[1] The biological pump affects atmospheric CO 2 levels and fuels most heterotrophic activity in the deep ocean. The efficiency of this pump depends on the rate of carbon fixation, export out of the euphotic zone and the depth of respiration. Here we study the depth dependence of respiration patterns, hence particulate carbon flux, using a compiled data set of sediment oxygen consumption rates. We show that the depth relationship can best be described by a double exponential model. For the upper part of the ocean, our resulting equation is similar to previous fluxdepth relationships but predicted fluxes are significantly larger in deeper waters. This implies a more efficient biological pump. Total oceanic respiration below the shelf break (200 m) is estimated to be 827 Tmol O 2 yr À1.
Benthic fluxes of nutrients and metals were measured in the coastal zone of the north-western Black Sea, which is influenced by the Danube and Dniestr rivers. The results from the benthic flux chambers deployed during two EROS 21 cruises in summer 1995 and in spring 1997 yield information on benthic nutrient cycling and diagenetic pathways at the sediment-water interface. This information is discussed in the light of benthic activity as well as pore-water data. The benthic recycling of nutrients varied seasonally due to the availability of oxygen and organic material, and spatially due to river influence. Areas of high benthic fluxes near-shore and of low benthic fluxes offshore on the shelf were distinguished. Nutrients and suspended particulate matter discharged by the Danube is kept nearshore by the coastal current. The oxygen concentrations in the bottom water varied strongly between the two seasons. In summer, the bottom water at the near-shore stations turned anoxic. In spring, the higher oxygen concentrations are related to more intense mixing of the water column due to stormy periods and high river discharge. Highest oxygen concentrations were found on the offshore continental shelf. However, the benthic oxygen consumption rates in spring (13-23 mmol m 2 day 1 ) were as high as in summer. Areas with highest nutrient concentrations in the overlaying bottom water were found at the Danube delta front and Danube prodelta. On the Danube delta front and the Dniestr mouth, ammonia fluxes were lower in spring (1·1-1·7 mmol m 2 day 1 ) than in the summer (2·6-4·4 mmol m 2 day 1 ) due to higher nitrification rates and the lower influx of organic matter. In spring, the concentrations of dissolved iron and manganese in the bottom water were one order of magnitude lower than during the summer. This decrease in flux rates is related to the higher oxygen concentrations in the bottom water in spring. A rough comparison of the river's nutrient load and the benthic recycling over a certain area showed that the near-shore benthic phosphate and silica recycling account for 50% and 35% of the Danube input in summer, respectively. The fluxes of ammonia from benthic recycling and from the Danube discharge were at the same order of magnitude in both seasons. Nitrogen is introduced by the rivers mostly as nitrate. Benthic recycling is the dominant source of ammonia. The Danube input is phosphate deficient. In brief, benthic nutrient recycling is an important factor in sustaining high productivity of the system.
The influence of the River Danube as a major source of nutrients and suspended solids to the continental shelf of the Black Sea has been analyzed. In the framework of the EC project EROS-2000, sediment cores from 33 stations on the northwestern continental shelf were sampled in August 1995. Spatial patterns in structural sediment characteristics, macrobenthos composition and benthic mineralization rates were examined using ordination techniques. Three general areas could be distinguished according to the distribution of abiotic sediment characteristics and macrobenthos community:(1) The area just in front of the Danube Delta where large amounts of nutrients and suspended solids are discharged. High sedimentation rates of fine-grained sediments and high benthic mineralization rates characterized this area. The macrobenthos community was dominated by deposit feeders.(2) The northern part of the continental shelf where an anticyclonic gyre is located. The majority of the Danube discharges are transported to this region. This area was characterized by low sedimentation rates. However, the deposited material contained a larger fraction of fresh organic matter compared to the delta area, resulting in high benthic mineralization rates. Suspension feeders dominated the macrobenthos community. (3) The southern part of the continental shelf was characterized by low sedimentation rates and low rates of benthic mineralization. In this area suspension feeders dominated the macrobenthos community. Oxygen fluxes into the sediment ranged between 2 and 52 mm01 0' m-' d-' (average 21 mm01 O2 m-' d-l) decreasing with water depth. Macrobenthos accounted for 20% of the total benthic oxygen consumption. In the northern part of the continental shelf and in the coastal stations, microorganisms, and micro-and meiobenthos dominated benthic community respiration, while macrobenthos became relatively more important in terms of oxygen consumption in the southern part of the continental shelf.
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