Macroinfauna of a Southern New England salt marsh: seasonal dynamics and production

The animal-habitat relationships and seasonal dynamics of the benthic macroinfauna were investigated from November 1986 to October 1988 in the Great Sippe-wissett salt marsh (Massachusetts, USA). Total macrofaunal abundance varied seasonally, displaying a peak in late spring and early summer, then declining sharply during late summer and recovering briefly in fall before collapsing in winter. Three macroinfaunal assemblages were found in the marsh, distributed along gradients of environmental factors. These included a sandy non-organic sediment assemblage, a sandy organic sediment assemblage and a muddy sediment assemblage. The species groups characteristic of unstable sandy non-organic sediments included the polychaetes Leitoscoloplos fragilis, Aricidea jefreyssi, Magelona rosea and Streptosyllis verrilli, the oligochaete Paranais litoralis, and the crustacean Acanthohaustorius millsi. Sandy organic sediments were characterized by the polychaetes Marenzelleria viridis, Capitella capitata, Neanthes succinea, N. arenaceodonta, Polydora ligni and Heteromastus filiformis, the oligochaete Lumbricillus sp., and the mollusc Gemma gemma. In muddy sites, the polychaete Streblospio benedicti and the oligochaetes Paranais litoralis and Monopylephorus evertus were the dominant species. Secondary production of benthic macroinfauna in each of these habitats was estimated. The highest values of biomass and production were recorded in the sandy organic sediments. Secondary production was estimated to be 1850 kJ m^-2 yr^-1 in sandy organic areas, but only 281 kJ m^-2 yr^-1 in sandy non-organic areas and 113 kJ m^-2 yr^-1 in muddy areas. This results in an area-weighted average production of 505 kJ m^-2 yr^-1 for the unvegetated areas of the marsh. The Great Sippewissett salt marsh has an area of 483800 m², the total secondary production of the macroinfauna for the whole unvegetated area of the marsh was estimated as 4651 kg dry wt yr^-1, expressed as somatic growth. This production value seems consistent with production data obtained for other intertidal North Atlantic environments.     

Phytoplankton in a temperate-zone salt marsh: Net production and exchanges with coastal waters

Phytoplankton production and associated variables were measured in Flax Pond, a 52 ha salt marsh on the north shore of Long Island, New York, from July 1972 to October 1973. Measurements made up to five times per day, once per week, yielded a mean annual net primary production, determined by the ¹⁴C technique, of 20.5 mg C/m³/h; daily means were as high as 60.0 mg C/m³/h. However, when productivity was calculated for the entire marsh ecosystem, the shallow water in the salt marsh produced only 11.7 g C/m² of marsh/year. There was a net flux of phytoplankton from the coastal waters into the marsh; during the summer up to 0.2 g chlorophy 11/m² of marsh was carried in with the tides daily and remained in the marsh. Analysis of the productivity data, as well as variables associated with productivity (pH, standing crop, nutrients, extinction coefficient), indicated that the aquatic portion of the marsh behaved more as a net consumer rather than a net producer of phytoplankton.Research carried out at Brookhaven National Laboratory, supported by the U.S. Atomic Energy Commission and the National Science Foundation under Grant No. AG-375.     

Heterotrophic nutrition of the marine pennate diatom Nitzschia angularis var. affinis

The nutritional pattern for heterotrophic growth of Nitzschia angularis var. affinis (Grun.) Perag. is more complex than for other diatom species studied previously. This species grew slowly in the dark in the presence of single amino acids, either glutamate or alanine; other amino acids when supplied singly were not used as substrates. Carbon from glutamate was converted to cell carbon with an efficiency of 43%. Glutamine was inhibitory both in the light and in the dark, and aspartate inhibited heterotrophic growth on glutamate. Glucose and tryptone supplied singly did not support heterotrophic growth, but when combined, together they allowed for rapid growth of N. angularis (generation time of 16 h). Glucose in combination with glutamate, alanine, aspartate, or asparagine (but not with any other amino acids) also supported growth in the dark, at a rate considerably more rapid than with glutamate alone. In the presence of excess glucose and limiting concentrations of glutamate, approximately 50% of the cell carbon for heterotrophic growth came from glucose, while in combination with tryptone about 25% of the cell carbon came from glucose. Amino acids were taken up by cells grown either photoautrophically or in the dark in the presence or absence of organic substrates; uptake rates were some-what higher for dark-grown than for light-grown cells. Glucose was taken up only by dark-grown cells; induction of a glucose uptake system in the dark required the presence of glutamate but not of glucose. The rates of uptake of glutamate and glucose by cells incubated in the dark with glutamate were sufficiently high to account for the observed rates of growth on these substrates in the dark. The uptake systems of N. angularis have relatively high affinities for glucose (K _s =0.03 mM) and glutamate (K _s =0.02 mM).Contribution No. 890 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Utilization of glutamate and glucose for heterotrophic growth by the marine pennate diatom Nitzschia laevis

Nitzschia laevis Hustedt grew in the dark in the presence of either glutamate or glucose as substrate. Complex mixtures of yeast extract or tryptone plus lactate also supported good heterotrophic growth, while tryptone alone only supported very slow growth in the dark. The observed growth rates of N. laevis in the dark at different concentrations of glutamate or glucose could be accounted for by the measured uptake rates of these compounds. The affinity of the uptake systems for glutamate and glucose (K _s =0.03 mM for each) was quite high, and similar for dark- and light-grown cells. The lack of a lag-phase when cells were transferred from photoautotrophic to heterotrophic growth conditions can be explained by the presence of uptake systems for glutamate and glucose in ligh--grown cells, as well as in dark-grown cells. However, the uptake capacity was generally higher in the latter than the former. N. laevis also took up alanine and lactate according to Michaelis-Menten kinetics, with a K _s for alanine of 0.02 mM and for lactate of 0.4 mM. Malate and glycerol were not taken up to a significant extent by the cells. Cells grown in continous light had a doubling time of 18 h. The shortest doubling time observed in the dark on glutamate was 48 h and on glucose 24 h. Glutamate was used for heterotrophic growth with an efficiency of 43% and glucose with an efficiency of 48%.Contribution No., 945 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Nutrient fluxes within a small north temperate salt marsh

The water exchange between a small (4.1 hectare) salt marsh adjoining the Great Bay Estuary system of New Hampshire, USA was sampled during 16 tidal cycles between July, 1976 and November, 1977. Tidal amplitude, temperature, salinity, nutrient concentrations (ammonia-N, nitrate-N, nitrite-N, orthophosphate-P, total-P, silicates) and suspended particulates were measured. Conspicuous tidal hydrographic patterns were observed. Mean concentrations of nitrate-N and silicates varied with season. The tidal information, combined with volume determinations, was extrapolated to determine the net flux of hydrographic parameters on monthly and yearly bases. Ammonia-N showed a pronounced seasonality of net exchange by regression analyses. Ammonia-N and suspended particulates had a statistically significant import into the marsh. However, the net fluxes of the other materials were not statistically different from O.Published with the approval of the Director of the New Hampshire Agricultural Experiment Station as Scientific Contribution No. 930Scientific Contribution No. 69 of the Jackson Estuarine Laboratory     

滨海盐沼及其植物群落的分布与多样性

滨海盐沼是广泛存在于世界中、高纬度地区的一种湿地生态系统,具有抵御风暴潮灾害、净化污染物和为珍稀濒危生物提供适宜生境等重要的生态和经济价值。滨海盐沼因随高程变化而急剧变化的环境梯度和植物带状分布现象而为生态学者阐释自然界物种的分布机制提供了理想系统。主要概述了滨海盐沼的定义、特点、类型、全球分布以及潮汐作用、土壤盐度等环境因子特征;阐述了不同尺度下滨海盐沼的植物群落分布和多样性特征。在滨海盐沼植物群落的分布特征上,重点阐述了中尺度下的植物带状分布,即植物群落往往在白海向陆渐高的不同高程梯度上表现出显著的分带分布,不同植物各自占据该梯度上的一定区域。通常认为,带状分布是植物竞争和物理性胁迫共同调控的结果,但其在不同地理区域的普适性仍存争议。滨海植物群落多样性往往较低,在中、小尺度上盐沼植物多样性受控于盐度、潮汐等物理性胁迫、植物间相互作用等因子的作用;在大尺度上盐沼植物多样性可能随纬度增大而增加。系统深入地认识滨海盐沼植物群落生态格局和过程,将为气候变化、生物入侵等人类影响下的滨海盐沼生态系统的管理和恢复提供有益经验。     

Bacteriastrum solitarium Mangin,a stage in the life history of the centric diatom Bacteriastrum hyalinum

The initial cell of Bacteriastrum hyalinum Lauder, which is the result of lateral auxospore formation, is identical with the solitary species B. solitarium Mangin. This evidence was obtained in laboratory cultures on B. hyalinum.     

Intertidal distribution,population dynamics and production of the amphipod Uhlorchestia spartinophila in a Georgia,USA, salt marsh

The talitrid amphipod Uhlorchestia spartinophila Bousfield and Heard occurs in close association with the smooth cordgrass Spartina alterniflora Loisel in salt marshes along the U.S. Atlantic coast. In order to assess its potential as a prey resource for secondary consumers, we followed the population and production dynamics of this amphipod from 3 November 1990 to 2 October 1991 in a salt marsh on Sapelo Island, Georgia. Semimonthly samples were taken along an intertidal transect, which extended from a vegetated creekbank levee landward 140 m to a site near the upland edge of the marsh. Amphipod densities ranged from 9 to 826 indm^-2 and were greatest in the levee and high marsh habitats at opposite ends of the transect. The highest densities occurred in March to May and the lowest in November and December. Specific growth rates, which ranged from 0.001 to 0.024 mm mm^-1 d^-1 length and 0.003 to 0.068 mg mg^-1 d^-1 AFDW (ash-free dry weight), were least in winter and decreased with increasing amphipod size. Although some reproduction occurred year-round, most of the population's reproductive output was from January to May, when adults had the greatest size-specific mass. Sex ratio usually favored females, which were larger than males. Minimum adult female size was 5.4 mm total length (TL). Broods included from 1 to 28 eggs or young and increased with increasing body size, averaging 7.4 young for an average-size female of 7.6 mm TL. Annual production, which ranged from 0.769 to 1.444 g AFDW m^-2, was least in the low marsh and greatest in the levee habitat. Production:biomass ratios were 15.4 to 17.3 in different habitats. Such high turnover rates, together with the population's broad intertidal distribution, suggest a greater contribution to trophic dynamics than is implied by the relatively low standing stock biomass of U. spartinophila in this marsh system.     

Consequences of omnivory for trophic interactions on a salt marsh shrub

Although omnivory is common in nature, its impact on trophic interactions is variable. Predicting the food web consequences of omnivory is complicated because omnivores can simultaneously produce conflicting direct and indirect effects on the same species or trophic level. We conducted field and laboratory experiments testing the top-down impacts of an omnivorous salt marsh crab, Armases cinereum, on the shrub Iva frutescens and its herbivorous and predatory arthropod fauna. Armases is a "true omnivore," consuming both Iva and arthropods living on Iva. We hypothesized that Armases would benefit Iva through a top-down trophic cascade, and that this benefit would be stronger than the direct negative effect of Armases on Iva. A field experiment on Sapelo Island, Georgia (USA), supported this hypothesis. Although Armases suppressed predators (spiders), it also suppressed herbivores (aphids), and benefited Iva, increasing leaf number, and reducing the proportion of dead shoots. A one-month laboratory experiment, focusing on the most common species in the food web, also supported this hypothesis. Armases strongly suppressed aphids and consumed fewer Iva leaves if aphids were available as an alternate diet. Armases gained more body mass if they could feed on aphids as well as on Iva. Although Armases had a negative effect on Iva when aphids were not present, Armases benefited Iva if aphids were present, because Armases controlled aphid populations, releasing Iva from herbivory. Although Armases is an omnivore, it produced strong top-down forces and a trophic cascade because it fed preferentially on herbivores rather than plants when both were available. At the same time, the ability of Armases to subsist on a plant diet allows it to persist in the food web when animal food is not available. Because omnivores feed on multiple trophic levels, their effects on food webs may differ from those predicted by standard trophic models that assume that each species feeds only on a single trophic level. To better understand the complexity of real food webs, the variable feeding habits and feeding preferences of different omnivorous species must be taken into consideration.     

Diel flux of dissolved carbohydrate in a salt marsh and a simulated estuarine ecosystem

The concentrations of total dissolved carbohydrate (TCHO), monosaccharide (MCHO) and polysaccharide (PCHO) were followed over a total of ten diel cycles in a salt marsh and a 13 m³ seawater tank simulating an estuarine ecosystem. Their patterns are compared to those for total dissolved organic carbon (DOC), CO₂, pH, O₂, chlorophyll a, phaeopigments and solar radiation. During 5 of the 6 marsh studies, PCHO underwent periods of sustained accumulation starting in the late morning or early afternoon and continuing into the early evening. These periods possibly represent release of recently synthesized PCHO from phototrophs. similar patterns were not found in the tank although direct associations between TCHO and phaeopigment dynamics suggest that zooplankton excretion was an important source of dissolved carbohydrate. The numbers of planktonic bacteria determined in one tank study increased rapidly during a late morning PCHO pulse and varied inversely with PCHO throughout the afternoon and evening, indicating that they were able to respond rapidly and control natural substrate concentrations on a time scale of a few hours. MCHO fluctuated to a much lesser extent than PCHO at both locations with levels possibly maintained near the bacterial uptake threshold or in a closely regulated steady state. TCHO concentration changes over 2-to 3-h sampling intervals suggest very rapid net system release and uptake with summer rates frequently exceeding 30 g C l^-1h^-1 in the marsh and 20 g C l^-1 h^-1 in the tank.     

Secondary succession on a high salt marsh at different grazing intensities

Succession occurs on a large part of German salt marshes following abandonment or reduction of grazing. Its speed and effect on the biodiversity of salt marshes has been discussed in the literature. Permanent plot studies show site-dependent differences in successional outcome. If grazing is to be continued, there is uncertainty about the stocking rates that are optimal for the conservation of plant diversity. We present an eight year permanent plot study with different grazing treatments on a high salt marsh at the marsh island of Hallig Langeness, Germany, which is enclosed by a summer dike. The study site is owned by WWF. Successional outcomes on the permanent plots depend on grazing intensity and depth of the groundwater table which is correlated with soil salinity. Intermediate stocking rates support a mixture of halophytes and glycophytes, the relative proportion of which depends on the depth of the groundwater table. Cattle grazing at stocking rates of 0.6 livestock units per ha were found to be optimal for plant diversity conservation. At sites with deeper groundwater tables and no grazing, dominantElymus spp. stands develop and diversity is strongly reduced. If the groundwater table is high, succession following grazing abandonment is retarded and small halophytes prevail.     

Latitudinal variation in top-down and bottom-up control of a salt marsh food web

The shrub Iva frutescens, which occupies the terrestrial border of U.S. Atlantic Coast salt marshes, supports a food web that varies strongly across latitude. We tested whether latitudinal variation in plant quality (higher at high latitudes), consumption by omnivores (a crab, present only at low latitudes), consumption by mesopredators (ladybugs, present at all latitudes), or the life history stage of an herbivorous beetle could explain continental-scale field patterns of herbivore density. In a mesocosm experiment, crabs exerted strong top-down control on herbivorous beetles, ladybugs exerted strong top-down control on aphids, and both predators benefited plants through trophic cascades. Latitude of plant origin had no effect on consumers. Herbivorous beetle density was greater if mesocosms were stocked with beetle adults rather than larvae, and aphid densities were reduced in the "adult beetle" treatment. Treatment combinations representing high and low latitudes produced patterns of herbivore density similar to those in the field. We conclude that latitudinal variation in plant quality is less important than latitudinal variation in top consumers and competition in mediating food web structure. Climate may also play a strong role in structuring high-latitude salt marshes by limiting the number of herbivore generations per growing season and causing high overwintering mortality.     

Twenty years of salt marsh succession on a Dutch coastal barrier island

After a formerly grazed salt marsh was released from cattle grazing, changes in plant species composition were monitored for 20 yr, using vegetation maps and permanent plots. Three areas, differing in age and nutrient status were compared. The number of plant species and plant communities decreased.Elymus athericus (Elytrigia pungens) became dominant in most plant communities after 5–20 yr on the oldest and most productive salt marsh. In younger areas it took more time forE. athericus to become dominant. At least 7 cm of clay seemed to be a prerequisite for this plant species to increase in dominance. The results from monitoring over decades are discussed in view of the knowledge on succession over centuries as derived from a chronosequence.     

Nitrogen exchange between a southeastern USA salt marsh ecosystem and the coastal ocean

The salt marsh ecosystem at North Inlet, South Carolina, USA consistently exported dissolved inorganic nitrogen via tidal exchange with the coastal Atlantic Ocean. Concentrations centrations of NH ₄ ⁺ and NO ₃ ^- +NO ₂ ^- displayed distinct tidal patterns with rising values during ebb flow. These patterns suggest the importance of biogeochemical processes in the flux of material from the salt marsh. NH ₄ ⁺ export peaked during the summer (15 to 20 mg m^-2 tide^-1) during a net balance of tidal water exchange. Remineralization of NH ₄ ⁺ within the salt marsh system appears to be contributing to the estimated annual net export of bout 4.7 g NH ₄ ⁺ -N m^-2 yr^-1. NO ₃ ^- +NO ₂ ^- exports were higher in the fall and winter of 1979 (2 to 4 mg N m^-2 tide^-1). The winter export coincided with a considerable net export of water with no distinctive concentration patterns, suggesting a simple advective export. However, the fall peak of NO ₃ ^- +NO ₂ ^- export occurred during a period of net water balance in tidal exchange and an insignificant freshwater input from the western, forested boundary. During the summer and fall, tidal concentration patterns were particularly apparent, suggesting that nitrification within the salt marsh system was contributing to the estimated annual net export of ca 0.6 g NO ₃ ^- +NO ₂ ^- -N m^-2 yr^-1.Contribution No. 637 from the Belle W. Baruch Institute of Marine Biology and Coastal Research     

Behaviour of horses and cattle at two stocking densities in a coastal salt marsh

Livestock grazing has been practiced in salt marshes in the Wadden Sea area since 600 B.C. Currently livestock grazing is also applied for conservation management. However, effects of such grazing management on salt marshes are likely to vary depending on the species of livestock and stocking density due to differences in the behaviour of the animals. Yet, little is known about the behaviour of different livestock species and stocking densities grazing in salt marshes. We studied the grazing behaviour of horses and cattle by focal observation in an experiment with four different grazing treatments on a coastal salt marsh. In all treatments we recorded diet choice, movement and grazing activity, and spatial distribution. Livestock species shared an overlap in diet choice. Yet, horses more often foraged on the short grass Puccinellia maritima, while the cattle diet contained a higher amount of Aster tripolium. Horses travelled longer distances per day and spent more time grazing than cattle. Spatial distribution of cattle was significantly clustered, while horses showed a random distribution utilizing the whole area. Animal behaviour differs between livestock species and stocking densities with respect to diet choice, activity and spatial distribution.     

Pigment composition and size distribution of phytoplankton in a confined Mediterranean salt marsh ecosystem

Pigment composition and size distribution of phytoplankton were analysed in a group of Mediterranean salt marshes, where hydrology is dominated by sudden inputs during sea storms, followed by long periods of confinement. These marshes are characterized by a low inorganic–organic nutrient ratio, and inorganic nitrogen is especially scarce due to denitrification. Nutrients were the main factor affecting phytoplankton biomass, while zooplankton grazing did not control either phytoplankton community composition, or their size distribution. The relative abundance of the different phytoplankton groups was analysed by correspondence analysis using the pigment composition measured by high-performance liquid chromatography (HPLC) and analysed with the CHEMTAX programme. In this analysis, phytoplankton pigment composition was correlated with two nutrient gradients. The first gradient was the ratio of nitrate–total nitrogen (TN), since the different phytoplankton groups were distributed according to their eco–physiological differences in nitrogen uptake. The second gradient was correlated with total nutrient loading. Biomass size distributions frequently showed a lack of intermediate sized nanophytoplankton (2.5–4 μm in diameter), and the importance of this lack of intermediate sizes correlated with dinoflagellate biomass. These results suggested that in confined environments, where nutrients are mainly in an organic form, dinoflagellates take advantage of their mixotrophy, by competing and grazing on smaller phytoplankters simultaneously.     

Tidal and diurnal influence on food consumption of a salt marsh killifish Fundulus heteroclitus

Feeding patterns during four 24-h periods, sampled at 3-h intervals, were investigated for the mummichog Fundulus heteroclitus, in a Delaware, USA tidal marsh. Two factors potentially influencing feeding patterns, time of day and tide height, were examined. On 2 of the sampling periods a low tide occurred in the morning, while on the other 2 sampling periods a high tide occurred in the morning. Results are reported as g-dry wt. of food per g-dry wt. of fish. F. heteroclitus is primarily a daytime feeder that most actively feeds at high tide, regardless of whether or not the high tide inundates marsh surface areas. When tide height was sufficient to inundate the marsh surface, fish invaded these areas and consumed prey characteristic of the marsh surface. F. heteroclitus is an important link in energy transfers between the marsh surface and subtidal systems, enhancing its own energy supplies by consuming marsh surface prey whenever available.     

Standing crop of foraminifera in sublittoral epiphytic communities of a Long Island salt marsh

During the summers of 1966 and 1967, 12 field trips were made to stations in North Sea Harbor, Southampton, Long Island (USA). From representative locations, 228 small samples of larger algae and their epiphytes (0.2 g dry weight) were taken aseptically. Enteromorpha intestinalis, the most widely distributed aquatic plant, was the most frequently collected. The large standing crop of Zostera marina and Zanichellia palustris was also sampled. Foraminifera were most abundant in epiphytic communities of Enteromorpha in early summer and later spread to Zostera, Zanichellia, Ulva, Polysiphonia, and Ceramium. Foraminifera were rarely found in epiphytic communities of Fucus or Codium. By summers' end Enteromorpha rarely had a standing crop of foraminifera. One of the most abundant foraminiferan species, Protelphidium tisburyensis, was found most frequently on Enteromorpha; Quinqueloculina spp occurring on Enteromorpha, less frequently. Ammonia beccarri and Elphidium spp were abundant in the environment, and showed little substrate preference. Patches of decaying Enteromorpha had the greatest standing crop of foraminifera and low species diversity index (0.581). Young green patches had a much higher species diversity index (0.94). Indices for Zostera, Zanichellia, Polysiphonia, Fucus, Ulva and Codium were, respectively, 0.82, 0.99, 0.86, 0.70, 0.77, and 0.196. No correlation was found between epiphytic community weight and total number of foraminifera recovered. The standing crop of epiphytes/g substrate dry weight was lower at some field stations; possibly explained by stronger current. Of the total samples, 50.4% were positive, with an average of 18 foraminifera/sample, or 40 foraminifera/g substrate plant and epiphytes. Twenty six samples were classified as bloom with 50 or more forams (57 to 425)/sample. Species diversity indices for these blooms ranged from 0.38 to 1.12. Ammonia beccarii was the dominant form in 18 blooms, Allogromia laticollaris and Protelphidium tisburyensis in 3; Elphidium incertum, Quinqueloculina seminulum and Trochammina inflata in 1 each.Supported by US AEC Contracts AT (30-1) 3396 and AT (30-1) 3995. Ref. numbers NYO 3396-17 and NYO 3995-2.Much of this study was carried out in the Living Foraminifera Laboratory, Department of Micropaleontology of the American Museum of Natural History.     

Acetylene reduction (nitrogen fixation) in a Delaware,USA salt marsh

Acetylene reduction (nitrogen fixation) was measured in several vegetational areas in a Delaware, USA salt marsh. Samples were collected for 1 yr and the results showed a seasonally variable pattern of acetylene reduction at all stations. Peak rates were generally recorded during the later summer and early fall (September–October). The seasonality was influenced mainly, although not exclusively, by the soil temperature. In addition, samples collected in short Spartina alterniflora stands exhibited rates which were up to 20-fold higher than those found in samples from tall S. alterniflora stands. Over 50% of the total yearly ethylene production occurred from mid-August until the beginning of December at the tall and short S. alterniflora stations. Maximum activity occurred at 5 cm depth for all stations. Surface activity accounted for only 3–4% of the total measured in the top 20 cm. Addition of glucose or mannitol resulted in considerable increases in activity, thus suggesting that heterotrophic acetylene reduction is carbon and/or energy limited. The results obtained in this study indicate that the measured rates are only potential rates and that considerable caution must be used in extrapolating from acetylene reduction rates to nitrogen fixation rates in situ.     

Sellafield waste radionuclides in Irish sea intertidal and salt marsh sediments

Low level liquid radioactive waste discharges from the Sellafield nuclear fuel reprocessing plant in north west England had generated environmental inventories of about 3 × 10¹⁶ Bq of¹³⁷Cs, 6.8 × 10¹⁴ Bq of^239,240Pu and 8.9 × 10¹⁴ Bq of²⁴¹Am by 1990. Most of the^239,240Pu and²⁴¹Am and about 10% of the¹³⁷Cs has been retained in a deposit of fine marine sediment close to the discharge point. The quantities of radionuclides discharged annually from Sellafield decreased by two orders of magnitude from the mid-1970s to 1990 but estimated critical group internal and external exposure decreased by less than one order of magnitude over this period. This indicates that during the period of reduced discharges, radionuclides already in the environment from previous releases continued to contribute to the critical group exposure and highlights the need to understand processes controlling the environmental distribution of the radionuclides. Redistribution of the contaminated marine sediment is potentially of major significance in this context, in particular if it results in transport of radionuclides to intertidal areas, where contact with the human population is relatively likely. A review is presented of published work relating to Sellafield waste radionuclides in Irish Sea sediments. Data on temporal and spatial trends in radionuclide concentrations and activity ratios are collated from a number of sources to show that the dominant mechanism of radionuclide supply to intertidal areas is by redistribution of the contaminated marine sediment. The implications of this mechanism of supply for trends in critical group radiation exposure are considered.