Iron addition as a measure to restore water quality: Implications for macrophyte growth
Introduction
High nutrient loading from agricultural runoff and wastewater discharge during the second half of the 20th century has led to eutrophication of many shallow lakes in north-western Europe. The excess input of phosphorus (P) and nitrogen (N) has resulted in (toxic) cyanobacterial blooms and subsequently turbid water, biodiversity loss, and a decline in submerged macrophytes (Tilman et al., 2001, Hilt et al., 2006, Hickey and Gibbs, 2009). Submerged macrophytes play a key role in the functioning of shallow water bodies by acting as a nutrient sink, providing a habitat for fauna and preventing resuspension of lake sediment. Through these actions macrophytes stabilize the clear water state of shallow lake ecosystems (Scheffer et al., 1993, Jeppesen et al., 1998, Bakker et al., 2010). After eutrophication, a strong reduction in P loading of a lake is required to restore a lake to this self-stabilizing clear water state (Cooke et al., 1993, Jaeger, 1994). However, internal loading of P from the sediment, particularly from nutrient rich organic lake sediment (Lamers et al., 2002), may significantly delay the recovery of aquatic ecosystems, even after the external loading has been reduced (Cooke et al., 1993, Jeppesen et al., 1998, Søndergaard et al., 2003, Søndergaard et al., 2013).
Before the intensification of agriculture, many peaty lakes would not suffer from high internal P loading, as iron in upwelling groundwater naturally binds to phosphorus (in the form of phosphates, PO4; Lamers et al., 2002). However, the upwelling of this iron-rich groundwater has declined due to changes in hydrological regimes and desiccation through extraction of groundwater for agricultural purposes, which consequently has led to a reduction in the amount of iron reaching the top layer of the sediment (Van der Welle et al., 2007b). Hence, one way to cope with internal P loading is by improving the P binding capacity of the lake sediment by adding iron (Fe) or other chemical P binding agents such as aluminium (Al), calcium (Ca), or lanthanum-enriched benthonite clay (Phoslock®) to the sediment (Cooke et al., 1993, Burley et al., 2001, Smolders et al., 2006, Hickey and Gibbs, 2009, Van Oosterhout and Lürling, 2011). These chemical binding agents, if added on a regular basis, will not only precipitate with the available PO4 in the sediment, but can potentially provide long-term control of internal P loading from the sediment (Boers et al., 1992, Boers et al., 1994, Cooke et al., 1993, Smolders et al., 2006, Kleeberg et al., 2013).
Various mesocosm and field experiments have shown that the addition of Fe to the sediment indeed results in lower total phosphorus (TP) concentration in the water column, which is why iron is often used to decrease P concentrations of lake inlet water before the water enters the lake (Klapwijk et al., 1982, Boers et al., 1992, Van Donk et al., 1994, Smolders et al., 1995, Kleeberg et al., 2013). High Fe concentrations in the sediment, however, can have deleterious effects on macrophytes (Kamal et al., 2004). Recent experiments have shown that growth of plants can be directly inhibited by high iron concentrations in the sediment for instance by the formation of necrotic leaf spots and iron plaques on roots (Lucassen et al., 2000, Van der Welle et al., 2007a). Evidently, iron can also have indirect effects on macrophytes by lowering the phosphorus concentration in the sediment, thereby decreasing the available nutrients for growth and by lowering the pH of the water (Boers et al., 1994). Moreover, the addition of iron to the sediment may be possible in mesocosms, but is a real challenge for a whole lake. Alternatively, iron could be added to the surface water. However, the effects of adding iron to the surface water on submerged macrophytes are not yet known, whereas they are directly exposed to the added iron when this is added in the surface water in contrast to addition in the sediment. The place of addition may affect macrophyte species differently, as macrophytes differ in rooting strategies (Jeppesen et al., 1998). Macrophytes depending for their growth on nutrients from the water column might be more affected by iron in the water column than rooting macrophytes, which generally take up nutrients from the sediment. Over time, rooting species may become affected as well, when the iron added in the water column precipitates and mixes with the sediment through macrofaunal activity or wind-driven sediment movement (Søndergaard et al., 2003).
The objective of this study was to test whether iron addition as a restoration measure affects the growth of submerged macrophytes. We hypothesized that this depends on the iron dosage, the application mode (surface water or sediment plus water) and the rooting strategy of the macrophytes (uptake of nutrients from sediment or water column). We experimentally tested potential negative effects of iron (Fe) on the growth of two submerged macrophytes, the facultative rooting species Elodea nuttallii (Planch.) H. St. John and the rooting species Potamogeton pectinatus L. as well as on the sprouting of propagules present in the sediment propagule bank. Furthermore, to simulate a condition in which wind driven sediment resuspension and subsequent sedimentation would lead to an accumulation of iron in the sediment, we added a treatment in which we, prior to the start of the experiment, mixed half of the total dosage of iron in the sediment. To study the effect of iron addition we focused on changes in macrophyte growth and appearance, biomass allocation, nutrient composition and sprouting of propagules from the sediment. The experiment is based upon planned restoration measures in peat lake Terra Nova, The Netherlands (Van de Haterd and Ter Heerdt, 2007), where water managers are proposing to add iron to the surface water.
Section snippets
Study species and study site
The study species E. nuttallii and Potamogeton species are often observed to be the first dominant species after lake restoration measures have been taken (Van Donk et al., 1994, Van Donk and Otte, 1996, Perrow et al., 1997, Irfanullah and Moss, 2004, Hilt et al., 2006). This was also the case in lake Terra Nova, where E. nuttallii became dominant and multiple Potamogeton species occurred (including P. pectinatus) after sediment disturbing fish were removed through biomanipulation in a
Macrophyte response
Adding iron to the water column or to both the water column and the sediment did not differentially affect macrophyte growth (Table 1), therefore we pooled these data for the analysis of macrophyte growth. Total macrophyte biomass (roots plus shoots) increased over time in all treatments, but iron addition induced a different response in the two macrophyte species (Table 1, Fig. 1). E. nuttallii biomass did not differ significantly between the three iron treatments with an average RGR of 0.034 ±
Macrophyte growth under iron addition
Iron addition in the sediment of various lakes in the Netherlands resulted in an improvement of the water quality by a decrease of the surface water PO4, chlorophyll-a and suspended solids concentrations (Boers et al., 1992, Smolders et al., 1995, Van der Welle et al., 2007a). By improving water transparency, iron addition can stimulate macrophyte growth, which is often light limited (Bornette and Puijalon, 2011). Epiphyton measurements from macrophyte shoots in our experimental units did not
Acknowledgements
We are grateful to Leon Lamers for his valuable theoretical insights and useful discussions. We would also like to thank Naomi Huig, Thijs de Boer and Koos Swart for their practical assistance in the field and Hans Kaper, Nico Helmsing and Harry Korthals for performing multiple chemical analyses. This study was funded by the Water Framework Directive Innovation Fund from Agentschap NL from the Dutch Ministry of Economic Affairs, Agriculture and Innovation.
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Present address: Institute F.-A. Forel, University of Geneva, 10 Route de Suisse, Versoix, Switzerland.