The conservation of British North Sea estuaries

Estuaries are amongst the most productive, but scarce, ecosystems in the world. Britain and the southern North Sea have many and extensive estuaries. Many British North Sea coast estuaries are nationally and internationally important for nature conservation for their variety of sand-dune, saltmarsh, sandflat and mudflat habitats, and the plants and animals that they support. The wildlife of estuaries are currently being affected by a great many of man's activities. Many, particularly land-claim, pollution and recreational activities cause loss and damage to this scarce wildlife habitat. Conservation of British estuaries is achieved mainly through site safeguard — the notification of Sites of Special Scientific Interest (SSSIs) and declaration of National Nature Reserves (NNRs) by the Nature Conservancy Council (NCC), and the designation of sites of international importance under the Ramsar Convention and the EC Directive on Wild Birds — and the amelioration of existing damage by sensitive management and pollution control. Several nationwide surveys, particularly NCC's Estuaries Review, are underway in Britain. These will provide sound comparable information on all estuarine sites, and will form the basis for developing an overall estuarine conservation strategy.     

Influence of pollution on the harpacticoid copepods of two North Sea estuaries

Seasonal monitoring of the meiobenthos in the Dutch estuaries revealed an anomaly in density and diversity of harpacticoid copepods in the Westerschelde.Another Dutch estuary, the Eems Dollard, has comparable hydrodynamical, physical and sedimentological, characteristics and a similar fauna, but even in the severely organically polluted oligohaline mudflats of this estuary, annual average density and diversity of endo-epibenthic harpacticoid communities are higher than at similar less enriched meso- to polyhaline biotopes of the Westerschelde.Besides the concentrations of inorganic pollutants, such as phosphates and nitrates, the concentrations of pesticides, cyanide, detergents, phenoles, oils, polychlorobenzenes, polycyclic aromatic hydrocarbons and heavy metals were compared in both estuaries and compared to suggested permissive levels when available. From this it appears that these pollutants are present in the Westerschelde either in too low concentrations to be considered dangerous or at concentrations comparable to those occurring in the Eems Dollard, except for heavy metals.The load of a.o. Zn, Cu and Pb is distinctly and persistently higher in sediments and suspensions of the Westerschelde than in the Eems Dollard and copper is continuously present in a concentration at which, according to bioassays, egg production and larval development of planktonic copepods are severely affected. The remarkable scarcity of harpacticoid life on nutrient rich mudflats of the Westerschelde is thus probably due to heavy metal pollution.Since no other hardbodied meio- and macrobenthic taxa nor the plankton of this estuary show such a marked impoverishment, benthic harpacticoids prove to be suitable as indicators for the first stages of ecosystem-breakdown in estuarine and coastal zones polluted by trace-metals.     

'Fish in hot water": the challenges facing fish and fisheries research in tropical estuaries

Tropical estuarine areas comprise small systems of a few km, larger estuaries, coastal lakes of hundreds of km 2 and vast shallow coastal waters that are contiguous with estuaries and have similar reduced salinities. Many of the world's great estuaries are in the tropics, e.g. the Amazon, Orinoco, Congo, Zambezi, Niger, Ganges and Mekong. The distribution of tropical and subtropical estuaries approximately follows that of mangroves. Like estuaries everywhere, they are a focus of human activity and are among the most exploited of ecosystems. In few other places do the activities of fishers, industrialists, shippers, farmers, conservationists, sports enthusiasts and biologists overlap to such an extent. Quite apart from the possible eects of all these activities, the fishes of subtropical and tropical estuaries already face one of the most rigorous of aquatic environments; but despite this, species diversity and productivity are high. Only in tropical estuaries are animals from such a wide range of taxa so closely associated, annelid worms, prawns, crocodiles, birds, ' hippos ', dolphins and of course fishes, all may form part of the overall community, often with functional ecological links. Unfortunately, the diculties of working in these often inhospitable environments, has meant that biologists have favoured projects in more appealing areas, such as coral reefs. While it is still true that most estuarine research is conducted in industrialized developed countries, nearly all of which are in cold or temperate regions, there has been a recent upsurge in tropical estuarine fish research. This is being driven by two imperatives, food security and the conservation and maintenance of biodiversity. Both these problems require knowledge of the ecology of tropical estuarine fishes, particularly their relationships with the environment and the extent to which they are dependent on estuaries or adjacent habitats for survival.     

Ichthyofaunal assemblages in estuaries: A South African case study

This review places the life-history styles of fishes associated with South African estuaries in a global context and presents a classification system incorporating all the major life-history categories for estuary-associated fish species around the world. In addition, it documents the early life histories of the major fish groups in South African estuaries, with particular emphasis on the differing modes of estuarine utilization by marine, estuarine and freshwater taxa.This review details factors influencing the ichthyofaunal community structure in South African estuaries. The availability of fish for recruitment into an estuary depends primarily upon the distributional range of euryhaline marine and estuarine species, with tropical and temperate taxa showing marked abundance trends. Within a particular biogeographic region, however, estuarine type and prevailing salinity regime have a major influence on the detailed ichthyofaunal structure that develops. There is an increasing preponderance of marine fish taxa when moving from a freshwater-dominated towards a seawater-dominated type of system, and a decline in species diversity between subtropical estuaries in the north-east and cool temperate systems in the south-west. Similar declines in fish species diversity between tropical and temperate estuaries in other parts of the world are highlighted.Fish assemblages in estuaries adjust constantly in response to changing seasons, salinities, turbidities, etc. Despite persistent fluctuations in both the biotic and abiotic environment, the basic ichthyofaunal structure appears to have an underlying stability and to be predictable in terms of the response of individual species to specific conditions. This stability seems to be governed by factors such as the dominance of eurytopic taxa within estuarine assemblages and the robust nature of food webs within these systems. The predictability arises from factors such as the seasonality associated with estuarine spawning cycles and juvenile fish recruitment patterns. These patterns, together with a well-documented resilience to a wide range of physico-chemical and biotic perturbations, appear to be an underlying feature of fish assemblages in estuaries around the world.In contrast to marine fish species, estuary-associated taxa have received little conservation attention. Apart from the designation of protected areas, the main direct means of conserving estuary-associated fish stocks include habitat conservation and controls over fishing methods, effort, efficiency and seasonality. Of these, the conservation of fish habitats, the most important, because healthy aquatic environments invariably support healthy fish populations. The use of estuarine sanctuaries for fish conservation is briefly reviewed, as well as the legislation governing the USA National Estuarine Research Reserve System (NERRS) and the Australian Marine and Estuarine Protected Area (MEPA) system. It is concluded that South Africa requires an expansion of the existing Estuarine Protected Area (EPA) network, as well as the upgrading of selected 'estuarine reserves' where fishing is permitted, into 'estuarine sanctuaries' where no exploitation of biological resources is allowed.     

Geomorphology and Late Quaternary development of Middleton and Elizabeth Reefs

Middleton and Elizabeth Reefs are two mid-latitude, annular reefs within the Lord Howe linear chain of volcanic islands and seamounts in the southwestern Pacific Ocean. Drilling, vibrocoring, seismic profiling, and dating indicate that each has a rim of Holocene reef framework, enclosing a lagoon partly filled by prograding sand sheets composed of fragments of coral, coralline algae, foraminifers, and other skeletal debris. The reefs lie close to the latitudinal limits for coral growth and the reef framework is very porous, dominated by branching rather than massive corals. Coralline algae are the principal binding agent in the upper reef framework. Holocene reef growth began on a foundation of Pleistocene reefal limestone encountered at a depth of 8 m in cores on the windward side of Middleton Reef. Holocene corals became established on this foundation around 6,700 radiocarbon yr B.P., implying little if any lag after inundation of the platform by the post-glacial sea-level rise. Windward reef growth tracked sea-level rise (keep-up mode), and a prominent reef crest was established on both reefs by 5,000 yr B.P. Leeward margins appear to have been characterized by catch-up growth. Development of cays is limited, and has been restricted by the paucity of coarse coralline debris or cemented conglomerate on which islands could become established. The morphology and development of Middleton and Elizabeth Reefs has been similar to that of tropical atolls, although the rate of subsidence appears to have been relatively slow reflecting their position on the margin of the foundered continental crust of the Lord Howe Rise.     

Comparative spring distribution of zooplankton in three macrotidal European estuaries

The zooplankton of three european estuaries (Ems, Gironde and Westerschelde) was investigated during spring 1992 by means of samples taken along the salinity gradient. The three estuaries are comparable in terms of total area, flushing time and salinity gradient but differ by their level of eutrophication (highest in the Westerschelde), suspended matter concentration (highest in the Gironde) and potential phytoplankton production (highest in the Ems). Copepods and meroplankton dominated the zooplankton in the three estuaries. The dominant copepod species were Eurytemora affinis and Acartia bifilosa. The distribution of E. affinis along the salinity gradient differed between the estuaries. Peaks of abundance were observed at 0 PSU in the Gironde, 6 PSU in the Ems and 9 PSU in the Westerschelde. The downstream shift of the population in the Westerschelde was likely due to anoxic conditions occurring in the oligohaline zone. In the Gironde the downstream distribution of E. affinis was limited by the very high suspended matter concentration found in the maximum turbidity zone. Whatever the estuary, the parameters of the population of E. affinis and maximum abundance values were similar. However, the influence of the better quality of the available food was suggested in the Ems where individual dry weights and egg production were higher than in the two other estuaries. The influence of a good quality of food in the Ems was confirmed by the development of a large population of Acartia bifilosa (as abundant as E. affinis) and highest values of adult individual weights.The meroplankton (essentially Polychaete and cirripede larvae) was much more developed in the Ems than in the Westerschelde and Gironde. This was likely due to the large extent of mudflats and hard substrates in the Ems favouring adult settlement and hence the number of larvae locally produced.     

Geographical variation in the distributions of macroalgae in estuaries

Assemblages of macroalgal species present in the upper reaches of estuaries were compared using published species lists for the North Atlantic, covering a north-south gradient in Europe from Iceland to northern Spain, and in North America between the St. Lawrence and Virginia. Patterns were sought using cluster analysis and ordination. Three groupings of estuaries were noted: Icelandic, mainland European and American. There was a slight trend of increase in the ratio of red to brown algae with decreasing latitude in Europe, though not as marked as for open coastal algae. This might be associated with the effect of temperature on salinity tolerance. Despite this biogeographic trend, some of the commonest species appear to be widely spread through estuaries, possibly including southern Atlantic and Pacific locations. Confirmation of the trends suggested requires further data from southern European estuaries.     

Comparative ecological analysis of the ostracod faunas from low- and high-polluted southwestern Spanish estuaries: a multivariate approach

The distribution and abundance of both living and total ostracod populations from the estuaries of the southwestern Spanish coast are discussed. A total of 68 species were obtained from 120 bottom samples collected in the channels, channel margins and salt marshes. Four assemblages were recognized in this area: (a) Urocythereis oblonga assemblage, with marine species well-represented in sandy substrates located near the mouths; (b) Loxoconcha rhomboidea assemblage, mainly developed in the quiet tidal channels conected with the main channels of the marine domains; (c) Loxoconcha elliptica assemblage, enclosing the most cosmopolitan and euryhaline species, very abundant in muddy sediments and low to moderate hydrodynamic conditions; and (d) Leptocythere porcellanea assemblage, only observed in the clayey-silty channel margins of the most protected ebb channels.In these transitional environments, very unfavorable conditions for the ostracod development were found in: (a) the erosional channels and channel margins; (b) most of the salt marsh samples with high subaerial exposure; and (c) sectors with high metallic pollution levels and acidic waters.     

A review of nutrient enrichment in the estuaries of Scotland: Implications for the natural heritage

The present nutrient status of 43 rivers entering Scottish estuaries and firths is reviewed using monitoring data for nitrogen concentrations from the river purification authorities and other sources. Nitrogen concentrations range from very low, with no evidence of elevated levels, to a small number of sites which are highly nitrogen-enriched. Only one estuary site, the Ythan in North-east Scotland, has been proposed as a Nitrate Vulnerable Zone, where a number of chemical and ecological criteria outlined in the EC Nitrate Directive have been met, with potential serious consequences for the natural heritage interests of the estuary. Evidence of nitrogen enrichment in the waters of other Scottish estuaries is discussed in relation to the nitrogen levels in the Ythan Estuary. In addition, the results of ecological studies are discussed for two sites of international importance for waders and waterfowl, viz. Montrose Basin and the Eden Estuary, where there is evidence that changes may be occurring in the intertidal ecology of these estuaries as a result of nutrient enrichment. Requirements for future monitoring and research on nutrient enrichment in the Ythan and other estuaries are discussed. A general approach to nitrogen reductions in the Ythan catchment is presented, in addition to alternative options which might have additional benefits for natural heritage interests.     

广东省入库河口N2O分布和通量的时空变化

本研究于2011年7月(丰水期)和12月(枯水期)分别对广东省13个水库的入库河口进行调查,使用吹扫捕集-气相色谱法测定了水中N2O的浓度并用Liss&Merlivat公式估算出入库河口向大气释放N2O的水-气交换通量。结果表明,河口丰水期N2O溶存浓度范围为15.37～175.22 nmol/L,平均值为(73.77±43.58)nmol/L;N2O水-气交换通量范围为0.26～5.40μmol/(m2.d),平均值是(2.53±1.94)μmol/(m2.d)。河口枯水期N2O溶存浓度范围为44.26～366.11 nmol/L,平均值是(126.61±102.74)nmol/L;N2O水-气交换通量范围是0.86～21.88μmol/(m2.d),平均值是(7.50±6.65)μmol/(m2.d)。河口丰水期N2O溶存浓度和水-气交换通量明显低于枯水期。与其他研究相比,本研究区N2O溶存浓度和水-气交换通量偏低。     

Estimation of fluxes to the ocean from mega-tidal estuaries under moderate climates and the problems they present

Evaluation of the quantity of estuarine flux material carried into the ocean, is difficult. High tides produce major oscillations in ocean waters and suspended matter (S.M.) near river mouths and, therefore, direct measurements at the river outlet do not represent real expulsions of S.M. The temperate climate, characterised by important seasonal variations throughout the year, induces significant hydrologic variations and consequently material fluxes into the ocean. Hence, it is difficult to represent measurements with respect to the time, season and especially the year, during which they take place.The Gironde estuary is a good example of this type of estuary, and shows that the evaluation of flux material can only be made from the following equation: expulsion = fluviatile inputs-deposits. Measurements are made from the fluxes of S.M. and from a few metals such as Zn, Cu, and Pb.It took three consecutive years (1976–1979) to carry out our measurements, which helped us to develop a general idea of the chief components of the system and to determine the flux variations.According to the results, important variations occur from year to year, particularly as far as S. M. is concerned. The estuary, thanks to its significant oscillating stock of S.M. (turbidity maximum), behaves as a regulator for the metals. In a surplus year it acts as a sink, and in a deficit year it acts as a source.     

Niche separation in Ambassis (Pisces: Perciformes) species co-existing in estuaries of south-east Africa

Ambassidae in the estuaries of south-east Africa are represented by three species which belong to the same trophic level and are similar in external appearance and feeding ecology. Investigations of osmoregulatory capabilities, salinity tolerances and temperature tolerances showed that, under local estuarine conditions, Ambassis productus is a euryhaline species with the ability to maintain a stable internal osmotic concentration in salinities less than 10%o. A. gymnocephalus is essentially stenohaline and is able to maintain internal stability only in salinities greater than 28%o. A. natalensis is moderately euryhaline and is incapable of maintaining a constant blood osmolality in salinities above 30%o and below about 7%o. Feeding ecology studies show that all three species feed mainly on zooplankton, but when physical conditions in estuaries allow overlap in the distribution of two or more species, certain mechanisms increase separation. A. productus feeds mainly on suspended and benthic organisms during the night and early morning, whereas A. natalensis feeds mainly during the day and early evening on insects at the water surface. A. gymnocephalus excludes insects from its diet and preys almost exclusively on zooplankton in the water column.     

Simulating vertical water mixing in homogeneous estuaries: [2pt] the SADO Estuary case

In this article, a three-dimensional primitive equation model is applied to Sado Estuary, which is a meso-tidal homogeneous estuary. Despite the homogeneity of its density field, the flow presents important vertical structures. This article shows that the common practice of using two-dimensional depth-integrated models in homogeneous estuaries can filter important vertical hydrodynamic features, such as secondary and circulating flows. The physical processes associated with these flows are analysed. The strong curvature of the channels and the strong bathymetry gradients were identified as the principal morphologic features of the estuary responsible for the generation of the flows. The model possesses a generic vertical mesh geometry approach, which is used to test the sensibility of the model to the vertical discretization when simulating these flows.     

Characteristics of the ichthyofaunas of southwestern Australian estuaries,including comparisons with holarctic estuaries and estuaries elsewhere in temperate Australia: A review

Data on the species compositions and the ages, sizes, reproductive biology, habitats and diets of the main species in the ichthyofaunas of seven estuaries in temperate southwestern Australia have been collated. Twenty-two species spawn in these estuaries, of which 21 complete their lifecycles in the estuary. The latter group, which includes several species of atherinids and gobies with short lifecycles, make far greater contributions to the total numbers of fish in the shallows of these estuaries than in those of holarctic estuaries, such as the Severn Estuary in the United Kingdom. This is presumably related in part to far less extreme tidal water movements and the maintenance of relatively high salinities during the dry summers, and thus to more favourable conditions for spawning and larval development. However, since estuaries in southwestern Australia have tended to become closed for periods, there would presumably also have been selection pressures in favour of any members of marine species that were able to spawn in an estuary when that estuary became landlocked. Furthermore, the deep saline waters, under the marked haloclines that form in certain regions during heavy freshwater discharge in winter, act as refugia for certain estuarine species. The contributions of estuarine-spawning species to total fish numbers in the shallows varied markedly from 33 or 34% in two permanently open estuaries to ≥ 95% in an intermittently open estuary, a seasonally closed estuary and a permanently open estuary on the south coast, in which recruitment of the 0 + age class of marine species was poor. The larger estuarine species can live for several years and reach total lengths of ~ 700 mm and some estuarine species move out into deeper waters as they increase in size. Several marine species use southwestern Australian estuaries as nursery areas for protracted periods. However, sudden, marked increases in freshwater discharge in winter and resultant precipitous declines in salinity in the shallows, and in other regions where haloclines are not formed, are frequently accompanied by rapid and pronounced changes in ichthyofaunal composition, partly due to the emigration of certain marine species. In contrast, the ichthyofaunal compositions of macrotidal holarctic estuaries undergo annual, cyclical changes, due largely to the sequential entry of the juveniles of different marine species for short periods. The ichthyofaunal compositions of the narrow entrance channels, wide basins and saline riverine reaches of large, permanently open southwestern Australian estuaries vary, reflecting the marked tendency for some species to be restricted mainly to one or two of these regions. Comparative data indicate that the characteristics determined for ichthyofaunas in southwestern Australian estuaries apply in general to estuaries elsewhere in temperate Australia.     

Australian wetlands: the monitoring challenge

Management of Australian wetlands is confounded by an absence of adequate environmental monitoring data. To a large extent we are still not sure of the exact location and extent of many wetlands. Inventory and monitoring information are intricately linked and at the heart of successful wetland management. Monitoring addresses the extent of change in the environment, yet, for wetlands, it is rarely based on valid scientific principles. Monitoring is also a research tool, but has not, until relatively recently, been accepted as integral to the research effort in environmental management, in part because the questions that may be answered have not been well defined or articulated. The absence of rigorous scientific input to monitoring design has contributed to the devaluing of past long-term monitoring effort. To turn this around and to ensure that wetland managers receive adequate and timely data we need to enhance the scientific rigor applied to monitoring. This requires long-term and strategic support. Management is a complex process and involves many sectors of society. Involving these sectors in monitoring has recently received some attention, but this is not universal and is not necessarily well supported. The challenge is to correct this situation and take advantage of community interest in wetland conservation that may provide mechanisms for more cost effective monitoring. A protocol for monitoring Australian wetlands is presented. This is a series of logical steps to assist in the development and implementation of monitoring programs that are effectively linked to management procedures (such as a formal management plan). The protocol includes a review process that could result in the termination of a program that has achieved its goals, or the abandonment or revamping of one that has failed.     

The intertidal soft-bottom infaunal macrobenthos in three Basque estuaries (Gulf of Biscay): a feeding guild approach

Feeding types of the infaunal macrobenthos of the intertidal soft-bottom communities from the estuaries of La Arena, Plentzia and Gernika, along the Basque coast (Gulf of Biscay) were studied in relation to habitat conditions, characteristics of each estuary and seasonal variation. General feeding structure observed in La Arena estuary was different from the other estuaries: omnivores were dominant, both in number of species and individuals, while surface deposit-feeders were the main feeding group in Plentzia and Gernika both in the finest sediment communities and in the sandy ones. Subsurface deposit-feeders were abundant in Plentzia and Gernika but scarce in La Arena estuary. Feeding types were related to the environmental variables. Sandy sediment communities presented a more balanced proportion among different feeding types and showed higher values of feeding diversity; suspension-feeders and predators were abundant only in that communities. By estuaries, feeding diversity was higher in Gernika and lower in La Arena. Seasonal variation in feeding diversity was observed, explained by changes in density of certain feeding groups; a common pattern of variation was found in Gernika communities, with an increase in June and a decrease in September. Oscillation of this index in Plentzia and La Arena depended on the estuary and the different communities; nevertheless a decrease of feeding diversity in September was commonly observed. Finally, feeding structure observed in our estuaries was compared to those described in coastal areas with organic enriched sediments, in order to assess the degree of perturbation of the environment based on the relative dominance of a certain feeding type in the macrobenthos.     

Vertical distribution and release characteristics of nitrogen fractions in sediments in the estuaries of Dianchi Lake,China

Columnar sediment samples were collected from five representative estuaries of Dianchi Lake, China. And the vertical distribution of each fraction of nitrogen (IEF-N, CF-N, IMOF-N, OSF-N) were tested. The results showed that the TN content in sediments from areas A, B, C, D and E gradually decreased with depth between 0 and 15 cm, then sharply decreased with depth between 15 and 30 cm and stabilized at depth below 30 cm, indicating the exogenous input of N in these areas has not been controlled effectively. The proportion of TN occupied by various N fractions in the sediments ranked as follows: OSF-N > IMOF > CF-N > IEF-N. Correlation analysis results showed both IEF-N and IMOF-N were significantly correlated with the content of TFe₂O₃ + MnO + Al₂O₃ in deeper sediments, while no correlation in superficial sediments. The areas A and B have extremely high release risks for N in superficial sediments. However, the N in the sediments of areas C, D and E were in relative equilibrium with the overlying water, indicating release potential risk was relatively low.     

Ecosystem Metabolism in Piedmont Streams: Reach Geomorphology Modulates the Influence of Riparian Vegetation

We measured the impact of riparian zone vegetation on ecosystem metabolism in paired forested and meadow reaches on 13 streams in southeastern Pennsylvania and Maryland, USA. Metabolism estimates were based on open-system measurements of dissolved oxygen changes, with reaeration determined from propane evasion. Daily gross primary productivity (GPP) in meadow and forested reaches averaged 2.85 and 0.86 g O₂ m⁻² d⁻¹, respectively, at water temperatures of 12°C or greater when the forest canopy was developed and 1.74 and 1.09 g O₂ m⁻² d⁻¹, respectively, at temperatures below 12°C when the canopy was bare. Community respiration (CR₂₄) also was greater in meadow reaches than in forested reaches, averaging 5.58 and 3.57 g O₂ m⁻² d⁻¹, respectively, in the warm season and 4.87 and 2.88 g O₂ m⁻² d⁻¹, respectively, during the cold season. Thus, both meadow and forested reaches were heterotrophic. Forested reaches were always wider and nearly always shallower than companion meadow reaches. When ecosystem function was assessed per unit of stream length, the difference in average GPP between meadow and forested reaches was reduced from three-fold to 1.9-fold in the warm season, and mean GPP was greater in the forested reaches during the cold season. Mean CR₂₄ per meter stream length was greater in forested reaches during both seasons. Even though riparian shading reduced primary productivity per unit area of streambed, the greater stream width of the forested reaches counteracted that reduction in part. Thus, when rates of ecosystem function were expressed per length of stream, differences between reaches were always smaller than when expressed per area, and activity per unit stream length was sometimes greater in forested reaches than in meadow reaches.