Petri Ekholm

Influence of dietary soy and phytase levels on performance and body composition of large rainbow trout (Oncorhynchus mykiss) and algal availability of phosphorus load

A feeding trial was designed to evaluate the influence of partial replacement of fish meal (FM) protein for soy-derived protein in large rainbow trout fed practical, high-energy diets with and without supplemental phytase. A 2×2 factorial arrangement with two soy levels (0% and 69.4% of the dietary protein from soybeans) and two phytase levels (0 and 1200 U kg−1) was used. Soy protein was derived from soy protein concentrate (SPC) and soybean meal (SBM) with a SPC:SBM protein ratio of 4:1. Diets were formulated to contain 36% and 28% crude protein and fat, respectively, and supplemented with lysine and methionine, but not with phosphorus (P). Consequently, the dietary P contents were 10.5 and 6.9 g P kg−1 for diets without and with soy-derived proteins, respectively. Three replicate groups of fish per treatment were hand-fed once daily to apparent satiety for 24 weeks. Fish grew from 0.25 kg to an average of 2.02 kg, with soy-fed fish being significantly larger at the end of the trial. Phytase had no influence on weight gain of fish. Dietary treatments did not affect feed efficiency, which averaged 0.90. Percent bone ash was statistically significantly lower in fish fed soy diets than in fish fed FM diets (means 56.4 and 57.7), but weight performance and whole body composition analyses did not confirm the modest P deficiency in fish fed soy without supplemental phytase. Phytase supplementation did not significantly increase bone ash of fish fed soy diets. P load significantly decreased from 8.5 to 4.6 g P kg−1 weight gain due to the partial replacement of FM protein for soy protein. Algal availability of P was lower with soy-based diet than with FM-based diet (23% vs. 35%), and lower in fecal matter of fish fed soy-based diets than in fish fed FM-based diets (9% vs. 27%). Results from this study show that a significant part of FM can be replaced by soy proteins for low-pollution diets, without compromising weight gain or feed efficiency in large rainbow trout fed practical diets.

Retaining agricultural nutrients in constructed wetlands—experiences under boreal conditions

Constructed wetlands are among the eutrophication abatement measures for which farmers may receive public subsidies in Finland. To assess the performance of constructed wetlands under boreal conditions, we monitored the retention of total suspended solids (TSS), total P (TP), dissolved reactive phosphorus (DRP), total N (TN), nitrate+nitrate-N (NOx-N) and ammonium-N (NH4-N) for 15–26 months in three constructed wetlands located in southern Finland. Annual retentions were −5–72% for TSS, −6–67% for TP, −33–33% for DRP, −7–40% for TN, −8–38% for NOx-N and −50–57% for NH4-N. The constructed wetland with the longest water residence time (WRT) showed the best performance, retaining annually about 25 kg of TP and 300 kg of TN per hectare. In contrast, the constructed wetland with the shortest WRT functioned only occasionally and was a net source for DRP and NOx-N. In addition to the WRT, high P sorption capacity of constructed wetland soil and high input concentrations appeared to promote retention. Vegetation had a limited effect, input load being transported mostly outside the growing season. When carefully designed and located, constructed wetlands may efficiently reduce nutrient loading from agriculture.

Evaluation of the accuracy and precision of annual phosphorus load estimates from two agricultural basins in Finland

Abstract The accuracy and precision of phosphorus load estimates from two agricultural drainage basins in western Finland were evaluated, based on continuous flow measurements and frequent flow-proportional sampling of total phosphorus concentration during a 2 year period. The objective was to compare different load calculation methods and to evaluate alternative sampling strategies. An hourly data set of concentrations was constructed by linear interpolation, and these data were used in Monte Carlo runs for producing replicate data sets for calculating the accuracy and precision of load estimates. All estimates were compared with reference values computed from the complete hourly data sets. The load calculation methods based on summing the products of regularly sampled flows and concentrations produced the best precision, whereas the best accuracy was achieved using methods based on multiplying annual flow by flow-weighted annual mean concentration. When comparing different sampling strategies, concentrating sampling in high runoff periods (spring and autumn) was found to give better accuracy and precision than strategies based on regular interval sampling throughout the year. However, the best result was obtained by taking samples flow-proportionally within the highest peak flows plus additional regular interval (e.g. biweekly) samples outside these flow peaks. Using this strategy, which calls for automatic sampling equipment, accuracies better than 5% and precisions better than 10% can be achieved with only 30–50 samples year−1.

Determining algal-available phosphorus of differing origin: routine phosphorus analyses versus algal assays

Eutrophication of surface waters can be accelerated by anthropogenic P-inputs, provided that P is in a form available to aquatic primary producers. Potentially algal-available P (Paa) under aerobic conditions was determined with a dual-culture assay from 172 samples representing P in point and nonpoint sources and in lacustrine matter. The availability of P – expressed as the proportion of Paa in total P (Tot-P) – ranged from 0 to 100%. In the different P sources, the mean availability ranged from 3.4 to 89% in descending order: wastewater of rural population > biologically treated urban wastewater > dairy house wastewater > biologically and chemically treated urban wastewater > field runoff > forest industrial effluent > fish fodder and feces > river water > field surface soil > forest runoff > lake settling matter > lake bottom sediments. Of the P fractions, dissolved reactive P (o-P) was highly available to algae, whereas particulate P (Ppart) and dissolved unreactive P (unr-Pdiss) contributed to Paa to a lower but varying degree. An approach based on source-dependent availability coefficients, derived from the algal assays, appeared promising in transforming the load of Tot-P into that of Paa. Although the values for Paa obtained by the dual-culture assay probably underestimate the true levels for ultimately available P, they may still give valuable information for eutrophication abatement.

Internal and external loading as regulators of nutrient concentrations in the agriculturally loaded Lake Pyhäjärvi (southwest Finland)

This study assesses the effects of external and internalloading on the nutrient concentrations in an agriculturallyloaded shallow lake. Using 13 years of observations of thelakes input and outflow, we calculated the long-term balancesof Tot-P and Tot-N. A more detailed balance, which includeddissolved nutrients and suspended solids, was estimated for anice-free period of one year. The contribution of the externalload was assessed using a mass-balance model. The internalload was estimated from the nutrient balances and on the basisof sedimentation measurements and bioassays. The drainagebasin of the lake provided most of the external nutrientinput; the remaining load was derived from atmosphericdeposition to the lake. The proportions of river-transported Pand N in dissolved form were 25% and 77%, respectively. Thelake retained >80% of the external load. Particulatenutrients settled to the bottom and were probably resuspendedseveral times before permanent sedimentation. Dissolvednutrients were bound by primary producers and a highproportion of dissolved P was removed with the fish catch.Dissolved N was also lost via denitrification. Themass-balance model showed that external loading only partlyregulated the mean annual nutrient concentrations in the lake.The regulation was probably due to internal loading, which washigh despite the efficient net retention of nutrients. Duringthe ice-free period, the temporal variations in nutrientconcentrations were controlled almost solely by internalprocesses, such as resuspension of inorganic and organicbottom matter. Although the internal load of bioavailable Pmay, under favourable conditions, exceed the external load,the mechanism by which bioavailable P is translocated from thebottom sediments to the water could not be fully identified.Abbreviations used in this paper follow the editorsrules.

Bioavailability of phosphorus in agriculturally loaded rivers in southern Finland

The potential bioavailability of phosphorus in agriculturally loaded rivers of southern Finland was determined by an algal bioassay and the release of the potentially bioavailable particulate P was estimated by sorption studies. According to the bioassay 0 to 13.2 per cent (mean 5.1%) of the particulate P in river water samples was potentially bioavailable. Dissolved reactive P (DRP) in river waters appeared to be totally bioavailable whereas the dissolved unreactive P appeared not to be utilized by algae. In addition to river waters two lake sediment samples were also assayed. In these samples 0 and 2.6% of the P was bioavailable. The potential bioavailability of particulate P in agriculturally loaded rivers obtained in this study was lower than that reported in studies from other countries. The difference was assumed to arise partly from methodological factors and partly from the nature of the Finnish soils. The EPC (equilibrium phosphate concentration) values indicated that during the period when most of the agricultural loading enters the lakes in Finland, potentially bioavailable P is not released from the particles because of the relatively high DRP concentration in the receiving waters. However, during the algal production period the DRP concentration in lakes decreases below the EPC and potentially bioavailable particulate P is desorbed. The increase in pH during this period may further enhance the desorption of P.

Coastal Eutrophication Thresholds: A Matter of Sediment Microbial Processes

Abstract In marine sediments, the major anaerobic mineralization processes are Fe(III) oxide reduction and sulfate reduction. In this article, we propose that the two alternative microbial mineralization pathways in sediments exert decisively different impacts on aquatic ecosystems. In systems where iron reduction dominates in the recently deposited sediment layers, the fraction of Fe(III) oxides that is dissolved to Fe(II) upon reduction will ultimately be transported to the oxic layer, where it will be reoxidized. Phosphorus, which is released from Fe(III) oxides and decomposing organic matter from the sediment, will be largely trapped by this newly formed Fe(III) oxide layer. Consequently, there are low concentrations of phosphorus in near-bottom and productive water layers and primary production tends to be limited by phosphorus (State 1). By contrast, in systems where sulfate reduction dominates, Fe(III) oxides are reduced by sulfides. This chemical reduction leads to the formation and permanent burial of iron as solid iron sulfides that are unable to capture phosphorus. In addition, the cycling of iron is blocked, and phosphorus is released to overlying water. Owing to the enrichment of phosphorus in water, the nitrogen : phosphorus ratio is lowered and nitrogen tends to limit algal growth, giving an advantage to nitrogen-fixing blue-green algae (State 2). A major factor causing a shift from State 1 to State 2 is an increase in the flux of labile organic carbon to the bottom sediments; upon accelerating eutrophication a critical point will be reached when the availability of Fe(III) oxides in sediments will be exhausted and sulfate reduction will become dominant. Because the reserves of Fe(III) oxides are replenished only slowly, reversal to State 1 may markedly exceed the time needed to reduce the flux of organic carbon to the sediment. A key factor affecting the sensitivity of a coastal system to such a regime shift is formed by the hydrodynamic alterations that decrease the transport of O2 to the near-bottom water, e.g., due to variations in salinity and temperature stratification.

Does control of soil erosion inhibit aquatic eutrophication

Much of the phosphorus (P) from erosive soils is transported to water bodies together with eroded soil. Studies clarifying the impact of soil erosion on eutrophication have sought largely to quantify the reserves of P in soil particles that can be desorbed in different types of receiving waters. Aquatic microbiology has revealed that the cycling of P is coupled to the availability of common electron acceptors, Fe oxides and SO₄, through anaerobic mineralization in sediments. Eroded soil is also rich in Fe oxides, and their effect on the coupled cycling of C, Fe, S, and P has been neglected in eutrophication research. Soil erosion, and its control, should therefore be studied by considering not only the processes occurring in the water phase but also those taking place after the soil particles have settled to the bottom. We propose that in SO₄-rich systems, Fe oxides transported by eroded soil may promote Fe cycling, inhibit microbial SO₄ reduction and maintain the ability of sediment to retain P. We discuss the mechanisms through which eroded soil may affect benthic mineralization processes and the manner in which soil erosion may contribute to or counteract eutrophication.

Influence of EU policy on agricultural nutrient losses and the state of receiving surface waters in Finland

In Finland, the first large-scale efforts to control nutrient loading from agriculture got under way with the introduction of the EU Agri-Environmental Program in 1995. We examined whether these efforts have decreased agricultural nutrient losses and improved the quality of receiving waters. To do so we used monitoring data on fluxes of nutrients and total suspended solids in agricultural catchments in 1990–2004 and on the water quality of agriculturally loaded rivers, lakes and estuaries in 1990–2005. No clear reduction in loading or improvement in water quality was detected. Hydrological fluctuations do not seem to have eclipsed the effects of the measures taken, since there was no systematic pattern in runoff in the period studied. The apparent inefficiency of the measures taken may be due to the large nutrient reserves of the soil, which slowed down nutrient reductions within the period studied. Simultaneous changes in agricultural production (e.g. regional specialisation) and in climate may also have counteracted the effects of agri-environmental measures. The actions to reduce agricultural loading might have been more successful had they focused specifically on the areas and actions that contribute most to the current loading.

Phosphorus in settling matter and bottom sediments in lakes loaded by agriculture

In shallow lakes, the cycling of P between water and bottom sediments is strongly influenced by wind-induced resuspension of particulate matter. The significance of this P flux as an algal nutrient source is unclear. We examined gross sedimentation in 3 open and shallow agriculturally loaded lakes. In addition, we estimated the potential P-release from settling and bottom matter by laboratory tests. The mean daily rate of gross sedimentation was 21–170 g m−2 d−1 of dry sediment, 0.04–0.18 g m−2 d−1 of P and 0.18–2.0 g m−2 d−1 of N; being the highest in the shallowest and most eutrophic lake. In Lake Karhijärvi, where the most intensive measurements were taken, wind explained the temporal variation in the gross sedimentation to some extent. The settling matter consisted of inorganic particles low in nutrients, especially during peak sedimentation periods. On average, 7.7 ± 3.1% (x ± 95% confidence interval) of the P in the settling matter in L. Karhijärvi was in an algal-available form according to 2–3 week bioassays. In the bottom matter of the three lakes, 3.0 ± 1.7% and 2.5 ± 3.6%, and 4.3 ± 3.7% of the P was utilized by the algae. In L. Karhijärvi, resuspension of the potentially available P exceeded 20 times the external loading during the open water season. According to sorption tests, P is released from the bottom matter only when the concentration of o-P is <2 μg l−1. Although such a low value cannot be determined with common analytical procedures, it seems probable that the P concentration allows P desorption during P-limited periods. However, the significance of resuspended matter as an algal nutrient source calls for further research.