Overview of biotic interactions

Table 2.2: Non-exhaustive overview of interactions among biological elements as reported in literature dealing with eutrophic, shallow water bodies. Interactions describe the effect of the biotic group on a specific row on a biotic group in a specific column. PhP: Phytoplankton, PeP:

Periphyton, ZP: Zooplankton, MI: Macroinvertebrates, MP: Macrophytes.

Phytoplankton Periphyton Zooplankton Macroinvertebrates Macrophytes Fish PhP - Cyanobacteria can

- Serve as food source¹⁷ - Indirectly reducing the competition with phytoplankton

- Serve as food source^{5, 17, 29}

(Continues on next page)

KEY ISSUES MULTIFUNCTIONAL WETLANDS

41 (Continued) Phytoplankton Periphyton Zooplankton Macroinvertebrates Macrophytes Fish

MI - Grazing³

CHAPTER 2

42

(Continued) Phytoplankton Periphyton Zooplankton Macroinvertebrates Macrophytes Fish

Fish - Resuspension Sand-Jensen and Borum (1991); ²⁰ Schrage and Downing (2004); ²¹ Spieles and Mitsch (2000); ²² Thomaz and Cunha (2010); ²³ Timms and Moss (1984); ²⁴ Travaini-Lima et al. (2016); ²⁵ van Donk and van de Bund (2002); ²⁶ Vanni (2002); ²⁷ Zhong et al. (2016); ²⁸ Zimmer et al. (2000); ²⁹ Zimmer et al. (2003)

KEY ISSUES MULTIFUNCTIONAL WETLANDS

43 2.3.2 Use of macrophytes to improve biodiversity

Macrophytes showed to provide a steering role regarding wetland community structure and functioning, affecting the physical, chemical and biological level. At the physical level, the presence of macrophytes reduces flow velocity and positively affects nutrient cycling and water storage. Moreover, in combination with macrophyte rooting, these reduced flow velocities cause less erosion and sediment resuspension, which positively affects transparency (Brix, 1997). However, under improved settling and decreased erosion, wetlands tend to be exposed to siltation and accretion, which can be further exacerbated by high transpiration rates of dense emergent communities (Angélibert et al., 2004; Zedler and Kercher, 2004).

The consequences of these physical changes on wetland community composition and functioning are case-dependent and situated along the positive-negative continuum. For instance, Rooth et al. (2003) showed that the invasion of wetlands occupied by Typha spp. and Panicum virgatum in the Chesapeake Bay by the invasive Phragmites australis caused higher sediment accretion rates within the areas invaded by P. australis.

Simultaneously, a reduction in total wetland area had occurred due to rising sea levels, yet the accretion caused by P. australis supported the continued existence of the invaded wetland. Hence, the invasion by P. australis caused the local disappearance of the native vegetation but avoided the complete loss of the wetland’s functionality.

Secondly, at the chemical level, nutrients are taken up directly from the water column, the sediment or a combination of both. This uptake supports biomass production, carbon sequestration and phytoremediation (see Box 2.2), with the latter being of main research interest for several decades (Brisson and Chazarenc, 2009; Rodríguez and Brisson, 2015; Tanner, 1996). Yet, this direct nutrient removal is estimated to represent maximally 10 % of the total provided load, though can be increased when frequent harvesting is applied and biomass-incorporated nutrients are completely removed from the aquatic system (Merlin et al., 2002; Park and Polprasert, 2008).

Within wetlands, oxygen is crucial for aerobic degradation and nitrification to occur (see Section 2.2.1). Emergent plants are known for providing root zone aeration within the (mostly anoxic) substrate, while being countered by an upward movement of methane (Bergström et al., 2007; Keddy, 2010; Vymazal, 2011b). This oxygen provision oxidises the reduced nitrogen compounds and drives a continuous diffusion of both reduced and oxidised nitrogen by altering the equilibrium between substrate and water column concentrations (Keddy, 2010). However, extensive surface coverage and dead plant material entering the water column cause additional oxygen consumption and the release of immobilised nutrients. For instance, duckweed species (Lemna spp.) can form dense mats under eutrophic conditions, which causes relatively high mortality rates and associated oxygen depletion underneath the mats (Janse and Van Puijenbroek, 1998).

CHAPTER 2

44

Box 2.2: Macrophytes providing phytoremediation to reduce pollutant levels Throughout the past decades, macrophytes have been frequently applied to counter the presence of pollutants in contaminated soil or water (i.e. phytoremediation) (Arthur et al., 2005; Dhir et al., 2009). Processes vary from degradation over immobilisation to extraction and are highly species- and environment-specific. For instance, Zhao et al. (2015a) studied the potential of several floating duckweed species to recover nutrients from wastewater and observed that Lemna japonica provided the highest nitrogen and phosphorus recovery and removal rates, while producing the most protein-rich biomass. In contrast, Amon et al. (2007) investigated the ability of various emergent macrophytes in supporting the dechlorination and mineralisation of perchloroethylene and observed significant improvements in pollutant removal.

Additional examples of phytoremediation being facilitated by aquatic macrophytes can be found in Carvalho et al. (2014), Dhir et al. (2009) and Rai (2009).

These effects of macrophyte presence on the physical and chemical conditions illustrate how species interact with their environment and create a framework for the development of biotic interactions (Vitousek et al., 1997). For instance, the development of a stable and biologically complex ecosystem is highly dependent on the presence of food, preferably provided by (a community of) primary producers, as autotrophic biomass production acts as a basis for the trophic cascade, feeding zooplankton, macroinvertebrates, amphibians, fish and birds (Balcombe et al., 2005b; Thomaz and Cunha, 2010; Worrall et al., 1997).

Moreover, during this primary production, nutrients are continuously taken up from the surrounding environment and converted into organic compounds to support cell growth. This causes pollutant levels to decrease towards the downstream sections of vegetated treatment systems, which creates different abiotic habitats along the flow path (Caraco and Cole, 2002). Due to these decreasing pollutant levels, the biotic diversity has the potential to increase towards the discharge point as the prevailing pollutant levels are less restrictive (Becerra-Jurado et al., 2012; Boets et al., 2011).

From this, it is clear that macrophyte occurrence represents an interesting starting point to support the conservation of wetlands, despite being determined by a range of species-specific preferences, interactions and functional traits, including the abiotic environment, dispersal capacity, temporary tolerance, resource competition, population dynamics, community ecology and evolution (Guisan and Thuiller, 2005; Pulliam, 2000; Sinclair et al., 2010). Appropriate wetland management requires that these aspects are considered into detail, with additional attention towards acceptable abiotic conditions for macrophyte presence.

KEY ISSUES MULTIFUNCTIONAL WETLANDS

45 2.3.2.1 Abiotic conditions for macrophyte presence

Field observations, laboratory experiments and expert knowledge contribute to an increased understanding of the preferred abiotic conditions (Hofstra et al., 2020). Based on field observations, suitable abiotic habitats for macrophyte presence can be derived, reflecting the niche concept as postulated by Hutchinson (1957) and re-evaluated by Pulliam (2000). This niche is a n-dimensional hypervolume in which every point represents an environmental condition that supports indefinite species survival and is generally referred to as the realised niche. The fundamental niche extends the realised niche as it excludes the effects of biotic interactions like resource competition and predator-prey interactions, thereby merely reflecting the suitable abiotic conditions (Pulliam, 2000).

Despite being unfit for inferring the fundamental niche, observations are often used within a data mining environment to derive suitable habitats, predict species distributions, define conservation value and restrict the spread of invasive alien species (Araújo and Guisan, 2006; Elith and Leathwick, 2009; McPherson et al., 2004).

Information obtained through these modelling exercises provides a valuable contribution to the delineation of a species’ realised niche, which allows its subsequent application as an overall filter, combining both abiotic and biotic influences (Anderson and Raza, 2010; Guisan and Rahbek, 2011). In contrast, experiments under controlled conditions allow to infer realistic species traits and population parameters, thereby aiding the development of process-based models with a more profound grounding in ecological theory (Gallien et al., 2010). Due to this approach, process-based models are better positioned than data-driven models when aiming to delineate the fundamental (abiotic) niche (Kearney and Porter, 2009).

Modelling techniques aiming to delineate species niches are intrinsically situated along a continuum between purely data-driven and completely knowledge-based (Dormann et al., 2012; Mount et al., 2016; Van Echelpoel et al., 2015), with observation-based habitat suitability models (HSMs) being highly data-dependent. Model performance and reliability rely on a plethora of variables, including the quality of the data and the applied model parameter settings, both of which require attention during model development (Everaert et al., 2016; Marvin and John, 2003; Zhang et al., 2003). Yet, despite their added value for ecosystem management, HSMs have been widely criticised in literature for a variety of reasons, including the limited consideration of species dispersal within the final model structure (Elith and Leathwick, 2009; Guisan and Thuiller, 2005;

Jarnevich et al., 2015). It is highly recommended to acknowledge these criticisms when assessing the applicability of modelling techniques.

CHAPTER 2

46