# Effects of Invasive Watermilfoil on Primary Production in Littoral Zones of North-Temperate Lakes
## 1 Introduction  
_Eurasian watermiloil_ (_Myriophyllum_ _spicatum_) is a globally important invasive aquatic macrophyte [1, 2], which has spread across North America over the past century [3] from source populations traced back to Asia [4]. _Eurasian watermilfoil_ invades the shallow-water habitats of lakes, called littoral zones, where it can grow rapidly and build a canopy of shoots, suppressing other aquatic plants (macrophytes) below [5, 6]. Although _Eurasian watermilfoil_ is perennial, it exhibits an annual pattern of growth where in spring shoots grow rapidly to the water's surface and branch profusely to establish dominance [7]. Plants remain evergreen in fall and over winter with substantial biomass, which allows this rapid spring growth [8]. The primary vector of spread within and between waterbodies is by vegetative reproduction and dispersal of shoot fragments [3]. Additionally, _Eurasian watermilfoil_ can hybridize with the native northern watermilfoil (_M. sibricum_) to create hybrids (_Myriophyllum_ _spicatum_ _x sibricum_) that exhibit increased growth vigor and increased resistance or tolerance to herbicides [9, 10, 11]. Both_Eurasian watermilfoil_ and its hybrids are present in Michigan [12] and nearly impossible to differentiate visually [13], so were considered collectively in this study as invasive watermilfoil (IWM). Dense, high biomass growth of IWM can alter populations of macroinvertebrates and fishes [3] and energy flow in lake food webs [14], but their potential effects on the distribution and production of all other primary producers in the littoral zone are not well understood. Submerged macrophytes directly and indirectly interact with other primary producers in lake littoral zones, and through these interactions can control lake ecosystem processes [15] and stability [16]. In a study of lakes in Wisconsin, littoral habitats with high macrophyte abundance contributed disproportionately to whole lake primary production, while primary production in littoral habitats with low abundance of macrophytes was similar to that of the open water (pelagic) habitats [17]. Macrophytes create physical structures and substrata while also modifying light and nutrient dynamics, which interact to affect other littoral primary producers (Figure 1). The vegetative structure and leaf area of macrophytes limit light penetration through the water column [18], which can reduce light availability for phytoplankton (free floating single cell and colonial algae), epiphytes (attached to macrophytes), and benthic periphyto (attached to bottom substrata). Macrophytes primarily take up nutrients from the sediment and can reduce sediment pools of nitrogen and phosphorus [19], while also using inorganic nutrients from the water column [20; 21]. In contrast, phytoplankton have a high affinity for dissolved nutrients and primarily obtain nutrients from the water column [22; 23]. Epiphytes and benthic periphyto have access to nutrients from their substratum, the water column, and internal recycling within their biofilm matrix [24; 25]. When large amounts of dissolved nutrients are available in eutrophic lakes, phytoplankton can dominate primary producer biomass, decreasing light penetration and strengthening light limitation of benthic periphyto [26], epiphytes, and macrophytes [16]. Further complicating interactions, epiphytes attached to macrophytes can take advantage of light availability higher in the water column, while simultaneously shading macrophytes they are using as a substratum and source of nutrients [15; 24; 27]. The direct and indirect interactions among different groups of primary producers in lake littoral zones are complicated, and they may be particularly sensitive to establishment of invasive macrophytes like IWM. Establishment of IWM in littoral zones may alter biomass and production of other primary producers either by altering competition for light and nutrients, or by creating a novel growth substratum for attached algae.