Hi David,

One way to do that:

(* Color and thickness for each line *)
lineColors = ColorData["SolarColors"] /@ Rescale[{1, 2, 3, 4}]
lineWidths = {Thickness[0.01], Automatic, Automatic, Automatic}

Plot[{4 t, 3 t, 2 t, t}, {t, 0, 3}, AxesOrigin -> {0, 0}, 
 FrameLabel -> {"t (years)", "C (mg/l)"}, PlotTheme -> "Scientific", 
 PerformanceGoal -> "Quality", PlotRange -> Full, 
 PlotLegends -> LineLegend[{"P", "N", "DO", "TSS"}, LegendFunction -> Frame], 
 PlotStyle -> Transpose[{lineColors, lineWidths}]]

David,

Is this what you mean?

Plot[{4 t, 3 t, 2 t, t}, {t, 0, 3}, AxesOrigin -> {0, 0}, 
 FrameLabel -> {"t (years)", "C (mg/l)"}, PlotTheme -> "Scientific", 
 PerformanceGoal -> "Quality", PlotRange -> Full, 
 PlotLegends -> 
  LineLegend[{"P", "N", "DO", "TSS"}, LegendFunction -> Frame], 
 PlotStyle -> {Thickness[0.01], Automatic, Automatic, Automatic}]