SUMMARY1. The impact of changes in subn\erged macrophyte abundance on fish-zooplanktonphytoplankton interactions was studied in eighteen large-scale (100 m-) enclosures in a shallow eutrophic lake. The submerged macrophytes comprised Potamotegon pectinatus L., P. pusillus L. and Callitriche hermaphroditica L. while the fish fry stock comprised threespined sticklebacks, Gasterosteus aculeatus L., and roach, Rutilus rutilus L. 2. In the absence of macrophytes zooplankton biomass was low and dominated by cyclopoid copepods regardless of fish density, while the phytoplankton biovolume was high (up to 38 mm' 1') and dominated by small pennate diatoms and chlorococcales. When the lake volume infested by submerged macrophytes (PVI) exceeded 15-20% and the fish density was below a catch per unit effort (CPUE) of 10 (approx. 2 fry m"^), planktonic cladoceran biomass was high and dominated by relatively large-sized specimens, while the phytoplankton biovolume was !ow and dominated by small fastgrowing flagellates. At higher fish densities, zooplankton biomass and average biomass of cladocerans decreased and a shift to cyclopoids occurred, while phytoplankton biovolume increased markedly and became dominated by cyanophytes and dinoflagellates. 3. Stepwise multiple linear regressions on log-trans formed data revealed that the biomass of Daphnia, Bosmina, Ceriodaphnia and Chydorus were all significantly positively related to PVI and negatively to the abundance of fish or PVI x fish. The average individual biomass of cladocerans was negatively related to fish, but unrelated to PVI. Calculated zooplankton grazing pressure on phytoplankton was positively related to PVI and negatively to PVI X fish. Accordingly the phytoplankton biovolume was negatively related to PVI and to PVI X zooplankton biomass. Cyanophytes and chryptophytes (% of biomass) were positively and Chlorococcales and diatoms negatively related to PVI, while cyanophytes and Chlorococcales were negatively related to PVI x zooplankton biomass. In contrast diatoms and cryptophytes were positively related to the zooplankton biomass or PVI x zooplankton. 4. The results suggest that fish predation has less impact on the zooplankton community in the more structured environment of macrophyte beds, particularly when the PVI exceeds 15-20%. They further suggest that the refuge capacity of macrophytes decreases markedly with increasing fish density (in our study above approximately 10 CPUE). Provided that the density of planktivorous fish is not high, even small improvements in submerged macrophyte abundance may have a substantial positive impact on the zooplankton, leading to a lower phytoplankton biovolume and higher water transparency. However, at high fish densities the refuge effect seems low and no major zooplankton mediated effects of enhanced growth of macrophytes are to be expected. 255 P. Schriver et alThis further emphasizes the usefulness of macrophyte refuges as a lake-restoration tool in shallow lakes, but also demonstrates the shortcomings of the method, if t...

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Quantifying P losses to surface waters at different scales and partitioning of the loads into P losses from point sources and diffuse sources are significant future challenges for river basin managers. The agricultural share of P losses to surface waters is, in many river basins, increasing and therefore becoming more important to quantify and analyse. The importance of phosphorus losses from agricultural land was analysed using monitoring data and two different models for 35 micro-catchments (<30 km 2 ) in the Nordic-Baltic region of Europe, 17 European macro-catchments (250-11 000 km 2 ) and 10 large European river basins (>50 000 km 2 ). Average annual phosphorus loss from agricultural land in the micro-catchments varied from 0.1 to 4.7 kg P ha )1 and showed no relationship with the short-term P surplus on agricultural land. The average annual total P loss from agricultural land showed equally large variation in the 17 macro-catchments (0.1-6.0 kg P ha )1 ), but the range was less for the 10 larger river basins (0.09-2.0 kg P ha )1 ). The annual P loss from the 35 micro-catchments was greatest in the micro-catchments characterized by soil erosion and a high proportion of surface run-off as in the Norwegian catchments. The same pattern was true for the 17 macro-catchments where the model-simulated total P loss from agricultural land was greatest in the catchments in northern and southern parts of Europe. The main diffuse pathways for total P loads in the 17 macro-catchments were simulated with the MONERIS model. On average, soil erosion and surface run-off was estimated to have contributed 53% (4.1-81%), groundwater 14% (0.2-41.7%) and tile drainage water 3% (0-14.0%).

We validated an existing physically based 3D MIKE SHE groundwater resource model (DK-model) at 175 Danish gauging stations covering different catchment sizes in order to calculate monthly water runoff in the 50% ungauged part of Denmark. Model performance was in most cases good (61% of gauging stations had a Nash-Sutcliffe (NS) coefficient >0.60) but nevertheless showed a large seasonal and georegion specific bias. Therefore, bias correction factors had to be developed before applying the DK-model simulations of runoff in the ungauged areas. Simulated monthly runoff from ungauged areas and the measured monthly runoff from 178 gauging stations were distributed to 2663 smaller Hydrological Units (ca. 15 km(2)) and linked with a new empirical model for flow-weighted monthly total nitrogen (TN) concentrations (R(2) = 0.43; P < 0.0001) developed based on 20 years of observations (1990-2009) in 83 small catchments for calculation of monthly gross diffuse TN-loads from HU's. Nitrogen retention was calculated in streams, lakes and wetlands utilising both lake specific models and rate coefficients to calculate N retention in surface water bodies. The whole model complex was linked in the DK-QN concept for simulation of monthly TN losses from point sources and diffuse sources, TN retention and resulting loadings to Danish coastal waters. The DK-QN model was validated in 118 gauged catchments and the model simulations had for >25% of the observations of monthly discharge weighted TN concentrations a NS larger than 0.26. Catchment specific monthly TN-loadings were modelled with a higher performance as 50% of the catchments had a NS greater than 0.75. The model concept allows calculation of N retention in streams, lakes and wetlands and the average annual model calculated N retention amounted to 21% of the modelled gross riverine TN loadings. The average annual gross TN loading to surface freshwater in Denmark derived from diffuse sources amounted to 97 000 tonnes N (91% of gross TN loadings) which is 54% of the total estimated N-leaching from the root zone on the Danish land area (212 000 tonnes N) during the period 1990-2009.

Quantitative knowledge of background nitrogen (N) concentrations and loadings is essential for as it determines the minimally disturbed conditions for N in surface waters as a near reference condition. To determine background N concentrations, a total of 39 smaller Danish streams draining relatively undisturbed catchments in different regions were selected and screened for N concentrations and discharge during a 1-year period (2004)(2005). Only 19 of the streams fulfilled the threshold set for least disturbed conditions (LDC) in their catchments (proportion of agricultural land <10%). Within the five sampled dominant landscape types in Denmark, the concentrations of ammonium N and total organic N in the LDC streams were found to be nearly constant: 0.05 ± 0.01 mg N L −1 and 0.53 ± 0.07 mg N L −1 , respectively. In contrast, the concentration of nitrate N differed significantly (P < 0.05) between the five landscape types, with variations between 0.06 and 0.83 mg N L −1 . An analysis of all the 39 streams sampled demonstrated significantly different relationships between the proportion of land use and the flow-weighted annual concentration of nitrate N for two dominant soil types with a Y-axis intercept equal to 0.12 mg N L −1 for streams draining coarse textural soils and 0.76 mg N L −1 for streams draining finer textural soils. A significant (P < 0.05) downward trend (18-41%) emerged for flow-weighted annual total N concentrations in four of the five streams that were sampled during the period 1990-2011. A 5 Â 5 km grid map of Denmark showing nitrate N and total N background concentrations was produced and used for estimating background N losses to Danish surface waters. Based on the grid map, the average annual background loss of total N was calculated to be 13,000 tonnes N or 19% of the total N loading to Danish coastal waters during the period 2004-2005.