Abundance and biomass of planktonic diatom Proboscia alata and associated species, chlorophyll a, organic carbon and nitrogen concentrations in waters over the Bering Sea middle shelf in August 2001

During most of the vegetation season from late May to early September large-sized diatom alga Proboscia alata forms local patches with high abundances and biomasses in different oceanographic domains of the eastern Bering Sea shelf. For 0-25 m layer average abundance and biomass of species in these patches are 700000 cells/l and 5 g/m**3 (wet weight), while corresponding estimates for the layer of maximal species concentrations are 40000000 cells/l and 38 g/m**3 (wet weight) or 1.6 g C/m**3. These levels of abundance and biomass are typical for the spring diatom bloom in the region. Outbursts of P. alata mass development are important for the carbon cycle in the pelagic zone of the shelf area in the summer season. The paradox of P. alata summertime blooms over the middle shelf lies in their occurrences against the background of the sharp seasonal pycnocline and deficiency in nutrients in the upper mixed layer. Duration of the outbursts in P. alata development is about two weeks and size of patches with high abundances can be as large as 200 km across. Degradation of the P. alata summertime outbursts may occur during 4-5 days. Rapid sinking of cells through the seasonal pycnocline results in intense transport of organic matter to bottom sediments. One of possible factors responsible for rapid degradation of the blooms is affect on the population by ectoparasitic flagellates. At terminal stages of the P. alata blooms percentage of infected cells can reach 70-99%.

Layer description, carbon and nitrogen content and stable isotope record of bulk sedimentary organic matter from Laguna Potrok Aike

An investigation of stable isotope (d13C TOC and d15N TN) and elemental parameters (TOC, TN contents and TOC/TN ratios) of bulk organic matter (<200 µm) from sediment cores recovered from the Patagonian lake Laguna Potrok Aike (Argentina) in the framework of the ICDP deep drilling project PASADO provided insights into past changes in lake primary productivity and environmental conditions in South Patagonia throughout the last Glacial-Interglacial transition. Stratigraphically constrained cluster analyses of all proxy parameters suggest four main phases. From ca 26,100 to 17,300 cal. years BP, lacustrine phytoplankton was presumably the predominant organic matter source in an aquatic environment with low primary productivity rates. At around 17,300 cal. years BP, abrupt and distinct shifts of isotopic and elemental values indicate that the lacustrine system underwent a rapid reorganization. Lake primary productivity (phytoplankton and aquatic macrophytes) shows higher levels albeit with large variations during most of the deglaciation until 13,000 cal. years BP. The main causes for this development can be seen in improved growing conditions for primary producers because of deglacial warming in combination with expedient availability of nutrients and likely calm wind conditions. After 13,000 cal. years BP, decreased d13C TOC values, TOC, TN contents and TOC/TN ratios indicate that the lake approached a new state with reduced primary productivity probably induced by unfavourable growing conditions for primary producers like strengthened winds and reduced nutrient availability. The steady increase in d15N TN values presumably suggests limitation of nitrate supply for growth of primary producers resulting from a nutrient shortage after the preceding phase with high productivity. Nitrate limitation and consequent decreased lacustrine primary productivity continued into the early Holocene (10,970-8400 cal. years BP) as reflected by isotopic and elemental values.