Estimates of Export Production in the Coastal Northern Baltic Sea

Abstract

Production and sedimentation of organic matter were studied during the spring and summer in 1983 and 1988, in the open sea zone off the SW coast of Finland. Net primary productivity was measured in situ (¹⁴C incorporation), and sedimentation rates of particulate organic carbon (POC) and nitrogen (PON) were measured using cylindrical sediment traps moored below the euphotic layer in a 50-meter deep basin. Export production (P_e) was measured directly as sedimentation of POC and PON. Measurements were compared to the P_e values calculated by the power model presented by Wassmann (1990) for the boreal coastal zone of the North Atlantic. Potential new production during the spring was assumed to be equivalent to the depletion of nitrate from the whole water column. The estimated f-ratio (P_e/P_total) based on sedimentation measurements and NO₃-N depletion (in spring) varied between 0.43 to 0.85 during the spring and about 0.37 to 0.43 for the whole study periods. P_e calculated by the power model (Wassmann, 1990) gave considerably lower values. The time scale of the study (5–6 months) might not have been sufficient for accomplishing steady-state of the pelagic system, hence explaining a part of the discrepancy. Moreover, the validity of the power model for the coastal area of Northern Baltic Sea may have been hampered by shallowness and topography of the coastal area. The weak density stratification of the water column and movements of water masses caused elevated resuspension of organic matter from the benthos. Also, methodological differences both in primary production and sedimentation measurements probably influenced the estimates of f-ratio. In conclusion, caution is advised when sedimentation measurements are used to estimate export production in the shallow coastal areas, which are often subjected to intense mixing and nutrient loading from terrestrial, anthropogenic sources.