Changes in spatial distribution of primary production, photosynthetic pigments and phytoplankton species composition during two surveys in the German bight

Abstract

A grid of 61 stations was sampled twice in the German Bight. At all stations dissolved nutrients (Si, P, N), fluorescence, temperature and salinity were measured along the vertical at surface, 5 m, 10 m, and 10 m intervals till the bottom. At selected stations photosynthetic rates were measured with 14C method (both surveys) and pigment with a HPLC method (first survey). Phytoplankton species composition was dominated by small diatoms, mainly Leptocylindrus minimus, flagellates, both dinoflagellates and choanoflagellates and near shore blooms of Phaeocystis pouchetii. Primary production and chlorophyll-a showed a linear correlation (r=0.90, n=23). Primary production showed highest rates along the North and East Frisian islands, where phytoplankton was dominated by Phaeocystis and Leptocylindrus. These coastal stations were completely mixed whereas a large part already showed stratification due to salinity and temperature. Thermocline and pycnocline were at about 10 m depth. No differences in oxygen content of surface or bottom waters were observed. Silicate and phophate concentrations were very low whereas nitrogen still showed very high concentrations. Low hypolimnetic oxygen concentrations are only possible when transport of primary produced organic matter is transported to stratified regions later in summer. Rapid growth of blooms was observed within a weeks period. More primary production measurements in the German Bight should be done to allow for budget calculations of oxygen consumption below the thermocline.