# Effects of Invasive Watermilfoil on Primary Production in Littoral Zones of North-Temperate Lakes
## 4 Discussion
IWM also tends to dominate under mesotrophic conditions and to flourish following disturbances [3]; these study lakes were oligo- to mesotrophic (Table A1) with no evidence of recent disturbance or active management for invasive aquatic plants near the study plots. Alternately, the low dominance of IWM in our study lakes may be consistent with the pattern that aquatic species, even invasive ones, are found in low abundances in most locations and abundant in a few [63]. For example, surveys of invasive plants of coastal wetlands of the Great Lakes found that _Eurasian watermilfoil_ was present in 61% of lakesbore segments surveyed with an average plant community dominance of 19% [64]; for comparison, +IWM plots had an average standardized percent abundance (plant community dominance) of 31% in our study. Moreover, IWM may be similar enough to other native macrophytes in our study lakes in physical structure and nutrient uptake patterns (e.g., _Creatophyllum_) that they are redundant, so that the main effects we observed were of total macrophyte standing crop on other primary producers [21; 29]. Although IWM did not uniformly alter standing crop or productivity of other primary producers between plots as we hypothesized, stepwise multiple linear regression across all plots suggested that increasing macrophyte standing crop decreased benthic periphyton GPP and chla, and increased epiphyte GPP and standing crops of all other primary producers, consistent with the complex direct and indirect interactions hypothesized in Figure 1. The negative effect of macrophytes on benthic periphyton is likely a result of shading of the benthos (e.g., [65]), while positive effects of macrophytes on epiphytes and phytoplankton could be due to increases in habitat for attachment and/or changes in nutrient availability mediated by macrophytes (Figure 1) [66; 67]. However, we must note that nutrient concentrations and light availability were not consistently identified as drivers of primary producer production and biomass across lakes; water clarity positively influenced benthic periphyton AFDM only. TDN was negatively associated with epiphyte GPP, chla, and AFDM, but positively associated with phytoplankton chla. The absence of these relationships could be because (1) our plot-level measurements of these water column parameters were too coarse to detect fine scale changes in light and water chemistry by macrophytes that affected interactions with other primary producers; or perhaps (2) because the macrophyte's effects on other primary producers was due to other mechanisms, such as changes in physical habitat, patterns of water turbulence, or disturbance [15]. Water temperature was also identified as a predictor of some biomass and productivity components, which was expected, as water temperature is a primary control on many biological and physiological processes in lake ecosystems, and water temperature is controlled in lakes by regional climate, landscape characteristics,and lake morphology; we did not observe nor did we hypothesize that macrophyte presence would alter this fundamental lake characteristic (Figure 1). The overall effects of macrophyte standing crops on other primary producers suggest that effects of IWM invasion on primary production may be stronger where IWM establishes in an area that was previously unvegetated, rather than when it replaces native species in established macrophyte stands. For example, invasions of IWM into unvegetated areas have been reported in the Tennessee Valley Association reservoirs and Lake Opinion, Ontario [3; 68]. In these situations, the creation of a macrophyte canopy where there was not one previously may be the primary effect of IWM invasion, instead of changes in the characteristics of the macrophyte assemblages.