Changes in the vertical distribution of primary production in response to land‐based nitrogen loading

Abstract

Anthropogenic nitrogen (N)‐loading has decreased significantly in the Baltic Sea Transition Zone over the past two decades. We show that the vertical distribution of primary production (PP) changed as a function of land‐based N‐loading using 1385 water column photosynthesis estimates, in which photosynthetic parameters were determined both in the surface water layer and in the pycnocline‐bottom layer (PBL) at six stations near the Danish coast between 1998 and 2012. Total annual PP and surface layer PP (SPP) correlate positively with land‐based N‐loading from Denmark (p < 0.003). The percentage of annual PP occurring in the PBL (denoted as deep primary production, DPP) varied annually between 6% and 30% (mean = 17%). The absolute magnitude of the DPP, as well as its relative proportion of total water column PP, correlates negatively with N‐loading (p < 0.009 and p < 0.0003, respectively). Thus, SPP decreases in response to decreased N‐loading, while DPP increases. Land‐based N‐loadings also correlate positively with the light attenuation coefficient (R2 = 0.39, p < 0.05), which may in part explain the response in DPP to changes in N‐loading. DPP occurs in active phytoplankton communities acclimated and/or adapted to low light and producing oxygen in the PBL water.