SE 9307/97 KEM Eseoti No 2/98 received JUN 2 9 m O&T I Arsenic in the aquatic environment - speciation and biological effects Exemption Substances Project Lars Landner Swedish Environmental Group DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED FOREIGN SALES PROHIBITED “PL- THE SWEDISH NATIONAL CHEMICALS INSPECTORATE DISCLAIMER Portions of this document may be illegible electronic image products. Images are produced from the best available original document. Arsenic in the aquatic environment - speciation and biological effects ISSN: 0284-1185 Order No. 360 599 Printed by: Printgraf, Stockholm, March 1998 Publisher: Swedish National Chemicals Inspectorate© Order address: P.O. Box 1384, S-171 27 Solna, Sweden Telefax 46 8-735 52 29, e-mail [email protected] Preface This report is a contribution to EC Commission's undertaking to review existing EC provisions on the substances for which Sweden has been granted transitional provisions*. The provisions imply that Sweden may maintain more stringent regulations on four substances until the end of 1998. The report is one in a series providing further facts to the Commission and Member states in the review process, and is produced within the Exemption Substances Project at the National Chemicals Inspectorate. Exempted substances are arsenic and organotin compounds (Directive 89/677/EEC), pentachlorophenol (91/171/EEC), cadmium (91/33 8/EEC), and fertilizers with regard to their cadmium content (76/1 16/EEC). The present report deals with speciation and biological effects of arsenic in three types of aquatic environments - marin water, estaurin or brackish-water and freshwater. The similarity between arsenate and phosphate and the interference in phosphorylation reactions is discussed. It is clear that in Scandinavian inland waters the concentration of phosphorus is on average lower than in most inland waters in continental Europe. However, in most inland waters phosphorus is the limiting actor for phytoplancton development and eutrofication, which means that there is a clear risk for detrimental effects in the great majority of inland waters, also eutrophic waters. The author alone is responsible for the contents of the report. Solna, Lars Gustafsson, Project Manager Exemption Substances Project *Act concerning the conditions of accession and the adjustments to the Treaties on which the Union is founded (OJ 94/C241/08) Table of Contents SUMMARY 1 SAMMANFATTNING 6 I. INTRODUCTION 11 II. SPECIATION OF ARSENIC IN THE AQUATIC ENVIRONMENT 13 1. Marine Environment 13 1.1 Inorganic arsenic species 13 1.2 Organic arsenic species 16 1.3 Biotransformation of arsenic - metabolic cycle 21 1.4 Sinks, mobilization and bioavailability of arsenic species 28 1.5 Uptake in organisms and bioaccumulation 30 1.6 Natural factors affecting speciation and bioavailability 30 2. Estuarine or Brackish Water Environment 32 2.1 Arsenic levels and speciation in water and sediments 32 2.2 Arsenic levels and speciation in biota 39 2.3 Biotransformation of arsenic 41 2.4 Sinks, mobilization and bioavailability of arsenic species 43 2.5 Uptake in organisms and bioaccumulation 46 2.6 Natural factors affecting speciation and bioavailability 51 3. Freshwater Environment(Lakes and Rivers) 53 3.1 Arsenic speciation in water and sediments 53 3.2 Arsenic levels and speciation in biota 57 3.3 Biotransformation of arsenic 58 3.4 Sinks, mobilization and bioavailability of arsenic species 60 3.5 Uptake in organisms and bioaccumulation 64 3.6 Natural factors affecting speciation and bioavailability 67 4. WaterQuality Characteristics in inland European Water Bodies of Relevance for Arsenic Speciation 68 III. EFFECTS OF VARIOUS ARSENIC SPECIES IN THE AQUATIC ENVIRONMENT 71 1. Marine Environment 71 1.1 Effects on microorganisms 71 1.2 Effects on macroalgae and other plants 76 1.3 Effects on invertebrates 76 1.4 Effects on marine fish 77 1.5 Effects at the ecosystem level 78 1.6 Arsenic criteria for protection of marine life 80 2. Estaurine or Brackish Water Environment 81 2.1 Effects on microorganisms 81 2.2 Effects on macroalgae and other plants 82 2.3 Effects on invertebrates 84 2.4 Effects on estuarine fish 86 2.5 Effects at the ecosystem level 86 3. Freshwater Environment (Lakes and Rivers) 87 3.1 Effects on microorganisms 87 3.2 Effects on aquatic plants 90 3.3 Effects on invertebrates 91 3.4 Effects on freshwater fish 92 3.5 Effects at the ecosystem level 93 3.6 Arsenic criteria for protection of freshwater aquatic life 93 4. Mechanisms of Toxicity - Interaction with Phosphorus 94 IV. LITERATURE REFERENCES 96 SUMMARY 1. Predominant Arsenic Species in Aquatic Environments Arsenate is the predominant form of arsenic in marine waters, because it is the thermo-dynamically most stable form in oxic waters. In addition to arsenate, three other arsenic species are commonly encountered in dissolved form in marine as well as brackish water: the reduced incorganic form, arsenite, and two methylated species, methylarsonic acid (MAA) and dimethylarsinic acid (DMAA). Other, more complex organic forms of arsenic may be present as well, but their concentrations in water are low, and they are not generally detected using conventional analytical methods. In reducing environments, such as anoxic water or sediments, arsenite is the more stable inorganic arsenic species, but it is also found in aerobic systems. However, the oxidation of arsenite to arsenate, in the presence of oxygen, is usually rapid. The rate of uptake of arsenate into autotrophs (phytoplankton, periphyton and macroalgae) in the marine environment is relatively high. The uptake is followed by a rapid transformation of arsenate to low- molecular, methylated arsenic species, such as MAA and DMAA, which are excreted back to the water phase, or - mainly - to complex organic arsenic species, arseno-sugars (dimethylarsinylribosides and trimethylarsonioribosides), which are stored inside the aquatic plants. The arsenosugars are then transformed in several steps to arsenocholine and arsenobetaine. Arsenobetaine is the predominant arsenic compound found in marine animals, including fish. The biogeochemistry of arsenic has been thoroughly studied and described in the marine environment, and most of our present knowledge about the metabolic transformations of arsenic in aquatic ecosystems has been obtained from marine studies. However, also in estuarine or brackish-water ecosystems, many similar studies have been conducted and it is now well established that the principle pattern of arsenic turnover is similar in these aquatic environments. Due to the usually higher aquatic productivity of estuaries and land-locked seas, as compared to the open ocean, the production and release by algae of arsenite and DMAA is often much higher than in the open sea. 1 While arsenic speciation has been thoroughly studied in marine ecosystems, and is also relatively well investigated in brackish-water ecosystems, much less data on the exact speciation and transformation of arsenic are available from freshwater ecosystems. In lakes and rivers, it appears that pH and the content of humic substances, as well as the existence of hydrous iron, manganese and aluminium oxides, play a relatively more important role for the form of occurrence and bioavail­ ability of arsenic. For example, in acid lakes with a high content of humic material, arsenic will form stable complexes with humic acid, which tend to increase the mobility of arsenic, but without increasing its bioavailability. Biomethylation of arsenic by freshwater organisms has been experimentally proven, but the exact nature and the chemical structure of the organic arsenic compounds present in the living cells of freshwater organisms still have to be revealed. However, at least traces of the comparatively non-toxic compound arsenobetaine have been positively identified in freshwater fish. It has also been demonstrated that the arsenic compounds that predominate in freshwater fish are trimethylated, i.e. they have the same number of methyl groups as arsenobetaine. At the present state of knowledge, it might be concluded that there seems to be a greater variation between freshwater biological species in their capacity to methylate arsenic and to synthesize complex organic arsenic compounds, as compared to marine species. 2. Ecotoxicology of Arsenic in the Aquatic Environment Elevated concentrations of arsenic in surface waters, due to geological anomalies or to pollution from industrial or mining activities, will generate a variety of complex interactions with biotic and abiotic factors which may affect the transport, bioavailability, metabolism and ecotoxicity of arsenic. Consequently, the ecotoxicity and hence the environmental risk of arsenic may be extremely variable depending on the natural factors existing in the water body being exposed to the elevated arsenic concentrations. As mentioned in the previous sections, the speciation of arsenic and the conversions between the different chemical forms are to a large extent determined by the biotic components of the ecosystem. Thus, the arsenic being released (e g from rocks and minerals) to the aquatic system will change its chemical form - and thereby its toxic properties - along its 2 route through the ecosystem in close relationship with the activity and the metabolic competence of the organisms through which it passes. Furthermore, one of the major inorganic arsenic species in water, arsenate, is chemically so similar to phosphate that organisms may be protected by phosphate from the toxicity of arsenate.