Seasonal and spatial variations of carbon and nitrogen distribution in the surface sediments of the Gulf of Riga, Baltic Sea Juris Aigars a,b , Rolf Carman a, * a Department of Geology and Geochemistry, Stockholm University, 106 91 Stockholm, Sweden b Institute of Aquatic Ecology, Latvian University, LV-2163 Salaspils, Latvia Received 30 August 1999; accepted 4 April 2000 Abstract The variations in concentrations of carbon and nitrogen in surface sediments of the Gulf of Riga were investigated between December 1993 and January 1995. The sediment samples were taken nine times during this period at two sampling sites. One sampling site, G5, exhibited high abundance of burrowing amphipods, whereas at the second site, T3, the number of macrozoobenthic organisms was comparatively small. Similar vertical pro®les of mean sediment dry-weight concentrations of total carbon TC) and total nitrogen TN) were obtained at both sites G5 and T3. However, during autumn±winter considerable dierences of TC and TN concentrations in surface sediments 0±2 cm) between sites were observed. This was probably the eect of dierences in the bioturbation level. During summer, when decline in numbers of amphipods were recorded, the vertical pro®les of TC and TN at site G5 were similar to those at site T3. Signi®cant dierences between months were detected for TN at site T3 re¯ecting sedimentation of spring and au- tumn blooms in April and late October±early November, respectively. This was supported also by lower atomic C/N ratios in surface sediments during corresponding sampling events. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Carbon; Nitrogen; Sediments; Baltic sea; Gulf of Riga 1. Introduction The fraction of the organic matter produced during primary production reaching the sea bottom is generally related to water-column depth and varies from 10% to 80% in coastal and estuarine sediments Suess and M uller, 1980; Suess, 1980). Subsequent remineralisation of organic material in the sediments and release of dis- solved nutrients into the overlying water column can supply a signi®cant portion of the nutrient requirements for primary producers in shallow coastal areas Zeitz- schel, 1980; Klump and Martens, 1987). However, sed- iments act not only as a source of nutrients but also as a sink. The sedimentation rate and quality of organic matter in¯uence to dierent degrees the abundance of macro- benthic organisms Cederwall and Elmgren, 1980) and microbial activity Graf, 1987), and can change the preservation of organic material in sediments due to altered redox conditions e.g., Jonsson et al., 1990). Many workers have examined the eects of organic matter addition on the sediment nutrient recycling pro- cesses e.g., Garber, 1984; Banta, 1992; Carey et al., 1993). The previous studies indicated a wide range of the half-life of added organic particles, most likely due to Chemosphere 43 2001) 313±320 * Corresponding author. Tel.: +1-46-8-164-738; fax: +1-46- 8-674-7855. E-mail address: rolf.carman@geo.su.se R. Carman). 0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII:S0045-653500)00150-8