# Effects of Invasive Watermilfoil on Primary Production in Littoral Zones of North-Temperate Lakes
## 4 Discussion  
We found that presence of IWM had limited effects on primary producer standing crops and rates of primary production in the littoral zones in our study of north-temperate lakes. Because we conducted this field study late in the summer, when IWM tends to reach seasonal peaks in biomass [61], we expected to find the greatest differences in standing crop and productivity between our +IWM and -IWM study plots. Contrary to our hypotheses, we found that standing crops, primary production by all primary producer groups, and plot-level GPP and ER rates were not different between our plots. However, regression analyses identified the macrophyte standing crop as a predictor of benthic periphyton GPP, epiphyte GPP, and standing crops of all other primary producers, along with water temperature, nutrient concentrations, and water clarity. These findings agree with other studies that have found that macrophyte standing crops can be an important control on whole-lake processes [17; 55]. Due to the phenology and growth characteristics of IWM, we predicted that IWM would alter productivity and standing crops of primary producer groups through a suite of direct and indirect interactions (Figure 1). Other studies have documented reduced macrophyte diversity or complete displacement of native species when IWM forms dense canopies [5; 6]. Yet we found that IWM was generally intermixed with native macrophytes in our study plots: IWM dominance averaged 31% of the standardized abundance of +IWM plots, and none of the +IWM plots had thick surface canopies characterized by sprawling shoots at the water's surface. It is important to note that although we found no uniform difference in the overall macrophyte standing crops or species diversity between +IWM  
Figure 7: Proportions of plot GPP by epiphyte, phytoplankton, benthic periphyton, and estimated macrophytes in paired +IWM and -IWM plots. Proportions of GPP from macrophytes that were set to zero are indicated with (*); see related methods in Section 2.7.  
and \(-\)IWM plots, average standing crops of native macrophytes were 1-fold lower in +IWM plots where IWM comprised 7-56% of the standardized macrophyte abundance, compared to \(-\)IWM plots where IWM comprised 0-2%. One important caveat of our study is that the paired-plot design could obscure subtle or gradual effects of IWM density or relative abundance within or among our study lakes. For example, proportions of total production contributed by benthic periphyton GPP were ca. 27% lower in +IWM vs. \(-\)IWM plots in Torch Lake, Iron Lake, and Islington Bay (Figure 7), which were the three lakes with IWM standardized abundance \(\geq\) 30% and dense IWM stems in the upper water column (Table 3). However, because of the paired study design, it is unclear if this pattern was driven by differences in IWM or total macrophyte biomass. Subsequent studies could employ a gradient of IWM density to more clearly separate the effects of IWM vs. overall macrophyte assemblage effects on littoral primary producers. Similarly, a gradient study could identify thresholds where IWM abundance and/or canopy structure becomes a strong control on direct and indirect interactions hypothesized in Figure 1.  
Lack of IWM dominance in our study plots could be due to our study lakes being located at the northern edge of the range of IWM, where it may be limited by light and annual water temperatures [62]. Because IWM is evergreen [8], it is possible that IWM could have larger effects on macrophyte assemblages or other primary producers during times of year that were not included in our study, although we have observed strong seasonality in the absolute and relative abundance of IWM in waterbodies of this study region. In particular, greater contributions of IWM to GPP might be expected during the rapid growth phase in spring and early summer.