dominated by the heavily silicified species Thalassiosira baltica (1991–1992; up to 5.5 mg ww L^− 1). T. baltica has been the principal spring bloom diatom in the Gulf of Riga since records began in 1975. DSi consumption and biomass yield experiments with cultured T. baltica suggest that intense blooms can potentially exhaust the DSi stock of the water column and exceed the annual Si dissolution in the Gulf of Riga. The phytoplankton time series reveals another exceptional T. baltica bloom period in 1981–1983 (up to 8 mg L^− 1), which, however, took place before the regular DSi measurements. These periods may be reflected in the conspicuous accumulation of T. baltica frustules in the sediment core corresponding to ca. 1975–1985.     

Seasonal carbon distribution of copepods in the eastern Weddell Sea, Antarctica

Copepods were sampled by a multiple opening-closing net in the eastern Weddell Sea during various seasons (late winter/early spring, summer, autumn). Total copepod biomass integrated over the upper 1000 m varied seasonally between 1.7 mg C m⁻³ in late winter/early spring and 3.7 mg C m⁻³ in autumn. After the dark season the copepods were rather evenly distributed vertically and highest biomass levels were found in the mid-water layers between about 200 m and 500 m. By contrast, especially in summer but also in autumn copepod biomass concentrated in the uppermost water layer. A total of 64 calanoid species were identified in the upper 1000 m with maximum species numbers in the deepest layer. The large calanoids Calanus propinquus, Calanoides acutus, Metridia gerlachei, Euchaeta antarctica and the small calanoid Microcalanus pygmaeus prevailed and accounted for 60–70% of total copepod biomass, while the small poecilostomatoid Oncaea and the cyclopoid Oithona species comprised about 20%. Hence, the distribution pattern of the entire copepod biomass is strongly influenced by the life cycles of a few dominant species.     

Seasonal variability in sea surface oceanographic conditions in the Aegean Sea (Eastern Mediterranean): an overview

Seasonal variability and the spatial distribution of sea surface temperatures (SST) and salinities (SSS) are reviewed, in relation to the prevailing climatological conditions, heat fluxes, water budget and general water circulation patterns. Within this context, consideration is given to: sea surface temperatures; air temperatures; precipitation; evaporation; wind speeds and directions; freshwater (mainly riverine) discharges throughout the Aegean; and the exchange of water masses with the Black Sea and eastern Mediterranean Sea. The investigation of satellite images, covering a 6-yr period (1988–1994), has enabled a synthesis of the monthly sea surface thermal distribution to be established.The climate of the Aegean Sea is characterised by annual air temperatures of 16–19.5°C, precipitation of about 500 mm yr⁻¹ and evaporation of some 4 mm d⁻¹. The Aegean has a negative heat budget (approximately −25 W m⁻²) and positive water balance (+ 1.0 m yr⁻¹), when inflow from the Black Sea is considered. During the summer, the (northerly) Etesians are the dominant winds over the Sea.Mean monthly sea surface temperatures (SST) vary from 8°C in the north during winter, up to 26°C in the south during summer. SST depends mainly upon air temperature; there is a month's delay between the former and latter maxima. The sea surface salinity (SSS) varies also spatially and seasonally, ranging from less than 31 psu, in the north, to more than 39 psu, in the southeast; lower values (< 25 psu) occur adjacent to the river mouths. SSSs present their maximum differences during summer, whilst during winter and autumn the distribution of SSS is more uniform. The overall spatial SST and SSS distribution pattern is controlled by: distribution of the (colder) Black Sea Waters; advection of the (warmer) Levantine Waters, from the southeastern part of the Aegean; upwelling and downwelling; and, to a lesser extent, but locally important, freshwater riverine inflows.     

Seasonality of picophytoplankton chlorophyll a and biomass in the central Cantabrian Sea, southern Bay of Biscay

Seasonal changes in the abundance and biomass of cyanobacteria (Synechococcus and Prochlorococcus) and picoeukaryotes were studied by flow cytometry in the upper layers of the central Cantabrian Sea continental shelf, from April 2002 to April 2006. The study area displayed the typical hydrographic conditions of temperate coastal zones. A marked seasonality of the relative contribution of prokaryotes and eukaryotes was found. While cyanobacteria were generally more abundant for most of the year (up to 2.4 10⁵ cells mL^− 1), picoeukaryotes dominated the community (up to 10⁴ cells mL^− 1) from February to May. The disappearance of Prochlorococcus from spring through summer is likely related to shifts in the prevailing current regime. The maximum total abundance of picophytoplankton was consistently found in late summer–early autumn. Mean photic-layer picoplanktonic chlorophyll a ranged from 0.06 to 0.53 µg L^− 1 with a relatively high mean contribution to total values (33 ± 2% SE), showing maxima around autumn and minima in spring. Biomass (range 0.58–40.16 mg C m^− 3) was generally dominated by picoeukaryotes (mean ± SE, 4.28 ± 0.27 mg C m^− 3) with an average contribution of cyanobacteria of 30 ± 2%. Different seasonality of pigment and biomass values resulted in a clear temporal pattern of picophytoplanktonic carbon to chlorophyll a ratio, which ranged from 10 (winter) to 140 (summer). This study highlights the important contribution of picoplanktonic chlorophyll a and carbon biomass in this coastal ecosystem.     

Microbial community structure in the marginal ice zone of the Bellingshausen Sea

Phytoplankton, bacteria and microzooplankton were investigated on a transect in the Bellingshausen Sea during the ice melt period in November–December 1992. The transect along the 85°W meridian comprised seven stations that progressed from solid pack-ice (70°S), through melting ice into open water (67°S). The abundance, biomass and taxonomic composition were determined for each component of the microbial community. The phytoplankton was mostly dominated by diatoms, particularly small (<20 μm) species. Diatom abundance ranged from 66 000 cells l⁻¹ under the ice to 410 000 cells l⁻¹ in open water. Phytoplankton biomass varied from <1 to 167 mg C m⁻³, with diatoms comprising 89–95% of the total biomass in open water and autotrophic nanoflagellates comprising 57% under the ice. The standing stocks of autotrophs in the mixed layer ranged from 95 mg C m⁻² under the pack-ice to 9478 mg C m⁻² in open waters. Bacterial abundance in ice-covered and open water stations varied from 1.1 to 5.5×10⁸ cells l⁻¹. Bacterial biomass ranged from 2.4 mg C m⁻³ under pack-ice to an average of 14 mg C m⁻³ in open water. The microzooplankton consisted mainly of aloricate oligotrich ciliates and heterotrophic dinoflagellates and these were most abundant in open waters. Their biomass varied between 0.2 and 54 mg C m⁻³ with a minimum at depth under the ice and maximum in open surface waters. Microheterotrophic standing stocks varied between 396 mg C m⁻² under pack-ice and 3677 mg C m⁻² in the open waters. The standing stocks of the total microbial community increased consistently from 491 mg C m⁻² at the ice station to 13 155 mg C m⁻² in open waters, reflecting the productive response of the community to ice-melt. The composition of the microbial community also shifted markedly from one dominated by heterotrophs (82% of microbial stocks) at the ice station to one dominated by autotrophs (73% of microbial stocks) in the open water. Our estimates suggest that the microbial community comprised >100% of the total particulate organic carbon (POC) under the ice and 62–66% of the measured POC in the open waters.     

Spatial and temporal variability of size-fractionated biomass and primary production in the Ross Sea (Antarctica) during austral spring and summer

The results of a study on the spatial and temporal dynamics of size-fractionated biomass and production of phytoplankton in the Ross Sea during the austral spring and summer are reported. The spring cruise took place in the offshore Ross Sea from 14 November to 14 December 1994. Sampling was carried out on a transect of 27 stations distributed from 76.5 to 72.0°S along 175°E, and covered the three main Antarctic environments of the polynya open waters, the marginal ice zone and the pack ice area. Three subsystems were identified. The subsystem of the polynya was characterised by the predominance of the micro- and nano-planktonic fractions, chlorophyll (Chl a) concentrations from 69.6 to 164.7 mg m⁻² and production rates from 0.68 to 1.14 g C m⁻² day⁻¹. The second subsystem, the marginal ice zone, showed a relative increase of the micro-planktonic fraction, high biomass levels (from 99.64 to 220 mg Chl m⁻²) and production rates from 0.99 to 2.7 g C m⁻² day⁻¹. The subsystem of the pack ice area had a phytoplankton community dominated by the pico-planktonic fraction and showed low biomasses (from 19.4 to 37.7 mg Chl m⁻²) and production rates (0.28 to 0.60 g C m⁻² day⁻¹). Selective grazing by krill is considered an important factor in determining the size structure of the phytoplankton communities. The summer study consisted of a time series carried out in inshore waters of Terra Nova Bay from 12 January to 8 February 1990. In a well stabilised water column and with high levels of PAR always available, the primary production rates of a community dominated by micro-plankton varied from 0.52 to 1.2 g C m⁻² day⁻¹ (average 0.84). A high P/B ratio, up to 3, and a remarkably elevated mean phaeopigment (Phaeo)/Chl a ratio of 2.4 indicated an active removal of biomass by grazing, confirmed by the presence of faecal pellets in quantities reaching 6000 m⁻³ in the upper 50 m. The peculiarities of the inshore versus offshore environments in terms of community size structure, production processes and their implications as regards the food web are discussed.     

Th-derived particulate organic carbon fluxes in the northern Barents Sea with comparison to drifting sediment trap fluxes

Large-volume sampling of ²³⁴Th was conducted to estimate particulate organic carbon (POC) export in conjunction with drifting sediment trap deployments in the northern Barents Sea in July 2003 and May 2005. ²³⁴Th-derived POC fluxes averaged 42.3 ± 39.7 mmol C m^− 2 d^− 1 in 2003 and 47.1 ± 30.6 mmol C m^− 2 d^− 1 in 2005. Sediment trap POC fluxes averaged 13.1 ± 8.2 mmol C m^− 2 d^− 1 in 2003 and 17.3 ± 11.4 mmol C m^− 2 d^− 1 in 2005, but better reflected the transient bloom conditions that were observed at each station within a season. Although ²³⁴Th fluxes agreed within a factor 2 at most stations and depths sampled, sediment trap POC fluxes were lower than large-volume POC flux estimates at almost every station. This may represent an under-collection of POC by the drifting sediment traps or, conversely, an over-collection of POC by the large-volume sampling of ²³⁴Th. It is hypothesized that the offset between the two methods is partly due to the presence of the prymnesiophyte Phaeocystis pouchetii, which potentially causes a large variation in > 53-μm POC/²³⁴Th ratios. Due to the large proportion of dissolved carbon or mucilage released by P. pouchetii, and because it is thought that P. pouchetii does not contribute significantly to the vertical export of biogenic matter in the Barents Sea, the application of large-volume sampling of ²³⁴Th may yield relatively high, and possibly inaccurate POC/²³⁴Th ratios. Hence, POC fluxes derived from ²³⁴Th sampling may be inappropriate and drifting sediment traps might be a more reliable method to measure the vertical export of biogenic matter in regions that have recurrent P. pouchetii blooms, such as the Barents Sea.     

Dynamics of inorganic nutrient species in the Bohai seawaters

Within the frame of a Sino-German Joint Research Program, two cruises of “R/V Dong Fang Hong 2” were carried out in September–October 1998 and April–May 1999, respectively, to understand the dynamics of nutrients in the Bohai. Nutrient species (NO₃⁻, NO₂⁻, NH₄⁺, PO₄³⁻ and SiO₃²⁻) are determined colorimetrically on board for five anchor and 30 grid stations. In situ incubation experiments are performed to determine planktonic nutrient uptake and benthic exchange flux. Nutrient concentrations display short-term variability and seasonal change in the Bohai, with higher levels in shallow coastal waters than in the Central Bohai. The influence of riverine discharge on nutrient levels can be seen from salinity isopleths, nutrient distribution and species ratios. Near-bottom (nb) waters have similar nutrient concentrations as to the surface waters in the Central Bohai, whereas stratification takes place in the Bohai Strait and North Yellow Sea. In situ incubation experiments provide evidence that the uptake ratio (i.e. N, P) by phytoplankton is proportional to the ratios among nutrient species in ambient waters. Based on the data of this study and previously publications, a preliminary estimate of nutrient budgets via riverine input and atmospheric deposition is established. The results indicate that atmospheric deposition gains importance over rivers in delivering nutrients into the Bohai and sustain the new production, following recent decrease in riverine inflow caused by drought periods in North China and damming practices. A historical review of nutrient data indicates that concentrations of nitrogen increase and phosphorus and silica decrease in the Central Bohai over last 40 years. This potentially has an important influence on the health of ecosystem in Bohai (e.g. food web and community structure), though further study is needed to examine the scenario in more detail.     

Variability of mesozooplankton spatial distribution in the North Aegean Sea, as influenced by the Black Sea waters outflow

The North Aegean Sea constitutes an important region of the Mediterranean Sea since in its eastern part the mesotrophic, low salinity and relatively cold water from the Black Sea (outflowing from the Dardanelles strait) meets the oligotrophic, warm and very saline water of Levantine origin, thus forming a thermohaline front. Mesozooplankton samples were collected at discrete layers according to the hydrology of the upper 100 m, during May 1997 and September 1998. In May highest biomass and abundance values (up to 66.82 mg m^− 3 and 14,157 ind m^− 3) were detected in the 10–20 m layer (within the halocline) of the stations positioned close to the Dardanelles strait. The front moved slightly southwards in September, characterized by high biomass and abundance values within the halocline layer. The areas moderately or non influenced by Black Sea water revealed lower standing stock values than the frontal area in both cruises and maxima were detected in the uppermost low salinity layer. Samples collected at the stations and/or layers more influenced by Black Sea water were distinguished from those collected at layers and/or stations more affected by Levantine waters in both periods. In May the former samples were characterized by the copepods Acartia clausi, Centropages typicus, Paracalanus parvus. The abundance of the above species decreased gradually with increasing salinity, in the horizontal and/or in the vertical dimension, with a parallel increase of the copepods Oithona plumifera, Oithona copepodites, Oncaea media, Ctenocalanus vanus, Farranula rostrata. During September the frontal area as well as that covered by the modified Black Sea water, were highly dominated by the cladoceran Penilia avirostris and doliolids. For both seasons, MDS plots, issued from the combination of mesozooplankton and water-type data, revealed the gradual differentiation of zooplankton composition from the frontal area towards the area covered by Levantine water, following the spreading and mixing of the Black sea water. The observed temporal and spatial variability in the distribution pattern of mesozooplankton standing stock and species composition seems to depend considerably on the variability of circulation and frontal flows.     

Changes in dissolved silicate loads to the Baltic Sea — The effects of lakes and reservoirs

We tested the hypothesis that dissolved silicate (DSi) yields [kg km^− 2 yr^− 1] of 82 major watersheds of the Baltic Sea can be expressed as a function of the hydraulic load (HL) as a measure of water residence time and the total organic carbon (TOC) concentration, both variables potentially increasing the DSi yield. Most boreal rivers fitted a linear regression model using HL as an independent variable to explain the DSi yield. Rivers with high HL, i.e., shortest residence times, showed highest DSi yields up to 2300 kg km^− 2 yr^− 1. This is most likely caused by an excess supply of DSi, i.e., the geochemical sources prevail over biological sinks in these boreal watersheds. The DSi yield for regulated and unregulated larger rivers of the boreal watersheds constituting about 40% of the total water discharge and of the total DSi load to the Baltic Sea, respectively, can be expressed as: DSi yield = 190 + 49.5 HL[m yr^− 1] + 0.346 TOC [µM] (R² = 0.80). Since both HL and TOC concentrations have decreased after damming, the DSi yields have decreased significantly in the regulated boreal watersheds, for the River Luleälven we estimated more than 30%. The larger eutrophic watersheds draining cultivated landscape of the southern catchment of the Baltic Sea and representing about 50% of the annual water discharge to the Baltic Sea, deviated from this pattern and showed lower DSi yields between 60–580 kg km^− 2 yr^− 1. DSi yields showed saturation curve like relationship to HL and it appears that DSi is retained in the watersheds efficiently through biogenic silica (BSi) production and subsequent sedimentation along the entire river network. The relationship between HL and DSi yields for all larger cultivated watersheds was best fitted by a Freundlich isotherm (DSi = 115.7HL¹⁰⁹; R² = 0.73), because once lake and reservoir area exceeds 10% of the watershed area, minimum DSi yields were reached. To estimate an uperturbed DSi yield for the larger eutrophic southeastern watersheds is still difficult, since no unperturbed watersheds for comparison were available. However, a rough estimate indicate that the DSi flux from the cultivated watersheds to the Baltic Sea is nowadays only half the uperturbed flux. Overall, the riverine DSi loads to the Baltic Sea might have dropped with 30–40% during the last century.     

The annual cycle of nanoflagellates in the Central Cantabrian Sea (Bay of Biscay)

The annual cycle of nanoflagellates (NF) including autotrophic (ANF), heterotrophic (HNF) and mixotropic (MNF) flagellates carried out in a temperate sea (Central Cantabrian Sea, southern Bay of Biscay) is presented. Three stations with characteristics ranging from coastal to oceanic conditions were analysed in order to compare NF response to this gradient. Samples were monthly collected at each station at three different depths between February 2002 and December 2002. CTD profiles were also taken at each station. NF were grouped according to their trophic status into ANF, HNF and MNF. Abundance and biomass were determined for each group. The annual cycle showed a general pattern consisting in a maximum in July with secondary maxima in March and October and minimum values in May. ANF were the most important fraction, making a major contribution (nearly 75%) to total NF biomass in all stations. HNF represented over 20% along the cycle, except for a peak in spring found in every station. MNF reached less than 5%, showing low seasonability. Small flagellates (2–5 µm) dominated throughout the cycle. Microplankton community was also analysed in terms of abundance and biomass. A significant positive correlation (r² = 0.49) was obtained between 2–5 µm NF and 10–20 µm HNF–MNF biomasses, suggesting a possible trophic relationship between these groups which should be cautiously taken. No significant relationships were found between microplankton and NF or between nutrients and ANF, indicating that the regulation of NF numbers is complex and probably implicates other groups. In addition to this, the unexpected 2002 Chl a concentration pattern and the misplacing of upwelling events render necessary to perform additional studies to fully understand the precise behaviour of NF in the Cantabrian Sea. To the best of our knowledge, this is the first study of a NF cycle in a temperate sea that considers all functional groups.     

The trophic role of the tunicate Salpa thompsoni in the Antarctic marine ecosystem

During a repeat grid survey and drogue study carried out in austral summer 1994/95, the abundance and feeding activity of salps were estimated in the Lazarev Sea region from net tows and in situ measurements of gut fluorescence. Throughout the survey area, Salpa thompsoni accounted for >95% of the total salp stock while Ihlea racovitzai was consistently represented in very low abundances. Maximum densities of S. thompsoni, with ≈4000 ind. 1000 m⁻³, were recorded in the Marginal Ice Zone (MIZ) in December when chlorophyll-a concentrations were well below 1 mg m⁻³. A dramatic decrease in salp stock was observed at the beginning of January, when S. thompsoni virtually disappeared from the most productive area of the MIZ where chlorophyll-a concentrations had by then reached bloom levels of 1.5–3 mg (Chl-a) m⁻³. In situ grazing measurements showed that throughout the cruise S. thompsoni exhibited the highest ingestion rates per individual of any of the most abundant components of the grazing pelagic community, with maxima of ≈160 μg (pigm) ind. ⁻¹ d⁻¹. These feeding rates are 3 to 5 times higher than those previously obtained using in vitro incubations. The total daily consumption of the population of S. thompsoni varied from 0.3 to 108% of daily primary production. We suggest that competitive removal of food by S. thompsoni, rather than direct predation, is responsible for the low krill abundances generally associated with salp swarms.     

Particle flux, and composition of sedimenting matter, in the Greenland Sea

Vertical flux of particulate material was recorded with moored sediment traps during 1988/1989 in the Greenland Sea at 72°N, 10°W. This region exhibits pronounced seasonal variability in ice cover. Annual fluxes at 500 m water depth were 22. 79, 8.55, 2.39, 3.81 and 0.51 g m⁻² for total flux (dry weight), carbonate particulate biogenic silicate, particulate organic carbon and nitrogen, respectively. Fluxes increased in April, maximum rates of all compounds occurred in May–June, and consistently high total flux rates of around 100 mg m⁻²d⁻¹ prevailed the summer. The increasing flux of biogenic particles measured in April is indicative of an early onset of algal growth in spring. Small pennate diatoms dominated in the trap collections during April, and were still numerous during the high flux period when Thalassiosira species were the most abundant diatoms. During May–June, up to 22% of the Thalassiosira cells collected were viable-looking cells. The faecal pellet flux increased after the May–June event. Therefore we conclude that the diatoms settled as phytodetritus, most likely in rapidly sinking aggregates. From seasonal nutrient profiles it is concluded that diatoms contribute 25% to new production during spring and 50% on an annual basis. More than 50% of newly produced silicate particles are dissolved above the 500 m horizon. High new production during spring does not lead to a pronounced sedimentation pulse of organic matter during spring but elevated vertical export is observed during the entire growth period.     

The Dead Sea hydrography from 1992 to 2000

The modern hydrological regime of the Dead Sea is strongly affected by anthropogenic activity. The natural fresh water budget has changed mainly due to the drastic reduction of runoff. Since 1977, the surface level of the Dead Sea has been lowered by an average rate of about 60 cm/year and for the period from 1998 to 2000, the lowering rate has reached about 100 cm/year. As a result of the runoff reduction, the upper layer salinity of the Dead Sea has increased and the gravitational stability of the water body was diminished. Eventually, during the winter of 1978–1979, the lake waters overturned, bringing to an end the long-term stable meromictic¹ hydrological regime. The lake entered a new phase in which its hydrological regime switches between holomictic and meromictic regimes, depending on the size of the runoff into the lake (i.e. the amount of precipitation in the lake's watershed). The first holomictic period, 1979–1980, lasted for 2 months only. It was succeeded by a 4-year meromictic period (1980–1983). The second holomictic period lasted for 9 years (1983–1991). The rainy winter of 1991–1992 resulted in an almost 2-m sea level rise. The upper layer with a relatively low salinity was restored and a new meromictic period persisted for 4 years, until winter 1995–1996. During the last meromictic period, the hydrological regime of the Dead Sea was characterized by following long-term trends: the depth of the summer thermocline increased from 12–15 to 25–30 m; the quasi-salinity of the upper layer, initially of about 164 kg/m³, increased rapidly at a rate of about 16–18 kg/m³/year; the quasi-salinity of the deep water, initially of about 235 kg/m³, decreased slowly at a rate of about 0.08–0.10 kg/m³/year (for the sake of comparison, a quasi salinity of 235 kg/m³ is the equivalent of 280‰ “usual” salinity); and the winter minimal temperature of the upper layer, initially of about 16 °C, increased rapidly at a rate of about 2 °C/year. In November 1995, the latest meromictic period of the Dead Sea came to an end. During the present holomictic period, 1996–2000, the hydrological regime of the Dead Sea is also characterized by long-term trends: the quasi-salinity of the entire Dead Sea increased at a rate of about 0.5 kg/m³/year, with practically no decrease during the winters; the temperature of the deep water mass increased with a rate of about 0.25 °C/year; and the period of vertical convection of the entire water column, initially about 3 months, increased at a rate of about 1 week/year. Moreover, we observed that the temperature and salinity of the bottom layer in the deepest part of the Dead Sea raised by about 0.5–0.6 °C and 0.15–0.25 kg/m³ during each holomictic summer.     

Remotely-sensed chlorophyll a observations of the northern Red Sea indicate seasonal variability and influence of coastal reefs

The biological dynamics of the open northern Red Sea (21.5°–27.5° N, 33.5°–40° E) have not been studied extensively, due in part to both the inaccessibility of this desert region and political considerations. Remotely-sensed chlorophyll a data therefore provide a framework to investigate the primary patterns of biological activity in this oceanic basin. Monthly chlorophyll a data from the 8-year Sea-viewing Wide Field-of-View sensor (SeaWiFS) mission, and data from the Moderate Resolution Imaging Spectroradiometer (MODIS), were analyzed with the Goddard Earth Sciences Data and Information Services Center (GES DISC) online data analysis system “Giovanni”. The data indicate that despite the normal low chlorophyll a concentrations (0.1–0.2 mg m^− 3) in these oligotrophic waters, there is a characteristic seasonal bloom in March–April in the northernmost open Red Sea (24° to 27.5° N) concurrent with minimum sea surface temperature. The location of the highest chlorophyll concentrations is consistent with a linear box model [Eshel, G., and Naik, N.H., 1997. Climatological coastal jet collision, intermediate water formation, and the general circulation of the Red Sea. J. Phys. Oceanogr. 27(7), 1233–1257.] of Red Sea circulation. Two years in the data set exhibited a different seasonal cycle consisting of a relatively weak northern spring bloom and elevated chlorophyll concentrations to the south (21.5° to 24° N).The data also indicate that large coral reef complexes may be sources of either nutrients or chlorophyll-rich detritus and sediment, enhancing chlorophyll a concentration in waters adjacent to the reefs.     

Air–sea CO2 fluxes in the Caribbean Sea from 2002–2004

Air–sea fluxes in the Caribbean Sea are presented based on measurements of partial pressure of CO₂ in surface seawater, pCO_2sw, from an automated system onboard the cruise ship Explorer of the Seas for 2002 through 2004. The pCO_2sw values are used to develop algorithms of pCO_2sw based on sea surface temperature (SST) and position. The algorithms are applied to assimilated SST data and remotely sensed winds on a 1° by 1° grid to estimate the fluxes on weekly timescales in the region. The positive relationship between pCO_2sw and SST is lower than the isochemical trend suggesting counteracting effects from biological processes. The relationship varies systematically with location with a stronger dependence further south. Furthermore, the southern area shows significantly lower pCO_2sw in the fall compared to the spring at the same SST, which is attributed to differences in salinity. The annual algorithms for the entire region show a slight trend between 2002 and 2004 suggesting an increase of pCO_2sw over time. This is in accord with the increasing pCO_2sw due the invasion of anthropogenic CO₂. The annual fluxes of CO₂ yield a net invasion of CO₂ to the ocean that ranges from − 0.04 to − 1.2 mol m^− 2 year^− 1 over the 3 years. There is a seasonal reversal in the direction of the flux with CO₂ entering into the ocean during the winter and an evasion during the summer. Year-to-year differences in flux are primarily caused by temperature anomalies in the late winter and spring period resulting in changes in invasion during these seasons. An analysis of pCO_2sw before and after hurricane Frances (September 4–6, 2004), and wind records during the storm suggest a large local enhancement of the flux but minimal influence on annual fluxes in the region.     

Present-day biosiliceous sedimentation in the northwestern Ross Sea, Antarctica

Sedimentological and oceanographic inferences have been obtained for the NW Ross Sea using sedimentary ²¹⁰Pb as a tracer together with determinations of biogenic silica and organic carbon. ²¹⁰Pb chronologies give apparent accumulation rates ranging between 14 and 80 mg cm⁻² yr⁻¹ (0.02–0.12 cm yr⁻¹) in the shelf basins. Even if a profile of ²¹⁰Pb is present in sediments from the top of the banks, here sediment accumulation rate is practically null, and physical mixing is responsible for the downward transport of fine particles and associated ²¹⁰Pb. The accuracy of ²¹⁰Pb-derived accumulation rates is discussed with respect to ¹⁴C dates. The annual rate of biogenic accumulation from ²¹⁰Pb appears to be ca. 8 times higher than the value derived using radiocarbon. Bioturbation is probably responsible for the discrepancy but also temporal and spatial variations in opal accumulation play a key-role. Contrasting measured and expected inventories of ²¹⁰Pb, a residence time of about 50 years has been tentatively estimated for the water in the NW Ross Sea. Furthermore, the data suggest that the pattern of present-day biosiliceous sediment accumulation in the Ross Sea is mainly driven by biogenic silica production in the water column, the SW area being the most productive part of the Ross Sea, by high sediment accumulation rate which enhances the seabed preservation, and by hydrodynamics, which is so effective to resuspend fine biogenic particles from the topographic highs. Resuspended particles are then deposited onto the flanks. The material which accumulates in the central part of the basins derives basically from primary production and settling through the water column.