Benthic nitrogen fluxes along a depth gradient in a microtidal fjord: The role of denitrification and microphytobenthos

Abstract

In littoral sediments, microphytobenthic (MPB) nitrogen assimilation often exceeds nitrogen removal by denitrification, partly because MPB activity suppresses denitrification. Little is known about the balance between these two processes at sublittoral depths. Benthic pigment composition, light and dark oxygen, and nutrient fluxes (NO3−, NH4+, dissolved organic nitrogen (DON), PO43−, Si(OH)4), as well as denitrification were measured between 1 and 15 m in depth in Gullmar Fjord (Skagerrak) in spring and autumn. The hypothesis was that the assimilation/denitrification ratio would decrease with depth, along with decreasing MPB activity caused by light limitation. MPB photosynthesis occurred along the entire depth gradient, although sediments were net autotrophic only above 5 m. Inorganic nitrogen (DIN) (and silica) flux changed along the depth gradient, the general pattern being sediment uptake at ≤5 m and efflux at ≥10 m depth. DON flux (~50% of total dissolved nitrogen flux) showed a less clear pattern. Two trends regarding DIN fluxes and denitrification—significant light effects and negative correlations with gross primary productivity—showed that MPB activity influenced nitrogen (N) turnover. Although denitrification increased with depth, rates remained low (<0.4 mmol N m−2 d−1), and MPB assimilation (0.2–3.6 mmol N m−2 d−1) exceeded or equaled denitrification. MPB incorporated ~35% of the remineralized N along the depth gradient, whereas denitrification removed ~20%. Thus, the influence of MPB on benthic nitrogen turnover, denitrification included, extends to sublittoral depths. Further, denitrification does not necessarily remove more N in the deeper, heterotrophic part of the photic zone, compared to the littoral, autotrophic zone.