Field data and sediment characteristics
Both OC and N content in the sediment were higher at the deeper stations, i.e. > 30 m water depth (6.5 ± 0.2 % OC and 0.9 ± 0.02 % N) when compared to the shallow stations, i.e. < 30 m water depth (1.8 ± 0.1 % OC and 0.2 ± 0.02 % N; Mann-Whitney U tests, U = 0, P = 0.000007 for both; Table 1). Hereafter the shallow stations will therefore be referred to as “Low OC” and the deep stations as “High OC”. Pore water NH4+ and PO43- extracted from the top 0–2 cm sediment layer at the seven sampled stations (Table 1 & Figure 1,n = 3 for each station) correlated positively with water depth (both P < 0.01, Pearson correlations, r = 0.83 and r = 0.64, respectively). NH4+ was significantly higher at High OC stations (308 ± 19.8 µg/l, n = 12) compared to the Low OC stations (196 ± 14.3 µg/l (mean ± SE, n = 9; Mann-Whitney U test,U = 10, P = 0.002). Similarly, pore water PO43- was significantly higher at High OC stations (32.2 ± 7.0 µg/l) compared to the Low OC (4.4 ± 0.5 µg/l; Mann-Whitney U test, U = 1, P = 0.000140). A full list of abiotic data for all stations is available in Supplementary Data 2.