Benthic Nutrient Cycling and Diagenetic Pathways in the North-western Black Sea

Abstract

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.

Nitrogen budget of the northwestern Black Sea shelf inferred from modeling studies and in situ benthic measurements

“…1): within the Danube river plume (BS 95/8/18, BS 97/5/11, BS 98/5/5), on the Danube Prodelta (BS 95/8/20,BS 97/5/14), in the Dniestr mouth (BS 95/8/14, BS 97/5/17), on the offshore shelf (BS 97/5/19, BS 98/5/6) and at the shelf edge (BS 95/8/9, BS 95/8/22, BS 97/5/24) using a benthic flux chamber lander and in situ dialysis porewater samplers. The 1995 benthic flux data were taken from Friedl et al (1998) and those for 1997 from Friedrich et al (2002).…”

“…The functioning of the benthic lander is described in detail in Tengberg et al (1995) and Friedrich et al (2002). The lander with the 2 stainless steel flux chambers was lowered to the bottom, tethered to a free floating buoy, and retrieved after 24 h. An electronic system triggers up to 15 mechanical functions.…”

“…These 2 data sets will be complemented by the results of an INTAS cruise conducted in May 1998. Measurements of benthic fluxes with in situ flux chambers during the EROS-21 and INTAS expeditions revealed large spatial differences in benthic nutrient regeneration (Friedl et al 1998, Wijsman et al 1999, Friedrich et al 2002 in response to the position of the river plume and the associated primary production, the intensity of the vertical mixing and the water depth.…”

“…Mass balance calculations of the nitrogen fluxes measured during EROS-21 indicate that denitrification may be an important remineralization pathway in summer, with denitrification rates in the range of 1 to 2 mmol N 2 m -2 d -1 (Friedrich et al 2002). This is confirmed by the N:P ratios of measured benthic fluxes (1.8 to 7.4) in summer which are far lower than the Redfield ratio (16) and the ratio of the Danube (25) (Friedl et al 1998).…”

Organic matter governs N and P balance in Danube Delta lakes

“…Using an average discharge of 6,460 m 3 s -1 (ICPDR 2004), transformation in the delta amounted in a reduction of the river's N and P load by *10,300 t N year -1 and *200 t P year -1 , representative for abstraction of 3% N and 0.7% P from the total riverine N and P load per year to the Black Sea (Teodoru et al 2007). Hence, the delta represents a sink for N, whereas a minor sink for P. The immobilization of N and P in the delta lakes was overall lower than expected from former studies (Friedrich et al 2002(Friedrich et al , 2003, because we accounted for the presence of organic and particulate N and P.…”