Applied Geochemistry

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1 Applied Geochemistry 25 (2010) Contents lists available at ScienceDirect Applied Geochemistry journal homepage: Nutrient availability correlates with bicarbonate accumulation in marine and freshwater sediments Empirical evidence from pore water analyses Tjisse van der Heide a,b,, Alfons J.P. Smolders b, Leon P.M. Lamers b, Marieke M. van Katwijk a, Jan G.M. Roelofs b a Department of Environmental Science, Institute for Wetland and Water Research, Radboud University Nijmegen, Faculty of Science, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands b Department of Environmental Biology, Institute for Water and Wetland Research, Radboud University Nijmegen, Faculty of Science, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands article info abstract Article history: Received 15 June 2009 Accepted 23 September 2010 Available online 26 September 2010 Editorial handling by R. Fuge In the last decades, eutrophication has become a major cause for concern in aquatic ecosystems worldwide. Apart from external nutrient loading, release of dissolved nutrients from sediment pore water into the water layer contributes importantly to eutrophication. Theory implies that pore water HCO 3 and nutrient mobility are linked, because anaerobic organic matter breakdown stimulates release of HCO 3 and nutrients, while HCO 3 stimulates decay rates by increasing buffer capacity and ph. An investigation was performed on how HCO 3 relates to commonly measured eutrophication-related variables in freshwater and marine sediments, by analyzing a database of 71 marine and 206 freshwater samples, using linear and non-linear regression. Results demonstrate that HCO 3 is indeed strongly related with NHþ and PO 3 in both freshwater and marine pore waters. Moreover, HCO 3 also correlated with sulfide levels in marine sediments. Contrastingly, no relationship was found with variables describing total nutrient content (i.e. organic matter, total P and N), suggesting that these do not reveal information on nutrient availability in aquatic sediments. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In the last 50 a, nutrient availability has increased in almost every aquatic ecosystem around the world (Tilman et al., 2001; Tilman and Lehman, 2001; Smith, 2003; Jeppesen et al., 2005). The source of this eutrophication is primarily anthropogenic. Most importantly, worldwide rapid intensification of agriculture and deforestation has increased nutrient and sediment runoff from terrestrial systems, causing a staged increase of N and P fluxes towards the majority of the world s freshwater and marine ecosystems (Tilman et al., 2001; Tilman and Lehman, 2001; Smith, 2003). Eutrophication has resulted in alterations in species compositions of aquatic ecosystems and a severe loss of biodiversity (Tilman and Lehman, 2001; Smith, 2003). In freshwater systems, where P is mostly the key growth-limiting nutrient (Reddy et al., 1999), its increased availability in the water layer has led to an increase in productivity and biomass of bloom-forming phytoplankton species, epiphytes, macro-algae, and floating macrophytes such as Lemnaceae, subsequently causing a strong decrease in abundance of rooting submerged macrophytes and associated animal species (Smith, 2003; Jeppesen et al., 2005). Although N instead Corresponding author. Present address: University of Groningen, The Netherlands. Tel.: ; fax: address: t.van.der.heide@rug.nl (T. van der Heide). of P is the primary limiting resource in most marine and estuarine ecosystems (Ryther and Dunstan, 1971), their general response to eutrophication has been similar to freshwater ecosystems (Smith, 2003). Effects of eutrophication are typically most directly visible in the water layer. However, decomposition of organic matter may result in an increased availability of nutrients in the sediment (Smolders and Roelofs, 1995; Lamers et al., 1998; Smolders et al., 2006b). Peaty sediments are naturally rich in organic matter while biomass production by algae or aquatic plants may lead to accumulation of organic matter in mineral sediments. In such systems, release of nutrients from the sediment back into the water layer, so-called internal eutrophication (Smolders et al., 2006a) can become important in the eutrophication process. Moreover, depending on organic matter degradation rates, internal eutrophication may persist even when external nutrient loading has been decreased (Smolders et al., 2006a). Pore water nutrient concentrations are important as mobile P and N pools, may be transported into the surface water (Geurts et al., 2008). Along with adsorption and precipitation of nutrients, turnover rates of organic matter are important to the mobility of nutrients. As long as O 2 is sufficiently present it serves as the primary terminal oxidant (electron acceptor) in the decay of organic matter. However, because O 2 penetration in underwater sediments /$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi: /j.apgeochem

2 1826 T. van der Heide et al. / Applied Geochemistry 25 (2010) A B C D Fig. 1. Relationship between HCO 3 (x-axes) and sediment eutrophication variables (y-axes) and the fitted equations for freshwater data. Variables plotted versus HCO 3 ;A, pore water NH þ ;B,PO3 ; C, sediment total P; and D, organic matter content of the sediment. is usually restricted to the first few millimetres, degradation of organic material will mostly occur anaerobically, by a series of reactions that represent successively lower redox (reduction oxidation) states (Gieskes, 1975; Drever, 1997; Wetzel, 2001; Golterman, 200). Once O 2 is depleted, NO 3 is used as the terminal electron acceptor by bacteria, if it is available. This reduction of NO 3 to N 2 gas (denitrification) or NH þ (ammonification or dissimilatory NO 3 reduction to ammonium, DNRA) involves a multitude of electron transfer steps (Smolders et al., 2006a). Alternatively, Fe(hydr)oxides and SO 2 become involved as alternative electron acceptors at even lower redox potentials (Eh). A lack of alternative electron acceptors may strongly hamper the decomposition of organic matter under anaerobic conditions (Lamers et al., 1998; Smolders et al., 2006a). However, the decomposition rate of organic matter also appears to be strongly related with ph and buffer capacity (Kok and van de Laar, 1991; Smolders et al., 2006a). Decomposition of organic matter is inhibited in acidic waters compared to alkaline waters (Traaen, 1980; McKinley and Vestal, 1982; Roelofs, 1991; Lamers et al., 1999; Smolders et al., 2002), since in the latter HCO 3 acts as ph buffer by neutralizing decay-inhibiting acids (Kok and van de Laar, 1991; Smolders et al., 2006a). Notably, inorganic C released during organic matter breakdown is predominantly released as HCO 3 in anaerobic conditions (Gieskes, 1975; Drever, 1997; Smolders et al., 2006a), thereby enhancing buffer capacity and potentially increasing decay rates again. Finally, HCO 3 can also mobilize PO3 because of the competition between (bi)carbonate and PO 3 for anion adsorption sites (Beltman et al., 2000; Smolders et al., 2006a). Thus, HCO 3 production and mobility of nutrients seem to be coupled in these processes. In this study, it is hypothesized that pore water concentrations of N and P correlate with pore water HCO 3, since release of HCO 3 and mobility of nutrients should theoretically be tightly linked together. How HCO 3 relates to a variety of commonly measured eutrophication-related variables, in both marine and freshwater bodies was examined. In the analyses, 71 marine and 206 freshwater samples were included. Correlations between HCO 3 and other included variables were analyzed using linear and non-linear regression techniques. 2. Materials and methods 2.1. Data collection Three replicates of pore water and sediment samples were collected from the top 10 cm of the sediment and pooled, during the growth seasons of Pore water was sampled using soil moisture samplers (Rhizon SMS-10 cm; Eijkelkamp Agrisearch Equipment). Sediments were collected with a core sampler (diameter: 5 cm). Marine samples were collected at sites scattered all over Western Europe. The database included 23 sites from the Baltic Sea, four sites from the Danish Limfjorden areas, 23 Wadden Sea sites, 13 Dutch brackish inland waters and eight sites from the French Atlantic coast. All 206 freshwater samples were collected from various water bodies (e.g., lakes, fens, ditches) scattered across the Netherlands and Belgium. Water depth at the sampled sites varied from 0.5 to 5 m Sample analyses In the pore water, total inorganic C was measured as CO 2 on an infrared carbon analyzer (PIR-2000, Horiba Instruments) after conversion of all TIC to CO 2 by H 3 PO. Bicarbonate levels were calculated from TIC and ph. Concentrations of PO 3 and NH þ were measured colorimetrically, using ammonium-molybdate (Hendriksen, 1965) and salicylate (Kempers and Zweers, 1986), respectively. Nitrate was determined by sulphanilamide after reduction

3 T. van der Heide et al. / Applied Geochemistry 25 (2010) Table 1 Results from regression analyses on freshwater sediment data. Equation type R 2 F p Intercept Slope Exponent Ammonium Linear < lmol L lmol meq 1 ortho-phosphate Exponential < lmol L L meq 1 Total phosphorus Linear ns ns Organic matter Linear < %.297 L meq 1 of NO 3 to NOÞ 2 in a cadmium column (Wood et al., 1967). Dissolved inorganic N was calculated as the sum of NH þ and NO 3. Dissolved P in the pore water was measured on an inductivelycoupled-plasma emission spectrophotometer (ICP) (Spectroflame, Spectro Inc., Littleton, USA). Organic matter content in freeze-dried sediments was estimated as weight loss by ignition at 550 C. Total P in the sediment was measured on ICP, after digestion with HNO 3 acid (Smolders et al., 2006b). For marine sites, the total N content of the sediment and sulfide concentration in the pore water were also determined. Total sulfide (TS) levels were determined immediately after sampling by measuring TS with an ion-selective silver/sulfide electrode in a mixed solution containing 50% Sulfide Anti-Oxidation buffer (SAOB) and 50% sample (Lamers et al., 1998). Nitrogen content of the sediment was determined from freeze-dried samples by a CNS analyzer (type NA1500; Carlo Erba Instruments, Milan, Italy). A B C D E F Fig. 2. Relationship between HCO 3 (x-axes) and pore water eutrophication variables (y-axes) and the fitted equations for marine water data. Variables plotted versus HCO 3 ; A, pore water NH þ ;B,PO3 ; C, pore water total dissolved sulfide; D, sediment total N; E, sediment total P: and F, organic matter content of the sediment.

4 1828 T. van der Heide et al. / Applied Geochemistry 25 (2010) Data analysis The relationships of all analyzed variables to HCO 3 were analyzed by linear and non-linear regression. Three types of 2-parameter functions were tested in the analyses. To start a simple linear equation was fitting to each relationship. Next, a test was made to see whether an exponential function produced a better fit: y ¼ ae bx With y as the y-axis variable, x as HCO 3 (meq L 1 ), a as the intercept at x = 0 and b as the exponent describing the rate of increase. Finally, a general saturation curve was fitted to the data. For this purpose, the Monod (or Michaelis Menten) type equation was used: x y ¼ y max ð2þ x þ x h In this equation, y max describes the maximum values for y and x h is the half-saturation constant. The equation that fitted best was adopted to each relationship. As a quality measure of the fit for each equation, the F-test and R 2 was used. 3. Results In both the freshwater and marine water dataset, NO 3 was only a small fraction compared to NH þ levels. Over 99% of dissolved inorganic N in all pore water samples consisted of NH þ. Similarly, PO 3 was by far the most important dissolved P species. In the samples, over 95% of all dissolved P was PO 3. Analyses of freshwater data demonstrated that HCO 3 showed strong correlation with NH and PO 3 levels in the pore water ð1þ Fig. 3. Schematic overview of processes leading to the observed relationship between pore water HCO 3, N and P concentrations. (Fig. 1). Contrastingly, HCO 3 did not relate to total P levels in the sediment and showed a relatively weak correlation with an inverse relationship to organic matter content. The relationship between ammonium and HCO 3 was best described by a linear equation. R 2 of the fit was 0.71, indicating that 71% of all variance in NH þ concentrations could be explained by HCO 3 through the fitted linear function (Table 1). An exponential equation could describe most of the variance in the relation between PO 3 and HCO 3. Seventy-six% of the spread in the data was explained by this function. In the marine water dataset, HCO 3 showed strong correlations to both NH þ (Fig. 2). Eighty% of all variance in NH þ levels could be explained by HCO 3 when fitting a linear function. The relationship between PO 3 and HCO 3 could be best explained by a monod function (R 2 = 0.76). Apart from PO 3 and NH þ, HCO 3 also correlated to sulfide levels in the pore water (R 2 = 0.66). Although the fits to sediment total N (linear), total P (monod) and organic matter (monod) were significant, HCO 3 describes only 33%, 25% and 3% of the variance in these variables, respectively.. Discussion The analyses reveal that HCO 3 correlated with NHþ in the pore waters of both marine and freshwater environments. Additionally, HCO 3 was related to sulfide levels in marine pore waters. Fig. 3 summarizes the processes, which lead to these observed relationships. In contrast, HCO 3 does not correlate well to commonly measured variables describing the total nutrient content (i.e. organic matter and total P) of the sediment. Indeed, these results also indicate that these variables did not correlate to free nutrient pools in the sediment, most likely because a large part of the N and P in sediments is present in the organic mater fraction and also because the sediment binding capacity for N and P may differ strongly between sediments. The latter is especially true for P in freshwater sediments, since a considerable fraction may be bound by Fe in these systems (Smolders et al., 2006a), while in marine systems Fe is mostly bound to sulfide (FeS x, which has a low affinity for P). The results, therefore, indicate that sediment variables like total P, total N or organic matter content do not disclose clear and reliable information on pore water nutrient concentrations and are not suitable for indicating the eutrophication state of marine or freshwater sediments (see Table 2). Although HCO 3 produced solid fits to dissolved pore water eutrophication variables in both marine and freshwater datasets, actual relationships turned out to be different for both environments. Ammonium has been observed to bind more strongly to sediment cation exchange or sorption sites in freshwater sediments than in saline sediments because of more competition for the sites by salt water cations and increased ion pairing of NH þ with negative anions in salt versus fresh water (Gardner et al., 1991; Seitzinger et al., 1991). Thus, NH þ tends to be more mobile in saline sediments than in freshwater sediments. However, in the present observations it was found that much more HCO 3 is released for any net amount of N or P (predominantly NH þ ) in marine sediments. This result can be explained by the fact that marine sediments are generally more calcareous than freshwater Table 2 Results from regression analyses on marine sediment data. Equation type R 2 F p Intercept Slope Maximum level Half-saturation Ammonium Linear < lmol L lmol meq 1 ortho-phosphate Monod < lmol L meq L 1 Sulfide Linear < lmol L lmol meq 1 Total nitrogen Linear < lmol g lmol g 1 meq 1 Total phosphorus Monod < lmol g meq L 1 Organic matter Monod < % meq L 1

5 T. van der Heide et al. / Applied Geochemistry 25 (2010) sediments (Gieskes, 1975). This results in a higher release of HCO 3 due to the interaction of CO 2 with (Ca)carbonates (reaction 1). CaCO 3 þ CO 2 þ H 2 O! Ca 2þ þ 2HCO 3 The fit of HCO 3 to NH þ was linear in both environments. However, the best fitting function for PO 3 to HCO 3 in freshwater was exponential, while this was a monod equation for the marine dataset. The lag-phase observed for PO 3 in the freshwater data, might be explained by the binding of PO 3 to Fe-hydroxides, Feoxides and Fe-humic complexes that may be available in soils where organic matter breakdown rates and related HCO 3 levels are relatively low. In freshwater soils with higher organic matter breakdown rates, SO 2 is the dominant electron acceptor in most cases (Lamers et al., 1998; Smolders et al., 2006a; Geurts et al., 2008). This results in the production of sulfide and the concomitant formation of Fe-sulfides (FeS x ), which have a low PO 3 adsorption capacity (Smolders et al., 2006a; Jordan et al., 2008). This lag-phase is absent in marine sediments, because most of the Fe fraction is present as Fe-sulfides in these S-rich environments (Blomqvist et al., 200; Jordan et al., 2008; Rodriguez et al., 2008). The typical rise to the maximum of PO 3 availability in marine waters could be explained by co-precipitation of PO 3 together with the precipitation of CaCO 3 at very high HCO 3 levels (Rodriguez et al., 2008). The poor correlations between organic matter content, HCO 3 and nutrient concentrations observed in the freshwater systems result from the fact that in most cases the availability of alternative electron acceptors determines the decomposition rates in the sediment (Smolders et al., 2006a). As SO 2 serves as an alternative electron acceptor in reducing sediments and stimulates the decomposition of organic matter, the increased input of SO 2 in freshwater ecosystems has led to a strong increase of N and P availability (Lamers et al., 1998; Smolders et al., 2006a). In marine systems SO 2 is typically present in high amounts and availability of degradable organic matter and potential electron donors produced by decomposition are more likely to affect net decomposition rates (Gieskes, 1975; Jorgensen, 1982). This idea is also reflected by the fact that organic matter content of marine sediments is generally lower compared to freshwater sediments and that HCO 3 also showed a good correlation with pore water sulfide concentrations in marine systems (Fig. 2). In summary, is empirically demonstrated that HCO 3 relates strongly to mobile nutrient pools in the pore waters of both marine and freshwater sediments, because nutrient mobilization and HCO 3 release are tightly linked. Decomposition rates of organic matter strongly determine pore water nutrient concentrations and thus the potential release of nutrients into the water layer. Moreover, the results confirm that decreasing the load of potential electron acceptors (SO 2 and NO 3 ) can be important in preventing eutrophication due to internal processes in freshwater systems (Smolders et al., 2006a). Acknowledgments We thank A. Vonk and B. Beltman for their valuable comments on an earlier version of this manuscript. R. Peters, M. van der Gaag and J. Eygensteyn are acknowledged for their help with chemical analyses. This study is financially supported by the Netherlands Organization of Scientific Research/Earth and Life Sciences (NWO-ALW). References Beltman, B., Rouwenhorst, T.G., Van Kerkhoven, M.B., Van Der Krift, T., Verhoeven, J.T.A., Internal eutrophication in peat soils through competition between chloride and sulphate with phosphate for binding sites. 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