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The high Arctic is considered a pristine environment compared with many other regions in the northern hemisphere. It is becoming increasingly vulnerable to invasion by invasive alien species (IAS), however, as climate change leads to rapid loss of sea ice, changes in ocean temperature and salinity, and enhanced human activities. These changes are likely to increase the incidence of arrival and the potential for establishment of IAS in the region. To predict the impact of IAS, a group of experts in taxonomy, invasion biology and Arctic ecology carried out a horizon scanning exercise using the Svalbard archipelago as a case study, to identify the species that present the highest risk to biodiversity, human health and the economy within the next 10 years. A total of 114 species, currently absent from Svalbard, recorded once and/or identified only from environmental DNA samples, were initially identified as relevant for review. Seven species were found to present a high invasion risk and to potentially cause a significant negative impact on biodiversity and five species had the potential to have an economic impact on Svalbard. Decapod crabs, ascidians and barnacles dominated the list of highest risk marine IAS. Potential pathways of invasion were also researched, the most common were found associated with vessel traffic. We recommend (i) use of this approach as a key tool within the application of biosecurity measures in the wider high Arctic, (ii) the addition of this tool to early warning systems for strengthening existing surveillance measures; and (iii) that this approach is used to identify high-risk terrestrial and freshwater IAS to understand the overall threat facing the high Arctic. Without the application of biosecurity measures, including horizon scanning, there is a greater risk that marine IAS invasions will increase, leading to unforeseen changes in the environment and economy of the high Arctic.
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Biodiversidade , Espécies Introduzidas , Humanos , Svalbard , Ecologia , Regiões Árticas , EcossistemaRESUMO
Polar cod (Boreogadus saida) is an important trophic link within Arctic marine food webs and is likely to experience diet shifts in response to climate change. One important tool for assessing organism diet is bulk stable isotope analysis. However, key parameters necessary for interpreting the temporal context of stable isotope values are lacking, especially for Arctic species. This study provides the first experimental determination of isotopic turnover (as half-life) and trophic discrimination factors (TDFs) of both δ13 C and δ15 N in adult polar cod muscle. Using a diet enriched in both 13 C and 15 N, we measured isotopic turnover times of 61 and 49 days for δ13 C and δ15 N, respectively, with metabolism accounting for >94% of the total turnover. These half-life estimates are valid for adult polar cod (>3 years) experiencing little somatic growth. We measured TDFs in our control of 2.6 and 3.9 for δ13 C and δ15 N, respectively, and we conclude that applying the commonly used TDF of ~1 for δ13 C for adult polar cod may lead to misrepresentation of dietary carbon source, while the use of 3.8 for δ15 N is appropriate. Based on these results, we recommend that studies investigating seasonal shifts in the diet of adult polar cod sample at temporal intervals of at least 60 days to account for isotopic turnover in polar cod muscle. Although isotopic equilibrium was reached by the fish in this study, it was at substantially lower isotope values than the diet. Additionally, the use of highly enriched algae in the experimental feed caused very high variability in diet isotope values which precluded accurate calculation of TDFs from the enriched fish. As a result of the challenges faced in this study, we discourage the use of highly enriched diets for similar experiments and provide recommendations to guide the design of future isotopic turnover experiments.
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Gadiformes , Músculos , Animais , Isótopos de Carbono/análise , Isótopos de Nitrogênio/análise , Músculos/química , Carbono , Estado Nutricional , Dieta , Peixes/metabolismo , Gadiformes/metabolismoRESUMO
The iconic picture of Arctic marine ecosystems shows an intense pulse of biological productivity around the spring bloom that is sustained while fresh organic matter (OM) is available, after which ecosystem activity declines to basal levels in autumn and winter. We investigated seasonality in benthic biogeochemical cycling at three stations in a high Arctic fjord that has recently lost much of its seasonal ice-cover. Unlike observations from other Arctic locations, we find little seasonality in sediment community respiration and bioturbation rates, although different sediment reworking modes varied through the year. Nutrient fluxes did vary, suggesting that, although OM was processed at similar rates, seasonality in its quality led to spring/summer peaks in inorganic nitrogen and silicate fluxes. These patterns correspond to published information on seasonality in vertical flux at the stations. Largely ice-free Kongsfjorden has a considerable detrital pool in soft sediments which sustain benthic communities over the year. Sources of this include macroalgae and terrestrial runoff. Climate change leading to less ice cover, higher light availability and expanded benthic habitat may lead to more detrital carbon in the system, dampening the quantitative importance of seasonal pulses of phytodetritus to seafloor communities in some areas of the Arctic. This article is part of the theme issue 'The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.
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Mudança Climática , Ecossistema , Regiões Árticas , Difusão , Sedimentos Geológicos/química , Camada de Gelo/química , Noruega , Oceanos e Mares , Compostos Orgânicos/análise , Oxigênio/análise , Estações do Ano , Água do Mar/química , Análise Espaço-TemporalRESUMO
Climate warming can trigger abrupt ecosystem changes in the Arctic. Despite the considerable interest in characterizing and understanding the ecological impact of rapid climate warming in the Arctic, few long time series exist that allow addressing these research goals. During a 30-y period (1980-2010) of gradually increasing seawater temperature and decreasing sea ice cover in Svalbard, we document rapid and extensive structural changes in the rocky-bottom communities of two Arctic fjords. The most striking component of the benthic reorganization was an abrupt fivefold increase in macroalgal cover in 1995 in Kongsfjord and an eightfold increase in 2000 in Smeerenburgfjord. Simultaneous changes in the abundance of benthic invertebrates suggest that the macroalgae played a key structuring role in these communities. The abrupt, substantial, and persistent nature of the changes observed is indicative of a climate-driven ecological regime shift. The ecological processes thought to drive the observed regime shifts are likely to promote the borealization of these Arctic marine communities in the coming years.
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Mudança Climática , Clima , Ecossistema , Biologia Marinha/métodos , Phaeophyceae/crescimento & desenvolvimento , Rodófitas/crescimento & desenvolvimento , Regiões Árticas , Monitoramento Ambiental/métodos , Camada de Gelo/microbiologia , Modelos Lineares , Água do Mar/microbiologia , TemperaturaRESUMO
On Arctic shelves, benthic food-webs are tightly linked to overlying primary production. In the seasonal ice zone, sympagic (ice-associated) primary production can be a major source of carbon for the benthos on productive inflow shelves. However, the role of sympagic organic matter is less well-understood in food webs of heavily ice-covered, less- productive outflow shelves, such as the northeast Greenland shelf. Highly branched isoprenoid biomarkers (HBIs) were used to track the relative distribution of sympagic and pelagic organic matter in the water column, sediments, and benthic fauna of the northeast Greenland shelf and fjords. Low pelagic HBI presence throughout the study area indicated a generally low production by pelagic diatoms (at the time of sampling). This was reflected in the benthos, as ~90% of their assimilated carbon was estimated to come from sympagic sources, indicating a benthic food-web highly reliant on sympagic production. This reliance was higher in coastal areas than on the open shelf, where the potentially higher pelagic productivity and shallower water on banks likely increased contributions of pelagic organic matter. As declining ice cover and reduced production of fast-sinking ice algae projected for Arctic shelves will likely result in weaker coupling between ice algae and the benthos, with possible consequences for future benthic-community structure and function.
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Cadeia Alimentar , Regiões Árticas , Camada de Gelo , Diatomáceas/metabolismo , Groenlândia , Sedimentos Geológicos , Animais , Biomarcadores/metabolismo , Carbono/metabolismoRESUMO
EcoQS assessment of the marine intertidal zone based on its fauna is challenging because the assemblages have a low diversity and consist of stress tolerant species. The new approach we propose is to pool foraminiferal diversity (effective number of species exp(H'bc)) across the whole intertidal zone including the salt marsh and tidal flat. In seven fjordheads studied in northern Fennoscandia, polycyclic aromatic hydrocarbon (PAH) concentrations indicated low levels of pollution (EcoQSPAH Excellent to Moderate). Jadammina or Balticammina dominated the salt marsh, Elphidium albiumbilicatum, Elphidium williamsoni, Elphidium clavatum, and Buccella frigida occurred in the tidal flat. Ovammina opaca thrived in both belts. While foraminiferal test abnormalities are often proposed to measure pollution impacts, we did not detect any correlation with PAHs. EcoQS based on foraminiferal diversity (EcoQSforam Excellent to Good) matched EcoQS based on PAHs suggesting that pooled foraminiferal diversity reliably measures intertidal EcoQS.
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Biodiversidade , Monitoramento Ambiental , Foraminíferos , Hidrocarbonetos Policíclicos Aromáticos , Foraminíferos/fisiologia , Hidrocarbonetos Policíclicos Aromáticos/análise , Monitoramento Ambiental/métodos , Regiões Árticas , Poluentes Químicos da Água/análiseRESUMO
Predation shapes marine benthic communities and affects prey species population dynamics in tropic and temperate coastal systems. However, information on its magnitude in systematically understudied Arctic coastal habitats is scarce. To test predation effects on the diversity and structure of Arctic benthic communities, we conducted caging experiments in which consumers were excluded from plots at two intertidal sedimentary sites in Svalbard (Longyearbyen and Thiisbukta) for 2.5 months. Unmanipulated areas served as controls and partial (open) cages were used to estimate potential cage effects. At the end of the experiment, we took one sediment core from each plot and quantified total biomass and the number of each encountered taxon. At both sites, the experimental exclusion of predators slightly changed the species composition of communities and had negligible effects on biomass, total abundance, species richness, evenness, and Shannon Index. In addition, we found evidence for cage effects, and spatial variability in the intensity of the predation effects was identified. Our study suggests that predators have limited effects on the structure of the studied intertidal macrobenthic Arctic communities, which is different from coastal soft-bottom ecosystems at lower latitudes.
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Pelagic-benthic coupling describes the connection between surface-water production and seafloor habitats via energy, nutrient and mass exchange. Massive ice loss and warming in the poorly studied Arctic Chukchi Borderland are hypothesized to affect this coupling. The strength of pelagic-benthic coupling was compared between 2 years varying in climate settings, 2005 and 2016, based on δ13C and δ15N stable isotopes of food-web end-members and pelagic and deep-sea benthic consumers. Considerably higher isotopic niche overlap and generally shorter isotopic distance were found between pelagic and benthic food web components in 2005 than in 2016, suggesting weaker coupling in the latter, low-ice year. δ15N values indicated more refractory food consumed by benthos in 2016 and fresher food reaching the seafloor in 2005. Higher δ13C values of zooplankton indirectly suggested a higher contribution of ice algae in 2005 than 2016. The difference in pelagic-benthic coupling between these years is consistent with higher energy retention within the pelagic system, perhaps due to strong stratification in the Amerasian Basin in the recent decade. Weaker coupling to the benthos can be expected to continue with ice loss in the study area, perhaps reducing benthic biomass and remineralization capacity; monitoring of the area is needed to confirm this prediction.
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Ecossistema , Camada de Gelo , Cadeia Alimentar , Regiões Árticas , BiomassaRESUMO
Amid the alarming atmospheric and oceanic warming rates taking place in the Arctic, western fjords around the Svalbard archipelago are experiencing an increased frequency of warm water intrusions in recent decades, causing ecological shifts in their ecosystems. However, hardly anything is known about their potential impacts on the until recently considered stable and colder northern fjords. We analyzed macrobenthic fauna from four locations in Rijpfjorden (a high-Arctic fjord in the north of Svalbard) along its axis, sampled intermittently in the years 2003, 2007, 2010, 2013 and 2017. After a strong seafloor warm water temperature anomaly (SfWWTA) in 2006, the abundance of individuals and species richness dropped significantly across the entire fjord in 2007, together with diversity declines at the outer parts (reflected in Shannon index drops) and increases in beta diversity between inner and outer parts of the fjord. After a period of three years with stable water temperatures and higher sea-ice cover, communities recovered through recolonization processes by 2010, leading to homogenization in community composition across the fjord and less beta diversity. For the last two periods (2010-2013 and 2013-2017), beta diversity between the inner and outer parts gradually increased again, and both the inner and outer sites started to re-assemble in different directions. A few taxa began to dominate the fjord from 2010 onwards at the outer parts, translating into evenness and diversity drops. The inner basin, however, although experiencing strong shifts in abundances, was partially protected by a fjordic sill from impacts of these temperature anomalies and remained comparatively more stable regarding community diversity after the disturbance event. Our results indicate that although shifts in abundances were behind important spatio-temporal community fluctuations, beta diversity variations were also driven by the occurrence-based macrofauna data, suggesting an important role of rare taxa. This is the first multidecadal time series of soft-bottom macrobenthic communities for a high-Arctic fjord, indicating that potential periodic marine heatwaves might drive shifts in community structure, either through direct effects from thermal stress on the communities or through changes in environmental regimes led by temperature fluctuations (i.e. sea ice cover and glacial runoff, which could lead to shifts in primary production and food supply to the benthos). Although high-Arctic macrobenthic communities might be resilient to some extent, sustained warm water anomalies could lead to permanent changes in cold-water fjordic benthic systems.
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Ecossistema , Estuários , Humanos , Temperatura , Água , Oceanos e Mares , Regiões ÁrticasRESUMO
We studied the depth distribution and production of kelp along the Greenland coast spanning Arctic to sub-Arctic conditions from 78 ºN to 64 ºN. This covers a wide range of sea ice conditions and water temperatures, with those presently realized in the south likely to move northwards in a warmer future. Kelp forests occurred along the entire latitudinal range, and their depth extension and production increased southwards presumably in response to longer annual ice-free periods and higher water temperature. The depth limit of 10% kelp cover was 9-14 m at the northernmost sites (77-78 ºN) with only 94-133 ice-free days per year, but extended to depths of 21-33 m further south (73 ºN-64 ºN) where >160 days per year were ice-free, and annual production of Saccharina longicruris and S. latissima, measured as the size of the annual blade, ranged up to sevenfold among sites. The duration of the open-water period, which integrates light and temperature conditions on an annual basis, was the best predictor (relative to summer water temperature) of kelp production along the latitude gradient, explaining up to 92% of the variation in depth extension and 80% of the variation in kelp production. In a decadal time series from a high Arctic site (74 ºN), inter-annual variation in sea ice cover also explained a major part (up to 47%) of the variation in kelp production. Both spatial and temporal data sets thereby support the prediction that northern kelps will play a larger role in the coastal marine ecosystem in a warmer future as the length of the open-water period increases. As kelps increase carbon-flow and habitat diversity, an expansion of kelp forests may exert cascading effects on the coastal Arctic ecosystem.
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Acoustic telemetry allows for high-resolution, long-term tracking of moving animals. Here, we describe data on the movement patterns of 37 adult red king crab (RKC, Paralithodes camtschaticus) obtained by means of acoustic telemetry. Acoustically tagged RKC were released in Gamvikfjorden (Sørøya, northern Norway) the 24th of May 2016 and tracked until the 1st of November 2016. Individual crabs resided in the fjord for 1-162 days and were recorded 16 - 11,501 times (mean number of records per crab: 2,851). In total, the data set consist of 105,484 pairs of accurate spatio-temporal coordinates. The acoustic receivers (n = 38) deployed close to the seabed were integrated with temperature sensors that continuously recorded the ambient seawater temperature, resulting in 174,154 water temperature recordings. These novel tracking data can be used to investigate the species' migratory behaviour, spatio-temporal habitat selection, and the relative role of their environment and their possible food sources. Moreover, the high-resolution seawater temperature dataset may serve independently as input data in physical-oceanographic models of this sub-Arctic sill fjord.
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Mesozooplankton research in high latitude ecosystems tends to focus on different life stages of Calanus spp. due to its biomass dominance and trophic roles. However, a complex seasonal succession of abundant smaller mesozooplankton taxa suggests that the ecological functioning of the mesozooplankton communities is more complicated. We studied the year-round taxon-specific biomass measurements and size distributions of mesozooplankton on a sub-Arctic continental shelf based on formalin preserved samples. Our results confirm that Calanus spp. dominate the mesozooplankton biomass (81%). We show that commonly used length-weight relationships underestimate Calanus biomass in autumn and winter, and accordingly, a strong seasonal bias was introduced in our understanding of sub-Arctic plankton communities. We observed two periods with considerable contribution of meroplankton, the planktonic larvae of benthic invertebrates, to the mesozooplankton biomass: (a) Cirripedia nauplii accounted for 17% of total biomass close to the coast in early April and (b) meroplankton comprised up to 12.7% of total biomass in late July. Based on these results, we suggest that meroplankton may play an ecologically important role in addition to their role in dispersal of benthic species. We conclude that the seasonal succession of the biomass of small-sized holoplankton and meroplankton, often obscured by patterns in the Calanus biomass, should receive more attention as these smaller individuals are likely an important functional component of the pelagic food web.
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The polar cod (Boreogadus saida) has a circumpolar distribution and is the most abundant planktivorous fish in the Arctic. Declining sea-ice coverage impacts polar cod directly and also facilitates expansion of human activities in the region leading to increasing anthropogenic pressures on biota. Here we summarize current data and knowledge on polar cod from the Russian sector of the Barents Sea and discuss knowledge needs for the management of polar cod under changing environmental conditions and anthropogenic impacts. We review 36 Russian historical (1935 - 2020) sources of data and knowledge largely unknown to western researchers, in addition to sources already published in the English language. This effort allowed for digitalization and visualization of 69 separate datasets on polar cod ecology, including maturation, fertility, feeding intensity, diet, lipid content, length-weight relationships and seasonal variation in larval size. Our review suggests that polar cod abundances are particularly large in the eastern Barents Sea and adjacent waters. Here, we identify and discuss key knowledge gaps. The review of polar cod in the eastern Barents Sea revealed 1) major variation in the timing and area of polar cod spawning, 2) uncertainty as to what degree the polar cod is dependent on sea ice, 3) deficient knowledge of juvenile (e.g., 0-group) distributions, particularly in the north-eastern Barents Sea, 4) deficient knowledge of the species' genetic structure and spatio-temporal distributions, and 5) insufficient understanding as to whether ongoing environmental change may induce phenological changes affecting the availability of potential food items for polar cod larvae and their match in space and time. Filling these knowledge gaps would provide an important step towards the reliable knowledge base needed in order to perform well-founded management and impact assessment under environmental changes and increasing anthropogenic impacts.
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Gadiformes , Animais , Regiões Árticas , Ecologia , Humanos , Camada de Gelo , Federação RussaRESUMO
Global warming is expected to cause reductions in organism body size, a fundamental biological unit important in determining biological processes. Possible effects of increasing temperature on biomass size spectra in coastal benthic communities were investigated. We hypothesized higher proportions of smaller size classes in warmer conditions. Soft bottom infauna samples were collected in six Norwegian and Svalbard fjords, spanning wide latitudinal (60-81°N) and bottom water temperature gradients (from -2 to 8 °C). Investigated fjords differed in terms of environmental settings (e.g., pigments or organic carbon in sediments). The slopes of normalised biomass size spectra (NBSS) did not differ among the fjords, while the benthic biomass and NBSS intercepts varied and were related to chlorophyll a and δ13C in sediments. The size spectra based on both abundance and biomass remained consistent, regardless of the strong variability in macrofauna taxonomic and functional trait composition. Variable relationships between temperature and body size were noted for particular taxa. Our results indicate that while benthic biomass depends on the nutritional quality of organic matter, its partitioning among size classes is consistent and independent of environmental and biological variability. The observed size structure remains a persistent feature of studied communities and may be resilient to major climatic changes.
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In the Arctic, seasonal patterns in seawater biochemical conditions are shaped by physical, chemical, and biological processes related to the alternation of seasons, i.e. winter polar night and summer midnight sun. In summertime, CO2 concentration is driven by photosynthetic activity of autotrophs which raises seawater pH and carbonate saturation state (Ω). In addition, restriction of photosynthetic activity to the euphotic zone and establishment of seasonal stratification often leads to depth gradients in pH and Ω. In winter, however, severely reduced primary production along with respiration processes lead to higher CO2 concentrations which consequently decrease seawater pH and Ω. Many calcifying invertebrates incorporate other metals, in addition to calcium, into their skeletons, with potential consequences for stability of the mineral matrix and vulnerability to abrasion of predators. We tested whether changes in seawater chemistry due to light-driven activities of marine biota can influence the uptake of Mg into calcified skeletons of Arctic Bryozoa, a dominant faunal group in polar hard-bottom habitats. Our results indicate no clear differences between summer and winter levels of skeletal MgCO3 in five bryozoan species despite differences in Ω between these two seasons. Furthermore, we could not detect any depth-related differences in MgCO3 content in skeletons of selected bryozoans. These results may indicate that Arctic bryozoans are able to control MgCO3 skeletal concentrations biologically. Yet recorded spatial variability in MgCO3 content in skeletons from stations exhibiting different seawater parameters suggests that environmental factors can also, to some extent, shape the skeletal chemistry of Arctic bryozoans.
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Briozoários , Animais , Regiões Árticas , Estações do Ano , Água do Mar , EsqueletoRESUMO
For organisms that remain active in one of the last undisturbed and pristine dark environments on the planet-the Arctic Polar Night-the moon, stars and aurora borealis may provide important cues to guide distribution and behaviours, including predator-prey interactions. With a changing climate and increased human activities in the Arctic, such natural light sources will in many places be masked by the much stronger illumination from artificial light. Here we show that normal working-light from a ship may disrupt fish and zooplankton behaviour down to at least 200 m depth across an area of >0.125 km2 around the ship. Both the quantitative and qualitative nature of the disturbance differed between the examined regions. We conclude that biological surveys in the dark from illuminated ships may introduce biases on biological sampling, bioacoustic surveys, and possibly stock assessments of commercial and non-commercial species.
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Comportamento Animal/efeitos da radiação , Peixes/fisiologia , Luz/efeitos adversos , Zooplâncton/fisiologia , Zooplâncton/efeitos da radiação , Animais , Regiões Árticas , Ritmo Circadiano/efeitos da radiação , Clima Frio , Ecossistema , Monitoramento Ambiental , Fotoperíodo , NaviosRESUMO
The aim of this study was to assess bioturbation rates in relation to macrozoobenthos and environmental variables in the Svalbard fjords, Barents Sea and Nansen Basin during spring to summer transition. The results showed differences in benthic community structure across sampled area in relation to sediment type and phytopigment content. Fjords, Barents Sea and the shallow parts of Nansen Basin (<400â¯m) were characterized by high functional groups diversity, and by biodiffusive and non-local rates ranging from 0.05 to 1.75â¯cm-2â¯y-1 and from 0.2 to 3.2 y-1, respectively. The deeper parts of Nansen Basin (>400m), dominated by conveyors species, showed only non-local transport rates (0.1-1 y-1). Both coefficients intensity varied with benthic biomass. Non-local transport increased with species richness and density and at stations with mud enriched by fresh phytopigments, whereas biodiffusion varied with sediment type and organic matter quantity. This study quantified for the first time the two modes of sediment mixing in the Arctic, each of which being driven by different environmental and biological situations.
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Invertebrados , Animais , Regiões Árticas , Biomassa , Oceanos e Mares , Dinâmica Populacional , Estações do Ano , SvalbardRESUMO
The recent intensification of energy resource exploration and human activities in the Barents Sea (BS) requires a more thorough assessment of the natural and anthropogenic impact of hydrocarbons on the environment. We analyzed a wide set of sensitive indicators, including hydrocarbon molecular markers and organic matter (OM) maturity parameters in the Holocene sediments from three regions of the BS: the Kola-Kanin Monocline (KKM), the Svalbard shelf, and the Shtokman gas-condensate field (GCF). An increase in pyrogenic polycyclic aromatic hydrocarbons toward the core surface traces the intensification of anthropogenic contamination in the KKM region during last century. An input of highly mature OM from the eroded coal rocks of Barentsburg were confirmed by comparison of biomarker distribution in sediments and coals. An increase in biogenic hopanes and hopenes content down-core, and a crude-oil stage of OM maturity in surface sediments of the Shtokman GCF attests to hydrocarbons migration from subsurface strata.
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Sedimentos Geológicos/análise , Hidrocarbonetos/análise , Regiões Árticas , Monitoramento Ambiental , Sedimentos Geológicos/química , Hidrocarbonetos/química , Campos de Petróleo e Gás , Petróleo , Hidrocarbonetos Policíclicos Aromáticos/análise , Hidrocarbonetos Policíclicos Aromáticos/química , Federação Russa , Svalbard , Poluentes Químicos da Água/análise , Poluentes Químicos da Água/químicaRESUMO
Intensive exploitation and transport of oil and derivatives are increasing the risk of coastal contamination by either dramatic disasters or diffuse sources. Tools for monitoring diffuse contamination, such as diesel oil that leaks from marine vessels are much needed. We experimentally tested the efficiency of antioxidant biomarkers as indicators of chronic exposure to diesel oil in a mudflat from the subtropical Bay of Paranaguá, in southern Brazil. We examined the effects of three successive diesel oil spills, with two weeks of recovery time between exposures, on the edible clam Anomalocardia flexuosa. Previous studies have highlighted its potential as a bioindicator species for diesel oil contamination in subtropical and tropical ecosystems. Endpoints measured in gill and digestive gland homogenates included the activity of antioxidant enzymes SOD, GPx, GST and levels of lipid peroxides. PAHs concentration in sediments and soft tissue were also quantified. GST and SOD were the most responsive biomarkers to the exposure. There were significant but non-cumulative departures from control levels in organisms from treated samples, which were, in all cases, more common 48 h after each experimental spill. Biomarker responses were more evident in the digestive gland than in gills. This work validated the short-term responsiveness of biomarkers as measures of repeated pulsed in situ exposure to low concentrations of diesel oil. For their routine implementation into monitoring programs for tropical estuaries our general recommendations are 1) to include several reference sites, 2) to analyze biomarker data using a logarithmic-scale and 3) to interpret deviations from "normal" activity as multiplicative interval differences.