Control of N Concentrations in Cape Cod Estuaries by Nitrogen Loads, Season, and Down-Estuary Transit: Assessment by Conventional and Effect-Size Statistics

To assess effects of N loads, season, and down-estuary transit on estuarine concentrations of nitrate (NO 3) , ammonium (NH 4 ), dissolved organic nitrogen (DON), and total dissolved nitrogen (TDN), we sampled three estuaries within Waquoit Bay, MA (USA), subject to different N loads, from spring to fall, at nine stations spanning the salinity range. Conventional statistical analysis on the basis of entire data for the three estuaries suggested significant effects of N loads on concentrations, but comparisons with effect size measures suggested that the effects were minor, largely an artifactual result from the large number of samples. Constraining variation by binning data into weekly means, and partitioning data by season (summer vs. spring-fall) and down-estuary transit (based on salinity) improved interpretations, allowing emergence of detectable increases in NO 3 concentrations in estuaries subject to larger N loads, confirmed interactions between N load and season (seasonal increases in NO 3 larger in estuaries subject to higher N loads), and down-estuary gradients (highest NO 3 concentrations in fresher reaches and significant down-estuary decreases in NO 3 ). NH 4 and DON were less responsive to N loads, with rapid uptake of NH 4 and seasonal trajectories of DON subject to within-estuary features. TDN concentrations decreased down-estuary, with 13–36% of loss attributable to within-estuary interception, a considerable effect in spite of short water residence times. Management implications include the need to sample seasonally and cover the salinity gradient, checking conventional statistical analyses of pooled data from large databases, using effect size measures to ensure meaningful biological and management results, and attention to the limited lability of DON often included in considering estuarine TDN and N loads.