Long term functional changes within the Oligochaeta communities within the Danube River Delta,Romania

Within the Danube River delta's lakes the Oligochaeta communities comprise between 7.9% and 36.2% of the total biomass of benthic fauna. Their importance in energy flow at the ecosystem level changed in relation to fast trophic transition of all shallow lakes to the hypertrophic state. The parameters of the energy budget of the dominant populations and the potential production of benthivorous fish species assessed during 1976–1994 interval support this conclusion. P/B ratio, K ₁ and K ₂ coefficients assessed for both the 1976–1980 and 1991–1994 intervals revealed different functional patterns of response of Potamothrix hammoniensis (Michaelsen, 1901) and Limnodrilus hoffmeisteri (Claparede, 1862) to varying trophic conditions. Changes in communities structure, size of the constituent populations and their age distribution, as well as the quantity and quality of food supply and level of hypoxia at the sediment/water interface were the main factors that affected the role of these populations as energy carrier from the huge energy pool represented by sedimented organic carbon to benthivorous fish species.

     

Ecological Significance of Nitrogen Cycling by Tubificid Communities in Shallow Eutrophic Lakes of the Danube Delta

Importance of tubificid populations on nitrogen cycle in two categories of shallow eutrophic lakes in the Danube Delta was quantitatively assessed for the 1992-1993 period. The structure of the primary producers in the studied lakes was used to discriminate between the two categories:(i) lakes dominated by macrophytes (A₁) and (ii) lakes dominated by phytoplankton (A₂). In both categories tubificid worms represented important fraction of the entire benthic community (35 and 32%, respectively, as number of individuals). They influence the sediment-water exchange of nutrients. The main processes involved are excretion of nutrients and their continuous release from sediments by molecular diffusion or through channels created by bioturbation. Inorganic nitrogen released from bottom sediments may regulate nitrogen load in the water body and thus, phytoplankton production. In 1992-1993, nitrogen stocks in tubificid biomass accounted for 5.3% in A₁ lakes and 15.6% in A₂ lakes of the amount stocked in phytoplankton, and only for 1.2 and 2.9% respectively, of the nitrogen load in water body. Nitrogen excretion rates ranged between 60.52 and 153.74 mg N m^–2 year^–1, and release rates from sediments between 378.26 and 960.87 mg N m^–2 year^–1, the lowest values being recorded for A₂ category. Differences are related to tubificid biomass, structure and abundance of primary producers and to nutrient load in different ecosystems. Ratios between release rate of inorganic nitrogen by tubificid worms and sedimentation rate of organic nitrogen in the two categories of lakes were 8.3 and 6.4% respectively. Contribution of nitrogen released daily from sediments to the dissolved inorganic nitrogen load in the water column was less than 0.5%. However, in A₁ and A₂ lakes, the released nitrogen had a potential to sustain 24.74 and 8.01%, respectively, of the annual phytoplankton production. These values suggest the significance of tubificids in keeping the eutrophication process at a high level, especially during the periods when nitrogen is the main limiting factor for phytoplankton production.     

Nitrogen and Phosphorous Excretion Rates by Tubificids from the Prahova River (Romania)

Nitrogen and phosphorous exchange at the water–sediment interface is controlled both by complex physico-chemical factors and biological processes. Zoobenthos excretion is one of the most important processes in the mineralization of sedimented organic mater. In polluted freshwaters, tubificid worms are among the dominant components of the benthic community. Rates of ammonium and inorganic phosphate excretion by tubificids were experimentally assessed. They were related to the tubificid abundance in a stream ecosystem polluted with municipal and industrial wastewater. The relationship between these rates and temperature were investigated within the range of 4–23 °C. Relatively constant excretion rates were obtained for both nutrients in the first 8 h of excretion, ranging between 0.076 and 0.226 μg N mg d.w.⁻¹ h⁻¹ and 0.0065–0.01 μg P mg d.w.⁻¹ h⁻¹_, respectively. Q₁₀ values of 2.52 for ammonium and 1.31 for phosphate were calculated. If we presume that all excreta eventually enters the water column, then we can calculate that these invertebrates potentially add 39.17 mg N m⁻² day⁻¹ and 0.49 mg P m⁻² day⁻¹. These values accounts for 17.16 and 7.56% of the nutrient load in the river water, respectively.