Bioenergetic responses in green lipped mussels (Perna viridis) as indicators of pollution stress in Xiamen coastal waters, China

Samples of green lipped mussels (Perna viridis) were collected from three sites (Huoshao, I’Maluan and Tong’an) around Xiamen coastal waters, where levels of various trace organic pollutants have been studied. Samples were also collected at a “cleaner” reference site near Dongshan Island for comparison. Clearance rate, absorption efficiency and oxygen consumption of the mussels were measured under controlled laboratory condition; organic pollutants in their tissues were also analyzed. Scope for growth (SFG) was employed as a general biomarker to evaluate the stress of pollutants. Specimens from Tong’an site had the lowest SFG values (1.14 J/h/g); while specimens from the I’Maluan site and Huoshao site had SFG values of 5.01 and 6.72 J/h/g, respectively. Specimens from Dongshan (reference site) had a relatively high SFG value of 10.96 J/h/g. There was a significant negative correlation between the SFG of the different populations of green lipped mussels and the concentration of DDT in their tissues.     

Toxic contaminants and their biological effects in coastal waters of Xiamen,China. I. Organic pollutants in mussel and fish tissues

Organochlorines and organophosphates were measured in four fish species (Lateolabrax japonicus, Pagrasomus major, Miichthys miiuy and Epinephalus awoara) and the mussel Perna viridis collected from aquaculture cages in coastal waters of Xiamen, China. Polycyclic aromatic hydrocarbon (PAH) metabolites were also measured in fish bile. Sites in Xiamen coastal waters (the harbour, Maluan and Tongan) are compared with a "cleaner" reference site at Dongshan Island. DDT was found in livers of all fish sampled (May 1998, 1999 and December 1999) ranging from 0.15 to 2.2 microg/g WW, but levels in muscle tissue (<0.5 ng/g to 0.22 microg/g WW) were at least an order of magnitude lower in the harbour, and were not detectable (<0.5 ng/g) in Tongan samples. All other pesticides examined were not detected in fish or mussels, except for dieldrin in one liver sample (0.07 microg/g WW). Food of the caged fish, small fish and dried pellets, had very low (0.015-0.027 microg/g WW) or non-detectable levels of DDT, indicating significant bioaccumulation of contaminants in caged fish and/or other possible sources of uptake (water and sediments). DDT levels in fish varied with species. DDT contamination in fish livers and whole mussels varied significantly with location. Highest concentrations in fish occurred in the harbour, and less consistently so in Tongan. Dongshan and Maluan had comparatively low levels of DDT. Relatively low residues of DDT in both fish and mussels at Dongshan likely reflect lower levels of pesticide input at this reference site. However, the low levels of DDT contamination in fish tissues from Maluan were unexpected in view of the close proximity of the harbour and possibly reflect the removal of particulate-bound contaminants by the intensive bivalve culture there. Consistently high proportions of DDT in the form of the parent compound (p,p'-DDT: fish 30-45%; mussels 40-65%) from all sites suggest recent releases of this chemical to the environment. Levels of two main groups of PAH metabolites (naphthalene and phenanthrene) in fish bile suggest a different environmental distribution pattern than that of pesticides, in that Dongshan fish appear to be no less exposed to hydrocarbons than those from Xiamen coastal waters.     

Alkylbenzenes in mussels from South and South East Asian coasts as a molecular tool to assess sewage impact

Alkylbenzenes, molecular markers of sewage, were measured in 34 green mussels collected from India, Indonesia, Malaysia, Thailand, Cambodia, Vietnam, and the Philippines together with blue mussels collected from Tokyo Bay, Japan. Linear alkylbenzene (LAB) concentrations in South and South East Asian countries ranged from 10 to 1640 ng-∑LAB/g-dry tissue. In some populous cities, LAB concentrations were similar or higher than those found in northern Tokyo Bay which is heavily impacted by sewage effluents. I/E ratios (a ratio of internal to external isomers of LABs) in the South and South East Asian countries (1–3) were much lower than those in Tokyo Bay (3–8), indicating sewage discharged in the coastal zone is poorly treated (e.g., raw sewage and/or primary effluents). Alkylbenzenes with branched alkyl chains, tetrapropylene-based alkylbenzenes, were also detected in mussels from Indonesia and Philippines. This “tell-tale” sign indicates that poorly degradable detergents are still in use in this area, although they have long been phased-out in many industrialized countries.     

Field validation of antioxidant enzyme biomarkers in mussels (Perna viridis) and clams (Ruditapes philippinarum) transplanted in Hong Kong coastal waters

Green-lipped mussels, Perna viridis, and Manila clams, Ruditapes philippinarum were sourced from “clean” sites in the Hong Kong region, depurated in a laboratory using uncontaminated filtered seawater for 8 days, and transplanted to a suspected gradient of chemically polluted sites in Hong Kong. After 14- and 28-days of field exposure, several antioxidant parameters including glutathione S transferase (GST), catalase (CAT), glutathione peroxidase (GPx), and glutathione (GSH) were quantified in gill and hepatopancreas tissues. Whole body tissue concentrations of polycyclic aromatic hydrocarbons (PAHs), petroleum hydrocarbons (PHCs), polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCs) were determined in pooled site samples. Chemical analyses indicated that: (a) clams had higher levels of PAHs, PHCs, DDTs and PCBs, whereas mussels had higher hexachlorocyclohexane (HCHs) and there was no difference between species for dieldrin and remaining OCs; (b) Kat O should not be continued as a “clean” reference site for Hong Kong, because of the levels of contaminants measured and (c) PAH concentrations in the current survey were similar to those previously measured. Toxicological conclusions were: (a) antioxidant responses were different between species; (b) CAT and GST have highest utility in clams for field use in Hong Kong, whereas CAT in both gill and hepatopancreas tissue showed most potential in mussels; (c) significant induction of antioxidant responses over day 0 (excluding GPx in both tissues, and GST in mussel hepatic tissue); (d) groups of contaminants do not consistently induce antioxidant responses and (e) organochlorines and PCBs correlated significantly with CAT and GST in clam hepatopancreas and with CAT in mussel gill and hepatic tissue. Multivariate statistical techniques indicated little relationship between the site patterns for antioxidant responses and the contaminant gradients identified in body burden analysis.     

Toxic contaminants and their biological effects in coastal waters of Xiamen,China. II. Biomarkers and embryo malformation rates as indicators of pollution stress in fish

Baseline information is presented on embryo malformation rate and biomarkers in fish as indicators of sub-lethal stress caused by pollution in coastal waters of Xiamen, PR China. Fish and eggs were sampled from several areas in Xiamen coastal waters (Xiamen Harbour, Maluan and Tongan Bays and East Channel), where varying levels of pollutant input have been documented. Comparative sampling was done at a "cleaner" reference site at Dongshan Island. Embryonic malformation rates, which indicate general water quality, varied with location and species of fish, and exceeded background levels for unpolluted waters (assumed approximately 5%) by up to eightfold at some sites. Generally, sites around Xiamen Harbour show signs of poor water quality having highest mean levels of embryo deformity (20-30%) and these decreased towards open waters (Tongan Bay, Eastern Channel) where abnormalities approached background levels. An indication that toxic contaminants may be having a localised effect in the region, particularly in the harbour was reinforced by the biomarker assays. However, activities of the biomarkers ethoxyresorufin O-deethylase (EROD) and glutathione S-transferase in fish livers indicate no clear pattern, and there is no evidence that fish from the four sampling areas have been more or less exposed to PAHs and other compounds that induce these biomarkers. Antioxidant biomarkers (glutathione peroxidase, catalase, superoxide dismutase, and reduced glutathione) suggest that exposure to xenobiotics appears to be lowest in Dongshan and Maluan and highest in the harbour and Tongan. Inhibition of acetylcholinesterase in fish muscle indicated possible effects by organophosphate and carbamate pesticides in Xiamen waters and these effects may be greatest in the area of the harbour.     

Organochlorines and dioxin-like compounds in green-lipped mussels Perna viridis from Hong Kong mariculture zones

Concentrations of persistent organic pollutants including polychlorinated biphenyls (PCBs), organochlorine (OC) pesticides and dioxin-like compounds were measured in green-lipped mussels, Perna viridis, collected from seven mariculture zones in Hong Kong between September and October in 2002 in order to evaluate the status, spatial distribution and potential sources of pollution in these areas. Concentrations ranged from 300 to 4400 ng/g lipid weight for total OCs and 170–1000 ng/g lipid weight for total PCBs (based on 28 congeners). Relatively smaller DDT concentrations in mussels compared with previous studies suggest reduced discharges of DDTs from nearby regions into Hong Kong waters. Detection of a mixture of HCH isomers in the mussels indicated that Hong Kong waters were predominantly contaminated by technical HCHs rather than lindane. Mussel samples from all sampling locations elicited significant dioxin-like activity in the H4IIE-luc bioassay. The greatest magnitude of dioxin-like response (39 pg TEQ/g wet wt.) was detected in mussels from Ma Wan in the western waters of Hong Kong, which is strongly influenced by the Pearl River discharge. Human health risk assessment was undertaken to evaluate potential risks associated with the consumption of the green-lipped mussels. Risk quotient (RQ) for dioxin-like compounds was greater than unity suggesting that adverse health effects may be associated with high mussel consumption.     

Metal accumulation in four molluscs

Specimens of mussels (Mytilus edulis), winkles (Littorina littorea), dogwhelks (Nucella lapillus) and limpets (Patella vulgata) were collected from the north-east coast of England in 1980–1981 in order to analyse the dried whole soft tissue for zinc, copper and iron. In each case, only a narrow size range of organisms was collected from a small environmentally homogeneous site and sex was taken into account. Most of the frequency distributions of metal concentrations obtained were found to show significant positive skewness. This skewness could not be explained either by the degree of mobility or type of feeding mechanism of the organisms or by any other known parameter, e.g. size, sex, habitat variation. Some individual mussels were found to be extremely good regulators of their zinc concentrations. On the other hand, other individual mussels had the ability to accumulate unusually high levels of zinc.     

Field validation of antioxidant enzyme biomarkers in mussels (Perna viridis) and clams (Ruditapes philippinarum) transplanted in Hong Kong coastal waters

Green-lipped mussels, Perna viridis, and Manila clams, Ruditapes philippinarum were sourced from “clean” sites in the Hong Kong region, depurated in a laboratory using uncontaminated filtered seawater for 8 days, and transplanted to a suspected gradient of chemically polluted sites in Hong Kong. After 14- and 28-days of field exposure, several antioxidant parameters including glutathione S transferase (GST), catalase (CAT), glutathione peroxidase (GPx), and glutathione (GSH) were quantified in gill and hepatopancreas tissues. Whole body tissue concentrations of polycyclic aromatic hydrocarbons (PAHs), petroleum hydrocarbons (PHCs), polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCs) were determined in pooled site samples. Chemical analyses indicated that: (a) clams had higher levels of PAHs, PHCs, DDTs and PCBs, whereas mussels had higher hexachlorocyclohexane (HCHs) and there was no difference between species for dieldrin and remaining OCs; (b) Kat O should not be continued as a “clean” reference site for Hong Kong, because of the levels of contaminants measured and (c) PAH concentrations in the current survey were similar to those previously measured. Toxicological conclusions were: (a) antioxidant responses were different between species; (b) CAT and GST have highest utility in clams for field use in Hong Kong, whereas CAT in both gill and hepatopancreas tissue showed most potential in mussels; (c) significant induction of antioxidant responses over day 0 (excluding GPx in both tissues, and GST in mussel hepatic tissue); (d) groups of contaminants do not consistently induce antioxidant responses and (e) organochlorines and PCBs correlated significantly with CAT and GST in clam hepatopancreas and with CAT in mussel gill and hepatic tissue. Multivariate statistical techniques indicated little relationship between the site patterns for antioxidant responses and the contaminant gradients identified in body burden analysis.     

Organochlorines and dioxin-like compounds in green-lipped mussels Perna viridis from Hong Kong mariculture zones

Concentrations of persistent organic pollutants including polychlorinated biphenyls (PCBs), organochlorine (OC) pesticides and dioxin-like compounds were measured in green-lipped mussels, Perna viridis, collected from seven mariculture zones in Hong Kong between September and October in 2002 in order to evaluate the status, spatial distribution and potential sources of pollution in these areas. Concentrations ranged from 300 to 4400 ng/g lipid weight for total OCs and 170–1000 ng/g lipid weight for total PCBs (based on 28 congeners). Relatively smaller DDT concentrations in mussels compared with previous studies suggest reduced discharges of DDTs from nearby regions into Hong Kong waters. Detection of a mixture of HCH isomers in the mussels indicated that Hong Kong waters were predominantly contaminated by technical HCHs rather than lindane. Mussel samples from all sampling locations elicited significant dioxin-like activity in the H4IIE-luc bioassay. The greatest magnitude of dioxin-like response (39 pg TEQ/g wet wt.) was detected in mussels from Ma Wan in the western waters of Hong Kong, which is strongly influenced by the Pearl River discharge. Human health risk assessment was undertaken to evaluate potential risks associated with the consumption of the green-lipped mussels. Risk quotient (RQ) for dioxin-like compounds was greater than unity suggesting that adverse health effects may be associated with high mussel consumption.     

Chemical record of the projectile in the graded fall-back sedimentary unit from the Ries Crater, Germany

The so-called “Graded Unit” comprises 17.2 m of the core from the research borehole Nördlingen 1973 and is thought to represent fall-back material, which has been sorted similar to tephra. Samples from six levels of these air-borne impact debris sediments in the Ries Crater have been analyzed for Ir, Ni, Co, Cr and Fe. The concentrations of these elements are negatively correlated with average grain size and accordingly the depth of the samples indicating the presence of a surface correlated component and are hence consistent with vapour deposition of meteoritic material on small dust grains. The largest Ir concentration found is 230 pg/g, a factor of 28 higher than the indigenous concentration. Enrichment factors in the uppermost layer of the “Graded Unit” are about 28 for Ir, approximately 2 for Fe, Co, Ni and 1.5 for Cr. If the enrichment of all elements determined were of meteoritic origin, then the “net meteoritic composition” is most closely matched by aubrites as suggested previously. However, it seems more likely that the observed concentration trends of Cr, Fe, Co and Ni are due to mineral sorting. Therefore the only as yet certain indication of meteoritic material is the presence of Ir, which is not sufficient to further constrain the nature of the projectile.     

Persistent organic pollutants and stable isotopes in biopsy samples (2004/2006) from Southern Resident killer whales

“Southern Resident” killer whales include three “pods” (J, K and L) that reside primarily in Puget Sound/Georgia Basin during the spring, summer and fall. This population was listed as “endangered” in the US and Canada following a 20% decline between 1996 and 2001. The current study, using blubber/epidermis biopsy samples, contributes contemporary information about potential factors (i.e., levels of pollutants or changes in diet) that could adversely affect Southern Residents. Carbon and nitrogen stable isotopes indicated J- and L-pod consumed prey from similar trophic levels in 2004/2006 and also showed no evidence for a large shift in the trophic level of prey consumed by L-pod between 1996 and 2004/2006. ∑PCBs decreased for Southern Residents biopsied in 2004/2006 compared to 1993–1995. Surprisingly, however, a three-year-old male whale (J39) had the highest concentrations of ∑PBDEs, ∑HCHs and HCB. POP ratio differences between J- and L-pod suggested that they occupy different ranges in winter.     

Observations of a Unique Cusp Signature at Low and Mid Altitudes

Observations of a unique cusp feature at low and mid altitudes are reported. This feature has a consistent double-peaked or “V”-shaped structure at the equatorward edge of high-latitude particle precipitation flux, and is predominantly present for high IMF B_y conditions. The observations are consistent with the Crooker (‘A split separator line merging model of the dayside magnetopause’, J. Geophys. Res. 90 (1985) 12104, ‘Mapping the merging potential from the magnetopause to the ionosphere through the dayside cusp’, J. Geophys. Res. (1988) 93 7338.) antiparallel merging model, which predicts a narrow wedge-shaped cusp whose geometry depends greatly on the dawn/dusk component of the IMF. Various observations are presented at low altitudes (DE-2, Astrid-2, Munin, UARS, DMSP) and at mid altitudes (DE-1, Cluster) that suggest a highly coherent cusp feature that is consistent with the narrow, wedge-shaped cusp of Crooker (1988), and contains persistent wave signatures that are compatible with previously reported high-altitude measurements. A statistical survey of Astrid-2 and DMSP satellite data is also presented, which shows this feature to be persistent and dependent on the IMF angle at the magnetopause, as expected. Thus, the cusp signatures observed at a wide range of altitudes present a coherent picture that may be interpreted in terms of a footprint of the magnetopause current layer.     

Contaminant exposures at the 4H shell mounds in the Santa Barbara Channel

Remobilization, bioavailability, and potential toxicity of chemical contaminants were evaluated at the 4H shell mounds – the site of abandoned offshore oil and gas production platforms in the Santa Barbara Channel region of the Southern California Bight. Evaluations used a weight-of-evidence approach based on results from bulk phase chemical analyses and laboratory toxicity testing of shell mound cores, in situ field bioassays using caged mussels, and surficial sediment chemistry. Shell mound cores contained elevated concentrations of metals associated with drilling wastes (e.g., Ba, Cr, Pb, and Zn), as well as monocyclic and polycyclic aromatic hydrocarbons (PAHs). The highest concentrations along with pockets of free oil were associated with the middle “cuttings” stratum. Sediments composited from all core strata caused significant acute toxicity and bioaccumulation of Ba and PAHs in test organisms during laboratory exposures. In contrast, caged mussels placed at each of the shell mounds for a period of 57–58 days had greater than 90% survival, and there were no significant differences in survival of mussels placed at the shell mounds and corresponding reference sites. While all mussel samples exhibited increases in shell length, whole animal weight, and tissue lipid content, in some cases growth metrics for the shell mound mussels were significantly higher than those for the reference sites. Concentrations of metals, PAHs, and polychlorinated biphenyls (PCBs) in tissues of the shell mound mussels were not significantly different from those at reference sites. The presence of labile aromatic hydrocarbons in shell mound cores and absence of significant contaminant accumulation of tissues of caged mussels indicated that chemical contaminants are not being remobilized from the 4H shell mounds. Surficial bottom sediments near the shell mounds contained elevated Ba concentrations that probably were associated with drilling wastes. However, concentrations did not exhibit clear spatial gradients with distance from the shell mounds. Despite a number of storm events during the mussel exposures, maximum currents were 34 cm s⁻¹ and unlikely to erode materials from the shell mounds. Thus, Ba distributions in bottom sediments probably were due to episodic disturbance such as platform removal or trawling rather than ongoing erosion and dispersion of shell mound solids by near-bottom currents. These results suggest that, in the absence of physical disturbances, contaminants are expected to remain sequestered in the shell mounds.     

Space physics coordinate transformations: the role of precession

Raw data on spacecraft orbits and attitude are usually supplied in “inertial” coordinates. The normal geocentric inertial coordinate system changes slowly in time owing to the effects of astronomical precession and the nutation of the Earth’s rotation axis. However, only precession produces a change that is significant compared with the errors in determining spacecraft position. We show that the transformations specified by Russell (1971) and Hapgood (1992) are strictly correct only if the epoch-of-date inertial system is used. We provide a simple formula for estimating the error in the calculated position if the inertial system for some other epoch is used. We also provide a formula for correcting inertial coordinates to the epoch-of-date from the standard fixed epoch of J2000.0.     

Occurrence of perfluorinated compounds in water, sediment and mussels from the Cantabrian Sea (North Spain)

Perfluorinated compounds (PFCs) have emerged as significant global environmental pollutants with persistent, bioaccumulative and toxic properties. The aim of this study was to determine the occurrence of PFCs in water (wastewater, submarine emissaries and port-waters), sediment and transplanted mussels in estuarine areas of high urban and industrial impact from Northern Spain. Five PFCs of industrial use were studied: perfluorooctanesulfonate, perfluorohexanesulfonate, perfluorobutanesulfonate, perfluorooctanoate acid and perfluorononanoate acid. After selective extraction, samples were analyzed by Ultra Performance Liquid Chromatography coupled to tandem mass spectrometry. ΣPFCs ranged from 0.06 to 10.9 ng/L in water, with higher levels in wastewater treatment plants effluents and port waters than in submarine emissaries. Little accumulation was observed in sediments and mussels with ΣPFCs ranging from 0.01-0.13 ng/g dw and 0.01-0.06 ng/g ww, respectively. Most ubiquitous compounds were PFOS and PFOA. Mass fluxes of PFCs to the Cantabrian Sea are estimated and the impact to the coastal ecosystem is discussed.     

Comparison of the effects of pollution on the marine bivalve Donax trunculus in the vicinity of polluted sites with specimens from a clean reference site (Mediterranean Sea)

The physiological and biochemical characteristics of tissue samples of the marine mussel, Donax trunculus, from an oil-polluted site (Qiryat Yam) and from a site adjacent to an industrial factory (Frutarom) producing polyvinyl chloride (PVC) were compared with samples from a clean reference site (Akko). All sites were located along the sandy shores of the Israeli Mediterranean Sea. The mussels from the oil-polluted site showed increased activity of the system of active transport of organic anions (SATOA) in the gills and the renocardial organ and also of the multixenobiotic resistance transporter (MXR) in the gills. In contrast, samples collected near the PVC factory showed a decrease in SATOA activity and no increase in the activity of MXR in the gills. Specimens from the reference site demonstrated a redox state of equilibrium between energy production and utilization, while in Donax from both the oil polluted and the PVC-polluted sites, the mitochondrial redox state reflected intensive consumption of energy. No significant changes were found in the activity of reduced glutathione s-transferase (GST) in the cytosolic fraction of the digestive gland of Donax collected from any of the three sites. The data demonstrate a differential increase in the anti-chemical defense systems and an intensification of energy metabolism in the mussels exposed to pollution.     

Okadaic acid, a causative toxin of diarrhetic shellfish poisoning, in green-lipped mussels Perna viridis from Hong Kong fish culture zones: Method development and monitoring

Green-lipped mussels (Perna viridis) were collected from seven fish culture zones (FCZs) in Hong Kong and analyzed for okadaic acid (OA). A conventional HPLC method was modified by incorporating a proteinase K digestion step. Results suggest that a higher recovery (2.5 times higher) of OA was obtained from spiked samples after the addition of 1.08 mg proteinase K in comparison with samples incubated without the proteolytic enzyme. For the hepatopancreas (HP) of individual field-collected mussels, the additional digestion step can enhance OA extraction by 3.1 times. Spatial and temporal variations in OA concentrations in the mussels from various FCZs were investigated. The highest concentration of OA in mussel HP samples was 1164.9 ng/g HP wet wt. With respect to OA concentrations in whole mussel tissues from seven sites and four seasonal samplings, the concentrations were between 70.0 and 131.0 ng/g wet wt., which did not exceed the generally recognized international regulatory criteria (>200 ng/g) for OA.     

Cytogenetic and cytotoxic effects in gill cells of the blue mussel Mytilus spp. from different zones of the Baltic Sea

Frequency of micronuclei (MN) and other nuclear abnormalities (nuclear buds, bi-nucleated and fragmented-apoptotic cells) was analysed in gill cells of the blue mussel (Mytilus spp.) from selected coastal sites in the Baltic Sea--Kv?d?fj?rden (Sweden), Klaipeda-Būtinge zone (Lithuania), Gulf of Gdansk (Poland) and Wismar Bay (Germany). Samples were collected from 650 specimens during bi-annual sampling campaigns in 2001 and 2002. The lowest frequency of MN (0.37 MN/1000 cells) was found in blue mussels from the reference site (Kv?d?fj?rden). The highest MN values (up to 6.7 MN/1000 cells) were registered in blue mussels from the Gulf of Gdansk in autumn 2001 and 2002, and at Wismar Bay in spring 2001 (up to 5.06 MN/1000 cells). Gradients of MN incidences were observed when comparing the three studied locations in Wismar Bay, and at the Lithuanian coast before the crude oil spill in the Būtinge oil terminal. Moreover, significant seasonal and inter-location differences in the responses were documented (P<0.0001). Nuclear abnormalities were observed most frequently in blue mussels from the Gulf of Gdansk.     

Evidence of nonlinear site response in HVSR from SMART1 (Taiwan) data

Deamplification of strong motion and the increase of the effective period of soil deposits are typical nonlinear effects; we seek them in SMART1-array data by applying the horizontal-to-vertical spectral ratio (HVSR) technique. The recordings, from four soil and one rock stations, represent 23 earthquakes (M_L 4.9–7.0); PGA varies between 20–260 cm/s². For each station, mean HVSR curves are calculated for two PGA ranges: <75 cm/s² and >100 cm/s² (weak and strong motion). At the soil stations, the “weak” (linear) and “strong” (nonlinear) responses are significantly different. Below 1–1.8 Hz, the nonlinear response exceeds the linear one. Above 2 Hz, the nonlinear response drops below the linear one and above 4–6 Hz below unity (deamplification). From 10 to 16 Hz, the two responses converge. One soil site shows significant negative correlation between resonance frequency and ground acceleration. Such behaviour agrees with other empirical studies and theoretical predictions. Our results imply that the HVSR technique is sensitive to ground-motion intensity and can be used to detect and study nonlinear site response.