Estuaries Of Major Rivers Research Articles

The characteristic of compound drought and saltwater intrusion events in the several major river estuaries worldwide

Compound Drought and Saltwater intrusion Events (CDSEs) refer to hydrologic drought and saltwater intrusion occurring simultaneously or consecutively in estuaries, and exacerbate the negative impacts resulting from an individual extreme event. CDSEs have been drawing increasing attention due to their potential adverse impacts on water resources, crop production, and food security. A new Standardized compound Drought and Saltwater intrusion Index (SDSI) was developed in this study to systematically detect changes in the severity of CDSEs in six estuaries (Little Back, Ebro, Rhine, Orange, Pearl River and Murray). The results illustrated that (1) compared to the Standardized Runoff Index (SRI), SDSI effectively characterizes and quantifies the occurrences and severity of CDSEs in major river estuaries worldwide. (2) Temporally, the SDSI trend varied across estuaries. Specifically, a decreasing trend was observed in the Little Back, Ebro, and Orange estuaries, with corresponding Zs values of −2.43, −3.63, and −3.23. (3) Spatially, moderate CDSEs occurred more frequently among different estuaries, and their frequency, duration and severity varied in different estuaries. Notably, Ebro, Rhine and Murray River estuaries had the highest probability of CDSEs, nearing 60%. Among them, the Murray Estuary had the longest average duration, spanning 7.68 months, and the highest severity was 5.94. (4) According to the contributions analysis, saltwater intrusion plays a dominant role in influencing SDSI severity, accounting for a substantial percentage (54%–95.30%) compared to runoff. Notably, the Orange Estuary experienced the greatest impact from saltwater intrusion (81.54%–95.30%), while the Murray Estuary had relatively equal contributions from hydrological drought and saltwater intrusion.

Water quality pollution indices to assess the heavy metal contamination: A case study of the estuarine waters in Cirebon City (West Java, Indonesia) Pre- and post- CARE COVID-19

The outbreak of Covid-19 has hindered societal activities, leading the government to implement lockdown measures aimed at minimizing transmission. The reduction in human activities, particularly among communities residing in estuarine areas, has the potential to impact the water quality of estuaries based on physical, chemical, and biological parameters. The impact of the Covid-19 pandemic on the coastal and marine environment is also evident in Cirebon, a port city renowned for trade and industry situated on the northern coast of Java Island.This study aims to analyse the water quality of three river estuaries in Cirebon before, during, and after the Covid-19 lockdown, specifically during the implementation of Community Activities Restrictions Enforcement (CARE) in Indonesia. Water samples from the Kesenden, Sukalila, and Kalijaga estuaries were collected in 2019, 2020, 2022, and 2023. These three stations were chosen because they are the three major river estuaries in the city of Cirebon, and each station is located in close proximity to coastal communities. These estuaries are adjacent to residential areas and serve as parking areas for small fishing boats owned by coastal communities residing around the estuaries. The river estuaries were selected as they represent transitional areas and the final discharge point of community activities that ultimately flow into the sea. Each parameter will be compared against the water quality standards of Class II as per Indonesian Government Regulation No. 22 of 2021. The Heavy Metal Pollution Index (HPI) method will be employed to classify the heavy metal index.The assessment of the heavy metal pollution index in 2019 ranged between 34.138 and 34.140, in 2020 the range was from 34.137 to 34.139, in 2022 it ranged from 29.561 to 31.86, and in 2023, the values for the heavy metal index were approximately ±34.116. Consequently, the water quality at the three observation stations exhibited improvement during the pandemic but regressed after the conclusion of the lockdown period.

Nitrate transformation and source tracking of rivers draining into the Bohai Sea using a multi-tracer approach combined with an optimized Bayesian stable isotope mixing model

Excessive levels of NO3- can result in multiple eco-environmental issues due to potential toxicity, especially in coastal areas. Accurate source tracing is crucial for effective pollutant control and policy development. Bayesian models have been widely employed to trace NO3- sources, while limited studies have utilized optimized Bayesian models for NO3- tracing in the coastal rivers. The Bohai Rim is highly susceptible to ecological disturbances, particularly N pollution, and has emerged as a critical area. Therefore, identification the N fate and understanding their sources contribution is urgent for pollution mitigation efforts. In addition, understanding the influenced key driven factors to source dynamic in the past ten years is also implication to environmental management. In this study, water samples were collected from 36 major river estuaries that drain into the Bohai Sea of North China. The main transformation processes were analyzed and quantified the sources of NO3- using a Bayesian stable isotope mixing model (MixSIAR) with isotopic approach (δ15N-NO3- and δ18O-NO3-). The overall isotopic composition of δ15N-NO3- and δ18O-NO3- in estuary waters ranged from −0.8–19.3‰ (9.3 ± 4.6‰) and from −7.1–10.5‰ (5.0 ± 4.3‰), respectively. The main sources of nitrate in most river estuaries were manure & sewage, and chemical fertilizer, while weak denitrification and mixed processes were observed in Bohai Rim region. A temporal decrease in the nitrogen load entering the Bohai Sea indicates an improvement in water quality in recent years. By incorporating informative priors and utilizing the calculated coefficients, the accuracy of sourcing results was significantly improved. This study highlighted the optimized MixSIAR model enhanced its accuracy for sourcing analysis and providing valuable insights for policy formulation. Future efforts should focus on improving management strategies to reduce nitrogen into the bay.

Migration and Estuarine Land Use/Land Cover (LULC) Change along Ghana’s Coast

Estuaries are among the world’s most biologically productive ecosystems and have since generations attracted migrants due to their high biodiversity, natural beauty, and source of livelihood. Growing coastal populations and immigration have led to the deterioration of many coastal ecosystems globally. Overfishing, habitat fragmentation, and degradation have significantly compromised these estuaries’ natural environments and subsequently affected their ability to serve as breeding grounds for aquatic organisms. Thus, understanding landscape patterns, changes, and human–nature​ interactions is critical for long-term monitoring of estuarine ecosystems. This study analysed changes in three major river estuaries (Pra, Ankobra and the Volta Rivers) in Ghana using Landsat satellite data from 1986 to 2020 and population data. The results revealed that coastal flooding and erosion had changed the area of bare land/settlements in both Pra Estuary (−13.7%) and Ankobra Estuary (−18.8%) into water areas, respectively. Likewise, anthropogenic activities resulted in a 12.6% loss in vegetation in the Ankobra Estuary and 42.8% in the Volta Estuary. However, mangrove reforestation interventions at the Pra Estuary increased the vegetation area (10.8%). These observed LULC changes in these areas explain the effect of migration and its impact on long-term resource management and the survival of estuarine communities. Associated with the increased population (immigration) of estuarine communities are mangrove destruction, sand mining, and indiscriminate waste disposal activities, driving the degradation of the estuaries. Consequently, migrant, and local community interests should be harnessed to avoid undesirable LULC dynamics. Thus, an integrated approach to the management of the coastal zone and associated wetlands are paramount for the sustainability of the environment and local livelihoods.

Effects of emerging contaminants and heavy metals on variation in bacterial communities in estuarine sediments

Emerging contaminants (ECs) and heavy metals (HMs) are universally present together in estuarine sediments; despite this, their effects on microbial communities have been widely studied separately, rather than in consort. In this study, the combined effects of ECs and HMs on microbial communities were investigated in sediments from 11 major river estuaries around the Bohai Sea, China. Proteobacteria, Bacteroidetes, and Firmicutes were the dominant phyla in the sediments. Using Shannon indices, total phosphorus and total organic carbon were shown to affect microbial community structure. Redundancy analysis of microbial variation implicated Cd and As as the greatest pollutants, followed by Mn, Fe, Zn and Cu; no impacts from galaxolide (HHCB) and tonalide (AHTN) were found. Correlation analysis demonstrated that the concentration of ECs increased the abundance of certain bacteria (e.g., Haliangium, Altererythrobacter, Gaiella and Erythrobacter), and therefore these can be used as potential contamination indicators. Shannon indices and Chao1 indices showed that there were differences in the richness and diversity of bacterial communities in the sediments of 11 rivers. The principal coordinate analysis displayed higher similarity of bacterial community composition in estuarine sediments in Liaoning province than other regions. The results can be used to predict changes in estuary ecosystems to maintain their ecological balance and health.

Physicochemical properties, total hydrocarbon content, and trace metals of water and sediments from major River Estuaries within the Niger Delta Region of Nigeria

Physicochemical properties, total hydrocarbon content, and trace metals of water and sediments from major River Estuaries within the Niger Delta Region of Nigeria

Physicochemical properties, total hydrocarbon content, and trace metals of water and sediments from major River Estuaries within the Niger Delta Region of Nigeria

Water and sediment samples from Cross River, Imo River, and Qua Iboe River Estuaries were collected and treated using standard analytical procedures. The samples were analyzed for their physicochemical properties, total hydrocarbon content (THC), and trace metals. Results obtained revealed that, the mean levels of total suspended solids (TSS), total dissolved solids (TDS), turbidity, colour, biochemical oxygen demand (BOD), THC, cadmium (Cd), iron (Fe), nickel (Ni), lead (Pb), and zinc (Zn) in water were higher than their acceptable limits in Nigeria. Whereas, the mean levels water pH, electrical conductivity (EC), dissolved oxygen (DO), nitrate, sulphate, phosphate, and copper (Cu) were within their acceptable limits. The results obtained also revealed that, the mean levels of nitrate, THC, and Fe in sediments were higher than their recommended limits. While the levels of pH, EC, sulphate, phosphate, Cd, Cu, Ni, Pb, and Zn in sediments were within their recommended limits. This study indicated that the anthropogenic activities by oil and oil-related companies within the Niger Delta Region of Nigeria may have contributed significantly to the high level of toxic substances in the water channels within the zone. The results obtained revealed that, the direct or indirect exposure to untreated water from the studied water channels may result in serious health challenges. Hence, the environment should be closely monitored to forestall a devastating situation in the region.

Diversity and Abundance of Phytoplankton in the Coastal Waters of South Sulawesi

Phytoplankton are primary producers that can be used as seawater condition indicators. Certain phytoplankton can proliferate, causing harmful algal blooms (HABs). The coastal waters of South Sulawesi, Indonesia are under pressure from land-based processes and activities resulting in inputs of organic and inorganic materials. This study analysed phytoplankton diversity and abundance in coastal waters around South Sulawesi. Phytoplankton were sampled and seawater parameters (salinity, temperature, turbidity, pH, nitrate concentration) measured in-situ at six stations around seven major river estuaries in three seaways (Makassar Strait, Flores Sea, Gulf of Bone). Phytoplankton taxonomic composition, abundance and indices of diversity (H’), evenness (E), and dominance (D) were analysed. Phytoplankton from 31 species and three classes (Bacillariophyceae, Cyanophyceae, Dinophyceae) were identified. Phytoplankton abundance and community structure differed significantly between sites and seaways but were not significantly correlated with water quality parameters although Dinophyceae abundance correlated significantly with observed pollution levels. Phytoplankton abundance was strongly influenced by the Dinophyceae, especially Ceratium furca, a potential HAB species; Cyanophyceae had the strongest influence on species richness but least on community structure. C. furca abundance was strongly correlated negatively with species richness, H’ and E, and positively with D, indicating negative impacts of this species on phytoplankton communities.

Sandpipers go with the flow: Correlations between estuarine conditions and shorebird abundance at an important stopover on the Pacific Flyway.

Estuaries of major rivers provide important stopover habitat for migratory birds throughout the world. These estuaries experience large amounts of freshwater inputs from spring runoff. Understanding how freshwater inputs affect food supply for migrating birds, and how birds respond to these changes will be essential for effective conservation of critical estuarine habitats. We estimated trends over time in counts of Western Sandpiper (Calidris mauri) and Pacific Dunlin (Calidris alpina pacifica) during northward migration on the Fraser River estuary, British Columbia, Canada, where shorebirds feed extensively on intertidal biofilm and invertebrates. We also examined whether counts were correlated with a suite of environmental variables related to local conditions (precipitation, temperature, wind speed and direction, solar radiation, tidal amplitude, and discharge rates from the Fraser River) during a total of 540 surveys from 1991 to 2019. Counts of Western Sandpiper declined ~54% (−2.0% per annum) over the entire study period, and 23% from 2009 to 2019 (−0.9% per annum). Counts of Pacific Dunlin did not show a statistically significant change over the study period. Counts of shorebirds were lower when discharge from the Fraser River was high, which we propose results from a complex interaction between the abrupt changes in salinity and the estuarine food web related to the quantity or quality of intertidal biofilm. Counts were also higher when tidal amplitude was lower (neap tides), potentially related to longer exposure times of the mudflats than during spring tides. Effects of wind are likely related to birds delaying departure from the stopover site during unfavorable wind conditions. The negative trend in migrating Western Sandpipers is consistent with declines in nonbreeding areas as observed in Christmas Bird Counts. Understanding causes of population change in migratory shorebirds highlights the need for research on mechanistic pathways in which freshwater inputs affect food resources at estuarine stopovers.

Short-term Estimate of Finfish Bycatch Discards in the Inshore Artisanal Shrimp Fishery of Guyana

The artisanal shrimp fishery in Guyana is important for livelihood and food security, involving around 300 vessels owned and crewed exclusively by Guyanese nationals. This fishery uses Chinese seines and operates in major river estuaries. It targets penaeid shrimp, but also retains some finfish and is known to discard a significant but undocumented quantity of smaller finfish bycatch. The lack of knowledge regarding the bycatch is a concern for fishery management and biodiversity conservation. In this study, we quantify for the first time the finfish bycatch discards through onboard observations (July—August 2016) of a single typical vessel operating in the Demerara estuary. Wet weights of the total catch, retained catch, and finfish discards were recorded separately for each of 76 seine hauls, and subsequently presented as catch rates per trip, catch rates per seine haul (kg/haul), and catch rates standardized per hour of seine net soak—time (kg/hr). A sub—sample of finfish discards was taken from every haul to determine taxonomic composition and species—specific length frequencies. Examination of 2,012 discarded finfish distributed among 32 species revealed high taxonomic diversity, none of which were considered vulnerable by the IUCN Red List. Most finfish discards were small (modal size class 5—7 cm fork length) and included juveniles of 15 species of importance to other fisheries in Guyana. On average, a standardized total catch rate of 14.8 kg was taken per hour of seine net soak—time, yielding 3.9 kg retained catch (shrimp and a few selected finfish), 10.3 kg of finfish bycatch discards, and 0.6 kg of miscellaneous invertebrate discards. This demonstrates significant wastage (finfish discards represent about 69% of the total catch weight) and potential for negative impact on biodiversity and other commercial fisheries. The information provided here addresses an important knowledge gap in the artisanal fisheries of Guyana.

Survey of selected personal care products in surface water of coral reefs in Kenting National Park, Taiwan

Kenting National Park (KNP) located in the Hengchun Peninsula in southern Taiwan is a popular tourist spot, annually attracting millions of visitors, who engage in water sport and amusement activities. In this region, sewage is directly discharged into the marine environment. In this study, the concentrations of five organic UV filters [benzophenone (BP), 2,4-dihydroxy benzophenone (BP-1), 2-hydroxy-4-methoxy benzophenone (BP-3), 2,2′-dihydroxy-4-methoxy benzophenone (BP-8), and 4-methylbenzylidene camphor], five preservatives [methylparaben (MeP), ethylparaben, propylparaben (PrP), butylparaben, and benzylparaben], one disinfectant [triclosan (TCS)], and twenty-four detergent derivatives [nonylphenol (NP), nonylphenol ethoxylates (NP2EO–NP12EO), octylphenol (OP) and octylphenol ethoxylates OP2EO–OP12EO] were detected in seawater and river water samples collected from eight beaches in KNP and two major river estuaries in the Hengchun Peninsula. BP-3 was detected at all sampling sites and was higher in concentration than the other organic UV filters. The highest concentration of BP-3 was 1233 ng/L collected from Wanlitong Beach. MeP and PrP were the main preservative components in seawater. The highest total content of preservative agents was 164 ng/L collected from Houwan Beach. Moreover, NP was detected at all sampling sites, with the highest concentration found at Sail Rock Beach (26.5 ng/L). The highest concentration of OP was 113 ng/L in the Boli River estuary. The widespread use of personal care products (PCPs) has resulted in the release of their major ingredients into natural ecosystems. Therefore, the potential long-term effects of multi-PCPs at low concentration exposure to on the coral reef ecosystem in KNP must be considered and monitored.

Nutrients and carbon fluxes in the estuaries of major rivers flowing into the tropical Atlantic

Knowledge of the seasonal variability of river discharge and the concentration of nutrients in the estuary waters of large rivers flowing into the tropical Atlantic contributes to a better understanding of the biogeochemical processes that occur in adjacent coastal and ocean systems. The monthly averaged variations of the physical and biogeochemical contributions of the Orinoco, Amazon, São Francisco, Paraíba do Sul (South America), Volta, Niger and Congo (Africa) Rivers are estimated from models or observations. The results indicate that these rivers deliver approximately 0.1 Pg C yr-1 in its dissolved organic (DOC 0.046 Pg C yr-1) and inorganic (DIC 0.053 Pg C yr-1) forms combined. These values represent 27.3% of the global DOC and 13.2% of the global DIC delivered by rivers into the world’s oceans. Estimations of the air-sea CO2 fluxes indicate a slightly higher atmospheric liberation for the African systems compared with the South American estuaries (+10.67 mmol m-2 day-1 and +5.48 mmol m-2 day-1, respectively). During the high river discharge periods, the fluxes remained positive in all of the analyzed systems (average +128 mmol m-2 day-1), except at the mouth of the Orinoco River, which continued to act as a sink for CO2. During the periods of low river discharges, the mean CO2 efflux decreased to +5.29 mmol m-2 day-1. The updated and detailed review presented here contributes to the accurate quantification of CO2 input into the atmosphere and to ongoing studies on the oceanic modeling of biogeochemical cycles in the tropical Atlantic.

Seasonal sea level variations in the gulf of Cadiz continental shelf from in-situ measurements and satellite altimetry

Long-term time series (1997–2008) of gridded multi-mission near shore altimeter data and in-situ tide gauge records have been analyzed to investigate the seasonal variability of sea level along the Gulf of Cadiz continental shelf and the Strait of Gibraltar. The contribution of different forcing mechanisms including atmospheric pressure, wind, steric anomalies and river runoff was also addressed. Comparison between altimeter and ground-truth data showed that the former can be a valuable tool to study the sea level seasonal cycle near the coast as well as to detect anomalous data from neighboring tide gauges. Overall, both sets of time series presented a similar mean seasonal cycle along the coast, dominated by the annual component and, to a lesser extent, by the semiannual one. Low-frequency atmospheric pressure and steric anomalies were the main forcing factors affecting the sea level seasonal cycle, their effect diminishing toward the Strait of Gibraltar. The combined effect of both forcing factors accounted for 55–61%/62–63% of the observed sea level variance at the tide gauge stations/ altimeter points located outside the Strait of Gibraltar and for only 27%/48% at those placed within the Strait. The influence of local wind was small in all stations except for the westernmost tide gauges. The effect of large river discharges was notable at the tide gauge located within the Guadalquivir river mouth, with 18% of the pressure-adjusted sea level variance explained, and negligible (< 0.1%) at the one placed within the Tinto–Odiel rivers mouth, indicating that such effects only need to be taken into account when dealing with tide gauges located within major river estuaries. Moreover, the effect of river runoff seems to be confined within a small area near the river mouth, thus not affecting the altimeter measurements. A small but significant influence of the wintertime North Atlantic Oscillation on sea level was observed, its effect being mainly reflected through changes in the atmospheric pressure, wind and river discharge.

Distribution and sources of organochlorine pesticides in sediments of the coastal East China Sea

Forty-two surface sediment samples collected from the southern coastal East China Sea (ECS) were analyzed for organochlorine pesticides (OCPs). The results showed high concentrations of DDTs and parent DDT fractions in the nearshore area, suggesting that there is still a fresh input of these species from non-point sources, probably due to the recent usage of relevant products. However, for HCHs, the high concentrations with elevated β-HCH fraction were focused at the local major river estuaries, indicating that soils-released HCH input was mainly from the major river runoffs. The distribution and composition of OCPs in the coastal ECS could be more prominently controlled by the direct regional inputs rather than the general natural sorption process on particulate organic carbon. DDTs and chlordane are more important for their potential ecotoxicological risk on neighboring benthic community in the nearshore region and the local major river estuaries.

Paleoshorelines and prehistory on the eastern Bothnian Bay coast (Finland): local environmental variability as a trigger for social change

Research on the relation between the natural environment and human society in northwest coastal Finland between 6500 and 4000 cal BP shows a strong relation between regional environmental variability and changes in human society. It is not only the environmental change but also the regional variability in shoreline displacement that triggered changes in hunter-gatherer society in Finland. Postglacial land uplift caused a reduction of the distance between the major river estuaries flowing into the Bothnian Bay due to its particular shape. This led to increased population densities, even without population growth. In combination with increasing variability in the westward displacement of the different river mouths, i.e. increased variability of river mouth stability, this resulted in increased sedentism and aggregation among the hunter-gatherers living on the Bothnian Bay coast of northern Finland around 5500–5000 cal BP.

海底堆積物を用いた有明海１００年変遷史の環境評価

Submarine sediments in Ariake Bay, Kyushu Japan, were examined to assess geoenvironmental changes in the area. 6 core samples from representative stations (Stas. 1-6) were analyzed for 16 minor elements (S, Cl, Ba, Rb, Th, Nb, Zr, Y, Ga, V, Zn, Cu, Ni, Co, Cr, and Pb) using XRF. Based on measurements of 210Pb radioactivity, sedimentation rates at 2 stations, one off the Chikugo River (Sta. 2) and the other at the mouth of Isahaya Bay (Sta. 4), were calculated, respectively, at 0.24 cm/year and a maximum of 0.15 cm/year.The concentrations of heavy metals, such as Cr, Ni, Zn, and Pb, in the marine sediments are about one-third of those of the highly polluted marine sediments recorded in Tokyo Bay around 1970. The vertical profile of a minor element can be divided into an upper section and a lower section. The upper section indicates various gradual changes, but the lower section is almost constant. The boundaries between the sections roughly correspond to the beginning of the 20th century, based on the sedimentation rate.Zn and Pb in the submarine sediments can be useful indicators to trace suspended mud derived from the Chikugo River. The complementary relationship of the vertical profiles in the spatial distribution suggests that the continuous migration of the sedimentary province may be governed by the geometrical arrangement of the estuaries of major rivers and by the tidal circulation pattern in Ariake Bay. Probably, large dams built on the major rivers have hampered the sedimentation process dominated by traction flow and have decreased the maximum drainage speed. As a result, such processes have promoted the expansion of the muddy area in Ariake Bay.Planktonic dinoflagellata (Sta. 2) and diatom (Sta. 4) fossils were also analyzed. Assuming such sedimentation rates at the stations, marine phytoplanktons (Brigantedinium spp., Rhizosorenia spp. and Skeletonema costatum), which indicate eutrophication of the sea water, have increased in relative abundance during the past 10 years.

Characterization and distribution of metals in surficial sediments in Southwestern Taiwan

This study analyses copper, zinc, lead, iron, nickel, chromium and cadmium concentrations in surficial sediments sampled from major river estuaries, harbours and ocean outfall fields in southwestern Taiwan. Grain size effect, normalization technique of iron concentration and guidelines of hazardous assessment suggested by Ontario Ministry of Environment, Canada are also discussed and/or compared. According to this comparison, the Kaohsiung coast appears to be the most polluted area with respect to enrichment of copper and zinc among all the stations studied. Metal contamination has reached a remote and undeveloped area, the Feng-Kung Chi estuary. The results demonstrate that normalization techniques of grain size and iron concentration can characterize polluted marine sediments.

Sources and distribution of chlorobenzenes and hexachlorobutadiene in surficial sediments along the coast of Southwestern Taiwan

The spatial distribution of chlorobenzenes in surficial sediments of the estuaries of major rivers along the coast of southwestern Taiwan was determined. Analytical results from sediment sampling stations indicate that the three outfall fields near Taiwan's largest industrial city, Kaohsiung, exhibit the highest concentrations of chlorobenzenes. More specifically, sediments from Tso-yin outfall field, which receives the wastewater from a very large petrochemical complex, had concentrations of 1,3,5-trichlorobenzene, pentachlorobenzene, hexachlorobenzene and hexachlorobutadiene 2–3 times higher than those from Chon-chou and Da-lin-pu outfall fields. Chlorobenzene congeners in sediments from the three outfall fields correlate closely and most of their correlation coefficients are even higher than those reported for Ise Bay, Japan. Results from undeveloped areas, estuaries of the Da-du, Tso-shui, and Chi-shui Chi, will be of value after the impact of planned industrial expansion development begins.

Critical review of selected heavy metal and chlorinated hydrocarbon concentrations in the marine environment

A critical review of existing concentrations of the potential pollutants Hg, Cd, Pb, PCBs and DDT in marine waters, sediments and biota of the global ocean indicates that the highest concentrations are usually found in the most densely populated and industrialized regions which are often located near major river estuaries. In general, present concentrations, particularly those in edible marine organisms, do not give rise for alarm; however, in some cases national and international concentration limits have been exceeded which has caused some concern for human health. In some ‘hot spots’ (e.g. Minamata Bay, Hudson-Raritan Estuary, Los Angeles Bight, Ems Estuary) where measures have been taken to eliminate the sources of contamination, a significant reduction in concentrations has occurred. Reliable temporal data are generally too sparse and have not been collected for a sufficient period to make accurate predictions about the environmental half-life of the contaminants; however, the widespread occurrence of persistent organochlorine residues of PCBs and DDT in remote areas far from known input sources suggests a long residence time in the ecosystem. Spatial data on a global scale are also limited; therefore, it is difficult to draw firm conclusions about long-term consequences of the shift in use of DDT and other chlorinated pesticides towards tropical areas and the southern hemisphere. However, the few reliable data available from these regions indicate concentrations of some organochlorine compounds in marine matrices that are as high or higher than those reported for the northern hemisphere. More data from a much wider area are needed in order to clarify this trend.

Regional distribution of Macoma balthica and Hydrobia minuta on the subarctic coasts of Hudson Bay and James Bay, Ontario, Canada

The distribution of Macoma balthica and Hydrobia minuta was studied regionally along a 1000 km stretch of subarctic coast from latitude 52°N to 57°N in western James Bay and Hudson Bay, Ontario. Macoma balthica is widely distributed, but absent from estuaries of major rivers and from intertidal transects periodically inundated by fluvial waters. The frequency of occurrence of Macoma balthica is highly correlated with fine to very fine sand and silt. It thrives in the lower and middle parts of the tidal flats. The high tidal flats and lower marshes have fine silt deposits: they may however, develop anoxic conditions due to decomposition of algal mats, and they are toxic environments for the survival and development of Macoma spats. The frequency of occurrence of Hydrobia minuta does not correlate closely with the average grain size of the substratum. Hydrobia is found in large numbers where clay drapes develop in sheltered locations and where relatively low-salinity and higher-temperature environments occur, as in southern James Bay. Hydrobia minuta reaches the northern limits of its range in James Bay, very few specimens having been found north of Akimiski Island (latitude 53°15′N).