THE ROLE OF ALLOCHTHONOUS AND AUTOCHTHONOUS CARBON IN LADOGA – THE RIVER – NEVA ESTUARY SYSTEM S.M. Golubkov1, M.S. Golubkov1, A.V. Tiunov2 1Zoological Institute of Russian Academy of Sciences, St. Petersburg, 199034, Russian Federation, [email protected] 2A.N. Severtsov Institute of Ecology and Evolution of Russian Academy of Sciences, Moscow, Russian Federation, [email protected]

It is well known, that in many water bodies local primary production is less than community respiration. This heterotrophy is sustained by supply of external (often terrestrial) resources in the form of dissolved and particulate organic carbon. The availability of two distinct sources of carbon (local pri- mary production and external subsidies) allows uncoupling of primary and secondary production and may have major consequences for food web dy- namics. However, there is an opinion that allochthonous carbon has a minor importance in grazing food webs due to its recalcitrance. Studies of the rela- tive importance of allochthonous vs. autochthonous carbon resources for food webs revealed major but variable terrestrial subsidy of the lake and estuarine food webs (Pace et al., 2004; Van den Meersche et al., 2009). In particular, the role of allochthonous carbon in lake metabolism and food webs can be negligible in eutrophic waters (e.g., Jones, 1992). The Neva Estuary receives water from the Neva River, a relatively short canal (74 km) between Lake La- doga and the Gulf of . Recent investigations show that present stage of development of the Ladoga ecosystem is characterized by a high intensity of decomposition of suspended and dissolved organic matter discharged by the drainage area to the system (Petrova et al., 2010). Thus, the discharge of a large amount of organic carbon of terrestrial origin from Lake Ladoga to the Neva Estuary is quite expected. On the other hand, primary productivity and biomasses of autotrophic and heterotrophic organisms in the Neva Estuary are among the highest in the Baltic (Golubkov M., 2008; Golubkov S., Alimov, 2010). Intensive cyanobacteria blooms are observed in the middle part of the estuary in last decade. The aim of the study was to evaluate the contribution of autochthonous vs. allochthonous carbon resources for community metabolism and benthic food webs in the terminal part of the Lake Ladoga – Neva River – eastern system: the Neva Estuary. We tested the hypothesis (1) that the contribution of allochthonous carbon leaching from watershed of the sys- tem may be important for benthic food webs even in highly eutrophic estua- rine waters and (2) that autochthonous carbon produced during cyanobacteria blooms may be effectively utilized by zoobenthos. We ascertained the origin and utilization of organic carbon in the benthic and pelagic habitats in the Ne-

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To remove this notice, visit: www.iceni.com/unlock.htm va Estuary by analyzing the metabolism in the pelagic zone and using stable isotope analysis (SIA) of C and N of zoobenthos and suspended matter (ses- ton) consisting of phytoplankton and detritus, both brought by Neva River waters and formed in the estuary. Isotopic signatures (δ13C) of seston and most of the dominant species of zoobenthos in Neva Bay (upper part of the estuary) were similar to the signature of terrestrial carbon (−27 ‰) coming from watershed. Decomposition of organic matter (OM) in Neva Bay was twice higher than autochthonous OM (primary) production. In the lower part of the Neva Estuary, Middle estuary, δ13C signature of seston was distinctly higher than in the Neva Bay. Especially high δ13C value (–21 ‰) was regis- tered in the local blooms of cyanobacteria. However, primary production in the middle part of the estuary was lower than the rate of decomposition of OM indicating considerable income of allochthonous OM to this part of the estuary. Most species of zoobenthos had δ13C values similar to those of seston in hypolimnion that were notably lower than δ13C values of cyanobacteria. The fraction of cyanobacteria in the zoobenthos diet estimated by SIA was only about 24 %. Therefore, OM created during cyanobacteria blooms was not very important as a food for zoobenthos. The pelagic-benthic decoupling is possibly related to the changes in the phytoplankton composition. Diatoms that dominated phytoplankton before late 1990s are fast sinking species, whose primary production supply benthic community by fresh organic matter. In contrast, cyanobacteria, which domi- nate nowadays, are very slow sinking species with adaptation to floating and movement in the water column. As a result, their primary production decom- posed mostly in pelagic zone. This was indicated by the four times higher concentration of cyanobacteria phycocyanine in the upper layer of water (epi- limnion) as compared to the lower layer (hypolimnion) and considerably low- er δ13C values of seston in the epilimnion than in the hypolimnion. As a re- sult, OM creating during cyanobacteria blooms was not very important as a source of food for zoobenthos. The study shows the importance of alloch- thonous OM in carbon cycling of the Neva Estuary.

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