Aquatic macrophytes are important primary producers and ecosystem engineers in freshwater systems (Koch, 2001). The majority of all aquatic vegetative growth occurs in the littoral zone; the transitional space between the profundal zone and the terrestrial landscape (Madsen et al., 2008). Aquatic macrophyte communities influence the structure and function of aquatic systems in many ways. Aquatic macrophytes provide food and habitat for waterfowl, fish, and macroinvertebrates (Waters & Giovanni, 2002; Wersal et al., 2005; Dibble & Pelicice, 2010). In addition to supporting animal communities, aquatic macrophytes inhibit the growth of phytoplankton allopathically, and provide habitat for important filter feeders that graze on phytoplankton; which can mitigate the frequency and intensity of algal blooms (Scheffer, 1999; Körner & Nicklisch, 2002; Takamura et al., 2003; Bakker et al., 2010). Aquatic vegetation can also improve water quality by promoting the settling of suspended sediment and inhibiting the resuspension of settled sediment by reducing wave action in the water column (Barko & Smart, 1986; James et al., 2004). The benefits of a diverse aquatic plant community affirm its value as an integral constituent of the freshwater lake system. Understanding the factors that affect the composition of the aquatic macrophyte community is important for managing aquatic systems, and preserving their structure and function.
Light availability is considered the principal limiting factor for the growth of aquatic macrophytes (Chambers & Kaiff, 1985; Barko et al., 1986; Lacoul & Freedman, 2006). Light is a rate limiting factor for the primary productivity of plants, and in aquatic systems, light is often limited by the attenuation of light by the water column (Lacoul & Freedman, 2006; Bornette & Puijalon, 2011). Light regime is the primary driver for the niche partitioning of aquatic macrophytes throughout the littoral zones of lakes (Barko et al., 1986). The typical structure of the littoral zone consists of angiosperms at shallow depths and bryophytes and charophytes at deeper depths (Chambers & Kaiff, 1985; Blindow, 1992). This zonation is primarily driven by the availability of light and the adaptations of plants to those light conditions. Light availability is also a strong determinant of macrophyte growth form. Lakes with very low light availability are often dominated by floating leaf and free-floating macrophytes that are adapted to grow leaves at the surface, where light is not limited (Lacoul & Freedman, 2006). Conversely, submersed macrophytes are generally more abundant in lakes where there is more light available in the water column (Lacoul & Freedman, 2006).
Water depth is also an important factor that affects distribution of macrophytes in lakes. Many lakes have a maximum depth of macrophyte colonization that is shallower than the maximum depth of the lake (Chambers & Kaiff, 1985; Rooney & Kalff, 2000). When water depths in the middle of the lake exceed the maximum depth of colonization, a profundal zone is present with the littoral zone found around the margin. The profundal zone is typical in deep lakes; however, in shallow lakes it may be absent entirely. Water depth is often considered an inhibiter of macrophyte growth because the water column attenuates more light as depth increases. This also explains why macrophytes at lower depths are often better adapted to lower light conditions than macrophytes at shallower depths. Overall, water depth has been found to have a negative relationship between the density and abundance of aquatic macrophytes (Barko et al., 1986; Cheruvelil & Soranno, 2008).
One of the major factors that limits the growth of aquatic macrophytes is water turbidity. Light availability in aquatic systems is primarily a function of turbidity and water depth (Barko et al., 1986; Lacoul & Freedman, 2006; Bornette & Puijalon, 2011). Turbidity in a lake system is mostly caused by suspension and resuspension of fine textured sediment (James et al., 2004). Suspended sediment can increase light attenuation and nutrients in the water column which reduces light availability and can promote algal blooms, thus inhibiting the growth of submersed macrophytes (James et al., 2004; Zhu et al., 2015). However, aquatic macrophytes can affect the turbidity of lake systems. Many studies have found that the presence of aquatic macrophytes reduces wave action and, consequently, reduces suspension and resuspension of fine sediments that contribute to turbidity (Barko et al., 1991; Madsen et al., 2001; Wu & Hua, 2014). The relationship between turbidity and aquatic macrophytes is complex, but abundant evidence implicates turbidity as a major limiting factor for plant growth through limiting light availability
Another important factor that affects the abundance and distribution of aquatic macrophytes is the fetch, the distance wind can travel unimpeded. In shallow lakes, one of the primary determinants of wave action is fetch (Andersson, 2001; Lacoul & Freedman, 2006). Depending on the intensity of the wave energy, the effects of wave action may be positive or negative. Macrophytes may respond to high wave action by changing their morphology, and moderate wave action may increase nutrient availability for the macrophyte community (Madsen et al., 2001; Lacoul & Freedman, 2006; Bornette & Puijalon, 2011). Wave action may also contribute to suspension and resuspension of fine textured sediment that may affect community structure in various ways (Madsen et al., 2001; James et al., 2004).
A lake’s sediment is highly influential on the macrophyte community, and interactions between sediment and macrophyte communities are highly complex (Barko & Smart, 1986; Barko et al., 1991). Fine sediments can contribute to turbidity, but sediment texture affects macrophytes in many other ways. For instance, Stuckenia pectinata (L.) Böerner (sago pondweed) has shown a proclivity for growth in sediments with abundant silt (Madsen et al., 1996; Koch, 2001; Case & Madsen, 2004). In Swan Lake and Middle Lake, Nicollet County, MN, USA, clayey sediment was positively related the presence of sago pondweed, but negatively related to the presence of Vallisneria americana Michx. (American eelgrass) (Madsen et al., 2006). Finer sediment like silts and clays can be both beneficial and detrimental to macrophytes, and effects are species specific. In finer sediments, macrophytes generally encounter a trade-off between nutrients and bulk density (Gerbersdorf et al., 2007). Finer sediment particles often have a higher activity, which improves cation exchange capacity (CEC), elevating nutrient availability. However, reduced porosity of finer sediments can inhibit root growth as bulk density is greater and generally results in more hypoxic sediments (Koch, 2001; Gerbersdorf et al., 2007). Evidence from numerous studies suggests that the interface between macrophytes and sediment is a major factor that affects macrophyte community structure.
Factors that affect the structure of the macrophyte community are highly influential on the structure and function of lake systems. Lakes of Minnesota are very diverse and this is largely due to landscape diversity across the state. State-wide, shallow lakes (max depth ≤ 4.5) are more common than deep lakes (max depth > 4.5m) (Radomski & Perleberg, 2012). In Minnesota, regions with deeper and more oligothrophic lakes, have lakes with much greater macrophyte richness than regions with shallow, eutrophic lakes (Radomski & Perleberg, 2012). Much of southern Minnesota is situated in the Prairie Pothole Region, where lakes are much shallower and more species poor than most other regions of Minnesota (Guntenspergen et al., 2002; Radomski & Perleberg, 2012). The ecology and management of shallow lakes is fundamentally different from typical lakes as they are generally warmer, more turbid, and more productive than deep lakes (Scheffer, 2004). Managing lakes in southern Minnesota requires an understanding of how certain physical and geographic factors affect the aquatic macrophyte community. The purpose of this study is to quantify the relationships between mean species richness, lake sediment, and geographic factors in five major lakes in Sibley County, MN, USA.