Effects of an estuarine plume-associated bloom on the carbonate system in the lower reaches of the Pearl River estuary and the coastal zone of the northern South China Sea

We observed a phytoplankton bloom downstream of a large estuarine plume induced by heavy precipitation during a cruise conducted in the Pearl River estuary and the northern South China Sea in May–June 2001. The plume delivered a significant amount of nutrients into the estuary and the adjacent coastal region, and enhanced stratification stimulating a phytoplankton bloom in the region near and offshore of Hong Kong. A several fold increase (0.2–1.8 μg Chl L⁻¹) in biomass (Chl a) was observed during the bloom. During the bloom event, the surface water phytoplankton community structure significantly shifted from a pico-phytoplankton dominated community to one dominated by micro-phytoplankton (>20 μm). In addition to increased Chl a, we observed a significant drawdown of pCO₂, biological uptake of dissolved inorganic carbon (DIC) and an associated enhancement of dissolved oxygen and pH, demonstrating enhanced photosynthesis during the bloom. During the bloom, we estimated a net DIC drawdown of 100–150 μmol kg⁻¹ and a TAlk increase of 0–50 μmol kg⁻¹. The mean sea–air CO₂ flux at the peak of the bloom was estimated to be as high as ∼−18 mmol m⁻² d⁻¹. For an average surface water depth of 5 m, a very high apparent biological CO₂ consumption rate of 70–110 mmol m⁻² d⁻¹ was estimated. This value is 2–6 times higher than the estimated air–sea exchange rate.     

Phytoplankton community variations in tropical coastal area affected where submarine groundwater occurs

The influence of subterranean water discharge on phytoplankton was studied at two localities (Progreso and Dzilam) on the northern coast of the Yucatan Peninsula. Hydrographic and phytoplankton samples were taken monthly between September 1998 and August 1999. High concentration of silicate (>65 μmol L⁻¹) and nitrate (>80 μmol L⁻¹) and low salinity showed the influence of submerged groundwater discharge (SGD) in the area. In Dzilam, hydrological conditions shows low salinity and high concentration of nitrate and silicate favored from the SGD. Meanwhile, high concentrations of ammonium, nitrite, and phosphate at Progreso (>150 000 inhabitants) suggest mixing of SGD and domestic waste waters. Thick-valve pennate diatoms dominated at Dzilam while dinoflagellates dominated in Progreso. Hydrological differences in both study zones suggest that local forcings, and interaction between coastal water masses and SGD plays an important role in hydrological conditions and primary productivity in the coastal zone of Yucatan. The anthropogenic modified SGD in Progreso may affect the nutrient regime and phytoplankton community structure, and may be used as indicator of eutrophication.     

The growth dynamics of Karenia brevis within discrete blooms on the West Florida Shelf

As part of the ECOHAB: Florida Program, we studied three large blooms of the harmful bloom forming dinoflagellate Karenia brevis. These blooms formed on the West Florida Shelf during Fall of 2000 off Panama City, and during Fall 2001 and Fall 2002 off the coastline between Tampa Bay and Charlotte Harbor. We suggest that these blooms represent two different stages of development, with the 2000 and 2001 blooms in an active growth or maintenance phase and the 2002 bloom in the early bloom initiation phase. Each bloom was highly productive with vertically integrated primary production values of 0.47–0.61, 0.39–1.33 and 0.65 g C m⁻² d⁻¹ for the 2000, 2001 and 2002 K. brevis blooms, respectively. Carbon specific growth rates were low during each of these blooms with values remaining fairly uniform with depth corresponding to generation times of 3–5 days. Nitrogen assimilation by K. brevis was highest during 2001 with values ranging from 0.15 to 2.14 μmol N L⁻¹ d⁻¹ and lower generally for 2000 and 2002 (0.01–0.64 and 0.66–0.76 μmol N L⁻¹ d⁻¹ for 2000 and 2002, respectively). The highest K. brevis cell densities occurred during the 2001 bloom and ranged from 400 to 800 cells mL⁻¹. Cell densities were lower for each of the 2000 and 2002 blooms relative to those for 2001 with densities ranging from 100 to 500 cells mL⁻¹. The 2000 and 2001 blooms were dominated by K. brevis in terms of its contribution to the total chlorophyll a (chl a) pool with K. brevis accounting generally for >70% of the observed chl a. For those populations that were dominated by K. brevis (e.g. 2000 and 2001), phytoplankton C biomass (C_p,0) constituted <30% of the total particulate organic carbon (POC). However, in 2002 when diatoms and K. brevis each contributed about the same to the total chl a, C_p,0 was >72% of the POC. The fraction of the total chl a that could be attributed to K. brevis was most highly correlated with POC, chl a and salinity. Nitrogen assimilation rate and primary production were highly correlated with a greater correlation coefficient than all other comparisons.     

Validation of SeaWiFS chlorophyll a in Massachusetts Bay

Massachusetts Bay, a semi-enclosed embayment (50×100 km) in the Northwest Atlantic, is the focus of a monitoring program designed to measure the effects of relocating the Boston Harbor sewage outfall to a site 15 km offshore in Massachusetts Bay. The Massachusetts Water Resources Authority (MWRA) in situ monitoring program samples selected stations up to 17 times per year to observe seasonal changes in phytoplankton biomass and other water quality variables. We investigated the feasibility of augmenting the monitoring data with satellite ocean color data to increase the spatial and temporal resolution of quantitative phytoplankton measurements. In coastal regions such as Massachusetts Bay, ocean color remote sensing can be complicated by in-water constituents whose concentrations vary independently of phytoplankton and by inaccurate modeling of absorbing aerosols that tend to be concentrated near the coast. An evaluation of in situ and sea-viewing wide field-of-view sensor (SeaWiFS) measurements from 1998 to 2005 demonstrated that SeaWiFS overestimated chlorophyll a mainly due to atmospheric correction errors that were amplified by absorption from elevated concentrations of chlorophyll a and colored dissolved organic matter. Negative water-leaving radiances in the 412 nm band, an obvious artifact of inadequate atmospheric correction, were recorded in approximately 60–80% of the cloud-free images along the coast, while the remaining portions of the Bay only experience negative radiances 35–55% of the time with a clear nearshore to offshore decrease in frequency. Seasonally, the greatest occurrences of negative 412 nm radiances were in November and December and the lowest were recorded during the summer months. Concentrations of suspended solids in Massachusetts Bay were low compared with other coastal regions and did not have a significant impact on SeaWiFS chlorophyll a measurements. A regional empirical algorithm was developed to correct the SeaWiFS data to agree with in situ observations. Monthly SeaWiFS composites illustrated the spatial extent of a bimodal seasonal pattern, including prominent spring and fall phytoplankton blooms; and the approximate 115 cloud-free scenes per year revealed interannual variations in the timing, magnitude and duration of phytoplankton blooms. Despite known artifacts of SeaWiFS in coastal regions, this study provided a viable chlorophyll a product in Massachusetts Bay that significantly increased the spatial and temporal synoptic coverage of phytoplankton biomass, which can be used to gain a comprehensive ecosystem-wide understanding of phytoplankton dynamics at event, seasonal and interannual timescales.     

Nutrient availability in support of Karenia brevis blooms on the central West Florida Shelf: What keeps Karenia blooming?

Identifying nutrient sources, primarily nitrogen (N) and phosphorus (P), sufficient to support high biomass blooms of the red tide dinoflagellate, Karenia brevis, has remained problematic. The West Florida Shelf is oligotrophic, yet populations >10⁶ cells L⁻¹ frequently occur and blooms can persist for months. Here we examine the magnitude and variety of sources for N and P that are available to support blooms. Annual average in situ or background concentrations of inorganic N in the region where blooms occur range 0.02–0.2 μM while inorganic P ranges 0.025–0.24 μM. Such concentrations would be sufficient to support the growth of populations up to ∼3×10⁴ cells L⁻¹ with at least a 1 d turnover rate. Organic N concentrations average 1–2 orders of magnitude greater than inorganic N, 8–14 μM while organic P concentrations average 0.2–0.5 μM. Concentrations of organic N are sufficient to support blooms >10⁵ cells L⁻¹ but the extent to which this complex mixture of N species is utilizable is unknown. Other sources of nutrients included in our analysis are aerial deposition, estuarine flux, benthic flux, zooplankton excretion, N₂-fixation, and subsequent release of organic and inorganic N by Trichodesmium spp., and release of N and P from dead and decaying fish killed by the blooms. Inputs based on atmospheric deposition, benthic flux, and N₂-fixation, were minor contributors to the flux required to support growth of populations >2.6×10⁴ cells L⁻¹. N and P from decaying fish could theoretically maintain populations at moderate concentrations but insufficient data on the flux and subsequent mixing rates does not allow us to calculate average values. Zooplankton excretion rates, based on measured zooplankton population estimates and excretion rates could also supply all of the N and P required to support populations of 10⁵ and 10⁶ cells L⁻¹, respectively, but excretion is considered as “regenerated” nutrient input and can only maintain biomass rather than contribute to “new” biomass. The combined estuarine flux from Tampa Bay, Charlotte Harbor, and the Caloosahatchee River can supply a varying, but at times significant level of N and P to meet growth and photosynthesis requirements for populations of approximately 10⁵ cells L⁻¹ or below. Estimates of remineralization of dead fish could supply a significant proportion of bloom maintenance requirements but the rate of supply must still be determined. Overall, a combination of sources is required to maintain populations >10⁶ cells L⁻¹.     

Recent carbon and nitrogen uptake rates of phytoplankton in Bering Strait and the Chukchi Sea

Cruises to Bering Strait and the Chukchi Sea in US waters from late June in 2002 to early September in 2004 and the Russian–American Long-term Census of the Arctic (RUSALCA) research cruise in 2004 covered all major water masses and contributed to a better understanding of the regional physics, nutrient dynamics, and biological systems. The integrated concentration of the high nitrate pool in the central Chukchi Sea was greater in this study than in previous studies, although the highest nitrate concentration (∼22 μM) in the Anadyr Water mass passing through the western side of Bering Strait was consistent with prior observations. The chlorophyll-a concentrations near the western side of the Diomede Islands ranged from 200 to 400 mg chl-a m⁻² and the range in the central Chukchi Sea was 200–500 mg chl-a m⁻² for the 2002–2004 Alpha Helix (HX) cruises. Chlorophyll-a concentrations for the 2004 RUSALCA cruise were lower than those from previous studies. The mean annual primary production of phytoplankton from this study, using a ¹³C–¹⁵N dual-isotope technique, was 55 g C m⁻² for the whole Chukchi Sea and 145 g C m⁻² for the plume of Anadyr–Bering Shelf Water in the central Chukchi Sea. In contrast, the averages of annual total nitrogen production were 13.9 g N m⁻² (S.D.=±16.2 g N m⁻²) and 33.8 g N m⁻² (S.D.=±14.1 g N m⁻²) for the Chukchi Sea and the plume, respectively. These carbon and nitrogen production rates of phytoplankton were consistently two-or three-fold lower than those from previous studies. We suggest that the lower rates in this study, and consequently more unused nitrate in the water column, were caused by lower phytoplankton biomass in the Bering Strait and the Chukchi Sea. However, we do not know if the lower rate of production from this study is a general decreasing trend or simply temporal variations in the Chukchi Sea, since temporal and geographical variations are substantially large and presently unpredictable.     

Determination of transport rates in the Yellow River–Bohai Sea mixing zone via natural geochemical tracers

In light of the current problems facing the Yellow River and surrounding areas (e.g., periods of zero river discharge, increasing nitrate concentrations of the Bohai Sea), we examined the coastal mixing dynamics around the mouth of the Yellow River. Naturally occurring radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra, and ²²⁸Ra) and other geochemical tracers (Ba, Si, and salinity) were employed to determine river plume transport scales and rates. Barium and radium exhibit elevated concentrations within the salinity gradient where they are desorbed from particles via ion-exchange. Once they are added to the system, they decrease offshore from dilution with lower concentration Bohai Sea water, and in the case of ²²⁴Ra and ²²³Ra, by radioactive decay. Using radium “ages” to assess the dissolved material transport scales and rates proved to be a useful tool in this environment. The ages based on the ²²⁴Ra/²²⁸Ra activity ratio increased gradually until salinities reached ∼25 when they rapidly increased due to decreased mixing at higher salinities. Integrated net transport rates through the salinity front ranged from 1.4 to 1.6 cm/s and did not vary significantly with river discharge. Thus, tidal mixing appears to dominate in this system, at least over the range of discharges investigated (80–600 m³/s). Determining the temporal scale of flow across the coastal zone in this region is a valuable first step toward examining whether the Yellow River is contributing to the increasing inorganic nitrogen concentrations in the central Bohai Sea.     

Physical–biological coupling in the Pearl River Estuary

The Pearl River Estuary is a subtropical estuary and the second largest in China based on discharge volume from the Pearl River. Processes in the estuary vary spatially and temporally (wet vs dry season). In the dry season at the head of the estuary, hypoxic and nearly anoxic conditions occur and NH₄ reaches >600 μM, NO₃ is ∼300 μM and nitrite is ∼60 μM indicating that nitrification and denitrification may be important dry season processes in the region extending 40 km upstream of the Humen outlet. There are very few biological studies conducted in this upper section of the estuary in either the dry or wet seasons and hence there is a need for further research in this region of the river. In the wet season, the salinity wedge extends to the Hongqimen outlet and oxygen is low (35–80% saturation). Nitrate is ∼100 μM, silicate ∼140 μM; and phosphate is relatively low at ∼0.5 μM, yielding an N:P ratio up to ∼200:1 in summer. Nutrients decrease in the lower estuary and primary productivity may become potentially P-limited. Eutrophication is not as severe as one would expect from the nutrient inputs from the Pearl River and from Hong Kong's sewage discharge. This estuary shows a remarkable capacity to cope with excessive nutrients. Physical processes such as river discharge, tidal flushing, turbulent dispersion, wind-induced mixing, and estuarine circulation play an important role in controlling the production and accumulation of algal blooms and the potential occurrence of hypoxia. Superimposed on the physical processes of the estuary are the chemical and biological processes involved in the production of the bloom. For example, the 100N:1P ratio indicates that P potentially limits the amount of algal biomass (and potential biological oxygen demand) in summer. While extended periods of hypoxia are rare in Hong Kong waters, episodic events have been reported to occur during late summer due to factors such as low wind, high rainfall and river discharge which result in strong density stratification that significantly dampens vertical mixing processes. Nutrient loads are likely to change over the next several decades and monitoring programs are essential to detect the response of the ecosystem due to the future changes in nutrient loading and the ratio of nutrients.     

Emerging organic contaminants in coastal waters: Anthropogenic impact,environmental release and ecological risk

This study provides a first estimate of the sources, distribution, and risk presented by emerging organic contaminants (EOCs) in coastal waters off southwestern Taiwan. Ten illicit drugs, seven nonsteroidal anti-inflammatory drugs (NSAIDs), five antibiotics, two blood lipid regulators, two antiepileptic drugs, two UV filters, caffeine, atenolol, and omeprazole were analyzed by solid-phase extraction and liquid chromatography coupled to tandem mass spectrometry (SPE–LC–MS/MS). Thirteen EOCs were detected in coastal waters, including four NSAIDs (acetaminophen, ibuprofen, ketoprofen, and codeine), three antibiotics (ampicillin, erythromycin, and cefalexin), three illicit drugs (ketamine, pseudoephedrine, and MDMA), caffeine, carbamazepine, and gemfibrozil. The median concentrations for the 13 EOCs ranged from 1.47 ng/L to 156 ng/L. Spatial variation in concentration of the 13 EOCs suggests discharge into coastal waters via ocean outfall pipes and rivers. Codeine and ampicillin have significant pollution risk quotients (RQ > 1), indicating potentially high risk to aquatic organisms in coastal waters.     

Trace metals associated with deep-sea tailings placement at the Batu Hijau copper–gold mine,Sumbawa, Indonesia

The Batu Hijau copper–gold mine on the island of Sumbawa, Indonesia operates a deep-sea tailings placement (DSTP) facility to dispose of the tailings within the offshore Senunu Canyon. The concentrations of trace metals in tailings, waters, and sediments from locations in the vicinity of the DSTP were determined during surveys in 2004 and 2009. In coastal and deep seawater samples from Alas Strait and the South Coast of Sumbawa, the dissolved concentrations of Ag, As, Cd, Cr, Hg, Pb and Zn were in the sub μg/L range. Dissolved copper concentrations ranged from 0.05 to 0.65 μg/L for all depths at these sites. Dissolved copper concentrations were the highest in the bottom-water from within the tailings plume inside Senunu Canyon, with up to 6.5 μg Cu/L measured in close proximity to the tailings discharge. In general, the concentrations of dissolved and particulate metals were similar in 2004 and 2009.     

Annual lamination and its sedimentary implications in the Yangtze River delta inferred from High-resolution biogenic silica and sensitive grain-size records

Annual sedimentary layers in core A from the Yangtze River (Changjiang) subaqueous delta were identified based on high-resolution biogenic silica (BSi) and grain-size records. The contents of BSi, the sediment grain-size and the sensitive grain-size fractions of <13.14 μm and 13.14-250 μm illustrated steady coincident fluctuations with the depth profile of core A. These fluctuations were inferred to reflect seasonal changes in physical and biological processes. For example, variations in the content of BSi likely represent changes in primary production in flood versus dry seasons. It in flood seasons was several times higher than that in dry seasons. The amounts of <13.14 and 13.14-250 μm fractions were complimentary to each other and co-varied with the BSi amount. Therefore, the BSi, grain-size and sensitive fractions can be used as proxies for the annual sedimentation. From 1992 to 2005, the estimated sedimentary rate based on the annual layer varied from 1.0 to 3.5  cm/a (mean of 2.07  cm/a), and these values varies with the annual sediment discharge from the Yangtze River to the sea. The significant seasonal variations in the water discharge, sediment load, ocean hydrodynamics and primary production, which are common in the area affected by the East Asian monsoon, are likely responsible for the formation of the annual sedimentary lamination.     

Intra-annual variability in nutrients in the Godavari estuary,India

Daily variations in nutrients were monitored for 15 months (September 2007–November 2008) in the Godavari estuary, Andhra Pradesh, India, at two fixed locations. River discharge has significant influence on nutrients loading to the estuary, which peaks during June–August (peak discharge period; monsoon) whereas exchanges at the sediment–water interface, groundwater and rainwater contribute significantly during other period. Despite significant amount of nutrients brought by discharge to the study region, phytoplankton biomass, in terms of chlorophyll-a (Chl a), did not increase significantly due to high suspended load and shallow photic depth. Nutrients showed downward gradient towards downstream of the estuary from upstream due to dilution by nutrient poor seawater and biological uptake. The N:P ratios were higher than Redfield ratio in both upstream and downstream of the estuary during no discharge period suggesting PO₄ to be a limiting nutrient for phytoplankton production, at levels <0.10 μmol L⁻¹. On the other hand, Si:N ratios were always more than unity during entire study period at both the stations indicating that Si(OH)₄ is not a limiting nutrient. Our results suggest that suspended matter limits phytoplankton biomass during peak discharge period whereas PO₄ during no discharge period.     

Total petroleum hydrocarbons and heavy metals in the surface sediments of Bohai Bay,China: Long-term variations in pollution status and adverse biological risk

Surface sediments collected from 2001 to 2011 were analyzed for total petroleum hydrocarbons (TPH) and five heavy metals. The sediment concentration ranges of TPH, Zn, Cu, Pb, Cd and Hg were 6.3–535 μg/g, 58–332 μg/g, 7.2–63 μg/g, 4.3–138 μg/g, 0–0.98 μg/g, and 0.10–0.68 μg/g, respectively. These results met the highest marine sediment quality standards in China, indicating that the sediment was fairly clean. However, based on the effects range-median (ERM) quotient method, the calculated values for all of the sampling sites were higher than 0.10, suggesting that there was a potential adverse biological risk in Bohai Bay. According to the calculated results, the biological risk decreased from 2001 to 2007 and increased afterwards. High-risk sites were mainly distributed along the coast. This study suggests that anthropogenic influences might be responsible for the potential risk of adverse biological effects from TPH and heavy metals in Bohai Bay.     

Phytoplankton dynamics in and near the highly eutrophic Pearl River Estuary,South China Sea

The dynamics of size-fractionated phytoplankton along the salinity gradient in the Pearl River Estuary and the adjacent near-shore oceanic water was investigated using microscopic, flow cytometric, and chlorophyll analyses in the early spring (March) and early autumn (September) of 2005. In the inner part of the estuary where salinity was less than 30, the phytoplankton community was dominated by micro- and nano-sized (3–200 μm) cells, particularly the diatom Skeletonema costatum, both in early spring and early autumn. In areas where salinity >30, including the mixing zone and nearshore oceanic water, micro- and nano-sized cell populations dominated the phytoplankton assemblage during early spring when influence of river discharge was minimal, whereas pico-sized (≤3 μm) cell populations were dominant during early autumn as a result of strong river discharge in the summer, with Synechococcus and pico-eukaryotes being predominant. Picophytoplankton were two orders of magnitude more abundant in early autumn (10⁶ cells mL⁻¹) than in early spring in the nearshore oceanic water. Nutrients delivered by freshwater input to the estuary were pushed toward high salinity (>30) areas as a result of short residence time, exerting a strong influence on phytoplankton abundance, especially picophytoplankton in the nearshore, otherwise oligotrophic, water. Influenced by high abundance of DIN and limitation in phosphorus, picophytoplankton in the adjacent nearshore oceanic water rose to prominence seasonally. Our results indicate that eutrophication in the Pearl River Estuary not only stimulates the growth of S. costatum in the nutrient-rich areas of the estuary but also appears to promote the growth of Synechococcus and pico-eukaryotes in the adjacent usually oligotrophic oceanic water at least during our autumn cruise.     

Seasonal variation of microzooplankton (20–200 μm) and its possible implications on the vertical carbon flux in the western Bay of Bengal

Stratification (throughout the year) and low solar radiation (during monsoon periods) have caused low chlorophyll a and primary production (seasonal average 13–18 mg m⁻² and 242–265 mg C m⁻² d⁻¹, respectively) in the western Bay of Bengal (BoB). The microzooplankton (MZP) community of BoB was numerically dominated by heterotrophic dinoflagellates (HDS) followed by ciliates (CTS). The highest MZP abundance (average 665±226×10⁴ m⁻²), biomass (average 260±145 mg C m⁻²) and species diversity (Shannon weaver index 2.8±0.42 for CTS and 2.6±0.35 for HDS) have occurred during the spring intermonsoon (SIM). This might be due to high abundance of smaller phytoplankton in the western BoB during SIM as a consequence of intense stratification and nitrate limitation (nitracline at 60 m depth). The strong stratification during SIM was biologically evidenced by intense blooms of Trichodesmium erythraeum and frequent Synechococcus–HDS associations. The high abundance of smaller phytoplankton favors microbial food webs where photosynthetic carbon is channeled to higher trophic levels through MZP. This causes less efficient transfer of primary organic carbon to higher trophic levels than through the traditional food web. The microbial food web dominant in the western BoB during SIM might be responsible for the lowest mesozooplankton biomass observed (average 223 mg C m⁻²). The long residence time of the organic carbon in the surface waters due to the active herbivorous pathways of the microbial food web could be a causative factor for the low vertical flux of biogenic carbon during SIM.     

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.     

Riverine input and air–sea CO2 exchanges near the Changjiang (Yangtze River) Estuary: Status quo and implication on possible future changes in metabolic status

Due to anthropogenic activities, the nutrient loadings of the Changjiang (Yangtze River) are strickly on the rise. The high nutrient concentrations notwithstanding, river water was pCO₂ supersaturated in the inner estuary during summer 2003 but decreased quickly in the mid-estuary due to mixing with low pCO₂ waters from offshore. In addition, settling of particles in the estuary resulted in better light conditions so that phytoplankton bloomed, driving down pCO₂ to ∼200 μatm. In the outer estuary and outside of the bloom area, pCO₂ increased again to near or just below saturation. Literature data also reveal that the mainstream of the Changjiang is always supersaturated with respect to CO₂ probably because the decomposition of terrestrial organic matter overwhelms the consumption of CO₂ due to biological production.     

Occurrence and distribution of phthalate esters in riverine sediments from the Pearl River Delta region,South China

Sixty-eight sediment samples collected from Dongjiang River, Xijiang River, Beijiang River and Zhujiang River in the Pearl River Delta (PRD) region, Southern China, were analyzed for 16 phthalate esters (PAEs). PAEs were detected in all riverine sediments analyzed, which indicate that PAEs are ubiquitous environmental contaminants. The Σ₁₆PAEs concentrations in riverine sediments in the PRD region ranged from 0.567 to 47.3 μg g⁻¹ dry weight (dw), with the mean and median concentrations of 5.34 μg g⁻¹ dw and 2.15 μg g⁻¹ dw, respectively. Elevated PAEs concentrations in riverine sediments in the PRD region were found in the highly urbanized and industrialized areas. Of the 16 PAEs, diisobutyl phthalate (DiBP), di-n-butyl phthalate (DnBP) and di(2-ethylhexyl) phthalate (DEHP) dominated the PAEs, with the mean and median concentrations of 1.12 μg g⁻¹ dw, 0.420 μg g⁻¹ dw and 3.72 μg g⁻¹ dw, and 0.429 μg g⁻¹ dw, 0.152 μg g⁻¹ dw and 1.55 μg g⁻¹ dw, respectively, and accounted for 94.2–99.7% of the Σ₁₆PAEs concentrations. Influenced by local sources and the properties of PAEs, a gradient trend of concentrations and a fractionation of composition from more to less industrialized and urbanized areas were discovered. As compared to the results from other studies, the riverine sediments in the PRD region were severely contaminated with PAEs. Information about PAEs contamination status and its effect on the aquatic organisms in the PRD region may deserve further attention.     

The relationship of Irgarol and its major metabolite to resident phytoplankton communities in a Maryland marina, river and reference area

The objectives of this study were to: (1) measure water column concentrations of Irgarol 1051 and its major metabolite GS26575 annually (2004-2006) during mid-June and mid-August at 14 sites in a study area comprised of three sub-regions chosen to reflect a gradient in Irgarol exposure (Port Annapolis marina, Severn River and Severn River reference area); (2) use a probabilistic approach to determine ecological risk of Irgarol and its major metabolite in the study area by comparing the distribution of exposure data with toxicity-effects endpoints; and (3) measure both functional and structural resident phytoplankton parameters concurrently with Irgarol and metabolite concentrations to assess relationships and determine ecological risk at six selected sites in the three study areas described above. The three-year summer mean Irgarol concentrations by site clearly showed a gradient in concentrations with greater values in Back Creek (400-500 ng/L range), lower values in the Severn River sites near the confluence with Back Creek (generally values less than 100 ng/L) and still lower values (<10 ng/L) at the Severn River reference sites at the confluence with Chesapeake Bay. A similar spatial trend, but with much lower concentrations, was also reported for GS26575. The probability of exceeding the Irgarol plant 10th centile of 193 ng/L and the microcosm NOEC (323 ng/L) suggested high ecological risk from Irgarol exposure at Port Annapolis marina sites but much lower risk at the other sites. There were no statistically significant differences among the three site types (marina, river and reference) with all years combined or among years within a site type for the following functional and structural phytoplankton endpoints: algal biomass, gross photosynthesis, biomass normalized photosynthesis, chlorophyll a, chlorophyll a normalized photosynthesis and taxa richness. Therefore, based on the above results, Irgarol adverse effects predicted from the plant 10th centile and the microcosm NOEC in the high Irgarol exposure area (Back Creek/Port Annapolis marina) were not confirmed with the actual field data for the receptor species (phytoplankton). These results also highlight the importance of unconfined field studies with a chemical gradient in providing valuable information regarding the responses of resident phytoplankton to herbicides.     

Microbial food web contributions to bottom water hypoxia in the northern Gulf of Mexico

Nutrients from the Mississippi/Atchafalaya Rivers greatly stimulate biological production in the ‘classical’ food web on the inner shelf of the northern Gulf of Mexico. Portions of this production, especially large diatoms and zooplankton fecal pellets, sink and decompose in the bottom water, consuming oxygen and contributing to the annual development of an extensive zone of bottom water hypoxia, typically >15,000 km² since 1993. The microbial food web is also active in the Mississippi River plume, but consists of small organisms that sink slowly. This ‘recycling’ food web has not been considered as a significant contributor to vertical flux and hypoxia. However, gelatinous zooplankton, especially pelagic appendicularians such as Oikopleura dioica, mediate the conversion of microbial web organisms to organic particles with high sinking rates. When pelagic appendicularians are abundant in coastal regions of the northern Gulf of Mexico, they stimulate the rapid vertical transfer of microbial web productivity in the surface layer, which is only 5–15 m thick in the coastal hypoxic region, to the sub-pycnocline layer that becomes hypoxic each summer. In this paper we present results from two studies examining the significance of this pathway. In both 2002 and 2004, we observed high production rates of appendicularians in coastal waters. Discarded gelatinous houses and fecal pellets from the appendicularian populations often provided more than 1 g m⁻² d⁻¹ of organic carbon for the establishment and maintenance of hypoxia in the northern Gulf of Mexico. This source of organic matter flux is especially important in regions far from the river plumes and during periods of low river discharge. Autotrophic elements of this food web are primarily supported by recycled inorganic nutrients originating in the Mississippi and Atchafalaya Rivers. Sources of dissolved organic matter (DOM) supporting the heterotrophic components of this microbial food web may include in situ production, the Mississippi/Atchafalaya Rivers, and Louisiana's coastal wetlands. If significant, the latter source provides a possible link between Louisiana's high rates of coastal land loss and the large hypoxic zone observed along the coast during summer. Both of the latter DOM sources are independent of phytoplankton production stimulated by inputs of riverine inorganic nutrients.