Variability of the subtropical shelf front off eastern South America: Winter 2003 and summer 2004

Hydrographic data collected during surveys carried out in austral winter 2003 and summer 2004 are used to analyze the distributions of temperature (T) and salinity (S) over the continental shelf and slope of eastern South America between 27°S and 39°S. The water mass structure and the characteristics of the transition between subantarctic and subtropical shelf water (STSW), referred to as the subtropical shelf front (STSF), as revealed by the vertical structure of temperature and salinity are discussed. During both surveys, the front intensifies downward and extends southwestward from the near coastal zone at 33°S to the shelf break at 36°S. In austral winter subantarctic shelf water (SASW), derived from the northern Patagonia shelf, forms a vertically coherent cold wedge of low salinity waters that locally separate the outer shelf STSW from the fresher inner shelf Plata Plume Water (PPW) derived from the Río de la Plata. Winter T–S diagrams and cross-shelf T and S distributions indicate that mixtures of PPW and tropical water only occur beyond the northernmost extent of pure SASW, and form STSW and an inverted thermocline characteristic of this region. In summer 2004, dilution of Tropical water (TW) occurs at two distinct levels: a warm near surface layer, associated to PPW–TW mixtures, similar to but significantly warmer than winter STSW, and a colder (T∼16 °C) salinity minimum layer at 40–50 m depth, created by SASW–STSW mixtures across the STSF. In winter, the salinity distribution controls the density structure creating a cross-shore density gradient, which prevents isopycnal mixing across the STSF. Temperature stratification in summer induces a sharp pycnocline providing cross-shelf isopycnal connections across the STSF. Cooling and freshening of the upper layer observed at stations collected along the western edge of the Brazil Current suggest offshore export of shelf waters. Low T and S filaments, evident along the shelf break in the winter data, suggest that submesoscale eddies may enhance the property exchange across the shelf break. These observations suggest that as the subsurface shelf waters converge at the STSF, they flow southward along the front and are expelled offshore, primarily along the front axis.     

Nutrient distributions over the Southwestern South Atlantic continental shelf from Mar del Plata (Argentina) to Itajaí (Brazil): Winter–summer aspects

Nutrient distributions observed at some depths along the continental shelf from 27°05′S (Brazil) to 39°31′S (Argentina) in winter, 2003 and summer, 2004 related to salinity and dissolved oxygen (mL L⁻¹) and saturation (%) data showed remarkable influences of fresh water discharge over the coastal region and in front of the La Plata estuary. In the southern portion of the study area different processes were verified. Upwelling processes caused by ocean dynamics typical of shelf break areas, eddies related to surface dynamics and regeneration processes confirmed by the increase of nutrients and the decrease of dissolved and saturation oxygen data were verified. High silicate concentrations in the surface waters were identified related to low salinities (minimum of 21.22 in winter and 21.96 in summer), confirming the importance of freshwater inputs in this region, especially in winter. Silicate concentration range showed values between 0.00 and 83.52 μM during winter and from 0.00 to 41.16 μM during summer. Phosphate concentrations worked as a secondary trace of terrestrial input and their values varied from 0.00 to 3.30 μM in winter and from 0.03 to 2.26 μM in summer; however, in shallow waters, phosphate indicated more clearly the fresh water influence. The most important information given by nitrate concentrations was the presence of water from SACW upwelling that represents a new source of nutrients for marine primary production. Nitrate maximum values reached 41.96 μM in winter and 33.10 μM in summer. At a depth ∼800 m, high nitrate, phosphate and silicate concentrations were related to Malvinas Current Waters, Subantarctic Shallow Waters and Antarctic Atlantic Intermediate Waters (AAIW). Dissolved oxygen varied from 3.41 to 7.06 mL L⁻¹ in winter and from 2.65 to 6.85 mL L⁻¹ in summer. The percentage of dissolved oxygen saturation in the waters showed values between 48% and 113% in winter and from 46% to135% in summer. The most important primary production was verified in the summer, and situations of undersaturation were mainly observed below 50 m depth and at some points near the coast. The anti-correlation between nutrients and dissolved oxygen which showed evident undersaturation also revealed important potential sites of remineralization processes. The nutrient behaviours showed some aspects of the processes that occur over the Southwestern South Atlantic continental shelf and in their land–sea interfaces between Mar del Plata and Itajaí.     

Uptake and resource allocation of ammonium and nitrate in temperate seagrasses Posidonia and Amphibolis

Ecologically relevant estimates of seasonal variability in nitrogen uptake and allocation in two species of temperate seagrasses were obtained using in situ isotope-labelling approach. Significantly higher uptake rates of ammonium by leaves, roots and epiphytes of Amphibolis than Posidonia were observed. Overall, root uptake rates were lower than other components. Effect of season was not significant for leaves, roots or epiphytes of the two species. However, plankton uptake varied seasonally with higher rates in winter (0.98 mg N g⁻¹ DW h⁻¹). In contrast, nitrate uptake rates for various components were significantly affected by seasons. Uptake rates by plankton were highest ranging from 0.003 mg N g⁻¹ DW h⁻¹ (summer, Amphibolis) to 0.69 mg N g⁻¹ DW h⁻¹ (winter, Posidonia). Uptake of nitrate by roots was negligible. Biotic uptake rates for nitrate were an order of magnitude slower than ammonium, demonstrating an affinity for ammonium over nitrate as a preferred inorganic nitrogen source. Adelaide coastal waters have lost over 5000 ha of seagrasses, much of this attributed to nutrient inputs from wastewater, industrial and stormwater. Managing these inputs into future requires better understanding of the fate of nutrients, particularly biological uptake. This study attempts to quantify uptake rates of nitrogen by seagrasses.     

Abundance distribution and seasonal variations of Calanus sinicus (Copepoda: Calanoida) in the northwest continental shelf of South China Sea

The effect of monsoon, coastal current and temperature on the distribution and seasonal variations of Calanus sinicus abundance were studied. The samples from the northwest continental shelf of South China Sea were collected with 505 μm planktonic nets from July 2006 to October 2007. The abundance of C. sinicus made up 34.28% and 12.34% of all copepods in spring and summer, respectively. The distribution of C. sinicus varied seasonally and regionally. The distribution of C. sinicus ranged between east inshore and offshore waters from the Leizhou Peninsula to Hainan Island, with a mean of 23.00 (±77.78) ind. m⁻³ in spring. In summer it had a mean of 13.74 (±45.10) ind. m⁻³ occurring only in the east inshore waters from Leizhou Peninsula to Hainan Island. C. sinicus was not abundant during autumn and winter seasons. The surveyed area was divided into three sub-regions based on topographical analysis and water mass, region I (included the east inshore waters of Leizhou Peninsula), region II (included the east inshore waters of Hainan Island) and region III (included the offshore waters from Leizhou Peninsula to Hainan Island). The average abundance of C. sinicus within region I was determined to be 115.63 (±145.93) and 68.12 (±84.00) ind. m⁻³ in spring and summer, respectively, values higher than those of regions II and III. Our findings suggested that C. sinicus was transported from the East China Sea to the northwest continental shelf of South China Sea by the Guangdong Coastal Current, which was driven by the northeast monsoon in spring. The presence of a cold eddy, in addition to coastal upwelling driven by the southwest monsoon, provided suitable survival conditions for C. sinicus in summer. This species disappeared in autumn due to high temperatures (>27 °C) and did not begin to enter into the northwest continental shelf of South China Sea from the East China Sea during the period of investigation in winter. The frequency of C. sinicus was low in region III during the year as a result of the South China Sea Warm Current and pelagic waters with high temperature during the spring and summer months.     

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.     

Seasonal contribution of living phytoplankton carbon to vertical fluxes in a coastal upwelling system (Ría de Vigo, NW Spain)

The aim of this study is to explore the contribution of living phytoplankton carbon to vertical fluxes in a coastal upwelling system as a key piece to understand the coupling between primary production in the photic layer and the transfer mechanisms of the organic material from the photic zone. Between April 2004 and January 2005, five campaigns were carried out in the Ría de Vigo (NW Iberian Peninsula) covering the most representative oceanographic conditions for this region. Measurements of particulate organic carbon (POC), chlorophyll-a (chl a), phaeopigments (phaeo), and identification of phytoplankton species were performed on the water column samples and on the organic material collected in sediment traps.The POC fluxes measured by the sediment traps presented no seasonal variation along the studied period ranging around a mean annual value of 1085±365 mg m⁻² d⁻¹, in the upper range of the previously reported values for other coastal systems. The fact that higher POC fluxes were registered during autumn and winter, when primary production rates were at their minimum levels points to a dominant contribution of organic carbon from resuspended sediments on the trap collected material. On the contrary, fluxes of living phytoplankton carbon (C_phyto) and chl a clearly presented a seasonal trend with maximum values during summer upwelling (546 mg m⁻² d⁻¹ and 22 mg chl a m⁻² d⁻¹, respectively) and minimum values during winter (22 mg m⁻² d⁻¹ and 0.1 mg chl a m⁻² d⁻¹, respectively). The contribution of C_phyto to the vertical flux of POC ranged between 2% and 49% in response to the pelagic phytoplankton community structure. Higher values of C_phyto fluxes were registered under upwelling conditions which favour the dominance of large chain-forming diatoms (Asterionellopsis glacialis and Detonula pumila) that were rapidly transferred to the sediments. By contrast, C_phyto fluxes decreased during the summer stratification associated with a pelagic phytoplankton community dominated by single-cell diatoms and flagellates. Minimal C_phyto fluxes were observed during the winter mixing conditions, when the presence of the benthic specie Paralia sulcata in the water column also points toward strong sediment resuspension.     

Respiration,growth and grazing rates of three ciliate species in hypoxic conditions

Marine hypoxic episodes are affecting both marine and freshwater bodies all over the world. Yet, limited data exists with regard to the effects of decreasing oxygen on protist metabolism. Three ciliate species were therefore isolated from Hong Kong coastal waters. Controlled hypoxic conditions were simulated in the lab environment, during which time growth, respiration and grazing rates were measured. Euplotes sp. and a Oxytrichidae-like ciliate showed decreased growth and respiration below 2.5 mg O₂ L⁻¹, however Uronema marinum kept steady growth and respiration until below 1.5 mg O₂ L⁻¹. Euplotes sp. and the Oxytrichidae-like ciliate had the highest ingestion rate, which dropped significantly below 3.0 mg O₂ L⁻¹. U.marinum grazing rates were affected at and below 1.5 mg O₂ L⁻¹, correlating with their drop in growth and respiration at this lower concentration. This study illustrates the slowing metabolism of key grazing protists, as well as species-specific tolerance in response to hypoxia.     

Seasonal variations and trophic ecology of microzooplankton in the southeastern Arabian Sea

The seasonal ecological response of microzooplankton in the southeastern Arabian Sea is presented. During the spring intermonsoon period, stratification and depletion of nitrate in the surface waters (nitracline was at 60 m depth) cause low integrated chlorophyll a (av. 19±11.3 mg m⁻²) and primary production (av. 164±91 mgC m⁻² d⁻¹). On the other hand, nutrient enrichment associated with coastal upwelling and river influx during the onset and peak summer monsoon resulted in high integrated chlorophyll a (av. 21±6 mg m⁻² and av. 29±21 mg m⁻², respectively) and primary production (av. 255±94 mgC m⁻² d⁻¹ and av. 335±278 mgC m⁻² d⁻¹, respectively). During all three periods, diazotropic cyanobacterium Trichodesmium erythraeum dominated in the nutrient depleted surface waters. A general increase in abundance of larger diatoms was evident in the surface waters of the inshore region during monsoon periods. The microzooplankton abundance was found to be significantly higher during the spring intermonsoon (av.241±113×10³ ind m⁻²) as compared to onset of summer monsoon (av. 105±89×10³ ind m⁻²) and peak summer monsoon (av.185±175×10³ ind m⁻²). Microzooplankton community during the spring intermonsoon was numerically dominated by ciliates while heterotrophic dinoflagellate was the dominant ones during the monsoon periods. The high abundance of ciliates during the spring intermonsoon could be attributed to the stratified environmental condition prevailed in the study area which favors high abundance of smaller phytoplankton and cyanobacteria, the most preferred food of ciliates. On the other hand, the dominance of heterotrophic dinoflagellates during the monsoon periods could be linked to their ability to graze larger diatoms which were abundant during the monsoon periods. The overall results show low abundance of microzooplankton in the eastern Arabian Sea during the monsoon periods mainly due to a decline in ciliates abundance. This decline during the monsoon periods could be the result of (a) low abundance of smaller phytoplankton and (b) high stock of mesozooplankton predators (av. 245 ml 100 m⁻³).     

Relationship of photosynthetic carbon fixation with environmental changes in the Jiulong River estuary of the South China Sea, with special reference to the effects of solar UV radiation

Phytoplankton cells in estuary waters usually experience drastic changes in chemical and physical environments due to mixing of fresh and seawaters. In order to see their photosynthetic performance in such dynamic waters, we measured the photosynthetic carbon fixation by natural phytoplankton assemblages in the Jiulong River estuary of the South China Sea during April 24-26 and July 24-26 of 2008, and investigated its relationship with environmental changes in the presence or the absence of UV radiation. Phytoplankton biomass (Chl a) decreased sharply from the river-mouth to seawards (17.3-2.1 μg L⁻¹), with the dominant species changed from chlorophytes to diatoms. The photosynthetic rate based on Chl a at noon time under PAR-alone increased from 1.9 μg C (μg Chl a)⁻¹ L⁻¹ in low salinity zone (SSS < 10) to 12.4 μg C (μg Chl a)⁻¹ L⁻¹ in turbidity front (SSS within 10-20), and then decreased to 2.1 μg C (μg Chl a)⁻¹ L⁻¹ in mixohaline zone (SSS > 20); accordingly, the carbon fixation per volume of seawater increased from 12.8 to 149 μg C L⁻¹ h⁻¹, and decreased to 14.3 μg C L⁻¹ h⁻¹. Solar UVR caused the inhibition of carbon fixation in surface water of all the investigated zones, by 39% in turbidity area and 7-10% in freshwater or mixohaline zones. In the turbidity zone, higher availability of CO₂ could have enhanced the photosynthetic performance; while osmotic stress might be responsible for the higher sensitivity of phytoplankton assemblages to solar UV radiation.     

Life history, growth and production of Theodoxus fluviatilis in Lake Esrom, Denmark

A population of Theodoxus fluviatilis L in the littoral zone of Lake Esrom was investigated from November 1977 to February 1979. The population was sampled every month in the winter period and twice during the rest of the year. Biomass was estimated as ash-free dry weight (AFDW) of the organic matter both of the soft parts of the animal and the shell itself. The relation between AFDW (c) and shell length (l) was log c=2.9509×log (l)−1.7120. The population comprised more than 1 year-class, which could be separated by shell length, by a narrow band on the shells and the growth of algae on the shell. The life cycle lasted years. The oldest animals had a shell length of 7.0-7.5 mm. A few individuals who were estimated to be years had a shell length up to 8.6 mm. Population density varied between 575 and 2115 individuals m⁻² on the stony substratum. The average was 1160 individuals m⁻². Mortality was low during the summer period. In winter many animals died due to the effect of ice and stormy weather on the stony substratum. Growth of the animals was estimated from the shell length. Maximum growth was observed from May to August with no growth during the winter. Egg capsules were found on the stones all year round. New capsules were found from late May to the middle of November. Most freshly laid capsules were observed in May-June and August-September. Capsules from the late summer hatched in spring and capsules laid in the spring hatched in August-September. The average annual net production for the whole population was estimated by three methods. The Allen curve method gave 1.895 AFDW m⁻², the growth-increment method gave 1.784 mg AFDW m⁻² and the Hynes method 2.284 mg AFDW m⁻². Corresponding estimated P/B ratios were 1.29, 1.30 and 1.57. Annual net-production of the four investigated year-classes was 16 mg AFDW m⁻² year⁻¹ for 1975, 224 mg AFDW m⁻² year⁻¹ for 1976, 1.258 mg AFDW m⁻² year⁻¹ for 1977 and 287 mg AFDW m⁻² year⁻¹ for 1978. P/B ratios for the three oldest year-classes were, respectively, 0.32, 0.50 and 1.67. A comparison with other investigations on gastropod life cycles, reproduction and P/B ratios is made and differences discussed. Variations are correlated to temperature, and food quality and quantity.     

Distribution,fluxes and bacterial consumption of total organic carbon in a populated Mediterranean Gulf

Saronikos Gulf (Greece) practically constitutes the sea border of the metropolitan city of Athens and the alongshore outskirts, and it receives the treated wastes of ∼4 million people from a point source that discharges on the sea bottom at ∼65 m water depth. Total organic carbon (TOC) was measured in 477 seawater samples collected in the Saronikos Gulf, during 10 cruises, from August 2001 to May 2004 and analyzed with the High Temperature Catalytic Oxidation method (HTCO). TOC concentrations ranged from 49 to 198 μmol C L⁻¹ in agreement with other Mediterranean coastal waters. The highest TOC concentrations were found in the upper waters (0–75 m), whereas in the deeper parts of the Gulf (between 100 and 400 m) TOC concentrations were kept constantly low (49–70 μmol C L⁻¹). A general pattern towards higher TOC concentrations during summer was also observed. Calculations of non-refractory (labile+semi-labile) organic material based on a one-dimensional (1D) conceptual model showed that it corresponds to 33% of the bulk TOC during summer and to 27% during winter. Bacterial production (BP) was measured at selected stations of ∼70–80 m depth using the [³H] leucine method. Depth integrated BP values varied from 2.8 to 10.9 mmol m⁻² d⁻¹, whereas extraordinary high integrated BP values (126 and 140 mmol m⁻² d⁻¹) were observed at the station over the treated sewage outflow. From the turnover time, τ, of the non-refractory TOC by bacteria it was implied that organic matter in the effluents is extremely labile (2–58 days). Moreover, τ values at the other sites showed that during summer non-refractory organic material resisted bacterial degradation (1–8 months), whereas during early spring it was easily degradable (20–50 days). The balance of TOC fluxes for the Inner Gulf for June and September 2003 showed that the Inner Gulf acts as a net producer of TOC during summer. Our results suggest that the presence of the Athens treated sewage outfall does not contribute to the observed summer accumulation of TOC in the Inner Gulf and other causes such as increased bacteria predation and/or nutrient limitation must be responsible.     

Stable Pb isotope ratios in aerosols, precipitation, and size-fractionated particulate matter in the Gulf of Maine, Scotian Shelf, and Labrador Sea

Measurements of Pb isotopes in aerosols, precipitation, and size-fractionated particulate matter from the Gulf of Maine, Scotian Shelf and Labrador Sea are used to investigate the source of Pb. The ²⁰⁶Pb/²⁰⁷Pb ratio in aerosols and precipitation collected at New Castle, NH suggests that anthropogenic Pb is a mixture of US and Canadian sources. ²⁰⁶Pb/²⁰⁷Pb ratios in >53 μm particulate matter from the Gulf of Maine and Scotian Shelf slope waters are consistent with contaminant Pb inputs predominantly from US and Canadian sources, while in shelf waters ²⁰⁶Pb/²⁰⁷Pb ratios in >0.4 μm and >53 μm particles are consistent with a mixture of US and Canadian sources, as well as Pb associated with resuspended surface sediment. ²⁰⁶Pb/²⁰⁷Pb ratios in particulate matter (>0.4 μm, 10-53 μm, and >53 μm) from Labrador Sea surface waters range from 1.165 to 1.211 and are a mixture of Pb derived from ore compositions consistent with Broken Hill, Australia and southeast Missouri, US sources.     

Effects of nitrogen and phosphorus on phytoplankton composition and biomass in 15 subtropical, urban shallow lakes in Wuhan, China

This study aims at investigating the composition and biomass of the phytoplankton community in 15 urban shallow eutrophic lakes as well as the effects of main environmental factors, including nutrient concentrations and the ratio of nitrogen to phosphorus, temperature, COD, BOD, water depth, etc. on the phytoplankton community structure. Lake water samples were taken and analyzed on a bimonthly basis during the period from March 2004 to March 2006. The redundancy analysis (RDA) and regression analysis (RA) were performed to identify the effects of nutrients on the phytoplankton community and biomass in these typical urban lakes. The results indicate that most of these urban lakes were hypertrophic due to high concentrations of total phosphorus (TP) and total nitrogen (TN), with mean levels of 490 and 5380 mg m⁻³, respectively. The phytoplankton community was dominated by Microcystis aeruginosa and Euglena caudate in summer and Cryptomonas ovata and Cyclotella meneghiniana in winter. The mean biomass of the phytoplankton reached 456.87 mg L⁻¹ in summer months and the annual level was 189.24 mg L⁻¹. Temperature and TP content were found to be the principal limiting factors for phytoplankton growth on an annual basis. On the other hand, the results of RDA and RA demonstrate that the dominant phytoplankton species were not nutrient-limited during summer months. Low TN:TP ratios (<10) were detected accompanied with fewer occurrences of N-fixing cyanobacteria and other filamentous algae in most lakes in summer, which implies that low N:P ratio does not always shifts the dominance of phytoplankton community to the N-fixing cyanobacteria. Moreover, TP always had higher correlation with chlorophyll a (Chl-a) than TN, even when the TN:TP ratios of most samples were lower than 10. Therefore, it is concluded that the TN:TP ratio is not always a suitable index to determine whether nitrogen or phosphorus limits the phytoplankton biomass in urban shallow eutrophic lakes.     

Seasonal variations in the inorganic carbon system in the Pearl River (Zhujiang) estuary

Seasonal variations in the inorganic carbon system in the Pearl River estuary are examined based on data from five surveys during the spring, summer, fall, and winter seasons. Both total dissolved inorganic carbon (DIC) and total alkalinity (TAlk) values in the freshwater end-members are high in the dry season (>2700 μmol kg⁻¹ for DIC and >2400 μmol kg⁻¹ for TAlk) and substantially lower in the wet season (DIC and TAlk were ∼1000 and 700 μmol kg⁻¹, respectively). Riverine DIC flux and drainage basin weathering rates, however, are significantly higher in the wet season (611×10⁹ mol yr⁻¹ and 13.6×10⁵ mol km⁻² yr⁻¹) than in the dry season (237×10⁹ mol yr⁻¹ and 5.3×10⁵ mol km⁻² yr⁻¹).     

Phytoplankton growth and microzooplankton grazing in the continental shelf area of northeastern South China Sea after Typhoon Fengshen

Dilution experiments were used to investigate the phytoplankton growth and microzooplankton grazing in the continental shelf area of northeastern South China Sea during 30 June and 7 July, 2008, occurring about a week after Typhoon Fengshen. We detected negative phytoplankton growth rates (−0.03 to −2.02 d⁻¹) and measured grazing rates of microzooplankton on phytoplankton in size-fractionations of 20-200 μm (1.25±0.44 d⁻¹), 3-20 μm (1.48±0.63 d⁻¹) and <3 μm (1.02±0.42 d⁻¹). Results showed significant correlations between phytoplankton growth and microzooplankton grazing rates, between phytoplankton and ciliate abundance, and between the dominant phytoplankton Thalassionema nitzschioides and the dominant ciliate Helicostomella longa (p<0.05). Phytoplankton decay, due to nutrient-limited conditions occurring with the fading of upwelling and spreading of freshwater plume after Typhoon Fengshen, may account for negative phytoplankton growth rates in this study. Synergism in the specific size-selective grazing of various species, including ciliates and heterotrophic dinoflagellates, may contribute to similar grazing rate on phytoplankton in different size-fractionations, at the integrated level. Interactions between phytoplankton and microzooplankton, including grazing selectivity, top-down and bottom-up control between phytoplankton and microzooplankton may contribute to these findings. Our results indicate that under conditions of negative phytoplankton growth microzooplankton grazing may reduce energy loss from the epipelagic waters by retrieving energy from the decaying phytoplankton community.     

Molecular and isotopic characterization of the particulate organic matter from an eutrophic coastal bay in SE Brazil

The present work aimed at studying the origin of particulate organic matter in Guanabara Bay and in some rivers of the Guanabara basin by using elemental composition, isotopic ratios (δ¹³C and δ¹⁵N) and molecular markers (sterols) in samples collected in two periods (winter and summer). Elemental and isotopic compositions were determined by dry combustion and mass spectrometry, respectively, while sterols were investigated by GC–FID and GC–MS. Higher sterol concentrations were present in the north-western part of the bay in winter (5.10–23.5 μg L^–1). The high abundance of algal sterols (26–57% of total sterols), the elemental composition (C/N=6–8) and the isotopic signatures (δ¹³C=−21.3‰ to −15.1‰ and δ¹⁵N=+7.3‰ to +11.1‰) suggested the predominance of autochthonous organic matter, as expected for an eutrophic bay, although seasonal variation in phytoplankton activity was observed. Coprostanol concentration (fecal sterol) was at least one order of magnitude higher in the particulate material from fluvial samples (4.65–55.98 μg L^–1) than in the bay waters (<0.33 μg L^–1). This could be ascribed to a combination of factors including efficient particle removal to sediments in the estuarine transition zone, dilution with bay water and bacterial degradation during particle transport in the water column.     

What drives the evolution of the sedimentary phosphorus cycle?

The ‘Chicken Creek’ artificial catchment area, Welzow-South, E Germany, created to study processes and structures of initial ecosystem development, discharges into a small experimental lake (A=3805 m², V=3992 m³, z_max=2.4 m). This lake was man-made in 2005 and filled by natural surface runoff until January 2006. In summer 2006 and 2008, the actual development of sediments and the evolution of the phosphorus (P) cycle were studied. 19.7% of the original lake volume was filled by sediment within the first 3 years. A fine-grained sediment representing silt (6.3-63 μm) accumulated at high accretion rates at the deepest point (200 mm a⁻¹, 0-24 mm week⁻¹) due to massive erosion in the catchment. The sediment is low in organic matter (2.5-5.2%) and total P (TP, 0.31-0.97 mg g⁻¹). Low amounts of P associated with degradable organic matter and surplus of metal hydroxides (Fe:P∼40, Al:P∼20) favor an efficient P binding and low dissolved P concentrations in pore water (1-107 μg l⁻¹). Hence, the mineral sediment quality and the low rates of P release (0.06 mg m⁻² d⁻¹) revealed that a lake at an initial stage of development has essentially no sedimentary P cycle compared to eutrophic shallow lakes. However, the increasing emersed and submersed macrophyte growth will control further lake succession by intensifying the internal nutrient cycling. The macrophytes drive the evolution of a sedimentary P cycle by mobilizing and translocating P, by accumulating carbon and thus by stimulating microbial and redox processes.     

Water quality in the Dickinson Bayou watershed (Texas, Gulf of Mexico) and health issues

The Dickinson Bayou watershed (near Houston, Texas, Gulf of Mexico) provides habitat for numerous coastally influenced communities of wildlife, including scores of birds and fish. Encroaching development and impervious surfaces are altering the habitat and degrading water quality. Herein we have defined the current health of the bayou using water quality data collected between 2000 and 2006. Elevated bacteria (fecal coliform, Escherichia coli and Enterococcus) and depressed dissolved oxygen concentrations (often <3 mg l⁻¹) are the two major impairments to this ecosystem. While nutrient ratios indicate primary productivity may be nitrogen limited, concerns of eutrophication persist because the bayou has a low intrinsic flushing rate. Consistent with this is the magnitude of algal blooms (ca. 100 μg chl l⁻¹) which often occur in spring/summer. The findings of this study will assist with the understanding of the influence of urban development on small watersheds.     

Pelagic production and respiration in the Gulf of Papua during May 2004

The metabolic balance between production and respiration in plankton communities of the Gulf of Papua was investigated in May 2004. Water samples taken at 19 stations were allocated to groups on the basis of physico-chemical characteristics. Oxygen consumption and production in flasks incubated in the dark and in the light was determined by micro-Winkler titration. Dark bottle respiration in samples influenced by the estuarine plume averaged 3.09±1.92 (SD) mmol O₂ m⁻³ d⁻¹ and production within surface light bottles averaged 7.63±3.36 (SD)  mmol O₂ m⁻³ d⁻¹. Corresponding values in stations more typical of the central Gulf of Papua were 1.68±1.30 (SD) mmol O₂ m⁻³ d⁻¹ and 1.08±2.25 (SD) mmol O₂ m⁻³ d⁻¹. Despite a shallow (<10 m) euphotic zone within the plume stations, phytoplankton production in the surface layers was sufficiently high to subsidise total water column respiration. Integrating production and respiration over the water column resulted in a calculation of net community production (NCP) of 626±504 (SD) mg C m⁻² d⁻¹, and community respiration (CR) of 712±492 mg C m⁻² d⁻¹ at the plume stations, with an average P:R ratio of 1.97. In the offshore group NCP was 157±450 (SD) mg C m⁻² d⁻¹ and CR was 1620±1576 mg C m⁻² d⁻¹. The average P:R ratio was 1.27. Three of the 7 stations allocated to the offshore group were net heterotrophic. In contrast to earlier studies in the area indicating that the Gulf of Papua waters is heterotrophic [Robertson, A.I., Dixon, P., Alongi, D.M., 1998. The influence of fluvial discharge on pelagic production in the Gulf of Papua, Northern Coral Sea. Estuarine, Coastal and Shelf Science 46, 319–331], our data indicate that in May 2004 the Gulf was in positive metabolic balance, but by only ∼120 mg C m⁻² d⁻¹. We conclude that waters of the Gulf of Papua under riverine influence are net autotrophic, but that within the central Gulf there is a fine metabolic balance alternating between autotrophy and heterotrophy.     

Oxygen consumption rates of fecal pellets produced by three coastal copepod species fed with a diatom Thalassiosira pseudonana

Fecal pellet production rate and oxygen consumption rate of copepod fecal pellets egested by Paracalanus sp., Acartia spinicauda and Centropages orsinii feeding on a diatom Thalassiosira pseudonana were measured. Fecal pellet production rates varied between 3.6 and 80.6 pellets ind⁻¹ d⁻¹ among three species, with fecal pellet production of Paracalanus sp. significantly higher than the other two copepod species. Average pellet size varied between 2.82 × 10⁵ μm³ for Paracalanus sp. and 5.36 × 10⁵ μm³ for A. spinicauda. Average volumetric oxygen consumption rates were 27.32, 5.84 and 45 fmol O₂ μm⁻³ d⁻¹ for fecal pellets produced by Paracalanus sp., A. spinicauda and C. orsinii, respectively. The potential oxygen consumption of fecal pellets egested from Paracalanus sp. was the highest due to its high fecal pellet production rate and high population abundance throughout the year. Overall, the approximate in situ oxygen consumption due to copepod fecal pellet degradation was significant in estuarine and coastal waters of Hong Kong. Results of this study give insight of the role of zooplankton in potential occurrence of hypoxia in coastal waters during summer.