Primary production, sedimentation and resuspension in large shallow Lake Võrtsjärv Peeter Nõges, Lea Tuvikene, Tiina Nõges and Anu Kisand Institute of Zoology and Botany of the Estonian Agricultural University, Võrtsjärv Limnological Station, Rannu, EE-2454 Tartumaa, Estonia and Institute of Zoology and Hydrobiology, Tartu University, Vanemuise 46,EE-2400 Tartu, Estonia Key words: Sedimentation, resuspension, nitrogen, phosphorus, suspended solids, shallow lake. ABSTRACT Phytoplankton primary production measurements and sediment trapping in the large (270 km 2 ) shallow (mean depth 2.8 m) Lake Võrtsjärv were performed during ice-free periods in 1995 – 1996. Sedimentation rate varied from 26 to 700 g DW m –2 d –1 (DW = dry weight), with a mean value of 170 g DW m –2 d –1 . Under the influence of prevailing westerly and south-westerly winds, the sedi- mentation rate was higher in the northern part, and lower in the western and southern parts of the lake. An increasing trend towards autumn could be followed in both years. The mean sedimenta- tion rates in the dry year 1996 with a low lake water level exceeded those of the “normal” 1995 by a factor of more than two. New sedimentation formed 10 – 15 % of the downward sediment flux in 1995 and 4 – 6 % in 1996, while the major part of entrapped matter originated from resuspended bottom sediments. The dynamics of gross sedimentation showed no relation to the primary pro- duction but correlated significantly with the concentration of suspended solids, with the average wind speed and with the mean depth of the lake during trap exposition. The composition of entrapped matter did not show seasonal variation but differed between the sampling stations. The amount of particulate organic matter (POM) per DW varied from 11 % at places where the bottom sediment consisted of moraine or sand to 28 % at places with muddy bottom. N : POM was rather stable (45 – 61 mg g –1 , C var = 6 %). The variability of P : POM (3.5 – 7.1, C var = 20 %) exceeded that of P:DW (0.75 – 1.31 mg g –1 , C var = 10 %) showing the essential role of mineral-bound P in the settling material. In these conditions, the N : P ratio was determined by the POM content of settling matter. POM : DW in settling material was generally higher than in the 0 – 2 cm layer of bottom sediments. Introduction In lakes dominated by large-sized or filamentous phytoplankton, sedimentation often forms the main loss process of particulate primary production. This can be termed fresh, new (Sanford, 1992), or primary sedimentation that contrasts with the repeated sedimentation of resuspended particles. New sedimentation plays an important role in the sea (Wassmann, 1990; Wassmann et al., 1990) in deep lakes Aquat.sci.61 (1999) 168 – 182 1015-1621/99/020168-15 $ 1.50+0.20/0 © Birkhäuser Verlag, Basel, 1999 Aquatic Sciences