## 3 Results  
### Nutrient Fluxes and Sediment Properties  
Plastics significantly and uniquely impacted key ecosystem processes (Table 1 and Figure 1). While GPP did not differ among treatments (Table 1, Figure 1A, \(p\) = 0.31) and all sediments were net heterotrophic and generally similar, PET-containing sediments had a lower NEM than the control (Table 1, Figure 1B, \(p\) = 0.02). Along with decreased NEM, sediments containing PET had marginally deeper sediment oxygen penetration (Table 1, Figure 1C). Sediments containing plastic had a more developed and distinct anoxic layer beneath the oxidized surface layer, especially in treatments containing SBR and PVC (Table 1, Figure 1D). The presence of SBR and PVC was also associated with unique effects on nutrient cycling: nitrate fluxes were negligible in all treatments except for SBR (Figure 1E, \(p\) = 0.04), and both ammonium (Figure 1F, \(p\) = 0.03) and phosphate (Figure 1G, \(p\) = 0.03) were released from the sediments to the water column in all treatments except for PVC, where sediment uptake was significant. Sediments containing PVC had decreased potential denitrification (Figure 1H; \(p\) = 0.58) and slightly higher benthic Chl \(a\) content (Figure 1I, \(p\) = 0.08). Tubificid worm abundance was substantially higher in PET treatments (Figure 1J, \(p\) = 0.01). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Variable** & **Plastic Type** & \\ \hline  & **F/X\({}^{2}\)** & \(p\) \\ \hline GPP & 1.32 & 0.31 \\ NEM & 4.97 & **0.02** \\ Oxic Depth & 3.38 & 0.05 \\ Anoxic Depth & 2.82 & 0.08 \\ Daily NO\({}_{\text{a}}^{-}\) & 7.91 * & **0.04** \\ Daily NH\({}_{\text{a}}^{+}\) & 8.93 & **0.03** \\ Daily PO\({}_{\text{a}}^{\text{3}-}\) & 3.93 * & **0.03** \\ PDNF & 2.84 & 0.58 \\ Chl \(a\) & 2.84 & 0.08 \\ Worm Count & 5.59 & **0.01** \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Results of one-way analysis of variance or Kruskal–Wallis tests examining the effect of plastic on net ecosystem metabolism (NEM), gross primary production (GPP), sediment oxygen penetration (oxic depth), sediment anoxic depth (anoxic depth), daily fluxes of nitrate, ammonium and phosphate, colonization of sediments by ubificid worms (Worm Count), microalgal chlorophyll \(a\) (Chl _a_) content, and potential denitrification (PDNF). Significant effects (_p_ < 0.05) are bolded. Values with asterisks indicate a Chi-square value from Kruskal–Wallis tests used for non-parametric data. All degrees of freedom are 3. Figure 1: Gross primary production (GPP; **A**); net ecosystem metabolism (NEM; **B**); sediment oxygen penetration (**C**); sediment anoxic depth (**D**); daily sediment-water column fluxes for NO\({}_{\text{3}}\)\({}^{-}\) (**E**); NH\({}_{\text{4}}^{+}\) (**F**); and PO\({}_{\text{4}}^{\text{3}-}\) (**G**); potential denitrification (**H**), benthic microalgal Chl \(a\) (**I**), and tubificid worm abundance (**J**), measured after 30 d. Values are mean \(\pm\) SE, n = 4. Negative flux values indicate the uptake of the solute by the sediments. Asterisks represent significant differences compared to the control based on post hoc testing. The impact of microalgal activity was demonstrated in some cases by differences in flux rates between light and dark. In control treatments only, the uptake of nitrate in the light was modestly higher than in the dark (\(p\) = 0.07). Ammonium uptake in the PVC treatment occurred only in the light, with release in the dark (Table 2, \(p\) = 0.08). No differences were observed between light and dark fluxes of phosphate (\(p\) > 0.1 for all). The PCA resulted in two main components that together explained 49% of the data variability (28.7% Component 1; 20.3% Component 2). The biplot of the PCA shows distinct grouping based on the plastic type, with PVC and PET separating from the control and SBR treatments (Figure 2). PET separates primarily on Component 1, which is defined primarily by reduced NEM, a deeper oxic layer, and worm abundance (Table 3).