# Effects of Invasive Watermilfoil on Primary Production in Littoral Zones of North-Temperate Lakes
## 3 Results
### Primary Production Rates  
GPP of primary producers was not different between +IWM and -IWM plots, and across all plots GPP for benthic periphyton and phytoplankton were higher in plots with warmer water temperature. GPP rates of epiphytes, phytoplankton, and benthic periphyton were not significantly different between +IWM and -IWM plots (epiphytes, \(t\) = 0.41, df = 5, \(p\) = 0.70; phytoplankton, \(t\) = 1.44, df = 5, \(p\) = 0.21; benthic periphyton, \(t\) = \(-\)1.25, df = 5, \(p\) = 0.27) (Figure 4). Benthic periphyton GPP rates were the most variable with high standard errors (on average 56.9 \(\pm\) 56.8, with SE up to 217.6 mg O\({}_{2}\) m\({}^{-2}\) h\({}^{-1}\)). Stepwise multiple linear regression identified significant models that explained about half of the variation in benthic periphyton and phytoplankton GPP (Table 4). Benthic periphyton GPP was negatively related to macrophyte standing crop and positively related to water temperature with R\({}^{2}_{\text{,adj}}\) = 0.50 and \(p\) = 0.02, while phytoplankton GPP was positively related to water temperature with R\({}^{2}_{\text{,adj}}\) = 0.46 and \(p\) = 0.009 (Table 4). Figure 4: Gross primary production (GPP) measured in light-dark bottles, and scaled per plot area of (**a**) epiphyte, (**b**) phytoplankton, and (**c**) benthic periphyton. Means \(\pm\) 1 standard error are represented by symbols with error bars. Shaded symbols represent +IWM plots, and unshaded symbols represent \(-\)IWM plots for each waterbody. Lines connecting symbols illustrate differences in paired plots. Symbols are offset horizontally to aid in interpretation of error bars. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Waterbody**} & \multirow{2}{*}{**Plot Type**} & \begin{tabular}{c} **IWM Canopy** \\ **Description 1** \\ \end{tabular} & \
\begin{tabular}{c} **Abundance of** \\ **IWM (\%)** \\ \end{tabular} &
\begin{tabular}{c} **Standardized Abundance** \\ **of IWM (\%)** 2** \\ \end{tabular} \\ \hline \multirow{2}{*}{Horsshoe Lake} & \(-\)IWM & None & 4 & 2 \\  & +IWM & Sparse & 84 & 7 \\ \hline \multirow{2}{*}{Lake St. Helen} & \(-\)IWM & None & 0 & 0 \\  & +IWM & Sparse & 35 & 23 \\ \hline \multirow{2}{*}{Islington Bay of Lake Huron} & \(-\)IWM & None & 0 & 0 \\  & +IWM & Dense & 65 & 30 \\ \hline \multirow{2}{*}{Sturgeon Sloughs of Portage Lake} & \(-\)IWM & None & 0 & 0 \\  & +IWM & Sparse & 27 & 15 \\ \hline \multirow{2}{*}{Iron Lake} & \(-\)IWM & None & 5 & 2 \\  & +IWM & Dense & 68 & 56 \\ \hline \multirow{2}{*}{Torch Lake} & \(-\)IWM & None & 0 & 0 \\  & +IWM & Dense & 89 & 52 \\ \hline \end{tabular}  
* None = no visually observed IWM. Dense = visually observed IWM stems crowded the upper water column. Sparse = visually observed low IWM stem densities in the upper water column. 2 Standardized abundance was calculated as standing crop of IWM divided by the maximum standing crop value of any macrophyte species measured in this study (Lake St. Helen \(-\)IWM, _Potamogton rob robinsii_, 294 g/m\({}^{2}\), Table S1).  
\end{table} 
Table 3: Dominance of IWM by abundance and standardized abundance in each plot.