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Seasonality of nearshore marine snow in the southern North Sea Michael Fettweis1, Matthias Baeye1, Dimitri Van der Zande1, Dries Van den Eynde1, Byung Joon Lee2 1 Royal Belgian Institute of Natural Sciences - Operational Directorate Natural Environment, Gulledelle 100, B-1200 Brussels, Belgium 2Kyungpook National University, School of Constructional and Environmental Engineering, 2559 Gyeongsang-daero, Sangju, Gyeongbuk, 742-711, South Korea s: Seasonal pattern in SPM and Chi concentration The suspended particulate matter (SPM) concentration in the Belgian coastal area (southern North Sea) is inversely correlated with chlorophyll (Chi) concentration. During winter SPM concentration is high and Chi concentration low and vice versa during summer. REMOTE SENSING DATA IN SITU DATA: MOW1 (Belgian coastal area) SPM (g/m3) 10 15 20 25 30 i 11 1 1 1 1 1 1 1 Ii I.I.MU 5 10 CHLfmg/m3) 15 SPM winter SPM summer TSo CHL winter CHli summer T d O © J Urbain (Belgian Navy) Hypothesis A: Physical Forcing The water clears as a result of reducing wind stirring and sediment re-suspension after winter storms. This physical clearing of the water induces higher light levels and contributes to the subsequent onset of the spring bloom. According to this hypothesis, the turbidity decreases BEFORE the spring bloom. Chicken or Egg ? ? Hypothesis B: Biology The turbidity decreases because of increased aggregation of particles brought about by biological activity (TEP concentration) during the bloom and biological activity in summer. According to this hypothesis, the bloom itself is the trigger for higher transparency and the water clears DURING and AFTER the spring bloom. Hypothesis testing: data 2011 The hypothesis have been tested using in situ data of SPM concentration, floe size, turbulence; remote sensing data of chlorophyll concentration and meteorological and wave data at MOW1 site (Belgian nearshore area). Physical forcing proxy: Kolmogorov microscale of turbulence (high values = low turbulence; low values = high turbulence) Biology proxy: Chi concentration as indication of TEP concentration Physical forcing: Kolmogorov microscale of turbulence O 0.6 y 0.4 SPM volume concentration c 0.5 2003-2011 2011 Biology: CHL concentration a? 10 H r e Size distribution: Frequen Primary Particles, Flocculi, Micro-,'MacrofIocs £ 50 ■I 20 actual data lowpass filtered 120 150 180 210 Day of the year 2011 240 270 300 330 360 PP Flocculi Micro Macro 15 o sz <D o- <D 10 - winter summer Primary Particles Microflocs 3 40 40 Flocculi Macroflocs all >0.65mm 0.65-0.25 <0.25mm all >0.65mm 0.65-0.25 <0.25mm - 3 0 - 20 10 0 40 30 - 20 - 10 0 Frequency of primary particles, flocculi, microflocs and macroflocs for the summer and winter season and according to Kolmogorov microscale Winter less macroflocs more PP and flocculi Summer less PP & flocculi more macroflocs Conclusions Wind strengths and wave heights have a seasonal signal, but these are not sufficient to explain the large differences observed in SPM concentration. Biomass effects increase the strength of macroflocs rather than their size. The results highlight the transformation of mainly microflocs and flocculi in winter towards more muddy marine snow with larger amounts of macroflocs in spring and summer. The larger fraction of macroflocs reduces the SPM concentrations as they settle faster, increases light condition in the surface layer and enhances algae growth. It is mainly the biological activity in spring and summer that lead to pattern in physical forcing. Further reading: Fettweis M, Baeye M, Van der Zande D, Van den Eynde D, Lee BJ. 2014. Ship Time RV Belgica was provided by BELSPO and RBINS-Operatlonal Directorate Natural Environment.
