Background N-compound levels (Table 5) were similar between treatments with insignificant differences in total-N (p = 0.380, average 107 mg N L−1, range 104–110), not related to mesh size. Nitrate levels were identical between and within treatments (p = 0.626)and did not change during the experimental period (105 ± 3 mgN L−1). During the experiment, the alkalinity measured before daily NaHCO3 addition decreased from 1.5 to 0.6 meq L−1 over time.Observed variations were in the same range through all systems and treatments (p = 0.963).