Jana Friedrich

Carbon export during the Spring Bloom at the Antarctic Polar Front, determined with the natural tracer 234Th

Profiles of particulate and dissolved 234Th were obtained during the JGOFS Southern Ocean expedition on R.V. Polarstern during October/November 1992. Measurements were made on three transects across the Antarctic Circumpolar Current, from the Polar Front (PF) in the north to the Weddell Sea/ACC boundary in the south. The dissolved 234Th238U ratio in surface waters gradually decreased during the development of the plankton bloom at the Polar Front. In the period between the first two transects, the 234Th activity removed from the dissolved phase had shifted to particles that had been produced, and as a result, the total activity ratio remained unchanged. The decrease in dissolved 234Th corresponds with decreases in dissolved nutrients and PC02, and with increases in chlorophyll and plankton biomass. Only during the third transect was the total activity of 234Th clearly reduced. The 234Th activity missing in the upper 100 m amounted to approximately 6 × 104 dpm m-2 at the Polar Front. In the 22-day period between the second and third transect, the 234Th export flux averaged 3200 dpm m−2 day−1. The ratio of organic carbon to 234Th on suspended particles was lower near the Polar Front than to the south, which we attribute to the higher abundance of empty diatom frustules at the Polar Front. With an average Corg/234Th ratio on suspended particles in surface water of about 20 μmol dpm−1, and assuming that the Corg/234 Th ratio on exported particles is 30–60% of this value, we estimate that 0.43–0.86 mol C m−2 had been removed over the 22-day period from the surface ocean by sinking particles. Export production was negligible in the Antarctic Zone including the ice edge, but during the later stage of the bloom in the Polar Front region, it amounted to 12–24% of primary production or 25–50% of the net CO2 uptake as estimated from a CO2 budget.

A two-tracer (210Po–234Th) approach to distinguish organic carbon and biogenic silica export flux in the Antarctic Circumpolar Current

We attempt to quantify and qualify the particle export from the surface water of the Antarctic Circumpolar Currentduring a spring phytoplankton bloom by the simultaneous use of the tracers 210Po and 234Th. We present data from theSouthern Ocean JGOFS expedition in 1992 at about 61W, from the marginal ice zone to the Polar Frontal region.Radionuclide export was calculated with a one-dimensional non-steady-state scavenging model. Rapidly changingactivities of 210Pb and 210Po during the phytoplankton bloom and the application of the tracer pair 210Po/210Pb withparticle-reactive parent and daughter required a new solution to the non-steady-state scavenging model. The observedfractionation of 210Po and 234Th on particles, dependent on particle composition (POC/biogenic silica ratio),corroborates the known preference of 210Po for cytoplasm. A combination of these two tracers can help to characterizethe nature (i.e. organic carbon and biogenic silica content) of the material settling out of the mixed layer, and thus toarrive at a more detailed interpretation of export fluxes than is possible with 234Th alone. In the Polar Front region,where diatoms are dominant in the phytoplankton and where the highest export rates were observed, we found apreferential settling of biogenic silica when heavily silicified diatom species occur. In contrast, POC and biogenic silicaare exported with comparable efficiency when diatom species with thinner frustules prevail. The export of biogenic opaland carbon is then closely coupled. In the southern Antarctic Circumpolar Current (sACC), where siliceous organismsare not dominant in the plankton, we found a preferential settling of siliceous material over POC.r2001 Elsevier ScienceLtd. All rights reserved.

Nutrient budgets for European seas: A measure of the effectiveness of nutrient reduction policies

Socio-economic development in Europe has exerted increasing pressure on the marine environment. Eutrophication, caused by nutrient enrichment, is evident in regions of all European seas. Its severity varies but has, in places, adversely impacted socio-economic activities. This paper aims to evaluate the effectiveness of recently adopted policies to reduce anthropogenic nutrient inputs to European seas. Nitrogen and phosphorus budgets were constructed for three different periods (prior to severe eutrophication, during severe eutrophication and contemporary) to capture changes in the relative importance of different nutrient sources in four European seas suffering from eutrophication (Baltic Proper, coastal North Sea, Northern Adriatic and North-Western Black Sea Shelf). Policy success is evident for point sources, notably for P in the Baltic and North Seas, but reduction of diffuse sources has been more problematic.

Nutrient uptake and benthic regeneration in Danube Delta Lakes

We investigated the nutrient uptake capacity of three lakes (Uzlina, Matita and Rosu) within the Danube Delta during high water level in June and low water level in September 1999. Special emphasis was placed on nutrient cycling at the sediment-water interface and on the self-purification capacity of the lakes in the Danube Delta. In order to estimate the nutrient uptake of selected lakes we present in this paper the results of water analyses, benthic flux chamber experiments and deck incubation experiments of 15N-labeled sediment cores at the inflow and the outlet of the lakes. The external input of dissolved inorganic nitrogen and silica into the lakes decreases with increasing distance to the main Danube branches whereas the total dissolved phosphorus input is independent of the hydrological distance to the main branches. The nutrient loading is highest in the inflow channels, and decreases towards the outflow of the lakes. In June, the uptake of NO3−, TDP and Si(OH)4 in the lakes was higher than in September. In contrast, NH4+ uptake was more intense in September, when benthic release was more intense as well. On average, about 76% of the external plus internal nitrogen and phosphorus input into the lakes was taken up by macrophytes and phytoplankton during the growing season, whereas the uptake of external nutrient input amounted to about 43%. The benthic release of ammonia and silica increases from June to September and indicates, that part of the nutrients taken up during the growing season might be released during winter. We estimate the net impact of the Delta on the nutrient reduction of the Danube during the growing season is about 4.3%, assuming 10% of the Danube water is flowing through the Delta.

Implications for chloro- and pheopigment synthesis and preservation from combined compound-specific δ 13 C, δ 15 N, and Δ 14 C analysis

Chloropigments and their derivative pheopig- ments preserved in sediments can directly be linked to pho- tosynthesis. Their carbon and nitrogen stable isotopic com- positions have been shown to be a good recorder of recent and past surface ocean environmental conditions tracing the carbon and nitrogen sources and dominant assimilation pro- cesses of the phytoplanktonic community. In this study we report results from combined compound-specific radiocar- bon and stable carbon and nitrogen isotope analysis to exam- ine the time-scales of synthesis and fate of chlorophyll-a and its degradation products pheophytin-a, pyropheophytin-a, and 13 2 ,17 3 -cyclopheophorbide-a-enol until burial in Black Sea core-top sediments. The pigments are mainly of ma- rine phytoplanktonic origin as implied by their stable isotopic compositions. Pigment 15 N values indicate nitrate as the major uptake substrate but 15 N-depletion towards the open marine setting indicates either contribution from N 2-fixation or direct uptake of ammonium from deeper waters. Ra- diocarbon concentrations translate into minimum and max- imum pigment ages of approximately 40 to 1200 years. This implies that protective mechanisms against decomposition such as association with minerals, storage in deltaic anoxic environments, or eutrophication-induced hypoxia and light limitation are much more efficient than previously thought.