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

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 landbased 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 landbased 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 5 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 5 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. Primary production (PP) is an important factor in structuring marine ecosystems, and changes in PP in response to increased nutrient loading have been identified as being responsible for symptoms of eutrophication in coastal marine systems (Nixon 1995; Cloern 2001; Smith 2003). After recognizing the relationship between anthropogenic nutrient loading and eutrophication, many countries have initiated programs to reduce nutrient enrichment (Boesch 2002) with the expectation that marine PP will respond to a reduction in land-based nutrient loading. In some areas, enough data have now been collected to allow researchers to examine the extent to which this expectation has been realized. The Baltic Sea Transition Zone (BSTZ), which comprises the Kattegat and the Belt Seas and thus forms the connection between the Baltic Sea and the Skagerrak, is such an area. It is a shallow, stratified, and temperate marine system with dynamic hydrography. Surface salinity varies in the region from 10 to 14 in the southern part of the region and from 20 to 25 in the northern Kattegat