Plankton and Primary Production in the Lena River Estuary and in the South-eastern Laptev Sea

Abstract

Primary production in the lower Lena River and its delta was high (50–160 mg Cm −3 day −1), while its integral values were moderate (0·1–0·3 g Cm −2 day −1) because of a low water transparency. In the river itself, phytoplankton was dominated by diatoms and picocyanobacteria with a biomass of 400–500 mg m −3. In waters of the Lena Estuary (Buor-Haya Bay) with surface salinity of 2–7, the surface primary production ranged from 75 to 640 mg Cm −3 day −1. The bulk of the surface primary production was contributed by the symbiotic ciliate Mesodinium rubrumwhich attained a biomass of 500–1500 mg m −3, together with a biomass of algae of about 100 mg m −3. In the open South-east Laptev Sea, influenced by the Lena and Yana Rivers, primary production at the surface was 37–98 mg Cm −3 day −1. To the west, off the Novosibirsk Islands, primary production was mod-erate; 11–23 mg C m −3 day −1, decreasing at the shelf edge to 3–6 mgC m −3 day −1. The total number of bacteria in the River and Estuary was over 1·5×10 6cells ml −1, the wet biomass was over 150 mg m −3, and daily production of bacteria was 10–50 mg m −3. In the more saline waters of the open Laptev Sea, the microbial biomass was 5–10 times less than in the Estuary. The planktonic protozoa were represented by zooflagellates (biomass 70–100 mg m −3), by Strombidiumand Tintinnopsis, genera of heterotrophic ciliates (biomass 10–70 mg m −3), and by the phototrophic symbiotic ciliate M. rubrum(biomass 300–1500 mg m −3). The mesozooplankton was scarce in the Lena River (20–60 mg m −3) while, in the brackish estuarine waters, its biomass rose to 300–1700 mg m −3. In the open Laptev Sea influenced by rivers, mesozooplankton biomass was also high; 300–400 mg m −3. The influence of river drainage upon the pelagic ecosystem of an arctic estuarine basin is discussed in the present paper.